Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2880 insertions, 0 deletions

diff --git a/intl/icu/source/common/utext.cpp b/intl/icu/source/common/utext.cpp
new file mode 100644
index 0000000000..eb0a6984bd
--- /dev/null
+++ b/intl/icu/source/common/utext.cpp
@@ -0,0 +1,2880 @@
+// © 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+/*
+*******************************************************************************
+*
+*   Copyright (C) 2005-2016, International Business Machines
+*   Corporation and others.  All Rights Reserved.
+*
+*******************************************************************************
+*   file name:  utext.cpp
+*   encoding:   UTF-8
+*   tab size:   8 (not used)
+*   indentation:4
+*
+*   created on: 2005apr12
+*   created by: Markus W. Scherer
+*/
+
+#include <cstddef>
+
+#include "unicode/utypes.h"
+#include "unicode/ustring.h"
+#include "unicode/unistr.h"
+#include "unicode/chariter.h"
+#include "unicode/utext.h"
+#include "unicode/utf.h"
+#include "unicode/utf8.h"
+#include "unicode/utf16.h"
+#include "ustr_imp.h"
+#include "cmemory.h"
+#include "cstring.h"
+#include "uassert.h"
+#include "putilimp.h"
+
+U_NAMESPACE_USE
+
+#define I32_FLAG(bitIndex) ((int32_t)1<<(bitIndex))
+
+
+static UBool
+utext_access(UText *ut, int64_t index, UBool forward) {
+    return ut->pFuncs->access(ut, index, forward);
+}
+
+
+
+U_CAPI UBool U_EXPORT2
+utext_moveIndex32(UText *ut, int32_t delta) {
+    UChar32  c;
+    if (delta > 0) {
+        do {
+            if(ut->chunkOffset>=ut->chunkLength && !utext_access(ut, ut->chunkNativeLimit, true)) {
+                return false;
+            }
+            c = ut->chunkContents[ut->chunkOffset];
+            if (U16_IS_SURROGATE(c)) {
+                c = utext_next32(ut);
+                if (c == U_SENTINEL) {
+                    return false;
+                }
+            } else {
+                ut->chunkOffset++;
+            }
+        } while(--delta>0);
+
+    } else if (delta<0) {
+        do {
+            if(ut->chunkOffset<=0 && !utext_access(ut, ut->chunkNativeStart, false)) {
+                return false;
+            }
+            c = ut->chunkContents[ut->chunkOffset-1];
+            if (U16_IS_SURROGATE(c)) {
+                c = utext_previous32(ut);
+                if (c == U_SENTINEL) {
+                    return false;
+                }
+            } else {
+                ut->chunkOffset--;
+            }
+        } while(++delta<0);
+    }
+
+    return true;
+}
+
+
+U_CAPI int64_t U_EXPORT2
+utext_nativeLength(UText *ut) {
+    return ut->pFuncs->nativeLength(ut);
+}
+
+
+U_CAPI UBool U_EXPORT2
+utext_isLengthExpensive(const UText *ut) {
+    UBool r = (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE)) != 0;
+    return r;
+}
+
+
+U_CAPI int64_t U_EXPORT2
+utext_getNativeIndex(const UText *ut) {
+    if(ut->chunkOffset <= ut->nativeIndexingLimit) {
+        return ut->chunkNativeStart+ut->chunkOffset;
+    } else {
+        return ut->pFuncs->mapOffsetToNative(ut);
+    }
+}
+
+
+U_CAPI void U_EXPORT2
+utext_setNativeIndex(UText *ut, int64_t index) {
+    if(index<ut->chunkNativeStart || index>=ut->chunkNativeLimit) {
+        // The desired position is outside of the current chunk.
+        // Access the new position.  Assume a forward iteration from here,
+        // which will also be optimimum for a single random access.
+        // Reverse iterations may suffer slightly.
+        ut->pFuncs->access(ut, index, true);
+    } else if((int32_t)(index - ut->chunkNativeStart) <= ut->nativeIndexingLimit) {
+        // utf-16 indexing.
+        ut->chunkOffset=(int32_t)(index-ut->chunkNativeStart);
+    } else {
+         ut->chunkOffset=ut->pFuncs->mapNativeIndexToUTF16(ut, index);
+    }
+    // The convention is that the index must always be on a code point boundary.
+    // Adjust the index position if it is in the middle of a surrogate pair.
+    if (ut->chunkOffset<ut->chunkLength) {
+        char16_t c= ut->chunkContents[ut->chunkOffset];
+        if (U16_IS_TRAIL(c)) {
+            if (ut->chunkOffset==0) {
+                ut->pFuncs->access(ut, ut->chunkNativeStart, false);
+            }
+            if (ut->chunkOffset>0) {
+                char16_t lead = ut->chunkContents[ut->chunkOffset-1];
+                if (U16_IS_LEAD(lead)) {
+                    ut->chunkOffset--;
+                }
+            }
+        }
+    }
+}
+
+
+
+U_CAPI int64_t U_EXPORT2
+utext_getPreviousNativeIndex(UText *ut) {
+    //
+    //  Fast-path the common case.
+    //     Common means current position is not at the beginning of a chunk
+    //     and the preceding character is not supplementary.
+    //
+    int32_t i = ut->chunkOffset - 1;
+    int64_t result;
+    if (i >= 0) {
+        char16_t c = ut->chunkContents[i];
+        if (U16_IS_TRAIL(c) == false) {
+            if (i <= ut->nativeIndexingLimit) {
+                result = ut->chunkNativeStart + i;
+            } else {
+                ut->chunkOffset = i;
+                result = ut->pFuncs->mapOffsetToNative(ut);
+                ut->chunkOffset++;
+            }
+            return result;
+        }
+    }
+
+    // If at the start of text, simply return 0.
+    if (ut->chunkOffset==0 && ut->chunkNativeStart==0) {
+        return 0;
+    }
+
+    // Harder, less common cases.  We are at a chunk boundary, or on a surrogate.
+    //    Keep it simple, use other functions to handle the edges.
+    //
+    utext_previous32(ut);
+    result = UTEXT_GETNATIVEINDEX(ut);
+    utext_next32(ut);
+    return result;
+}
+
+
+//
+//  utext_current32.  Get the UChar32 at the current position.
+//                    UText iteration position is always on a code point boundary,
+//                    never on the trail half of a surrogate pair.
+//
+U_CAPI UChar32 U_EXPORT2
+utext_current32(UText *ut) {
+    UChar32  c;
+    if (ut->chunkOffset==ut->chunkLength) {
+        // Current position is just off the end of the chunk.
+        if (ut->pFuncs->access(ut, ut->chunkNativeLimit, true) == false) {
+            // Off the end of the text.
+            return U_SENTINEL;
+        }
+    }
+
+    c = ut->chunkContents[ut->chunkOffset];
+    if (U16_IS_LEAD(c) == false) {
+        // Normal, non-supplementary case.
+        return c;
+    }
+
+    //
+    //  Possible supplementary char.
+    //
+    UChar32   trail = 0;
+    UChar32   supplementaryC = c;
+    if ((ut->chunkOffset+1) < ut->chunkLength) {
+        // The trail surrogate is in the same chunk.
+        trail = ut->chunkContents[ut->chunkOffset+1];
+    } else {
+        //  The trail surrogate is in a different chunk.
+        //     Because we must maintain the iteration position, we need to switch forward
+        //     into the new chunk, get the trail surrogate, then revert the chunk back to the
+        //     original one.
+        //     An edge case to be careful of:  the entire text may end with an unpaired
+        //        leading surrogate.  The attempt to access the trail will fail, but
+        //        the original position before the unpaired lead still needs to be restored.
+        int64_t  nativePosition = ut->chunkNativeLimit;
+        if (ut->pFuncs->access(ut, nativePosition, true)) {
+            trail = ut->chunkContents[ut->chunkOffset];
+        }
+        UBool r = ut->pFuncs->access(ut, nativePosition, false);  // reverse iteration flag loads preceding chunk
+        U_ASSERT(r);
+        // Here we need to restore chunkOffset since the access functions were called with
+        // chunkNativeLimit but that is not where we were (we were 1 code unit before the
+        // limit). Restoring was originally added in ICU-4669 but did not support access
+        // functions that changed the chunk size, the following does.
+        ut->chunkOffset = ut->chunkLength - 1;
+        if(!r) {
+            return U_SENTINEL;
+        }
+    }
+
+    if (U16_IS_TRAIL(trail)) {
+        supplementaryC = U16_GET_SUPPLEMENTARY(c, trail);
+    }
+    return supplementaryC;
+
+}
+
+
+U_CAPI UChar32 U_EXPORT2
+utext_char32At(UText *ut, int64_t nativeIndex) {
+    UChar32 c = U_SENTINEL;
+
+    // Fast path the common case.
+    if (nativeIndex>=ut->chunkNativeStart && nativeIndex < ut->chunkNativeStart + ut->nativeIndexingLimit) {
+        ut->chunkOffset = (int32_t)(nativeIndex - ut->chunkNativeStart);
+        c = ut->chunkContents[ut->chunkOffset];
+        if (U16_IS_SURROGATE(c) == false) {
+            return c;
+        }
+    }
+
+
+    utext_setNativeIndex(ut, nativeIndex);
+    if (nativeIndex>=ut->chunkNativeStart && ut->chunkOffset<ut->chunkLength) {
+        c = ut->chunkContents[ut->chunkOffset];
+        if (U16_IS_SURROGATE(c)) {
+            // For surrogates, let current32() deal with the complications
+            //    of supplementaries that may span chunk boundaries.
+            c = utext_current32(ut);
+        }
+    }
+    return c;
+}
+
+
+U_CAPI UChar32 U_EXPORT2
+utext_next32(UText *ut) {
+    UChar32       c;
+
+    if (ut->chunkOffset >= ut->chunkLength) {
+        if (ut->pFuncs->access(ut, ut->chunkNativeLimit, true) == false) {
+            return U_SENTINEL;
+        }
+    }
+
+    c = ut->chunkContents[ut->chunkOffset++];
+    if (U16_IS_LEAD(c) == false) {
+        // Normal case, not supplementary.
+        //   (A trail surrogate seen here is just returned as is, as a surrogate value.
+        //    It cannot be part of a pair.)
+        return c;
+    }
+
+    if (ut->chunkOffset >= ut->chunkLength) {
+        if (ut->pFuncs->access(ut, ut->chunkNativeLimit, true) == false) {
+            // c is an unpaired lead surrogate at the end of the text.
+            // return it as it is.
+            return c;
+        }
+    }
+    UChar32 trail = ut->chunkContents[ut->chunkOffset];
+    if (U16_IS_TRAIL(trail) == false) {
+        // c was an unpaired lead surrogate, not at the end of the text.
+        // return it as it is (unpaired).  Iteration position is on the
+        // following character, possibly in the next chunk, where the
+        //  trail surrogate would have been if it had existed.
+        return c;
+    }
+
+    UChar32 supplementary = U16_GET_SUPPLEMENTARY(c, trail);
+    ut->chunkOffset++;   // move iteration position over the trail surrogate.
+    return supplementary;
+    }
+
+
+U_CAPI UChar32 U_EXPORT2
+utext_previous32(UText *ut) {
+    UChar32       c;
+
+    if (ut->chunkOffset <= 0) {
+        if (ut->pFuncs->access(ut, ut->chunkNativeStart, false) == false) {
+            return U_SENTINEL;
+        }
+    }
+    ut->chunkOffset--;
+    c = ut->chunkContents[ut->chunkOffset];
+    if (U16_IS_TRAIL(c) == false) {
+        // Normal case, not supplementary.
+        //   (A lead surrogate seen here is just returned as is, as a surrogate value.
+        //    It cannot be part of a pair.)
+        return c;
+    }
+
+    if (ut->chunkOffset <= 0) {
+        if (ut->pFuncs->access(ut, ut->chunkNativeStart, false) == false) {
+            // c is an unpaired trail surrogate at the start of the text.
+            // return it as it is.
+            return c;
+        }
+    }
+
+    UChar32 lead = ut->chunkContents[ut->chunkOffset-1];
+    if (U16_IS_LEAD(lead) == false) {
+        // c was an unpaired trail surrogate, not at the end of the text.
+        // return it as it is (unpaired).  Iteration position is at c
+        return c;
+    }
+
+    UChar32 supplementary = U16_GET_SUPPLEMENTARY(lead, c);
+    ut->chunkOffset--;   // move iteration position over the lead surrogate.
+    return supplementary;
+}
+
+
+
+U_CAPI UChar32 U_EXPORT2
+utext_next32From(UText *ut, int64_t index) {
+    UChar32       c      = U_SENTINEL;
+
+    if(index<ut->chunkNativeStart || index>=ut->chunkNativeLimit) {
+        // Desired position is outside of the current chunk.
+        if(!ut->pFuncs->access(ut, index, true)) {
+            // no chunk available here
+            return U_SENTINEL;
+        }
+    } else if (index - ut->chunkNativeStart  <= (int64_t)ut->nativeIndexingLimit) {
+        // Desired position is in chunk, with direct 1:1 native to UTF16 indexing
+        ut->chunkOffset = (int32_t)(index - ut->chunkNativeStart);
+    } else {
+        // Desired position is in chunk, with non-UTF16 indexing.
+        ut->chunkOffset = ut->pFuncs->mapNativeIndexToUTF16(ut, index);
+    }
+
+    c = ut->chunkContents[ut->chunkOffset++];
+    if (U16_IS_SURROGATE(c)) {
+        // Surrogates.  Many edge cases.  Use other functions that already
+        //              deal with the problems.
+        utext_setNativeIndex(ut, index);
+        c = utext_next32(ut);
+    }
+    return c;
+}
+
+
+U_CAPI UChar32 U_EXPORT2
+utext_previous32From(UText *ut, int64_t index) {
+    //
+    //  Return the character preceding the specified index.
+    //  Leave the iteration position at the start of the character that was returned.
+    //
+    UChar32     cPrev;    // The character preceding cCurr, which is what we will return.
+
+    // Address the chunk containing the position preceding the incoming index
+    // A tricky edge case:
+    //   We try to test the requested native index against the chunkNativeStart to determine
+    //    whether the character preceding the one at the index is in the current chunk.
+    //    BUT, this test can fail with UTF-8 (or any other multibyte encoding), when the
+    //    requested index is on something other than the first position of the first char.
+    //
+    if(index<=ut->chunkNativeStart || index>ut->chunkNativeLimit) {
+        // Requested native index is outside of the current chunk.
+        if(!ut->pFuncs->access(ut, index, false)) {
+            // no chunk available here
+            return U_SENTINEL;
+        }
+    } else if(index - ut->chunkNativeStart <= (int64_t)ut->nativeIndexingLimit) {
+        // Direct UTF-16 indexing.
+        ut->chunkOffset = (int32_t)(index - ut->chunkNativeStart);
+    } else {
+        ut->chunkOffset=ut->pFuncs->mapNativeIndexToUTF16(ut, index);
+        if (ut->chunkOffset==0 && !ut->pFuncs->access(ut, index, false)) {
+            // no chunk available here
+            return U_SENTINEL;
+        }
+    }
+
+    //
+    // Simple case with no surrogates.
+    //
+    ut->chunkOffset--;
+    cPrev = ut->chunkContents[ut->chunkOffset];
+
+    if (U16_IS_SURROGATE(cPrev)) {
+        // Possible supplementary.  Many edge cases.
+        // Let other functions do the heavy lifting.
+        utext_setNativeIndex(ut, index);
+        cPrev = utext_previous32(ut);
+    }
+    return cPrev;
+}
+
+
+U_CAPI int32_t U_EXPORT2
+utext_extract(UText *ut,
+             int64_t start, int64_t limit,
+             char16_t *dest, int32_t destCapacity,
+             UErrorCode *status) {
+                 return ut->pFuncs->extract(ut, start, limit, dest, destCapacity, status);
+             }
+
+
+
+U_CAPI UBool U_EXPORT2
+utext_equals(const UText *a, const UText *b) {
+    if (a==nullptr || b==nullptr ||
+        a->magic != UTEXT_MAGIC ||
+        b->magic != UTEXT_MAGIC) {
+            // Null or invalid arguments don't compare equal to anything.
+            return false;
+    }
+
+    if (a->pFuncs != b->pFuncs) {
+        // Different types of text providers.
+        return false;
+    }
+
+    if (a->context != b->context) {
+        // Different sources (different strings)
+        return false;
+    }
+    if (utext_getNativeIndex(a) != utext_getNativeIndex(b)) {
+        // Different current position in the string.
+        return false;
+    }
+
+    return true;
+}
+
+U_CAPI UBool U_EXPORT2
+utext_isWritable(const UText *ut)
+{
+    UBool b = (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_WRITABLE)) != 0;
+    return b;
+}
+
+
+U_CAPI void U_EXPORT2
+utext_freeze(UText *ut) {
+    // Zero out the WRITABLE flag.
+    ut->providerProperties &= ~(I32_FLAG(UTEXT_PROVIDER_WRITABLE));
+}
+
+
+U_CAPI UBool U_EXPORT2
+utext_hasMetaData(const UText *ut)
+{
+    UBool b = (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_HAS_META_DATA)) != 0;
+    return b;
+}
+
+
+
+U_CAPI int32_t U_EXPORT2
+utext_replace(UText *ut,
+             int64_t nativeStart, int64_t nativeLimit,
+             const char16_t *replacementText, int32_t replacementLength,
+             UErrorCode *status)
+{
+    if (U_FAILURE(*status)) {
+        return 0;
+    }
+    if ((ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_WRITABLE)) == 0) {
+        *status = U_NO_WRITE_PERMISSION;
+        return 0;
+    }
+    int32_t i = ut->pFuncs->replace(ut, nativeStart, nativeLimit, replacementText, replacementLength, status);
+    return i;
+}
+
+U_CAPI void U_EXPORT2
+utext_copy(UText *ut,
+          int64_t nativeStart, int64_t nativeLimit,
+          int64_t destIndex,
+          UBool move,
+          UErrorCode *status)
+{
+    if (U_FAILURE(*status)) {
+        return;
+    }
+    if ((ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_WRITABLE)) == 0) {
+        *status = U_NO_WRITE_PERMISSION;
+        return;
+    }
+    ut->pFuncs->copy(ut, nativeStart, nativeLimit, destIndex, move, status);
+}
+
+
+
+U_CAPI UText * U_EXPORT2
+utext_clone(UText *dest, const UText *src, UBool deep, UBool readOnly, UErrorCode *status) {
+    if (U_FAILURE(*status)) {
+        return dest;
+    }
+    UText *result = src->pFuncs->clone(dest, src, deep, status);
+    if (U_FAILURE(*status)) {
+        return result;
+    }
+    if (result == nullptr) {
+        *status = U_MEMORY_ALLOCATION_ERROR;
+        return result;
+    }
+    if (readOnly) {
+        utext_freeze(result);
+    }
+    return result;
+}
+
+
+
+//------------------------------------------------------------------------------
+//
+//   UText common functions implementation
+//
+//------------------------------------------------------------------------------
+
+//
+//  UText.flags bit definitions
+//
+enum {
+    UTEXT_HEAP_ALLOCATED  = 1,      //  1 if ICU has allocated this UText struct on the heap.
+                                    //  0 if caller provided storage for the UText.
+
+    UTEXT_EXTRA_HEAP_ALLOCATED = 2, //  1 if ICU has allocated extra storage as a separate
+                                    //     heap block.
+                                    //  0 if there is no separate allocation.  Either no extra
+                                    //     storage was requested, or it is appended to the end
+                                    //     of the main UText storage.
+
+    UTEXT_OPEN = 4                  //  1 if this UText is currently open
+                                    //  0 if this UText is not open.
+};
+
+
+//
+//  Extended form of a UText.  The purpose is to aid in computing the total size required
+//    when a provider asks for a UText to be allocated with extra storage.
+
+struct ExtendedUText {
+    UText               ut;
+    std::max_align_t    extension;
+};
+
+static const UText emptyText = UTEXT_INITIALIZER;
+
+U_CAPI UText * U_EXPORT2
+utext_setup(UText *ut, int32_t extraSpace, UErrorCode *status) {
+    if (U_FAILURE(*status)) {
+        return ut;
+    }
+
+    if (ut == nullptr) {
+        // We need to heap-allocate storage for the new UText
+        int32_t spaceRequired = sizeof(UText);
+        if (extraSpace > 0) {
+            spaceRequired = sizeof(ExtendedUText) + extraSpace - sizeof(std::max_align_t);
+        }
+        ut = (UText *)uprv_malloc(spaceRequired);
+        if (ut == nullptr) {
+            *status = U_MEMORY_ALLOCATION_ERROR;
+            return nullptr;
+        } else {
+            *ut = emptyText;
+            ut->flags |= UTEXT_HEAP_ALLOCATED;
+            if (spaceRequired>0) {
+                ut->extraSize = extraSpace;
+                ut->pExtra    = &((ExtendedUText *)ut)->extension;
+            }
+        }
+    } else {
+        // We have been supplied with an already existing UText.
+        // Verify that it really appears to be a UText.
+        if (ut->magic != UTEXT_MAGIC) {
+            *status = U_ILLEGAL_ARGUMENT_ERROR;
+            return ut;
+        }
+        // If the ut is already open and there's a provider supplied close
+        //   function, call it.
+        if ((ut->flags & UTEXT_OPEN) && ut->pFuncs->close != nullptr)  {
+            ut->pFuncs->close(ut);
+        }
+        ut->flags &= ~UTEXT_OPEN;
+
+        // If extra space was requested by our caller, check whether
+        //   sufficient already exists, and allocate new if needed.
+        if (extraSpace > ut->extraSize) {
+            // Need more space.  If there is existing separately allocated space,
+            //   delete it first, then allocate new space.
+            if (ut->flags & UTEXT_EXTRA_HEAP_ALLOCATED) {
+                uprv_free(ut->pExtra);
+                ut->extraSize = 0;
+            }
+            ut->pExtra = uprv_malloc(extraSpace);
+            if (ut->pExtra == nullptr) {
+                *status = U_MEMORY_ALLOCATION_ERROR;
+            } else {
+                ut->extraSize = extraSpace;
+                ut->flags |= UTEXT_EXTRA_HEAP_ALLOCATED;
+            }
+        }
+    }
+    if (U_SUCCESS(*status)) {
+        ut->flags |= UTEXT_OPEN;
+
+        // Initialize all remaining fields of the UText.
+        //
+        ut->context             = nullptr;
+        ut->chunkContents       = nullptr;
+        ut->p                   = nullptr;
+        ut->q                   = nullptr;
+        ut->r                   = nullptr;
+        ut->a                   = 0;
+        ut->b                   = 0;
+        ut->c                   = 0;
+        ut->chunkOffset         = 0;
+        ut->chunkLength         = 0;
+        ut->chunkNativeStart    = 0;
+        ut->chunkNativeLimit    = 0;
+        ut->nativeIndexingLimit = 0;
+        ut->providerProperties  = 0;
+        ut->privA               = 0;
+        ut->privB               = 0;
+        ut->privC               = 0;
+        ut->privP               = nullptr;
+        if (ut->pExtra!=nullptr && ut->extraSize>0)
+            uprv_memset(ut->pExtra, 0, ut->extraSize);
+
+    }
+    return ut;
+}
+
+
+U_CAPI UText * U_EXPORT2
+utext_close(UText *ut) {
+    if (ut==nullptr ||
+        ut->magic != UTEXT_MAGIC ||
+        (ut->flags & UTEXT_OPEN) == 0)
+    {
+        // The supplied ut is not an open UText.
+        // Do nothing.
+        return ut;
+    }
+
+    // If the provider gave us a close function, call it now.
+    // This will clean up anything allocated specifically by the provider.
+    if (ut->pFuncs->close != nullptr) {
+        ut->pFuncs->close(ut);
+    }
+    ut->flags &= ~UTEXT_OPEN;
+
+    // If we (the framework) allocated the UText or subsidiary storage,
+    //   delete it.
+    if (ut->flags & UTEXT_EXTRA_HEAP_ALLOCATED) {
+        uprv_free(ut->pExtra);
+        ut->pExtra = nullptr;
+        ut->flags &= ~UTEXT_EXTRA_HEAP_ALLOCATED;
+        ut->extraSize = 0;
+    }
+
+    // Zero out function table of the closed UText.  This is a defensive move,
+    //   intended to cause applications that inadvertently use a closed
+    //   utext to crash with null pointer errors.
+    ut->pFuncs        = nullptr;
+
+    if (ut->flags & UTEXT_HEAP_ALLOCATED) {
+        // This UText was allocated by UText setup.  We need to free it.
+        // Clear magic, so we can detect if the user messes up and immediately
+        //  tries to reopen another UText using the deleted storage.
+        ut->magic = 0;
+        uprv_free(ut);
+        ut = nullptr;
+    }
+    return ut;
+}
+
+
+
+
+//
+// invalidateChunk   Reset a chunk to have no contents, so that the next call
+//                   to access will cause new data to load.
+//                   This is needed when copy/move/replace operate directly on the
+//                   backing text, potentially putting it out of sync with the
+//                   contents in the chunk.
+//
+static void
+invalidateChunk(UText *ut) {
+    ut->chunkLength = 0;
+    ut->chunkNativeLimit = 0;
+    ut->chunkNativeStart = 0;
+    ut->chunkOffset = 0;
+    ut->nativeIndexingLimit = 0;
+}
+
+//
+// pinIndex        Do range pinning on a native index parameter.
+//                 64 bit pinning is done in place.
+//                 32 bit truncated result is returned as a convenience for
+//                        use in providers that don't need 64 bits.
+static int32_t
+pinIndex(int64_t &index, int64_t limit) {
+    if (index<0) {
+        index = 0;
+    } else if (index > limit) {
+        index = limit;
+    }
+    return (int32_t)index;
+}
+
+
+U_CDECL_BEGIN
+
+//
+// Pointer relocation function,
+//   a utility used by shallow clone.
+//   Adjust a pointer that refers to something within one UText (the source)
+//   to refer to the same relative offset within a another UText (the target)
+//
+static void adjustPointer(UText *dest, const void **destPtr, const UText *src) {
+    // convert all pointers to (char *) so that byte address arithmetic will work.
+    char  *dptr = (char *)*destPtr;
+    char  *dUText = (char *)dest;
+    char  *sUText = (char *)src;
+
+    if (dptr >= (char *)src->pExtra && dptr < ((char*)src->pExtra)+src->extraSize) {
+        // target ptr was to something within the src UText's pExtra storage.
+        //   relocate it into the target UText's pExtra region.
+        *destPtr = ((char *)dest->pExtra) + (dptr - (char *)src->pExtra);
+    } else if (dptr>=sUText && dptr < sUText+src->sizeOfStruct) {
+        // target ptr was pointing to somewhere within the source UText itself.
+        //   Move it to the same offset within the target UText.
+        *destPtr = dUText + (dptr-sUText);
+    }
+}
+
+
+//
+//  Clone.  This is a generic copy-the-utext-by-value clone function that can be
+//          used as-is with some utext types, and as a helper by other clones.
+//
+static UText * U_CALLCONV
+shallowTextClone(UText * dest, const UText * src, UErrorCode * status) {
+    if (U_FAILURE(*status)) {
+        return nullptr;
+    }
+    int32_t  srcExtraSize = src->extraSize;
+
+    //
+    // Use the generic text_setup to allocate storage if required.
+    //
+    dest = utext_setup(dest, srcExtraSize, status);
+    if (U_FAILURE(*status)) {
+        return dest;
+    }
+
+    //
+    //  flags (how the UText was allocated) and the pointer to the
+    //   extra storage must retain the values in the cloned utext that
+    //   were set up by utext_setup.  Save them separately before
+    //   copying the whole struct.
+    //
+    void *destExtra = dest->pExtra;
+    int32_t flags   = dest->flags;
+
+
+    //
+    //  Copy the whole UText struct by value.
+    //  Any "Extra" storage is copied also.
+    //
+    int sizeToCopy = src->sizeOfStruct;
+    if (sizeToCopy > dest->sizeOfStruct) {
+        sizeToCopy = dest->sizeOfStruct;
+    }
+    uprv_memcpy(dest, src, sizeToCopy);
+    dest->pExtra = destExtra;
+    dest->flags  = flags;
+    if (srcExtraSize > 0) {
+        uprv_memcpy(dest->pExtra, src->pExtra, srcExtraSize);
+    }
+
+    //
+    // Relocate any pointers in the target that refer to the UText itself
+    //   to point to the cloned copy rather than the original source.
+    //
+    adjustPointer(dest, &dest->context, src);
+    adjustPointer(dest, &dest->p, src);
+    adjustPointer(dest, &dest->q, src);
+    adjustPointer(dest, &dest->r, src);
+    adjustPointer(dest, (const void **)&dest->chunkContents, src);
+
+    // The newly shallow-cloned UText does _not_ own the underlying storage for the text.
+    // (The source for the clone may or may not have owned the text.)
+
+    dest->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT);
+
+    return dest;
+}
+
+
+U_CDECL_END
+
+
+
+//------------------------------------------------------------------------------
+//
+//     UText implementation for UTF-8 char * strings (read-only)
+//     Limitation:  string length must be <= 0x7fffffff in length.
+//                  (length must for in an int32_t variable)
+//
+//         Use of UText data members:
+//              context    pointer to UTF-8 string
+//              utext.b    is the input string length (bytes).
+//              utext.c    Length scanned so far in string
+//                           (for optimizing finding length of zero terminated strings.)
+//              utext.p    pointer to the current buffer
+//              utext.q    pointer to the other buffer.
+//
+//------------------------------------------------------------------------------
+
+// Chunk size.
+//     Must be less than 85 (256/3), because of byte mapping from char16_t indexes to native indexes.
+//     Worst case is three native bytes to one char16_t.  (Supplemenaries are 4 native bytes
+//     to two UChars.)
+//     The longest illegal byte sequence treated as a single error (and converted to U+FFFD)
+//     is a three-byte sequence (truncated four-byte sequence).
+//
+enum { UTF8_TEXT_CHUNK_SIZE=32 };
+
+//
+// UTF8Buf  Two of these structs will be set up in the UText's extra allocated space.
+//          Each contains the char16_t chunk buffer, the to and from native maps, and
+//          header info.
+//
+//     because backwards iteration fills the buffers starting at the end and
+//     working towards the front, the filled part of the buffers may not begin
+//     at the start of the available storage for the buffers.
+//
+//     Buffer size is one bigger than the specified UTF8_TEXT_CHUNK_SIZE to allow for
+//     the last character added being a supplementary, and thus requiring a surrogate
+//     pair.  Doing this is simpler than checking for the edge case.
+//
+
+struct UTF8Buf {
+    int32_t   bufNativeStart;                        // Native index of first char in char16_t buf
+    int32_t   bufNativeLimit;                        // Native index following last char in buf.
+    int32_t   bufStartIdx;                           // First filled position in buf.
+    int32_t   bufLimitIdx;                           // Limit of filled range in buf.
+    int32_t   bufNILimit;                            // Limit of native indexing part of buf
+    int32_t   toUCharsMapStart;                      // Native index corresponding to
+                                                     //   mapToUChars[0].
+                                                     //   Set to bufNativeStart when filling forwards.
+                                                     //   Set to computed value when filling backwards.
+
+    char16_t  buf[UTF8_TEXT_CHUNK_SIZE+4];           // The char16_t buffer.  Requires one extra position beyond the
+                                                     //   the chunk size, to allow for surrogate at the end.
+                                                     //   Length must be identical to mapToNative array, below,
+                                                     //   because of the way indexing works when the array is
+                                                     //   filled backwards during a reverse iteration.  Thus,
+                                                     //   the additional extra size.
+    uint8_t   mapToNative[UTF8_TEXT_CHUNK_SIZE+4];   // map char16_t index in buf to
+                                                     //  native offset from bufNativeStart.
+                                                     //  Requires two extra slots,
+                                                     //    one for a supplementary starting in the last normal position,
+                                                     //    and one for an entry for the buffer limit position.
+    uint8_t   mapToUChars[UTF8_TEXT_CHUNK_SIZE*3+6]; // Map native offset from bufNativeStart to
+                                                     //   corresponding offset in filled part of buf.
+    int32_t   align;
+};
+
+U_CDECL_BEGIN
+
+//
+//   utf8TextLength
+//
+//        Get the length of the string.  If we don't already know it,
+//              we'll need to scan for the trailing  nul.
+//
+static int64_t U_CALLCONV
+utf8TextLength(UText *ut) {
+    if (ut->b < 0) {
+        // Zero terminated string, and we haven't scanned to the end yet.
+        // Scan it now.
+        const char *r = (const char *)ut->context + ut->c;
+        while (*r != 0) {
+            r++;
+        }
+        if ((r - (const char *)ut->context) < 0x7fffffff) {
+            ut->b = (int32_t)(r - (const char *)ut->context);
+        } else {
+            // Actual string was bigger (more than 2 gig) than we
+            //   can handle.  Clip it to 2 GB.
+            ut->b = 0x7fffffff;
+        }
+        ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+    }
+    return ut->b;
+}
+
+
+
+
+
+
+static UBool U_CALLCONV
+utf8TextAccess(UText *ut, int64_t index, UBool forward) {
+    //
+    //  Apologies to those who are allergic to goto statements.
+    //    Consider each goto to a labelled block to be the equivalent of
+    //         call the named block as if it were a function();
+    //         return;
+    //
+    const uint8_t *s8=(const uint8_t *)ut->context;
+    UTF8Buf *u8b = nullptr;
+    int32_t  length = ut->b;         // Length of original utf-8
+    int32_t  ix= (int32_t)index;     // Requested index, trimmed to 32 bits.
+    int32_t  mapIndex = 0;
+    if (index<0) {
+        ix=0;
+    } else if (index > 0x7fffffff) {
+        // Strings with 64 bit lengths not supported by this UTF-8 provider.
+        ix = 0x7fffffff;
+    }
+
+    // Pin requested index to the string length.
+    if (ix>length) {
+        if (length>=0) {
+            ix=length;
+        } else if (ix>=ut->c) {
+            // Zero terminated string, and requested index is beyond
+            //   the region that has already been scanned.
+            //   Scan up to either the end of the string or to the
+            //   requested position, whichever comes first.
+            while (ut->c<ix && s8[ut->c]!=0) {
+                ut->c++;
+            }
+            //  TODO:  support for null terminated string length > 32 bits.
+            if (s8[ut->c] == 0) {
+                // We just found the actual length of the string.
+                //  Trim the requested index back to that.
+                ix     = ut->c;
+                ut->b  = ut->c;
+                length = ut->c;
+                ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+            }
+        }
+    }
+
+    //
+    // Dispatch to the appropriate action for a forward iteration request.
+    //
+    if (forward) {
+        if (ix==ut->chunkNativeLimit) {
+            // Check for normal sequential iteration cases first.
+            if (ix==length) {
+                // Just reached end of string
+                // Don't swap buffers, but do set the
+                //   current buffer position.
+                ut->chunkOffset = ut->chunkLength;
+                return false;
+            } else {
+                // End of current buffer.
+                //   check whether other buffer already has what we need.
+                UTF8Buf *altB = (UTF8Buf *)ut->q;
+                if (ix>=altB->bufNativeStart && ix<altB->bufNativeLimit) {
+                    goto swapBuffers;
+                }
+            }
+        }
+
+        // A random access.  Desired index could be in either or niether buf.
+        // For optimizing the order of testing, first check for the index
+        //    being in the other buffer.  This will be the case for uses that
+        //    move back and forth over a fairly limited range
+        {
+            u8b = (UTF8Buf *)ut->q;   // the alternate buffer
+            if (ix>=u8b->bufNativeStart && ix<u8b->bufNativeLimit) {
+                // Requested index is in the other buffer.
+                goto swapBuffers;
+            }
+            if (ix == length) {
+                // Requested index is end-of-string.
+                //   (this is the case of randomly seeking to the end.
+                //    The case of iterating off the end is handled earlier.)
+                if (ix == ut->chunkNativeLimit) {
+                    // Current buffer extends up to the end of the string.
+                    //   Leave it as the current buffer.
+                    ut->chunkOffset = ut->chunkLength;
+                    return false;
+                }
+                if (ix == u8b->bufNativeLimit) {
+                    // Alternate buffer extends to the end of string.
+                    //   Swap it in as the current buffer.
+                    goto swapBuffersAndFail;
+                }
+
+                // Neither existing buffer extends to the end of the string.
+                goto makeStubBuffer;
+            }
+
+            if (ix<ut->chunkNativeStart || ix>=ut->chunkNativeLimit) {
+                // Requested index is in neither buffer.
+                goto fillForward;
+            }
+
+            // Requested index is in this buffer.
+            u8b = (UTF8Buf *)ut->p;   // the current buffer
+            mapIndex = ix - u8b->toUCharsMapStart;
+            U_ASSERT(mapIndex < (int32_t)sizeof(UTF8Buf::mapToUChars));
+            ut->chunkOffset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
+            return true;
+
+        }
+    }
+
+
+    //
+    // Dispatch to the appropriate action for a
+    //   Backwards Direction iteration request.
+    //
+    if (ix==ut->chunkNativeStart) {
+        // Check for normal sequential iteration cases first.
+        if (ix==0) {
+            // Just reached the start of string
+            // Don't swap buffers, but do set the
+            //   current buffer position.
+            ut->chunkOffset = 0;
+            return false;
+        } else {
+            // Start of current buffer.
+            //   check whether other buffer already has what we need.
+            UTF8Buf *altB = (UTF8Buf *)ut->q;
+            if (ix>altB->bufNativeStart && ix<=altB->bufNativeLimit) {
+                goto swapBuffers;
+            }
+        }
+    }
+
+    // A random access.  Desired index could be in either or niether buf.
+    // For optimizing the order of testing,
+    //    Most likely case:  in the other buffer.
+    //    Second most likely: in neither buffer.
+    //    Unlikely, but must work:  in the current buffer.
+    u8b = (UTF8Buf *)ut->q;   // the alternate buffer
+    if (ix>u8b->bufNativeStart && ix<=u8b->bufNativeLimit) {
+        // Requested index is in the other buffer.
+        goto swapBuffers;
+    }
+    // Requested index is start-of-string.
+    //   (this is the case of randomly seeking to the start.
+    //    The case of iterating off the start is handled earlier.)
+    if (ix==0) {
+        if (u8b->bufNativeStart==0) {
+            // Alternate buffer contains the data for the start string.
+            // Make it be the current buffer.
+            goto swapBuffersAndFail;
+        } else {
+            // Request for data before the start of string,
+            //   neither buffer is usable.
+            //   set up a zero-length buffer.
+            goto makeStubBuffer;
+        }
+    }
+
+    if (ix<=ut->chunkNativeStart || ix>ut->chunkNativeLimit) {
+        // Requested index is in neither buffer.
+        goto fillReverse;
+    }
+
+    // Requested index is in this buffer.
+    //   Set the utf16 buffer index.
+    u8b = (UTF8Buf *)ut->p;
+    mapIndex = ix - u8b->toUCharsMapStart;
+    ut->chunkOffset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
+    if (ut->chunkOffset==0) {
+        // This occurs when the first character in the text is
+        //   a multi-byte UTF-8 char, and the requested index is to
+        //   one of the trailing bytes.  Because there is no preceding ,
+        //   character, this access fails.  We can't pick up on the
+        //   situation sooner because the requested index is not zero.
+        return false;
+    } else {
+        return true;
+    }
+
+
+
+swapBuffers:
+    //  The alternate buffer (ut->q) has the string data that was requested.
+    //  Swap the primary and alternate buffers, and set the
+    //   chunk index into the new primary buffer.
+    {
+        u8b   = (UTF8Buf *)ut->q;
+        ut->q = ut->p;
+        ut->p = u8b;
+        ut->chunkContents       = &u8b->buf[u8b->bufStartIdx];
+        ut->chunkLength         = u8b->bufLimitIdx - u8b->bufStartIdx;
+        ut->chunkNativeStart    = u8b->bufNativeStart;
+        ut->chunkNativeLimit    = u8b->bufNativeLimit;
+        ut->nativeIndexingLimit = u8b->bufNILimit;
+
+        // Index into the (now current) chunk
+        // Use the map to set the chunk index.  It's more trouble than it's worth
+        //    to check whether native indexing can be used.
+        U_ASSERT(ix>=u8b->bufNativeStart);
+        U_ASSERT(ix<=u8b->bufNativeLimit);
+        mapIndex = ix - u8b->toUCharsMapStart;
+        U_ASSERT(mapIndex>=0);
+        U_ASSERT(mapIndex<(int32_t)sizeof(u8b->mapToUChars));
+        ut->chunkOffset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
+
+        return true;
+    }
+
+
+ swapBuffersAndFail:
+    // We got a request for either the start or end of the string,
+    //  with iteration continuing in the out-of-bounds direction.
+    // The alternate buffer already contains the data up to the
+    //  start/end.
+    // Swap the buffers, then return failure, indicating that we couldn't
+    //  make things correct for continuing the iteration in the requested
+    //  direction.  The position & buffer are correct should the
+    //  user decide to iterate in the opposite direction.
+    u8b   = (UTF8Buf *)ut->q;
+    ut->q = ut->p;
+    ut->p = u8b;
+    ut->chunkContents       = &u8b->buf[u8b->bufStartIdx];
+    ut->chunkLength         = u8b->bufLimitIdx - u8b->bufStartIdx;
+    ut->chunkNativeStart    = u8b->bufNativeStart;
+    ut->chunkNativeLimit    = u8b->bufNativeLimit;
+    ut->nativeIndexingLimit = u8b->bufNILimit;
+
+    // Index into the (now current) chunk
+    //  For this function  (swapBuffersAndFail), the requested index
+    //    will always be at either the start or end of the chunk.
+    if (ix==u8b->bufNativeLimit) {
+        ut->chunkOffset = ut->chunkLength;
+    } else  {
+        ut->chunkOffset = 0;
+        U_ASSERT(ix == u8b->bufNativeStart);
+    }
+    return false;
+
+makeStubBuffer:
+    //   The user has done a seek/access past the start or end
+    //   of the string.  Rather than loading data that is likely
+    //   to never be used, just set up a zero-length buffer at
+    //   the position.
+    u8b = (UTF8Buf *)ut->q;
+    u8b->bufNativeStart   = ix;
+    u8b->bufNativeLimit   = ix;
+    u8b->bufStartIdx      = 0;
+    u8b->bufLimitIdx      = 0;
+    u8b->bufNILimit       = 0;
+    u8b->toUCharsMapStart = ix;
+    u8b->mapToNative[0]   = 0;
+    u8b->mapToUChars[0]   = 0;
+    goto swapBuffersAndFail;
+
+
+
+fillForward:
+    {
+        // Move the incoming index to a code point boundary.
+        U8_SET_CP_START(s8, 0, ix);
+
+        // Swap the UText buffers.
+        //  We want to fill what was previously the alternate buffer,
+        //  and make what was the current buffer be the new alternate.
+        UTF8Buf *u8b_swap = (UTF8Buf *)ut->q;
+        ut->q = ut->p;
+        ut->p = u8b_swap;
+
+        int32_t strLen = ut->b;
+        UBool   nulTerminated = false;
+        if (strLen < 0) {
+            strLen = 0x7fffffff;
+            nulTerminated = true;
+        }
+
+        char16_t   *buf = u8b_swap->buf;
+        uint8_t *mapToNative  = u8b_swap->mapToNative;
+        uint8_t *mapToUChars  = u8b_swap->mapToUChars;
+        int32_t  destIx       = 0;
+        int32_t  srcIx        = ix;
+        UBool    seenNonAscii = false;
+        UChar32  c = 0;
+
+        // Fill the chunk buffer and mapping arrays.
+        while (destIx<UTF8_TEXT_CHUNK_SIZE) {
+            c = s8[srcIx];
+            if (c>0 && c<0x80) {
+                // Special case ASCII range for speed.
+                //   zero is excluded to simplify bounds checking.
+                buf[destIx] = (char16_t)c;
+                mapToNative[destIx]    = (uint8_t)(srcIx - ix);
+                mapToUChars[srcIx-ix]  = (uint8_t)destIx;
+                srcIx++;
+                destIx++;
+            } else {
+                // General case, handle everything.
+                if (seenNonAscii == false) {
+                    seenNonAscii = true;
+                    u8b_swap->bufNILimit = destIx;
+                }
+
+                int32_t  cIx      = srcIx;
+                int32_t  dIx      = destIx;
+                int32_t  dIxSaved = destIx;
+                U8_NEXT_OR_FFFD(s8, srcIx, strLen, c);
+                if (c==0 && nulTerminated) {
+                    srcIx--;
+                    break;
+                }
+
+                U16_APPEND_UNSAFE(buf, destIx, c);
+                do {
+                    mapToNative[dIx++] = (uint8_t)(cIx - ix);
+                } while (dIx < destIx);
+
+                do {
+                    mapToUChars[cIx++ - ix] = (uint8_t)dIxSaved;
+                } while (cIx < srcIx);
+            }
+            if (srcIx>=strLen) {
+                break;
+            }
+
+        }
+
+        //  store Native <--> Chunk Map entries for the end of the buffer.
+        //    There is no actual character here, but the index position is valid.
+        mapToNative[destIx]     = (uint8_t)(srcIx - ix);
+        mapToUChars[srcIx - ix] = (uint8_t)destIx;
+
+        //  fill in Buffer descriptor
+        u8b_swap->bufNativeStart     = ix;
+        u8b_swap->bufNativeLimit     = srcIx;
+        u8b_swap->bufStartIdx        = 0;
+        u8b_swap->bufLimitIdx        = destIx;
+        if (seenNonAscii == false) {
+            u8b_swap->bufNILimit     = destIx;
+        }
+        u8b_swap->toUCharsMapStart   = u8b_swap->bufNativeStart;
+
+        // Set UText chunk to refer to this buffer.
+        ut->chunkContents       = buf;
+        ut->chunkOffset         = 0;
+        ut->chunkLength         = u8b_swap->bufLimitIdx;
+        ut->chunkNativeStart    = u8b_swap->bufNativeStart;
+        ut->chunkNativeLimit    = u8b_swap->bufNativeLimit;
+        ut->nativeIndexingLimit = u8b_swap->bufNILimit;
+
+        // For zero terminated strings, keep track of the maximum point
+        //   scanned so far.
+        if (nulTerminated && srcIx>ut->c) {
+            ut->c = srcIx;
+            if (c==0) {
+                // We scanned to the end.
+                //   Remember the actual length.
+                ut->b = srcIx;
+                ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+            }
+        }
+        return true;
+    }
+
+
+fillReverse:
+    {
+        // Move the incoming index to a code point boundary.
+        // Can only do this if the incoming index is somewhere in the interior of the string.
+        //   If index is at the end, there is no character there to look at.
+        if (ix != ut->b) {
+            // Note: this function will only move the index back if it is on a trail byte
+            //       and there is a preceding lead byte and the sequence from the lead 
+            //       through this trail could be part of a valid UTF-8 sequence
+            //       Otherwise the index remains unchanged.
+            U8_SET_CP_START(s8, 0, ix);
+        }
+
+        // Swap the UText buffers.
+        //  We want to fill what was previously the alternate buffer,
+        //  and make what was the current buffer be the new alternate.
+        UTF8Buf *u8b_swap = (UTF8Buf *)ut->q;
+        ut->q = ut->p;
+        ut->p = u8b_swap;
+
+        char16_t   *buf = u8b_swap->buf;
+        uint8_t *mapToNative = u8b_swap->mapToNative;
+        uint8_t *mapToUChars = u8b_swap->mapToUChars;
+        int32_t  toUCharsMapStart = ix - sizeof(UTF8Buf::mapToUChars) + 1;
+        // Note that toUCharsMapStart can be negative. Happens when the remaining
+        // text from current position to the beginning is less than the buffer size.
+        // + 1 because mapToUChars must have a slot at the end for the bufNativeLimit entry.
+        int32_t  destIx = UTF8_TEXT_CHUNK_SIZE+2;   // Start in the overflow region
+                                                    //   at end of buffer to leave room
+                                                    //   for a surrogate pair at the
+                                                    //   buffer start.
+        int32_t  srcIx  = ix;
+        int32_t  bufNILimit = destIx;
+        UChar32   c;
+
+        // Map to/from Native Indexes, fill in for the position at the end of
+        //   the buffer.
+        //
+        mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
+        mapToUChars[srcIx - toUCharsMapStart] = (uint8_t)destIx;
+
+        // Fill the chunk buffer
+        // Work backwards, filling from the end of the buffer towards the front.
+        //
+        while (destIx>2 && (srcIx - toUCharsMapStart > 5) && (srcIx > 0)) {
+            srcIx--;
+            destIx--;
+
+            // Get last byte of the UTF-8 character
+            c = s8[srcIx];
+            if (c<0x80) {
+                // Special case ASCII range for speed.
+                buf[destIx] = (char16_t)c;
+                U_ASSERT(toUCharsMapStart <= srcIx);
+                mapToUChars[srcIx - toUCharsMapStart] = (uint8_t)destIx;
+                mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
+            } else {
+                // General case, handle everything non-ASCII.
+
+                int32_t  sIx      = srcIx;  // ix of last byte of multi-byte u8 char
+
+                // Get the full character from the UTF8 string.
+                //   use code derived from the macros in utf8.h
+                //   Leaves srcIx pointing at the first byte of the UTF-8 char.
+                //
+                c=utf8_prevCharSafeBody(s8, 0, &srcIx, c, -3);
+                // leaves srcIx at first byte of the multi-byte char.
+
+                // Store the character in UTF-16 buffer.
+                if (c<0x10000) {
+                    buf[destIx] = (char16_t)c;
+                    mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
+                } else {
+                    buf[destIx]         = U16_TRAIL(c);
+                    mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
+                    buf[--destIx]       = U16_LEAD(c);
+                    mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
+                }
+
+                // Fill in the map from native indexes to UChars buf index.
+                do {
+                    mapToUChars[sIx-- - toUCharsMapStart] = (uint8_t)destIx;
+                } while (sIx >= srcIx);
+                U_ASSERT(toUCharsMapStart <= (srcIx+1));
+
+                // Set native indexing limit to be the current position.
+                //   We are processing a non-ascii, non-native-indexing char now;
+                //     the limit will be here if the rest of the chars to be
+                //     added to this buffer are ascii.
+                bufNILimit = destIx;
+            }
+        }
+        u8b_swap->bufNativeStart     = srcIx;
+        u8b_swap->bufNativeLimit     = ix;
+        u8b_swap->bufStartIdx        = destIx;
+        u8b_swap->bufLimitIdx        = UTF8_TEXT_CHUNK_SIZE+2;
+        u8b_swap->bufNILimit         = bufNILimit - u8b_swap->bufStartIdx;
+        u8b_swap->toUCharsMapStart   = toUCharsMapStart;
+
+        ut->chunkContents       = &buf[u8b_swap->bufStartIdx];
+        ut->chunkLength         = u8b_swap->bufLimitIdx - u8b_swap->bufStartIdx;
+        ut->chunkOffset         = ut->chunkLength;
+        ut->chunkNativeStart    = u8b_swap->bufNativeStart;
+        ut->chunkNativeLimit    = u8b_swap->bufNativeLimit;
+        ut->nativeIndexingLimit = u8b_swap->bufNILimit;
+        return true;
+    }
+
+}
+
+
+
+//
+//  This is a slightly modified copy of u_strFromUTF8,
+//     Inserts a Replacement Char rather than failing on invalid UTF-8
+//     Removes unnecessary features.
+//
+static char16_t*
+utext_strFromUTF8(char16_t *dest,
+              int32_t destCapacity,
+              int32_t *pDestLength,
+              const char* src,
+              int32_t srcLength,        // required.  NUL terminated not supported.
+              UErrorCode *pErrorCode
+              )
+{
+
+    char16_t *pDest = dest;
+    char16_t *pDestLimit = (dest!=nullptr)?(dest+destCapacity):nullptr;
+    UChar32 ch=0;
+    int32_t index = 0;
+    int32_t reqLength = 0;
+    uint8_t* pSrc = (uint8_t*) src;
+
+
+    while((index < srcLength)&&(pDest<pDestLimit)){
+        ch = pSrc[index++];
+        if(ch <=0x7f){
+            *pDest++=(char16_t)ch;
+        }else{
+            ch=utf8_nextCharSafeBody(pSrc, &index, srcLength, ch, -3);
+            if(U_IS_BMP(ch)){
+                *(pDest++)=(char16_t)ch;
+            }else{
+                *(pDest++)=U16_LEAD(ch);
+                if(pDest<pDestLimit){
+                    *(pDest++)=U16_TRAIL(ch);
+                }else{
+                    reqLength++;
+                    break;
+                }
+            }
+        }
+    }
+    /* donot fill the dest buffer just count the UChars needed */
+    while(index < srcLength){
+        ch = pSrc[index++];
+        if(ch <= 0x7f){
+            reqLength++;
+        }else{
+            ch=utf8_nextCharSafeBody(pSrc, &index, srcLength, ch, -3);
+            reqLength+=U16_LENGTH(ch);
+        }
+    }
+
+    reqLength+=(int32_t)(pDest - dest);
+
+    if(pDestLength){
+        *pDestLength = reqLength;
+    }
+
+    /* Terminate the buffer */
+    u_terminateUChars(dest,destCapacity,reqLength,pErrorCode);
+
+    return dest;
+}
+
+
+
+static int32_t U_CALLCONV
+utf8TextExtract(UText *ut,
+                int64_t start, int64_t limit,
+                char16_t *dest, int32_t destCapacity,
+                UErrorCode *pErrorCode) {
+    if(U_FAILURE(*pErrorCode)) {
+        return 0;
+    }
+    if(destCapacity<0 || (dest==nullptr && destCapacity>0)) {
+        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
+        return 0;
+    }
+    int32_t  length  = ut->b;
+    int32_t  start32 = pinIndex(start, length);
+    int32_t  limit32 = pinIndex(limit, length);
+
+    if(start32>limit32) {
+        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
+        return 0;
+    }
+
+
+    // adjust the incoming indexes to land on code point boundaries if needed.
+    //    adjust by no more than three, because that is the largest number of trail bytes
+    //    in a well formed UTF8 character.
+    const uint8_t *buf = (const uint8_t *)ut->context;
+    int i;
+    if (start32 < ut->chunkNativeLimit) {
+        for (i=0; i<3; i++) {
+            if (U8_IS_SINGLE(buf[start32]) || U8_IS_LEAD(buf[start32]) || start32==0) {
+                break;
+            }
+            start32--;
+        }
+    }
+
+    if (limit32 < ut->chunkNativeLimit) {
+        for (i=0; i<3; i++) {
+            if (U8_IS_SINGLE(buf[limit32]) || U8_IS_LEAD(buf[limit32]) || limit32==0) {
+                break;
+            }
+            limit32--;
+        }
+    }
+
+    // Do the actual extract.
+    int32_t destLength=0;
+    utext_strFromUTF8(dest, destCapacity, &destLength,
+                    (const char *)ut->context+start32, limit32-start32,
+                    pErrorCode);
+    utf8TextAccess(ut, limit32, true);
+    return destLength;
+}
+
+//
+// utf8TextMapOffsetToNative
+//
+// Map a chunk (UTF-16) offset to a native index.
+static int64_t U_CALLCONV
+utf8TextMapOffsetToNative(const UText *ut) {
+    //
+    UTF8Buf *u8b = (UTF8Buf *)ut->p;
+    U_ASSERT(ut->chunkOffset>ut->nativeIndexingLimit && ut->chunkOffset<=ut->chunkLength);
+    int32_t nativeOffset = u8b->mapToNative[ut->chunkOffset + u8b->bufStartIdx] + u8b->toUCharsMapStart;
+    U_ASSERT(nativeOffset >= ut->chunkNativeStart && nativeOffset <= ut->chunkNativeLimit);
+    return nativeOffset;
+}
+
+//
+// Map a native index to the corresponding chunk offset
+//
+static int32_t U_CALLCONV
+utf8TextMapIndexToUTF16(const UText *ut, int64_t index64) {
+    U_ASSERT(index64 <= 0x7fffffff);
+    int32_t index = (int32_t)index64;
+    UTF8Buf *u8b = (UTF8Buf *)ut->p;
+    U_ASSERT(index>=ut->chunkNativeStart+ut->nativeIndexingLimit);
+    U_ASSERT(index<=ut->chunkNativeLimit);
+    int32_t mapIndex = index - u8b->toUCharsMapStart;
+    U_ASSERT(mapIndex < (int32_t)sizeof(UTF8Buf::mapToUChars));
+    int32_t offset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
+    U_ASSERT(offset>=0 && offset<=ut->chunkLength);
+    return offset;
+}
+
+static UText * U_CALLCONV
+utf8TextClone(UText *dest, const UText *src, UBool deep, UErrorCode *status)
+{
+    // First do a generic shallow clone.  Does everything needed for the UText struct itself.
+    dest = shallowTextClone(dest, src, status);
+
+    // For deep clones, make a copy of the string.
+    //  The copied storage is owned by the newly created clone.
+    //
+    // TODO:  There is an issue with using utext_nativeLength().
+    //        That function is non-const in cases where the input was NUL terminated
+    //          and the length has not yet been determined.
+    //        This function (clone()) is const.
+    //        There potentially a thread safety issue lurking here.
+    //
+    if (deep && U_SUCCESS(*status)) {
+        int32_t  len = (int32_t)utext_nativeLength((UText *)src);
+        char *copyStr = (char *)uprv_malloc(len+1);
+        if (copyStr == nullptr) {
+            *status = U_MEMORY_ALLOCATION_ERROR;
+        } else {
+            uprv_memcpy(copyStr, src->context, len+1);
+            dest->context = copyStr;
+            dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT);
+        }
+    }
+    return dest;
+}
+
+
+static void U_CALLCONV
+utf8TextClose(UText *ut) {
+    // Most of the work of close is done by the generic UText framework close.
+    // All that needs to be done here is to delete the UTF8 string if the UText
+    //  owns it.  This occurs if the UText was created by cloning.
+    if (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT)) {
+        char *s = (char *)ut->context;
+        uprv_free(s);
+        ut->context = nullptr;
+    }
+}
+
+U_CDECL_END
+
+
+static const struct UTextFuncs utf8Funcs =
+{
+    sizeof(UTextFuncs),
+    0, 0, 0,             // Reserved alignment padding
+    utf8TextClone,
+    utf8TextLength,
+    utf8TextAccess,
+    utf8TextExtract,
+    nullptr,                /* replace*/
+    nullptr,                /* copy   */
+    utf8TextMapOffsetToNative,
+    utf8TextMapIndexToUTF16,
+    utf8TextClose,
+    nullptr,                // spare 1
+    nullptr,                // spare 2
+    nullptr                 // spare 3
+};
+
+
+static const char gEmptyString[] = {0};
+
+U_CAPI UText * U_EXPORT2
+utext_openUTF8(UText *ut, const char *s, int64_t length, UErrorCode *status) {
+    if(U_FAILURE(*status)) {
+        return nullptr;
+    }
+    if(s==nullptr && length==0) {
+        s = gEmptyString;
+    }
+
+    if(s==nullptr || length<-1 || length>INT32_MAX) {
+        *status=U_ILLEGAL_ARGUMENT_ERROR;
+        return nullptr;
+    }
+
+    ut = utext_setup(ut, sizeof(UTF8Buf) * 2, status);
+    if (U_FAILURE(*status)) {
+        return ut;
+    }
+
+    ut->pFuncs  = &utf8Funcs;
+    ut->context = s;
+    ut->b       = (int32_t)length;
+    ut->c       = (int32_t)length;
+    if (ut->c < 0) {
+        ut->c = 0;
+        ut->providerProperties |= I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+    }
+    ut->p = ut->pExtra;
+    ut->q = (char *)ut->pExtra + sizeof(UTF8Buf);
+    return ut;
+
+}
+
+
+
+
+
+
+
+
+//------------------------------------------------------------------------------
+//
+//     UText implementation wrapper for Replaceable (read/write)
+//
+//         Use of UText data members:
+//            context    pointer to Replaceable.
+//            p          pointer to Replaceable if it is owned by the UText.
+//
+//------------------------------------------------------------------------------
+
+
+
+// minimum chunk size for this implementation: 3
+// to allow for possible trimming for code point boundaries
+enum { REP_TEXT_CHUNK_SIZE=10 };
+
+struct ReplExtra {
+    /*
+     * Chunk UChars.
+     * +1 to simplify filling with surrogate pair at the end.
+     */
+    char16_t s[REP_TEXT_CHUNK_SIZE+1];
+};
+
+
+U_CDECL_BEGIN
+
+static UText * U_CALLCONV
+repTextClone(UText *dest, const UText *src, UBool deep, UErrorCode *status) {
+    // First do a generic shallow clone.  Does everything needed for the UText struct itself.
+    dest = shallowTextClone(dest, src, status);
+
+    // For deep clones, make a copy of the Replaceable.
+    //  The copied Replaceable storage is owned by the newly created UText clone.
+    //  A non-nullptr pointer in UText.p is the signal to the close() function to delete
+    //    it.
+    //
+    if (deep && U_SUCCESS(*status)) {
+        const Replaceable *replSrc = (const Replaceable *)src->context;
+        dest->context = replSrc->clone();
+        dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT);
+
+        // with deep clone, the copy is writable, even when the source is not.
+        dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_WRITABLE);
+    }
+    return dest;
+}
+
+
+static void U_CALLCONV
+repTextClose(UText *ut) {
+    // Most of the work of close is done by the generic UText framework close.
+    // All that needs to be done here is delete the Replaceable if the UText
+    //  owns it.  This occurs if the UText was created by cloning.
+    if (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT)) {
+        Replaceable *rep = (Replaceable *)ut->context;
+        delete rep;
+        ut->context = nullptr;
+    }
+}
+
+
+static int64_t U_CALLCONV
+repTextLength(UText *ut) {
+    const Replaceable *replSrc = (const Replaceable *)ut->context;
+    int32_t  len = replSrc->length();
+    return len;
+}
+
+
+static UBool U_CALLCONV
+repTextAccess(UText *ut, int64_t index, UBool forward) {
+    const Replaceable *rep=(const Replaceable *)ut->context;
+    int32_t length=rep->length();   // Full length of the input text (bigger than a chunk)
+
+    // clip the requested index to the limits of the text.
+    int32_t index32 = pinIndex(index, length);
+    U_ASSERT(index<=INT32_MAX);
+
+
+    /*
+     * Compute start/limit boundaries around index, for a segment of text
+     * to be extracted.
+     * To allow for the possibility that our user gave an index to the trailing
+     * half of a surrogate pair, we must request one extra preceding char16_t when
+     * going in the forward direction.  This will ensure that the buffer has the
+     * entire code point at the specified index.
+     */
+    if(forward) {
+
+        if (index32>=ut->chunkNativeStart && index32<ut->chunkNativeLimit) {
+            // Buffer already contains the requested position.
+            ut->chunkOffset = (int32_t)(index - ut->chunkNativeStart);
+            return true;
+        }
+        if (index32>=length && ut->chunkNativeLimit==length) {
+            // Request for end of string, and buffer already extends up to it.
+            // Can't get the data, but don't change the buffer.
+            ut->chunkOffset = length - (int32_t)ut->chunkNativeStart;
+            return false;
+        }
+
+        ut->chunkNativeLimit = index + REP_TEXT_CHUNK_SIZE - 1;
+        // Going forward, so we want to have the buffer with stuff at and beyond
+        //   the requested index.  The -1 gets us one code point before the
+        //   requested index also, to handle the case of the index being on
+        //   a trail surrogate of a surrogate pair.
+        if(ut->chunkNativeLimit > length) {
+            ut->chunkNativeLimit = length;
+        }
+        // unless buffer ran off end, start is index-1.
+        ut->chunkNativeStart = ut->chunkNativeLimit - REP_TEXT_CHUNK_SIZE;
+        if(ut->chunkNativeStart < 0) {
+            ut->chunkNativeStart = 0;
+        }
+    } else {
+        // Reverse iteration.  Fill buffer with data preceding the requested index.
+        if (index32>ut->chunkNativeStart && index32<=ut->chunkNativeLimit) {
+            // Requested position already in buffer.
+            ut->chunkOffset = index32 - (int32_t)ut->chunkNativeStart;
+            return true;
+        }
+        if (index32==0 && ut->chunkNativeStart==0) {
+            // Request for start, buffer already begins at start.
+            //  No data, but keep the buffer as is.
+            ut->chunkOffset = 0;
+            return false;
+        }
+
+        // Figure out the bounds of the chunk to extract for reverse iteration.
+        // Need to worry about chunk not splitting surrogate pairs, and while still
+        // containing the data we need.
+        // Fix by requesting a chunk that includes an extra char16_t at the end.
+        // If this turns out to be a lead surrogate, we can lop it off and still have
+        //   the data we wanted.
+        ut->chunkNativeStart = index32 + 1 - REP_TEXT_CHUNK_SIZE;
+        if (ut->chunkNativeStart < 0) {
+            ut->chunkNativeStart = 0;
+        }
+
+        ut->chunkNativeLimit = index32 + 1;
+        if (ut->chunkNativeLimit > length) {
+            ut->chunkNativeLimit = length;
+        }
+    }
+
+    // Extract the new chunk of text from the Replaceable source.
+    ReplExtra *ex = (ReplExtra *)ut->pExtra;
+    // UnicodeString with its buffer a writable alias to the chunk buffer
+    UnicodeString buffer(ex->s, 0 /*buffer length*/, REP_TEXT_CHUNK_SIZE /*buffer capacity*/);
+    rep->extractBetween((int32_t)ut->chunkNativeStart, (int32_t)ut->chunkNativeLimit, buffer);
+
+    ut->chunkContents  = ex->s;
+    ut->chunkLength    = (int32_t)(ut->chunkNativeLimit - ut->chunkNativeStart);
+    ut->chunkOffset    = (int32_t)(index32 - ut->chunkNativeStart);
+
+    // Surrogate pairs from the input text must not span chunk boundaries.
+    // If end of chunk could be the start of a surrogate, trim it off.
+    if (ut->chunkNativeLimit < length &&
+        U16_IS_LEAD(ex->s[ut->chunkLength-1])) {
+            ut->chunkLength--;
+            ut->chunkNativeLimit--;
+            if (ut->chunkOffset > ut->chunkLength) {
+                ut->chunkOffset = ut->chunkLength;
+            }
+        }
+
+    // if the first char16_t in the chunk could be the trailing half of a surrogate pair,
+    // trim it off.
+    if(ut->chunkNativeStart>0 && U16_IS_TRAIL(ex->s[0])) {
+        ++(ut->chunkContents);
+        ++(ut->chunkNativeStart);
+        --(ut->chunkLength);
+        --(ut->chunkOffset);
+    }
+
+    // adjust the index/chunkOffset to a code point boundary
+    U16_SET_CP_START(ut->chunkContents, 0, ut->chunkOffset);
+
+    // Use fast indexing for get/setNativeIndex()
+    ut->nativeIndexingLimit = ut->chunkLength;
+
+    return true;
+}
+
+
+
+static int32_t U_CALLCONV
+repTextExtract(UText *ut,
+               int64_t start, int64_t limit,
+               char16_t *dest, int32_t destCapacity,
+               UErrorCode *status) {
+    const Replaceable *rep=(const Replaceable *)ut->context;
+    int32_t  length=rep->length();
+
+    if(U_FAILURE(*status)) {
+        return 0;
+    }
+    if(destCapacity<0 || (dest==nullptr && destCapacity>0)) {
+        *status=U_ILLEGAL_ARGUMENT_ERROR;
+    }
+    if(start>limit) {
+        *status=U_INDEX_OUTOFBOUNDS_ERROR;
+        return 0;
+    }
+
+    int32_t  start32 = pinIndex(start, length);
+    int32_t  limit32 = pinIndex(limit, length);
+
+    // adjust start, limit if they point to trail half of surrogates
+    if (start32<length && U16_IS_TRAIL(rep->charAt(start32)) &&
+        U_IS_SUPPLEMENTARY(rep->char32At(start32))){
+            start32--;
+    }
+    if (limit32<length && U16_IS_TRAIL(rep->charAt(limit32)) &&
+        U_IS_SUPPLEMENTARY(rep->char32At(limit32))){
+            limit32--;
+    }
+
+    length=limit32-start32;
+    if(length>destCapacity) {
+        limit32 = start32 + destCapacity;
+    }
+    UnicodeString buffer(dest, 0, destCapacity); // writable alias
+    rep->extractBetween(start32, limit32, buffer);
+    repTextAccess(ut, limit32, true);
+
+    return u_terminateUChars(dest, destCapacity, length, status);
+}
+
+static int32_t U_CALLCONV
+repTextReplace(UText *ut,
+               int64_t start, int64_t limit,
+               const char16_t *src, int32_t length,
+               UErrorCode *status) {
+    Replaceable *rep=(Replaceable *)ut->context;
+    int32_t oldLength;
+
+    if(U_FAILURE(*status)) {
+        return 0;
+    }
+    if(src==nullptr && length!=0) {
+        *status=U_ILLEGAL_ARGUMENT_ERROR;
+        return 0;
+    }
+    oldLength=rep->length(); // will subtract from new length
+    if(start>limit ) {
+        *status=U_INDEX_OUTOFBOUNDS_ERROR;
+        return 0;
+    }
+
+    int32_t start32 = pinIndex(start, oldLength);
+    int32_t limit32 = pinIndex(limit, oldLength);
+
+    // Snap start & limit to code point boundaries.
+    if (start32<oldLength && U16_IS_TRAIL(rep->charAt(start32)) &&
+        start32>0 && U16_IS_LEAD(rep->charAt(start32-1)))
+    {
+            start32--;
+    }
+    if (limit32<oldLength && U16_IS_LEAD(rep->charAt(limit32-1)) &&
+        U16_IS_TRAIL(rep->charAt(limit32)))
+    {
+            limit32++;
+    }
+
+    // Do the actual replace operation using methods of the Replaceable class
+    UnicodeString replStr((UBool)(length<0), src, length); // read-only alias
+    rep->handleReplaceBetween(start32, limit32, replStr);
+    int32_t newLength = rep->length();
+    int32_t lengthDelta = newLength - oldLength;
+
+    // Is the UText chunk buffer OK?
+    if (ut->chunkNativeLimit > start32) {
+        // this replace operation may have impacted the current chunk.
+        // invalidate it, which will force a reload on the next access.
+        invalidateChunk(ut);
+    }
+
+    // set the iteration position to the end of the newly inserted replacement text.
+    int32_t newIndexPos = limit32 + lengthDelta;
+    repTextAccess(ut, newIndexPos, true);
+
+    return lengthDelta;
+}
+
+
+static void U_CALLCONV
+repTextCopy(UText *ut,
+                int64_t start, int64_t limit,
+                int64_t destIndex,
+                UBool move,
+                UErrorCode *status)
+{
+    Replaceable *rep=(Replaceable *)ut->context;
+    int32_t length=rep->length();
+
+    if(U_FAILURE(*status)) {
+        return;
+    }
+    if (start>limit || (start<destIndex && destIndex<limit))
+    {
+        *status=U_INDEX_OUTOFBOUNDS_ERROR;
+        return;
+    }
+
+    int32_t start32     = pinIndex(start, length);
+    int32_t limit32     = pinIndex(limit, length);
+    int32_t destIndex32 = pinIndex(destIndex, length);
+
+    // TODO:  snap input parameters to code point boundaries.
+
+    if(move) {
+        // move: copy to destIndex, then replace original with nothing
+        int32_t segLength=limit32-start32;
+        rep->copy(start32, limit32, destIndex32);
+        if(destIndex32<start32) {
+            start32+=segLength;
+            limit32+=segLength;
+        }
+        rep->handleReplaceBetween(start32, limit32, UnicodeString());
+    } else {
+        // copy
+        rep->copy(start32, limit32, destIndex32);
+    }
+
+    // If the change to the text touched the region in the chunk buffer,
+    //  invalidate the buffer.
+    int32_t firstAffectedIndex = destIndex32;
+    if (move && start32<firstAffectedIndex) {
+        firstAffectedIndex = start32;
+    }
+    if (firstAffectedIndex < ut->chunkNativeLimit) {
+        // changes may have affected range covered by the chunk
+        invalidateChunk(ut);
+    }
+
+    // Put iteration position at the newly inserted (moved) block,
+    int32_t  nativeIterIndex = destIndex32 + limit32 - start32;
+    if (move && destIndex32>start32) {
+        // moved a block of text towards the end of the string.
+        nativeIterIndex = destIndex32;
+    }
+
+    // Set position, reload chunk if needed.
+    repTextAccess(ut, nativeIterIndex, true);
+}
+
+static const struct UTextFuncs repFuncs =
+{
+    sizeof(UTextFuncs),
+    0, 0, 0,           // Reserved alignment padding
+    repTextClone,
+    repTextLength,
+    repTextAccess,
+    repTextExtract,
+    repTextReplace,
+    repTextCopy,
+    nullptr,              // MapOffsetToNative,
+    nullptr,              // MapIndexToUTF16,
+    repTextClose,
+    nullptr,              // spare 1
+    nullptr,              // spare 2
+    nullptr               // spare 3
+};
+
+
+U_CAPI UText * U_EXPORT2
+utext_openReplaceable(UText *ut, Replaceable *rep, UErrorCode *status)
+{
+    if(U_FAILURE(*status)) {
+        return nullptr;
+    }
+    if(rep==nullptr) {
+        *status=U_ILLEGAL_ARGUMENT_ERROR;
+        return nullptr;
+    }
+    ut = utext_setup(ut, sizeof(ReplExtra), status);
+    if(U_FAILURE(*status)) {
+        return ut;
+    }
+
+    ut->providerProperties = I32_FLAG(UTEXT_PROVIDER_WRITABLE);
+    if(rep->hasMetaData()) {
+        ut->providerProperties |=I32_FLAG(UTEXT_PROVIDER_HAS_META_DATA);
+    }
+
+    ut->pFuncs  = &repFuncs;
+    ut->context =  rep;
+    return ut;
+}
+
+U_CDECL_END
+
+
+
+
+
+
+
+
+//------------------------------------------------------------------------------
+//
+//     UText implementation for UnicodeString (read/write)  and
+//                    for const UnicodeString (read only)
+//             (same implementation, only the flags are different)
+//
+//         Use of UText data members:
+//            context    pointer to UnicodeString
+//            p          pointer to UnicodeString IF this UText owns the string
+//                       and it must be deleted on close().  nullptr otherwise.
+//
+//------------------------------------------------------------------------------
+
+U_CDECL_BEGIN
+
+
+static UText * U_CALLCONV
+unistrTextClone(UText *dest, const UText *src, UBool deep, UErrorCode *status) {
+    // First do a generic shallow clone.  Does everything needed for the UText struct itself.
+    dest = shallowTextClone(dest, src, status);
+
+    // For deep clones, make a copy of the UnicodeSring.
+    //  The copied UnicodeString storage is owned by the newly created UText clone.
+    //  A non-nullptr pointer in UText.p is the signal to the close() function to delete
+    //    the UText.
+    //
+    if (deep && U_SUCCESS(*status)) {
+        const UnicodeString *srcString = (const UnicodeString *)src->context;
+        dest->context = new UnicodeString(*srcString);
+        dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT);
+
+        // with deep clone, the copy is writable, even when the source is not.
+        dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_WRITABLE);
+    }
+    return dest;
+}
+
+static void U_CALLCONV
+unistrTextClose(UText *ut) {
+    // Most of the work of close is done by the generic UText framework close.
+    // All that needs to be done here is delete the UnicodeString if the UText
+    //  owns it.  This occurs if the UText was created by cloning.
+    if (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT)) {
+        UnicodeString *str = (UnicodeString *)ut->context;
+        delete str;
+        ut->context = nullptr;
+    }
+}
+
+
+static int64_t U_CALLCONV
+unistrTextLength(UText *t) {
+    return ((const UnicodeString *)t->context)->length();
+}
+
+
+static UBool U_CALLCONV
+unistrTextAccess(UText *ut, int64_t index, UBool  forward) {
+    int32_t length  = ut->chunkLength;
+    ut->chunkOffset = pinIndex(index, length);
+
+    // Check whether request is at the start or end
+    UBool retVal = (forward && index<length) || (!forward && index>0);
+    return retVal;
+}
+
+
+
+static int32_t U_CALLCONV
+unistrTextExtract(UText *t,
+                  int64_t start, int64_t limit,
+                  char16_t *dest, int32_t destCapacity,
+                  UErrorCode *pErrorCode) {
+    const UnicodeString *us=(const UnicodeString *)t->context;
+    int32_t length=us->length();
+
+    if(U_FAILURE(*pErrorCode)) {
+        return 0;
+    }
+    if(destCapacity<0 || (dest==nullptr && destCapacity>0)) {
+        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
+    }
+    if(start<0 || start>limit) {
+        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
+        return 0;
+    }
+
+    int32_t start32 = start<length ? us->getChar32Start((int32_t)start) : length;
+    int32_t limit32 = limit<length ? us->getChar32Start((int32_t)limit) : length;
+
+    length=limit32-start32;
+    if (destCapacity>0 && dest!=nullptr) {
+        int32_t trimmedLength = length;
+        if(trimmedLength>destCapacity) {
+            trimmedLength=destCapacity;
+        }
+        us->extract(start32, trimmedLength, dest);
+        t->chunkOffset = start32+trimmedLength;
+    } else {
+        t->chunkOffset = start32;
+    }
+    u_terminateUChars(dest, destCapacity, length, pErrorCode);
+    return length;
+}
+
+static int32_t U_CALLCONV
+unistrTextReplace(UText *ut,
+                  int64_t start, int64_t limit,
+                  const char16_t *src, int32_t length,
+                  UErrorCode *pErrorCode) {
+    UnicodeString *us=(UnicodeString *)ut->context;
+    int32_t oldLength;
+
+    if(U_FAILURE(*pErrorCode)) {
+        return 0;
+    }
+    if(src==nullptr && length!=0) {
+        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
+    }
+    if(start>limit) {
+        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
+        return 0;
+    }
+    oldLength=us->length();
+    int32_t start32 = pinIndex(start, oldLength);
+    int32_t limit32 = pinIndex(limit, oldLength);
+    if (start32 < oldLength) {
+        start32 = us->getChar32Start(start32);
+    }
+    if (limit32 < oldLength) {
+        limit32 = us->getChar32Start(limit32);
+    }
+
+    // replace
+    us->replace(start32, limit32-start32, src, length);
+    int32_t newLength = us->length();
+
+    // Update the chunk description.
+    ut->chunkContents    = us->getBuffer();
+    ut->chunkLength      = newLength;
+    ut->chunkNativeLimit = newLength;
+    ut->nativeIndexingLimit = newLength;
+
+    // Set iteration position to the point just following the newly inserted text.
+    int32_t lengthDelta = newLength - oldLength;
+    ut->chunkOffset = limit32 + lengthDelta;
+
+    return lengthDelta;
+}
+
+static void U_CALLCONV
+unistrTextCopy(UText *ut,
+               int64_t start, int64_t limit,
+               int64_t destIndex,
+               UBool move,
+               UErrorCode *pErrorCode) {
+    UnicodeString *us=(UnicodeString *)ut->context;
+    int32_t length=us->length();
+
+    if(U_FAILURE(*pErrorCode)) {
+        return;
+    }
+    int32_t start32 = pinIndex(start, length);
+    int32_t limit32 = pinIndex(limit, length);
+    int32_t destIndex32 = pinIndex(destIndex, length);
+
+    if( start32>limit32 || (start32<destIndex32 && destIndex32<limit32)) {
+        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
+        return;
+    }
+
+    if(move) {
+        // move: copy to destIndex, then remove original
+        int32_t segLength=limit32-start32;
+        us->copy(start32, limit32, destIndex32);
+        if(destIndex32<start32) {
+            start32+=segLength;
+        }
+        us->remove(start32, segLength);
+    } else {
+        // copy
+        us->copy(start32, limit32, destIndex32);
+    }
+
+    // update chunk description, set iteration position.
+    ut->chunkContents = us->getBuffer();
+    if (move==false) {
+        // copy operation, string length grows
+        ut->chunkLength += limit32-start32;
+        ut->chunkNativeLimit = ut->chunkLength;
+        ut->nativeIndexingLimit = ut->chunkLength;
+    }
+
+    // Iteration position to end of the newly inserted text.
+    ut->chunkOffset = destIndex32+limit32-start32;
+    if (move && destIndex32>start32) {
+        ut->chunkOffset = destIndex32;
+    }
+
+}
+
+static const struct UTextFuncs unistrFuncs =
+{
+    sizeof(UTextFuncs),
+    0, 0, 0,             // Reserved alignment padding
+    unistrTextClone,
+    unistrTextLength,
+    unistrTextAccess,
+    unistrTextExtract,
+    unistrTextReplace,
+    unistrTextCopy,
+    nullptr,                // MapOffsetToNative,
+    nullptr,                // MapIndexToUTF16,
+    unistrTextClose,
+    nullptr,                // spare 1
+    nullptr,                // spare 2
+    nullptr                 // spare 3
+};
+
+
+
+U_CDECL_END
+
+
+U_CAPI UText * U_EXPORT2
+utext_openUnicodeString(UText *ut, UnicodeString *s, UErrorCode *status) {
+    ut = utext_openConstUnicodeString(ut, s, status);
+    if (U_SUCCESS(*status)) {
+        ut->providerProperties |= I32_FLAG(UTEXT_PROVIDER_WRITABLE);
+    }
+    return ut;
+}
+
+
+
+U_CAPI UText * U_EXPORT2
+utext_openConstUnicodeString(UText *ut, const UnicodeString *s, UErrorCode *status) {
+    if (U_SUCCESS(*status) && s->isBogus()) {
+        // The UnicodeString is bogus, but we still need to detach the UText
+        //   from whatever it was hooked to before, if anything.
+        utext_openUChars(ut, nullptr, 0, status);
+        *status = U_ILLEGAL_ARGUMENT_ERROR;
+        return ut;
+    }
+    ut = utext_setup(ut, 0, status);
+    //    note:  use the standard (writable) function table for UnicodeString.
+    //           The flag settings disable writing, so having the functions in
+    //           the table is harmless.
+    if (U_SUCCESS(*status)) {
+        ut->pFuncs              = &unistrFuncs;
+        ut->context             = s;
+        ut->providerProperties  = I32_FLAG(UTEXT_PROVIDER_STABLE_CHUNKS);
+        ut->chunkContents       = s->getBuffer();
+        ut->chunkLength         = s->length();
+        ut->chunkNativeStart    = 0;
+        ut->chunkNativeLimit    = ut->chunkLength;
+        ut->nativeIndexingLimit = ut->chunkLength;
+    }
+    return ut;
+}
+
+//------------------------------------------------------------------------------
+//
+//     UText implementation for const char16_t * strings
+//
+//         Use of UText data members:
+//            context    pointer to UnicodeString
+//            a          length.  -1 if not yet known.
+//
+//         TODO:  support 64 bit lengths.
+//
+//------------------------------------------------------------------------------
+
+U_CDECL_BEGIN
+
+
+static UText * U_CALLCONV
+ucstrTextClone(UText *dest, const UText * src, UBool deep, UErrorCode * status) {
+    // First do a generic shallow clone.
+    dest = shallowTextClone(dest, src, status);
+
+    // For deep clones, make a copy of the string.
+    //  The copied storage is owned by the newly created clone.
+    //  A non-nullptr pointer in UText.p is the signal to the close() function to delete
+    //    it.
+    //
+    if (deep && U_SUCCESS(*status)) {
+        U_ASSERT(utext_nativeLength(dest) < INT32_MAX);
+        int32_t  len = (int32_t)utext_nativeLength(dest);
+
+        // The cloned string IS going to be NUL terminated, whether or not the original was.
+        const char16_t *srcStr = (const char16_t *)src->context;
+        char16_t *copyStr = (char16_t *)uprv_malloc((len+1) * sizeof(char16_t));
+        if (copyStr == nullptr) {
+            *status = U_MEMORY_ALLOCATION_ERROR;
+        } else {
+            int64_t i;
+            for (i=0; i<len; i++) {
+                copyStr[i] = srcStr[i];
+            }
+            copyStr[len] = 0;
+            dest->context = copyStr;
+            dest->providerProperties |= I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT);
+        }
+    }
+    return dest;
+}
+
+
+static void U_CALLCONV
+ucstrTextClose(UText *ut) {
+    // Most of the work of close is done by the generic UText framework close.
+    // All that needs to be done here is delete the string if the UText
+    //  owns it.  This occurs if the UText was created by cloning.
+    if (ut->providerProperties & I32_FLAG(UTEXT_PROVIDER_OWNS_TEXT)) {
+        char16_t *s = (char16_t *)ut->context;
+        uprv_free(s);
+        ut->context = nullptr;
+    }
+}
+
+
+
+static int64_t U_CALLCONV
+ucstrTextLength(UText *ut) {
+    if (ut->a < 0) {
+        // null terminated, we don't yet know the length. Scan for it.
+        //    Access is not convenient for doing this
+        //    because the current iteration position can't be changed.
+        const char16_t  *str = (const char16_t *)ut->context;
+        for (;;) {
+            if (str[ut->chunkNativeLimit] == 0) {
+                break;
+            }
+            ut->chunkNativeLimit++;
+        }
+        ut->a = ut->chunkNativeLimit;
+        ut->chunkLength = (int32_t)ut->chunkNativeLimit;
+        ut->nativeIndexingLimit = ut->chunkLength;
+        ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+    }
+    return ut->a;
+}
+
+
+static UBool U_CALLCONV
+ucstrTextAccess(UText *ut, int64_t index, UBool  forward) {
+    const char16_t *str   = (const char16_t *)ut->context;
+
+    // pin the requested index to the bounds of the string,
+    //  and set current iteration position.
+    if (index<0) {
+        index = 0;
+    } else if (index < ut->chunkNativeLimit) {
+        // The request data is within the chunk as it is known so far.
+        // Put index on a code point boundary.
+        U16_SET_CP_START(str, 0, index);
+    } else if (ut->a >= 0) {
+        // We know the length of this string, and the user is requesting something
+        // at or beyond the length.  Pin the requested index to the length.
+        index = ut->a;
+    } else {
+        // Null terminated string, length not yet known, and the requested index
+        //  is beyond where we have scanned so far.
+        //  Scan to 32 UChars beyond the requested index.  The strategy here is
+        //  to avoid fully scanning a long string when the caller only wants to
+        //  see a few characters at its beginning.
+        int32_t scanLimit = (int32_t)index + 32;
+        if ((index + 32)>INT32_MAX || (index + 32)<0 ) {   // note: int64 expression
+            scanLimit = INT32_MAX;
+        }
+
+        int32_t chunkLimit = (int32_t)ut->chunkNativeLimit;
+        for (; chunkLimit<scanLimit; chunkLimit++) {
+            if (str[chunkLimit] == 0) {
+                // We found the end of the string.  Remember it, pin the requested index to it,
+                //  and bail out of here.
+                ut->a = chunkLimit;
+                ut->chunkLength = chunkLimit;
+                ut->nativeIndexingLimit = chunkLimit;
+                if (index >= chunkLimit) {
+                    index = chunkLimit;
+                } else {
+                    U16_SET_CP_START(str, 0, index);
+                }
+
+                ut->chunkNativeLimit = chunkLimit;
+                ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+                goto breakout;
+            }
+        }
+        // We scanned through the next batch of UChars without finding the end.
+        U16_SET_CP_START(str, 0, index);
+        if (chunkLimit == INT32_MAX) {
+            // Scanned to the limit of a 32 bit length.
+            // Forceably trim the overlength string back so length fits in int32
+            //  TODO:  add support for 64 bit strings.
+            ut->a = chunkLimit;
+            ut->chunkLength = chunkLimit;
+            ut->nativeIndexingLimit = chunkLimit;
+            if (index > chunkLimit) {
+                index = chunkLimit;
+            }
+            ut->chunkNativeLimit = chunkLimit;
+            ut->providerProperties &= ~I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+        } else {
+            // The endpoint of a chunk must not be left in the middle of a surrogate pair.
+            // If the current end is on a lead surrogate, back the end up by one.
+            // It doesn't matter if the end char happens to be an unpaired surrogate,
+            //    and it's simpler not to worry about it.
+            if (U16_IS_LEAD(str[chunkLimit-1])) {
+                --chunkLimit;
+            }
+            // Null-terminated chunk with end still unknown.
+            // Update the chunk length to reflect what has been scanned thus far.
+            // That the full length is still unknown is (still) flagged by
+            //    ut->a being < 0.
+            ut->chunkNativeLimit = chunkLimit;
+            ut->nativeIndexingLimit = chunkLimit;
+            ut->chunkLength = chunkLimit;
+        }
+
+    }
+breakout:
+    U_ASSERT(index<=INT32_MAX);
+    ut->chunkOffset = (int32_t)index;
+
+    // Check whether request is at the start or end
+    UBool retVal = (forward && index<ut->chunkNativeLimit) || (!forward && index>0);
+    return retVal;
+}
+
+
+
+static int32_t U_CALLCONV
+ucstrTextExtract(UText *ut,
+                  int64_t start, int64_t limit,
+                  char16_t *dest, int32_t destCapacity,
+                  UErrorCode *pErrorCode)
+{
+    if(U_FAILURE(*pErrorCode)) {
+        return 0;
+    }
+    if(destCapacity<0 || (dest==nullptr && destCapacity>0) || start>limit) {
+        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
+        return 0;
+    }
+
+    //const char16_t *s=(const char16_t *)ut->context;
+    int32_t si, di;
+
+    int32_t start32;
+    int32_t limit32;
+
+    // Access the start.  Does two things we need:
+    //   Pins 'start' to the length of the string, if it came in out-of-bounds.
+    //   Snaps 'start' to the beginning of a code point.
+    ucstrTextAccess(ut, start, true);
+    const char16_t *s=ut->chunkContents;
+    start32 = ut->chunkOffset;
+
+    int32_t strLength=(int32_t)ut->a;
+    if (strLength >= 0) {
+        limit32 = pinIndex(limit, strLength);
+    } else {
+        limit32 = pinIndex(limit, INT32_MAX);
+    }
+    di = 0;
+    for (si=start32; si<limit32; si++) {
+        if (strLength<0 && s[si]==0) {
+            // Just hit the end of a null-terminated string.
+            ut->a = si;               // set string length for this UText
+            ut->chunkNativeLimit    = si;
+            ut->chunkLength         = si;
+            ut->nativeIndexingLimit = si;
+            strLength               = si;
+            limit32                 = si;
+            break;
+        }
+        U_ASSERT(di>=0); /* to ensure di never exceeds INT32_MAX, which must not happen logically */
+        if (di<destCapacity) {
+            // only store if there is space.
+            dest[di] = s[si];
+        } else {
+            if (strLength>=0) {
+                // We have filled the destination buffer, and the string length is known.
+                //  Cut the loop short.  There is no need to scan string termination.
+                di = limit32 - start32;
+                si = limit32;
+                break;
+            }
+        }
+        di++;
+    }
+
+    // If the limit index points to a lead surrogate of a pair,
+    //   add the corresponding trail surrogate to the destination.
+    if (si>0 && U16_IS_LEAD(s[si-1]) &&
+            ((si<strLength || strLength<0)  && U16_IS_TRAIL(s[si])))
+    {
+        if (di<destCapacity) {
+            // store only if there is space in the output buffer.
+            dest[di++] = s[si];
+        }
+        si++;
+    }
+
+    // Put iteration position at the point just following the extracted text
+    if (si <= ut->chunkNativeLimit) {
+        ut->chunkOffset = si;
+    } else {
+        ucstrTextAccess(ut, si, true);
+    }
+
+    // Add a terminating NUL if space in the buffer permits,
+    // and set the error status as required.
+    u_terminateUChars(dest, destCapacity, di, pErrorCode);
+    return di;
+}
+
+static const struct UTextFuncs ucstrFuncs =
+{
+    sizeof(UTextFuncs),
+    0, 0, 0,           // Reserved alignment padding
+    ucstrTextClone,
+    ucstrTextLength,
+    ucstrTextAccess,
+    ucstrTextExtract,
+    nullptr,              // Replace
+    nullptr,              // Copy
+    nullptr,              // MapOffsetToNative,
+    nullptr,              // MapIndexToUTF16,
+    ucstrTextClose,
+    nullptr,              // spare 1
+    nullptr,              // spare 2
+    nullptr,              // spare 3
+};
+
+U_CDECL_END
+
+static const char16_t gEmptyUString[] = {0};
+
+U_CAPI UText * U_EXPORT2
+utext_openUChars(UText *ut, const char16_t *s, int64_t length, UErrorCode *status) {
+    if (U_FAILURE(*status)) {
+        return nullptr;
+    }
+    if(s==nullptr && length==0) {
+        s = gEmptyUString;
+    }
+    if (s==nullptr || length < -1 || length>INT32_MAX) {
+        *status = U_ILLEGAL_ARGUMENT_ERROR;
+        return nullptr;
+    }
+    ut = utext_setup(ut, 0, status);
+    if (U_SUCCESS(*status)) {
+        ut->pFuncs               = &ucstrFuncs;
+        ut->context              = s;
+        ut->providerProperties   = I32_FLAG(UTEXT_PROVIDER_STABLE_CHUNKS);
+        if (length==-1) {
+            ut->providerProperties |= I32_FLAG(UTEXT_PROVIDER_LENGTH_IS_EXPENSIVE);
+        }
+        ut->a                    = length;
+        ut->chunkContents        = s;
+        ut->chunkNativeStart     = 0;
+        ut->chunkNativeLimit     = length>=0? length : 0;
+        ut->chunkLength          = (int32_t)ut->chunkNativeLimit;
+        ut->chunkOffset          = 0;
+        ut->nativeIndexingLimit  = ut->chunkLength;
+    }
+    return ut;
+}
+
+
+//------------------------------------------------------------------------------
+//
+//     UText implementation for text from ICU CharacterIterators
+//
+//         Use of UText data members:
+//            context    pointer to the CharacterIterator
+//            a          length of the full text.
+//            p          pointer to  buffer 1
+//            b          start index of local buffer 1 contents
+//            q          pointer to buffer 2
+//            c          start index of local buffer 2 contents
+//            r          pointer to the character iterator if the UText owns it.
+//                       Null otherwise.
+//
+//------------------------------------------------------------------------------
+#define CIBufSize 16
+
+U_CDECL_BEGIN
+static void U_CALLCONV
+charIterTextClose(UText *ut) {
+    // Most of the work of close is done by the generic UText framework close.
+    // All that needs to be done here is delete the CharacterIterator if the UText
+    //  owns it.  This occurs if the UText was created by cloning.
+    CharacterIterator *ci = (CharacterIterator *)ut->r;
+    delete ci;
+    ut->r = nullptr;
+}
+
+static int64_t U_CALLCONV
+charIterTextLength(UText *ut) {
+    return (int32_t)ut->a;
+}
+
+static UBool U_CALLCONV
+charIterTextAccess(UText *ut, int64_t index, UBool  forward) {
+    CharacterIterator *ci   = (CharacterIterator *)ut->context;
+
+    int32_t clippedIndex = (int32_t)index;
+    if (clippedIndex<0) {
+        clippedIndex=0;
+    } else if (clippedIndex>=ut->a) {
+        clippedIndex=(int32_t)ut->a;
+    }
+    int32_t neededIndex = clippedIndex;
+    if (!forward && neededIndex>0) {
+        // reverse iteration, want the position just before what was asked for.
+        neededIndex--;
+    } else if (forward && neededIndex==ut->a && neededIndex>0) {
+        // Forward iteration, don't ask for something past the end of the text.
+        neededIndex--;
+    }
+
+    // Find the native index of the start of the buffer containing what we want.
+    neededIndex -= neededIndex % CIBufSize;
+
+    char16_t *buf = nullptr;
+    UBool  needChunkSetup = true;
+    int    i;
+    if (ut->chunkNativeStart == neededIndex) {
+        // The buffer we want is already the current chunk.
+        needChunkSetup = false;
+    } else if (ut->b == neededIndex) {
+        // The first buffer (buffer p) has what we need.
+        buf = (char16_t *)ut->p;
+    } else if (ut->c == neededIndex) {
+        // The second buffer (buffer q) has what we need.
+        buf = (char16_t *)ut->q;
+    } else {
+        // Neither buffer already has what we need.
+        // Load new data from the character iterator.
+        // Use the buf that is not the current buffer.
+        buf = (char16_t *)ut->p;
+        if (ut->p == ut->chunkContents) {
+            buf = (char16_t *)ut->q;
+        }
+        ci->setIndex(neededIndex);
+        for (i=0; i<CIBufSize; i++) {
+            buf[i] = ci->nextPostInc();
+            if (i+neededIndex > ut->a) {
+                break;
+            }
+        }
+    }
+
+    // We have a buffer with the data we need.
+    // Set it up as the current chunk, if it wasn't already.
+    if (needChunkSetup) {
+        ut->chunkContents = buf;
+        ut->chunkLength   = CIBufSize;
+        ut->chunkNativeStart = neededIndex;
+        ut->chunkNativeLimit = neededIndex + CIBufSize;
+        if (ut->chunkNativeLimit > ut->a) {
+            ut->chunkNativeLimit = ut->a;
+            ut->chunkLength  = (int32_t)(ut->chunkNativeLimit)-(int32_t)(ut->chunkNativeStart);
+        }
+        ut->nativeIndexingLimit = ut->chunkLength;
+        U_ASSERT(ut->chunkOffset>=0 && ut->chunkOffset<=CIBufSize);
+    }
+    ut->chunkOffset = clippedIndex - (int32_t)ut->chunkNativeStart;
+    UBool success = (forward? ut->chunkOffset<ut->chunkLength : ut->chunkOffset>0);
+    return success;
+}
+
+static UText * U_CALLCONV
+charIterTextClone(UText *dest, const UText *src, UBool deep, UErrorCode * status) {
+    if (U_FAILURE(*status)) {
+        return nullptr;
+    }
+
+    if (deep) {
+        // There is no CharacterIterator API for cloning the underlying text storage.
+        *status = U_UNSUPPORTED_ERROR;
+        return nullptr;
+    } else {
+        CharacterIterator *srcCI =(CharacterIterator *)src->context;
+        srcCI = srcCI->clone();
+        dest = utext_openCharacterIterator(dest, srcCI, status);
+        if (U_FAILURE(*status)) {
+            return dest;
+        }
+        // cast off const on getNativeIndex.
+        //   For CharacterIterator based UTexts, this is safe, the operation is const.
+        int64_t  ix = utext_getNativeIndex((UText *)src);
+        utext_setNativeIndex(dest, ix);
+        dest->r = srcCI;    // flags that this UText owns the CharacterIterator
+    }
+    return dest;
+}
+
+static int32_t U_CALLCONV
+charIterTextExtract(UText *ut,
+                  int64_t start, int64_t limit,
+                  char16_t *dest, int32_t destCapacity,
+                  UErrorCode *status)
+{
+    if(U_FAILURE(*status)) {
+        return 0;
+    }
+    if(destCapacity<0 || (dest==nullptr && destCapacity>0) || start>limit) {
+        *status=U_ILLEGAL_ARGUMENT_ERROR;
+        return 0;
+    }
+    int32_t  length  = (int32_t)ut->a;
+    int32_t  start32 = pinIndex(start, length);
+    int32_t  limit32 = pinIndex(limit, length);
+    int32_t  desti   = 0;
+    int32_t  srci;
+    int32_t  copyLimit;
+
+    CharacterIterator *ci = (CharacterIterator *)ut->context;
+    ci->setIndex32(start32);   // Moves ix to lead of surrogate pair, if needed.
+    srci = ci->getIndex();
+    copyLimit = srci;
+    while (srci<limit32) {
+        UChar32 c = ci->next32PostInc();
+        int32_t  len = U16_LENGTH(c);
+        U_ASSERT(desti+len>0); /* to ensure desti+len never exceeds MAX_INT32, which must not happen logically */
+        if (desti+len <= destCapacity) {
+            U16_APPEND_UNSAFE(dest, desti, c);
+            copyLimit = srci+len;
+        } else {
+            desti += len;
+            *status = U_BUFFER_OVERFLOW_ERROR;
+        }
+        srci += len;
+    }
+
+    charIterTextAccess(ut, copyLimit, true);
+
+    u_terminateUChars(dest, destCapacity, desti, status);
+    return desti;
+}
+
+static const struct UTextFuncs charIterFuncs =
+{
+    sizeof(UTextFuncs),
+    0, 0, 0,             // Reserved alignment padding
+    charIterTextClone,
+    charIterTextLength,
+    charIterTextAccess,
+    charIterTextExtract,
+    nullptr,                // Replace
+    nullptr,                // Copy
+    nullptr,                // MapOffsetToNative,
+    nullptr,                // MapIndexToUTF16,
+    charIterTextClose,
+    nullptr,                // spare 1
+    nullptr,                // spare 2
+    nullptr                 // spare 3
+};
+U_CDECL_END
+
+
+U_CAPI UText * U_EXPORT2
+utext_openCharacterIterator(UText *ut, CharacterIterator *ci, UErrorCode *status) {
+    if (U_FAILURE(*status)) {
+        return nullptr;
+    }
+
+    if (ci->startIndex() > 0) {
+        // No support for CharacterIterators that do not start indexing from zero.
+        *status = U_UNSUPPORTED_ERROR;
+        return nullptr;
+    }
+
+    // Extra space in UText for 2 buffers of CIBufSize UChars each.
+    int32_t  extraSpace = 2 * CIBufSize * sizeof(char16_t);
+    ut = utext_setup(ut, extraSpace, status);
+    if (U_SUCCESS(*status)) {
+        ut->pFuncs                = &charIterFuncs;
+        ut->context              = ci;
+        ut->providerProperties   = 0;
+        ut->a                    = ci->endIndex();        // Length of text
+        ut->p                    = ut->pExtra;            // First buffer
+        ut->b                    = -1;                    // Native index of first buffer contents
+        ut->q                    = (char16_t*)ut->pExtra+CIBufSize;  // Second buffer
+        ut->c                    = -1;                    // Native index of second buffer contents
+
+        // Initialize current chunk contents to be empty.
+        //   First access will fault something in.
+        //   Note:  The initial nativeStart and chunkOffset must sum to zero
+        //          so that getNativeIndex() will correctly compute to zero
+        //          if no call to Access() has ever been made.  They can't be both
+        //          zero without Access() thinking that the chunk is valid.
+        ut->chunkContents        = (char16_t *)ut->p;
+        ut->chunkNativeStart     = -1;
+        ut->chunkOffset          = 1;
+        ut->chunkNativeLimit     = 0;
+        ut->chunkLength          = 0;
+        ut->nativeIndexingLimit  = ut->chunkOffset;  // enables native indexing
+    }
+    return ut;
+}