From fe472f30e1b82f762993cbc5376ff9b25c605aa9 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Tue, 16 May 2023 10:48:50 -0400
Subject: int: Backport C23's endian utilities

---
 src/int.h   | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/int.c | 10 +++++++++
 2 files changed, 78 insertions(+)

diff --git a/src/int.h b/src/int.h
index 56fabee..1cd455a 100644
--- a/src/int.h
+++ b/src/int.h
@@ -11,6 +11,10 @@
 #include <limits.h>
 #include <stdint.h>
 
+#if __STDC_VERSION__ >= 202311L
+#  include <stdbit.h>
+#endif
+
 // C23 polyfill: _WIDTH macros
 
 // The U*_MAX macros are of the form 2**n - 1, and we want to extract the n.
@@ -97,4 +101,68 @@
 #  define INTMAX_WIDTH UINTMAX_WIDTH
 #endif
 
+// C23 polyfill: byte order
+
+#ifdef __STDC_ENDIAN_LITTLE__
+#  define ENDIAN_LITTLE __STDC_ENDIAN_LITTLE__
+#elif defined(__ORDER_LITTLE_ENDIAN__)
+#  define ENDIAN_LITTLE __ORDER_LITTLE_ENDIAN__
+#else
+#  define ENDIAN_LITTLE 1234
+#endif
+
+#ifdef __STDC_ENDIAN_BIG__
+#  define ENDIAN_BIG __STDC_ENDIAN_BIG__
+#elif defined(__ORDER_BIG_ENDIAN__)
+#  define ENDIAN_BIG __ORDER_BIG_ENDIAN__
+#else
+#  define ENDIAN_BIG 4321
+#endif
+
+#ifdef __STDC_ENDIAN_NATIVE__
+#  define ENDIAN_NATIVE __STDC_ENDIAN_NATIVE__
+#elif defined(__ORDER_NATIVE_ENDIAN__)
+#  define ENDIAN_NATIVE __ORDER_NATIVE_ENDIAN__
+#else
+#  define ENDIAN_NATIVE 0
+#endif
+
+#if __STDC_VERSION__ >= 202311L
+#  define bswap16 stdc_memreverse8u16
+#  define bswap32 stdc_memreverse8u32
+#  define bswap64 stdc_memreverse8u64
+#elif __GNUC__
+#  define bswap16 __builtin_bswap16
+#  define bswap32 __builtin_bswap32
+#  define bswap64 __builtin_bswap64
+#else
+
+static inline uint16_t bswap16(uint16_t n) {
+	return (n << 8) | (n >> 8);
+}
+
+static inline uint32_t bswap32(uint32_t n) {
+	return ((uint32_t)bswap16(n) << 16) | bswap16(n >> 16);
+}
+
+static inline uint64_t bswap64(uint64_t n) {
+	return ((uint64_t)bswap32(n) << 32) | bswap32(n >> 32);
+}
+
+#endif
+
+static inline uint8_t bswap8(uint8_t n) {
+	return n;
+}
+
+/**
+ * Reverse the byte order of an integer.
+ */
+#define bswap(n) \
+	_Generic((n), \
+		uint8_t: bswap8, \
+		uint16_t: bswap16, \
+		uint32_t: bswap32, \
+		uint64_t: bswap64)(n)
+
 #endif // BFS_INT_H
diff --git a/tests/int.c b/tests/int.c
index db59e90..0039862 100644
--- a/tests/int.c
+++ b/tests/int.c
@@ -1,10 +1,14 @@
 // Copyright © Tavian Barnes <tavianator@tavianator.com>
 // SPDX-License-Identifier: 0BSD
 
+#undef NDEBUG
+
 #include "../src/int.h"
 #include "../src/diag.h"
+#include <assert.h>
 #include <limits.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 bfs_static_assert(UMAX_WIDTH(0x1) == 1);
 bfs_static_assert(UMAX_WIDTH(0x3) == 2);
@@ -51,4 +55,10 @@ bfs_static_assert(INTMAX_MIN == IWIDTH_MIN(INTMAX_WIDTH));
 bfs_static_assert(INTMAX_MAX == IWIDTH_MAX(INTMAX_WIDTH));
 
 int main(void) {
+	assert(bswap((uint8_t)0x12) == 0x12);
+	assert(bswap((uint16_t)0x1234) == 0x3412);
+	assert(bswap((uint32_t)0x12345678) == 0x78563412);
+	assert(bswap((uint64_t)0x1234567812345678) == 0x7856341278563412);
+
+	return EXIT_SUCCESS;
 }
-- 
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