feat: add string manipulation functions derived from musl libc

This commit introduces a variety of string manipulation functions, including:
- `memmem`: Searches for a substring in a memory area.
- `memmove`: Safely copies memory areas, handling overlaps.
- `mempcpy`: Copies memory and returns a pointer to the end of the destination.
- `memrchr`: Finds the last occurrence of a character in a memory area.
- `memset`: Fills a memory area with a specified value.
- `rindex`: Returns the last occurrence of a character in a string.
- `stpcpy` and `stpncpy`: Copies strings and returns a pointer to the end.
- `strcat`, `strchr`, `strcspn`, `strlcat`, `strlcpy`, `strlen`, `strncat`, `strncmp`, `strncpy`, `strnlen`, `strpbrk`, `strrchr`, `strstr`, `strtok`, and `strtok_r`: Various string manipulation functions for concatenation, searching, and tokenization.

Assembly implementations for `memcpy`, `memmove`, and `memset` are also included for optimized performance on x86_64 architectures.

1 files changed, 30 insertions, 0 deletions

diff --git a/na/lib/string/memchr.c b/na/lib/string/memchr.c
new file mode 100644
index 0000000..d0f670f
--- /dev/null
+++ b/na/lib/string/memchr.c
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: MIT */
+/* Derived from musl libc (Copyright © 2005-2020 Rich Felker, et al.) */
+
+#include <string.h>
+#include <stdint.h>
+#include <limits.h>
+
+#define SS (sizeof(size_t))
+#define ALIGN (sizeof(size_t)-1)
+#define ONES ((size_t)-1/UCHAR_MAX)
+#define HIGHS (ONES * (UCHAR_MAX/2+1))
+#define HASZERO(x) ((x)-ONES & ~(x) & HIGHS)
+
+void *memchr(const void *src, int c, size_t n)
+{
+	const unsigned char *s = src;
+	c = (unsigned char)c;
+#ifdef __GNUC__
+	for (; ((uintptr_t)s & ALIGN) && n && *s != c; s++, n--);
+	if (n && *s != c) {
+		typedef size_t __attribute__((__may_alias__)) word;
+		const word *w;
+		size_t k = ONES * c;
+		for (w = (const void *)s; n>=SS && !HASZERO(*w^k); w++, n-=SS);
+		s = (const void *)w;
+	}
+#endif
+	for (; n && *s != c; s++, n--);
+	return n ? (void *)s : 0;
+}