int: Backport C23's _WIDTH macros

1 files changed, 100 insertions, 0 deletions

diff --git a/src/int.h b/src/int.h
new file mode 100644
index 0000000..56fabee
--- /dev/null
+++ b/src/int.h
@@ -0,0 +1,100 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/**
+ * Bits & bytes.
+ */
+
+#ifndef BFS_INT_H
+#define BFS_INT_H
+
+#include <limits.h>
+#include <stdint.h>
+
+// C23 polyfill: _WIDTH macros
+
+// The U*_MAX macros are of the form 2**n - 1, and we want to extract the n.
+// One way would be *_WIDTH = popcount(*_MAX).  Alternatively, we can use
+// Hallvard B. Furuseth's technique from [1], which is shorter.
+//
+// [1]: https://groups.google.com/g/comp.lang.c/c/NfedEFBFJ0k
+
+// Let mask be of the form 2**m - 1, e.g. 0b111, and let n range over
+// [0b0, 0b1, 0b11, 0b111, 0b1111, ...].  Then we have
+//
+//     n % 0b111
+//         == [0b0, 0b1, 0b11, 0b0, 0b1, 0b11, ...]
+//     n / (n % 0b111 + 1)
+//         == [0b0 (x3), 0b111 (x3), 0b111111 (x3), ...]
+//     n / (n % 0b111 + 1) / 0b111
+//         == [0b0 (x3), 0b1 (x3), 0b1001 (x3), 0b1001001 (x3), ...]
+//     n / (n % 0b111 + 1) / 0b111 % 0b111
+//         == [0 (x3), 1 (x3), 2 (x3), ...]
+//         == UMAX_CHUNK(n, 0b111)
+#define UMAX_CHUNK(n, mask) (n / (n % mask + 1) / mask % mask)
+
+// 8 * UMAX_CHUNK(n, 255) gives [0 (x8), 8 (x8), 16 (x8), ...].  To that we add
+// [0, 1, 2, ..., 6, 7, 0, 1, ...], which we get from a linear interpolation on
+// n % 255:
+//
+//     n % 255
+//         == [0, 1, 3, 7, 15, 31, 63, 127, 0, ...]
+//     86 / (n % 255 + 12)
+//         == [7, 6, 5, 4, 3, 2, 1, 0, 7, ...]
+#define UMAX_INTERP(n) (7 - 86 / (n % 255 + 12))
+
+#define UMAX_WIDTH(n) (8 * UMAX_CHUNK(n, 255) + UMAX_INTERP(n))
+
+#ifndef CHAR_WIDTH
+#  define CHAR_WIDTH CHAR_BIT
+#endif
+#ifndef UCHAR_WIDTH
+#  define UCHAR_WIDTH CHAR_WIDTH
+#endif
+#ifndef SCHAR_WIDTH
+#  define SCHAR_WIDTH CHAR_WIDTH
+#endif
+#ifndef USHRT_WIDTH
+#  define USHRT_WIDTH UMAX_WIDTH(USHRT_MAX)
+#endif
+#ifndef SHRT_WIDTH
+#  define SHRT_WIDTH USHRT_WIDTH
+#endif
+#ifndef UINT_WIDTH
+#  define UINT_WIDTH UMAX_WIDTH(UINT_MAX)
+#endif
+#ifndef INT_WIDTH
+#  define INT_WIDTH UINT_WIDTH
+#endif
+#ifndef ULONG_WIDTH
+#  define ULONG_WIDTH UMAX_WIDTH(ULONG_MAX)
+#endif
+#ifndef LONG_WIDTH
+#  define LONG_WIDTH ULONG_WIDTH
+#endif
+#ifndef ULLONG_WIDTH
+#  define ULLONG_WIDTH UMAX_WIDTH(ULLONG_MAX)
+#endif
+#ifndef LLONG_WIDTH
+#  define LLONG_WIDTH ULLONG_WIDTH
+#endif
+#ifndef SIZE_WIDTH
+#  define SIZE_WIDTH UMAX_WIDTH(SIZE_MAX)
+#endif
+#ifndef PTRDIFF_WIDTH
+#  define PTRDIFF_WIDTH (UMAX_WIDTH(PTRDIFF_MAX) + 1)
+#endif
+#ifndef UINTPTR_WIDTH
+#  define UINTPTR_WIDTH UMAX_WIDTH(UINTPTR_MAX)
+#endif
+#ifndef INTPTR_WIDTH
+#  define INTPTR_WIDTH UINTPTR_WIDTH
+#endif
+#ifndef UINTMAX_WIDTH
+#  define UINTMAX_WIDTH UMAX_WIDTH(UINTMAX_MAX)
+#endif
+#ifndef INTMAX_WIDTH
+#  define INTMAX_WIDTH UINTMAX_WIDTH
+#endif
+
+#endif // BFS_INT_H