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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ModuloBuffer_h
+#define ModuloBuffer_h
+
+#include "mozilla/leb128iterator.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/PowerOfTwo.h"
+#include "mozilla/ProfileBufferEntrySerialization.h"
+#include "mozilla/UniquePtr.h"
+
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <type_traits>
+
+namespace mozilla {
+
+// The ModuloBuffer class is a circular buffer that holds raw byte values, with
+// data-read/write helpers.
+//
+// OffsetT: Type of the internal offset into the buffer of bytes, it should be
+// large enough to access all bytes of the buffer. It will also be used as
+// Length (in bytes) of the buffer and of any subset. Default uint32_t
+// IndexT: Type of the external index, it should be large enough that overflows
+// should not happen during the lifetime of the ModuloBuffer.
+//
+// The basic usage is to create an iterator-like object with `ReaderAt(Index)`
+// or `WriterAt(Index)`, and use it to read/write data blobs. Iterators
+// automatically manage the wrap-around (through "Modulo", which is effectively
+// an AND-masking with the PowerOfTwo buffer size.)
+//
+// There is zero safety: No thread safety, no checks that iterators may be
+// overwriting data that's still to be read, etc. It's up to the caller to add
+// adequate checks.
+// The intended use is as an underlying buffer for a safer container.
+template <typename OffsetT = uint32_t, typename IndexT = uint64_t>
+class ModuloBuffer {
+ public:
+ using Byte = uint8_t;
+ static_assert(sizeof(Byte) == 1, "ModuloBuffer::Byte must be 1 byte");
+ using Offset = OffsetT;
+ static_assert(!std::numeric_limits<Offset>::is_signed,
+ "ModuloBuffer::Offset must be an unsigned integral type");
+ using Length = Offset;
+ using Index = IndexT;
+ static_assert(!std::numeric_limits<Index>::is_signed,
+ "ModuloBuffer::Index must be an unsigned integral type");
+ static_assert(sizeof(Index) >= sizeof(Offset),
+ "ModuloBuffer::Index size must >= Offset");
+
+ // Create a buffer of the given length.
+ explicit ModuloBuffer(PowerOfTwo<Length> aLength)
+ : mMask(aLength.Mask()),
+ mBuffer(WrapNotNull(new Byte[aLength.Value()])),
+ mBufferDeleter([](Byte* aBuffer) { delete[] aBuffer; }) {}
+
+ // Take ownership of an existing buffer. Existing contents is ignored.
+ // Done by extracting the raw pointer from UniquePtr<Byte[]>, and adding
+ // an equivalent `delete[]` in `mBufferDeleter`.
+ ModuloBuffer(UniquePtr<Byte[]> aExistingBuffer, PowerOfTwo<Length> aLength)
+ : mMask(aLength.Mask()),
+ mBuffer(WrapNotNull(aExistingBuffer.release())),
+ mBufferDeleter([](Byte* aBuffer) { delete[] aBuffer; }) {}
+
+ // Use an externally-owned buffer. Existing contents is ignored.
+ ModuloBuffer(Byte* aExternalBuffer, PowerOfTwo<Length> aLength)
+ : mMask(aLength.Mask()), mBuffer(WrapNotNull(aExternalBuffer)) {}
+
+ // Disallow copying, as we may uniquely own the resource.
+ ModuloBuffer(const ModuloBuffer& aOther) = delete;
+ ModuloBuffer& operator=(const ModuloBuffer& aOther) = delete;
+
+ // Allow move-construction. Stealing ownership if the original had it.
+ // This effectively prevents copy construction, and all assignments; needed so
+ // that a ModuloBuffer may be initialized from a separate construction.
+ // The moved-from ModuloBuffer still points at the resource but doesn't own
+ // it, so it won't try to free it; but accesses are not guaranteed, so it
+ // should not be used anymore.
+ ModuloBuffer(ModuloBuffer&& aOther)
+ : mMask(std::move(aOther.mMask)),
+ mBuffer(std::move(aOther.mBuffer)),
+ mBufferDeleter(std::move(aOther.mBufferDeleter)) {
+ // The above move leaves `aOther.mBufferDeleter` in a valid state but with
+ // an unspecified value, so it could theoretically still contain the
+ // original function, which would be bad because we don't want aOther to
+ // delete the resource that `this` now owns.
+ if (aOther.mBufferDeleter) {
+ // `aOther` still had a non-empty deleter, reset it.
+ aOther.mBufferDeleter = nullptr;
+ }
+ }
+
+ // Disallow assignment, as we have some `const` members.
+ ModuloBuffer& operator=(ModuloBuffer&& aOther) = delete;
+
+ // Destructor, deletes the resource if we uniquely own it.
+ ~ModuloBuffer() {
+ if (mBufferDeleter) {
+ mBufferDeleter(mBuffer);
+ }
+ }
+
+ PowerOfTwo<Length> BufferLength() const {
+ return PowerOfTwo<Length>(mMask.MaskValue() + 1);
+ }
+
+ // Size of external resources.
+ // Note: `mBufferDeleter`'s potential external data (for its captures) is not
+ // included, as it's hidden in the `std::function` implementation.
+ size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+ if (!mBufferDeleter) {
+ // If we don't have a buffer deleter, assume we don't own the data, so
+ // it's probably on the stack, or should be reported by its owner.
+ return 0;
+ }
+ return aMallocSizeOf(mBuffer);
+ }
+
+ size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+ return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+ }
+
+ ProfileBufferEntryReader EntryReaderFromTo(
+ Index aStart, Index aEnd, ProfileBufferBlockIndex aBlockIndex,
+ ProfileBufferBlockIndex aNextBlockIndex) const {
+ using EntrySpan = Span<const ProfileBufferEntryReader::Byte>;
+ if (aStart == aEnd) {
+ return ProfileBufferEntryReader{};
+ }
+ // Don't allow over-wrapping.
+ MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+ // Start offset in 0 .. (buffer size - 1)
+ Offset start = static_cast<Offset>(aStart) & mMask;
+ // End offset in 1 .. (buffer size)
+ Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+ if (start < end) {
+ // Segment doesn't cross buffer threshold, one span is enough.
+ return ProfileBufferEntryReader{EntrySpan(&mBuffer[start], end - start),
+ aBlockIndex, aNextBlockIndex};
+ }
+ // Segment crosses buffer threshold, we need one span until the end and one
+ // span restarting at the beginning of the buffer.
+ return ProfileBufferEntryReader{
+ EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+ EntrySpan(&mBuffer[0], end), aBlockIndex, aNextBlockIndex};
+ }
+
+ // Return an entry writer for the given range.
+ ProfileBufferEntryWriter EntryWriterFromTo(Index aStart, Index aEnd) const {
+ using EntrySpan = Span<ProfileBufferEntryReader::Byte>;
+ if (aStart == aEnd) {
+ return ProfileBufferEntryWriter{};
+ }
+ MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+ // Start offset in 0 .. (buffer size - 1)
+ Offset start = static_cast<Offset>(aStart) & mMask;
+ // End offset in 1 .. (buffer size)
+ Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+ if (start < end) {
+ // Segment doesn't cross buffer threshold, one span is enough.
+ return ProfileBufferEntryWriter{
+ EntrySpan(&mBuffer[start], end - start),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd)};
+ }
+ // Segment crosses buffer threshold, we need one span until the end and one
+ // span restarting at the beginning of the buffer.
+ return ProfileBufferEntryWriter{
+ EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+ EntrySpan(&mBuffer[0], end),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd)};
+ }
+
+ // Emplace an entry writer into `aMaybeEntryWriter` for the given range.
+ void EntryWriterFromTo(Maybe<ProfileBufferEntryWriter>& aMaybeEntryWriter,
+ Index aStart, Index aEnd) const {
+ MOZ_ASSERT(aMaybeEntryWriter.isNothing(),
+ "Reference entry writer should be Nothing.");
+ using EntrySpan = Span<ProfileBufferEntryReader::Byte>;
+ if (aStart == aEnd) {
+ return;
+ }
+ MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+ // Start offset in 0 .. (buffer size - 1)
+ Offset start = static_cast<Offset>(aStart) & mMask;
+ // End offset in 1 .. (buffer size)
+ Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+ if (start < end) {
+ // Segment doesn't cross buffer threshold, one span is enough.
+ aMaybeEntryWriter.emplace(
+ EntrySpan(&mBuffer[start], end - start),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd));
+ } else {
+ // Segment crosses buffer threshold, we need one span until the end and
+ // one span restarting at the beginning of the buffer.
+ aMaybeEntryWriter.emplace(
+ EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+ EntrySpan(&mBuffer[0], end),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+ ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd));
+ }
+ }
+
+ // All ModuloBuffer operations should be done through this iterator, which has
+ // an effectively infinite range. The underlying wrapping-around is hidden.
+ // Use `ReaderAt(Index)` or `WriterAt(Index)` to create it.
+ //
+ // `const Iterator<...>` means the iterator itself cannot change, i.e., it
+ // cannot move, and only its const methods are available. Note that these
+ // const methods may still be used to modify the buffer contents (e.g.:
+ // `operator*()`, `Poke()`).
+ //
+ // `Iterator</*IsBufferConst=*/true>` means the buffer contents cannot be
+ // modified, i.e., write operations are forbidden, but the iterator may still
+ // move if non-const itself.
+ template <bool IsBufferConst>
+ class Iterator {
+ // Alias to const- or mutable-`ModuloBuffer` depending on `IsBufferConst`.
+ using ConstOrMutableBuffer =
+ std::conditional_t<IsBufferConst, const ModuloBuffer, ModuloBuffer>;
+
+ // Implementation note about the strange enable-if's below:
+ // `template <bool NotIBC = !IsBufferConst> enable_if_t<NotIBC>`
+ // which intuitively could be simplified to:
+ // `enable_if_t<!IsBufferConst>`
+ // The former extra-templated syntax is in fact necessary to delay
+ // instantiation of these functions until they are actually needed.
+ //
+ // If we were just doing `enable_if_t<!IsBufferConst>`, this would only
+ // depend on the *class* (`ModuloBuffer<...>::Iterator`), which gets
+ // instantiated when a `ModuloBuffer` is created with some template
+ // arguments; at that point, all non-templated methods get instantiated, so
+ // there's no "SFINAE" happening, and `enable_if_t<...>` is actually doing
+ // `typename enable_if<...>::type` on the spot, but there is no `type` if
+ // `IsBufferConst` is true, so it just fails right away. E.g.:
+ // error: no type named 'type' in 'std::enable_if<false, void>';
+ // 'enable_if' cannot be used to disable this declaration
+ // note: in instantiation of template type alias 'enable_if_t'
+ // > std::enable_if_t<!IsBufferConst> WriteObject(const T& aObject) {
+ // in instantiation of template class
+ // 'mozilla::ModuloBuffer<...>::Iterator<true>'
+ // > auto it = mb.ReaderAt(1);
+ //
+ // By adding another template level `template <bool NotIsBufferConst =
+ // !IsBufferConst>`, the instantiation is delayed until the function is
+ // actually invoked somewhere, e.g. `it.Poke(...);`.
+ // So at that invocation point, the compiler looks for a "Poke" name in it,
+ // and considers potential template instantiations that could work. The
+ // `enable_if_t` is *now* attempted, with `NotIsBufferConst` taking its
+ // value from `!IsBufferConst`:
+ // - If `IsBufferConst` is false, `NotIsBufferConst` is true,
+ // `enable_if<NotIsBufferConst>` does define a `type` (`void` by default),
+ // so `enable_if_t` happily becomes `void`, the function exists and may be
+ // called.
+ // - Otherwise if `IsBufferConst` is true, `NotIsBufferConst` is false,
+ // `enable_if<NotIsBufferConst>` does *not* define a `type`, therefore
+ // `enable_if_t` produces an error because there is no `type`. Now "SFINAE"
+ // happens: This "Substitution Failure Is Not An Error" (by itself)... But
+ // then, there are no other functions named "Poke" as requested in the
+ // `it.Poke(...);` call, so we are now getting an error (can't find
+ // function), as expected because `it` had `IsBufferConst`==true. (But at
+ // least the compiler waited until this invocation attempt before outputting
+ // an error.)
+ //
+ // C++ is fun!
+
+ public:
+ // These definitions are expected by std functions, to recognize this as an
+ // iterator. See https://en.cppreference.com/w/cpp/iterator/iterator_traits
+ using difference_type = Index;
+ using value_type = Byte;
+ using pointer = std::conditional_t<IsBufferConst, const Byte*, Byte*>;
+ using reference = std::conditional_t<IsBufferConst, const Byte&, Byte&>;
+ using iterator_category = std::random_access_iterator_tag;
+
+ // Can always copy/assign from the same kind of iterator.
+ Iterator(const Iterator& aRhs) = default;
+ Iterator& operator=(const Iterator& aRhs) = default;
+
+ // Can implicitly copy an Iterator-to-mutable (reader+writer) to
+ // Iterator-to-const (reader-only), but not the reverse.
+ template <bool IsRhsBufferConst,
+ typename = std::enable_if_t<(!IsRhsBufferConst) && IsBufferConst>>
+ MOZ_IMPLICIT Iterator(const Iterator<IsRhsBufferConst>& aRhs)
+ : mModuloBuffer(aRhs.mModuloBuffer), mIndex(aRhs.mIndex) {}
+
+ // Can implicitly assign from an Iterator-to-mutable (reader+writer) to
+ // Iterator-to-const (reader-only), but not the reverse.
+ template <bool IsRhsBufferConst,
+ typename = std::enable_if_t<(!IsRhsBufferConst) && IsBufferConst>>
+ Iterator& operator=(const Iterator<IsRhsBufferConst>& aRhs) {
+ mModuloBuffer = aRhs.mModuloBuffer;
+ mIndex = aRhs.mIndex;
+ return *this;
+ }
+
+ // Current location of the iterator in the `Index` range.
+ // Note that due to wrapping, multiple indices may effectively point at the
+ // same byte in the buffer.
+ Index CurrentIndex() const { return mIndex; }
+
+ // Location comparison in the `Index` range. I.e., two `Iterator`s may look
+ // unequal, but refer to the same buffer location.
+ // Must be on the same buffer.
+ bool operator==(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex == aRhs.mIndex;
+ }
+ bool operator!=(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex != aRhs.mIndex;
+ }
+ bool operator<(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex < aRhs.mIndex;
+ }
+ bool operator<=(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex <= aRhs.mIndex;
+ }
+ bool operator>(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex > aRhs.mIndex;
+ }
+ bool operator>=(const Iterator& aRhs) const {
+ MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+ return mIndex >= aRhs.mIndex;
+ }
+
+ // Movement in the `Index` range.
+ Iterator& operator++() {
+ ++mIndex;
+ return *this;
+ }
+ Iterator operator++(int) {
+ Iterator here(*mModuloBuffer, mIndex);
+ ++mIndex;
+ return here;
+ }
+ Iterator& operator--() {
+ --mIndex;
+ return *this;
+ }
+ Iterator operator--(int) {
+ Iterator here(*mModuloBuffer, mIndex);
+ --mIndex;
+ return here;
+ }
+ Iterator& operator+=(Length aLength) {
+ mIndex += aLength;
+ return *this;
+ }
+ Iterator operator+(Length aLength) const {
+ return Iterator(*mModuloBuffer, mIndex + aLength);
+ }
+ friend Iterator operator+(Length aLength, const Iterator& aIt) {
+ return aIt + aLength;
+ }
+ Iterator& operator-=(Length aLength) {
+ mIndex -= aLength;
+ return *this;
+ }
+ Iterator operator-(Length aLength) const {
+ return Iterator(*mModuloBuffer, mIndex - aLength);
+ }
+
+ // Distance from `aRef` to here in the `Index` range.
+ // May be negative (as 2's complement) if `aRef > *this`.
+ Index operator-(const Iterator& aRef) const {
+ MOZ_ASSERT(mModuloBuffer == aRef.mModuloBuffer);
+ return mIndex - aRef.mIndex;
+ }
+
+ // Dereference a single byte (read-only if `IsBufferConst` is true).
+ reference operator*() const {
+ return mModuloBuffer->mBuffer[OffsetInBuffer()];
+ }
+
+ // Random-access dereference.
+ reference operator[](Length aLength) const { return *(*this + aLength); }
+
+ // Write data (if `IsBufferConst` is false) but don't move iterator.
+ template <bool NotIsBufferConst = !IsBufferConst>
+ std::enable_if_t<NotIsBufferConst> Poke(const void* aSrc,
+ Length aLength) const {
+ // Don't allow data larger than the buffer.
+ MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+ // Offset inside the buffer (corresponding to our Index).
+ Offset offset = OffsetInBuffer();
+ // Compute remaining bytes between this offset and the end of the buffer.
+ Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+ if (MOZ_LIKELY(remaining >= aLength)) {
+ // Enough space to write everything before the end.
+ memcpy(&mModuloBuffer->mBuffer[offset], aSrc, aLength);
+ } else {
+ // Not enough space. Write as much as possible before the end.
+ memcpy(&mModuloBuffer->mBuffer[offset], aSrc, remaining);
+ // And then continue from the beginning of the buffer.
+ memcpy(&mModuloBuffer->mBuffer[0],
+ static_cast<const Byte*>(aSrc) + remaining,
+ (aLength - remaining));
+ }
+ }
+
+ // Write object data (if `IsBufferConst` is false) but don't move iterator.
+ // Note that this copies bytes from the object, with the intent to read them
+ // back later. Restricted to trivially-copyable types, which support this
+ // without Undefined Behavior!
+ template <typename T, bool NotIsBufferConst = !IsBufferConst>
+ std::enable_if_t<NotIsBufferConst> PokeObject(const T& aObject) const {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "PokeObject<T> - T must be trivially copyable");
+ return Poke(&aObject, sizeof(T));
+ }
+
+ // Write data (if `IsBufferConst` is false) and move iterator ahead.
+ template <bool NotIsBufferConst = !IsBufferConst>
+ std::enable_if_t<NotIsBufferConst> Write(const void* aSrc, Length aLength) {
+ Poke(aSrc, aLength);
+ mIndex += aLength;
+ }
+
+ // Write object data (if `IsBufferConst` is false) and move iterator ahead.
+ // Note that this copies bytes from the object, with the intent to read them
+ // back later. Restricted to trivially-copyable types, which support this
+ // without Undefined Behavior!
+ template <typename T, bool NotIsBufferConst = !IsBufferConst>
+ std::enable_if_t<NotIsBufferConst> WriteObject(const T& aObject) {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "WriteObject<T> - T must be trivially copyable");
+ return Write(&aObject, sizeof(T));
+ }
+
+ // Number of bytes needed to represent `aValue` in unsigned LEB128.
+ template <typename T>
+ static unsigned ULEB128Size(T aValue) {
+ return ::mozilla::ULEB128Size(aValue);
+ }
+
+ // Write number as unsigned LEB128 (if `IsBufferConst` is false) and move
+ // iterator ahead.
+ template <typename T, bool NotIsBufferConst = !IsBufferConst>
+ std::enable_if_t<NotIsBufferConst> WriteULEB128(T aValue) {
+ ::mozilla::WriteULEB128(aValue, *this);
+ }
+
+ // Read data but don't move iterator.
+ void Peek(void* aDst, Length aLength) const {
+ // Don't allow data larger than the buffer.
+ MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+ // Offset inside the buffer (corresponding to our Index).
+ Offset offset = OffsetInBuffer();
+ // Compute remaining bytes between this offset and the end of the buffer.
+ Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+ if (MOZ_LIKELY(remaining >= aLength)) {
+ // Can read everything we need before the end of the buffer.
+ memcpy(aDst, &mModuloBuffer->mBuffer[offset], aLength);
+ } else {
+ // Read as much as possible before the end of the buffer.
+ memcpy(aDst, &mModuloBuffer->mBuffer[offset], remaining);
+ // And then continue from the beginning of the buffer.
+ memcpy(static_cast<Byte*>(aDst) + remaining, &mModuloBuffer->mBuffer[0],
+ (aLength - remaining));
+ }
+ }
+
+ // Read data into an object but don't move iterator.
+ // Note that this overwrites `aObject` with bytes from the buffer.
+ // Restricted to trivially-copyable types, which support this without
+ // Undefined Behavior!
+ template <typename T>
+ void PeekIntoObject(T& aObject) const {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "PeekIntoObject<T> - T must be trivially copyable");
+ Peek(&aObject, sizeof(T));
+ }
+
+ // Read data as an object but don't move iterator.
+ // Note that this creates an default `T` first, and then overwrites it with
+ // bytes from the buffer. Restricted to trivially-copyable types, which
+ // support this without Undefined Behavior!
+ template <typename T>
+ T PeekObject() const {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "PeekObject<T> - T must be trivially copyable");
+ T object;
+ PeekIntoObject(object);
+ return object;
+ }
+
+ // Read data and move iterator ahead.
+ void Read(void* aDst, Length aLength) {
+ Peek(aDst, aLength);
+ mIndex += aLength;
+ }
+
+ // Read data into a mutable iterator and move both iterators ahead.
+ void ReadInto(Iterator</* IsBufferConst */ false>& aDst, Length aLength) {
+ // Don't allow data larger than the buffer.
+ MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+ MOZ_ASSERT(aLength <= aDst.mModuloBuffer->BufferLength().Value());
+ // Offset inside the buffer (corresponding to our Index).
+ Offset offset = OffsetInBuffer();
+ // Compute remaining bytes between this offset and the end of the buffer.
+ Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+ if (MOZ_LIKELY(remaining >= aLength)) {
+ // Can read everything we need before the end of the buffer.
+ aDst.Write(&mModuloBuffer->mBuffer[offset], aLength);
+ } else {
+ // Read as much as possible before the end of the buffer.
+ aDst.Write(&mModuloBuffer->mBuffer[offset], remaining);
+ // And then continue from the beginning of the buffer.
+ aDst.Write(&mModuloBuffer->mBuffer[0], (aLength - remaining));
+ }
+ mIndex += aLength;
+ }
+
+ // Read data into an object and move iterator ahead.
+ // Note that this overwrites `aObject` with bytes from the buffer.
+ // Restricted to trivially-copyable types, which support this without
+ // Undefined Behavior!
+ template <typename T>
+ void ReadIntoObject(T& aObject) {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "ReadIntoObject<T> - T must be trivially copyable");
+ Read(&aObject, sizeof(T));
+ }
+
+ // Read data as an object and move iterator ahead.
+ // Note that this creates an default `T` first, and then overwrites it with
+ // bytes from the buffer. Restricted to trivially-copyable types, which
+ // support this without Undefined Behavior!
+ template <typename T>
+ T ReadObject() {
+ static_assert(std::is_trivially_copyable<T>::value,
+ "ReadObject<T> - T must be trivially copyable");
+ T object;
+ ReadIntoObject(object);
+ return object;
+ }
+
+ // Read an unsigned LEB128 number and move iterator ahead.
+ template <typename T>
+ T ReadULEB128() {
+ return ::mozilla::ReadULEB128<T>(*this);
+ }
+
+ private:
+ // Only a ModuloBuffer can instantiate its iterator.
+ friend class ModuloBuffer;
+
+ Iterator(ConstOrMutableBuffer& aBuffer, Index aIndex)
+ : mModuloBuffer(WrapNotNull(&aBuffer)), mIndex(aIndex) {}
+
+ // Convert the Iterator's mIndex into an offset inside the byte buffer.
+ Offset OffsetInBuffer() const {
+ return static_cast<Offset>(mIndex) & mModuloBuffer->mMask;
+ }
+
+ // ModuloBuffer that this Iterator operates on.
+ // Using a non-null pointer instead of a reference, to allow re-assignment
+ // of an Iterator variable.
+ NotNull<ConstOrMutableBuffer*> mModuloBuffer;
+
+ // Position of this iterator in the wider `Index` range. (Will be wrapped
+ // around as needed when actually accessing bytes from the buffer.)
+ Index mIndex;
+ };
+
+ // Shortcut to iterator to const (read-only) data.
+ using Reader = Iterator<true>;
+ // Shortcut to iterator to non-const (read/write) data.
+ using Writer = Iterator<false>;
+
+ // Create an iterator to const data at the given index.
+ Reader ReaderAt(Index aIndex) const { return Reader(*this, aIndex); }
+
+ // Create an iterator to non-const data at the given index.
+ Writer WriterAt(Index aIndex) { return Writer(*this, aIndex); }
+
+#ifdef DEBUG
+ void Dump() const {
+ Length len = BufferLength().Value();
+ if (len > 128) {
+ len = 128;
+ }
+ for (Length i = 0; i < len; ++i) {
+ printf("%02x ", mBuffer[i]);
+ }
+ printf("\n");
+ }
+#endif // DEBUG
+
+ private:
+ // Mask used to convert an index to an offset in `mBuffer`
+ const PowerOfTwoMask<Offset> mMask;
+
+ // Buffer data. `const NotNull<...>` shows that `mBuffer is `const`, and
+ // `Byte* const` shows that the pointer cannot be changed to point at
+ // something else, but the pointed-at `Byte`s are writable.
+ const NotNull<Byte* const> mBuffer;
+
+ // Function used to release the buffer resource (if needed).
+ std::function<void(Byte*)> mBufferDeleter;
+};
+
+} // namespace mozilla
+
+#endif // ModuloBuffer_h