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+/* A type for indices and sizes.
+ Copyright (C) 2020-2023 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#ifndef _IDX_H
+#define _IDX_H
+
+/* Get ptrdiff_t. */
+#include <stddef.h>
+
+/* Get PTRDIFF_MAX. */
+#include <stdint.h>
+
+/* The type 'idx_t' holds an (array) index or an (object) size.
+ Its implementation promotes to a signed integer type,
+ which can hold the values
+ 0..2^63-1 (on 64-bit platforms) or
+ 0..2^31-1 (on 32-bit platforms).
+
+ Why a signed integer type?
+
+ * Security: Signed types can be checked for overflow via
+ '-fsanitize=undefined', but unsigned types cannot.
+
+ * Comparisons without surprises: ISO C99 § 6.3.1.8 specifies a few
+ surprising results for comparisons, such as
+
+ (int) -3 < (unsigned long) 7 => false
+ (int) -3 < (unsigned int) 7 => false
+ and on 32-bit machines:
+ (long) -3 < (unsigned int) 7 => false
+
+ This is surprising because the natural comparison order is by
+ value in the realm of infinite-precision signed integers (ℤ).
+
+ The best way to get rid of such surprises is to use signed types
+ for numerical integer values, and use unsigned types only for
+ bit masks and enums.
+
+ Why not use 'size_t' directly?
+
+ * Because 'size_t' is an unsigned type, and a signed type is better.
+ See above.
+
+ Why not use 'ssize_t'?
+
+ * 'ptrdiff_t' is more portable; it is standardized by ISO C
+ whereas 'ssize_t' is standardized only by POSIX.
+
+ * 'ssize_t' is not required to be as wide as 'size_t', and some
+ now-obsolete POSIX platforms had 'size_t' wider than 'ssize_t'.
+
+ * Conversely, some now-obsolete platforms had 'ptrdiff_t' wider
+ than 'size_t', which can be a win and conforms to POSIX.
+
+ Won't this cause a problem with objects larger than PTRDIFF_MAX?
+
+ * Typical modern or large platforms do not allocate such objects,
+ so this is not much of a problem in practice; for example, you
+ can safely write 'idx_t len = strlen (s);'. To port to older
+ small platforms where allocations larger than PTRDIFF_MAX could
+ in theory be a problem, you can use Gnulib's ialloc module, or
+ functions like ximalloc in Gnulib's xalloc module.
+
+ Why not use 'ptrdiff_t' directly?
+
+ * Maintainability: When reading and modifying code, it helps to know that
+ a certain variable cannot have negative values. For example, when you
+ have a loop
+
+ int n = ...;
+ for (int i = 0; i < n; i++) ...
+
+ or
+
+ ptrdiff_t n = ...;
+ for (ptrdiff_t i = 0; i < n; i++) ...
+
+ you have to ask yourself "what if n < 0?". Whereas in
+
+ idx_t n = ...;
+ for (idx_t i = 0; i < n; i++) ...
+
+ you know that this case cannot happen.
+
+ Similarly, when a programmer writes
+
+ idx_t = ptr2 - ptr1;
+
+ there is an implied assertion that ptr1 and ptr2 point into the same
+ object and that ptr1 <= ptr2.
+
+ * Being future-proof: In the future, range types (integers which are
+ constrained to a certain range of values) may be added to C compilers
+ or to the C standard. Several programming languages (Ada, Haskell,
+ Common Lisp, Pascal) already have range types. Such range types may
+ help producing good code and good warnings. The type 'idx_t' could
+ then be typedef'ed to a range type that is signed after promotion. */
+
+/* In the future, idx_t could be typedef'ed to a signed range type.
+ The clang "extended integer types", supported in Clang 11 or newer
+ <https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types>,
+ are a special case of range types. However, these types don't support binary
+ operators with plain integer types (e.g. expressions such as x > 1).
+ Therefore, they don't behave like signed types (and not like unsigned types
+ either). So, we cannot use them here. */
+
+/* Use the signed type 'ptrdiff_t'. */
+/* Note: ISO C does not mandate that 'size_t' and 'ptrdiff_t' have the same
+ size, but it is so on all platforms we have seen since 1990. */
+typedef ptrdiff_t idx_t;
+
+/* IDX_MAX is the maximum value of an idx_t. */
+#define IDX_MAX PTRDIFF_MAX
+
+/* So far no need has been found for an IDX_WIDTH macro.
+ Perhaps there should be another macro IDX_VALUE_BITS that does not
+ count the sign bit and is therefore one less than PTRDIFF_WIDTH. */
+
+#endif /* _IDX_H */