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, 23 insertions, 0 deletions

diff --git a/na/lib/string/strspn.c b/na/lib/string/strspn.c
new file mode 100644
index 0000000..f27710d
--- /dev/null
+++ b/na/lib/string/strspn.c
@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: MIT */
+/* Derived from musl libc (Copyright © 2005-2020 Rich Felker, et al.) */
+
+#include <string.h>
+
+#define BITOP(a,b,op) \
+ ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
+
+size_t strspn(const char *s, const char *c)
+{
+	const char *a = s;
+	size_t byteset[32/sizeof(size_t)] = { 0 };
+
+	if (!c[0]) return 0;
+	if (!c[1]) {
+		for (; *s == *c; s++);
+		return s-a;
+	}
+
+	for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
+	for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
+	return s-a;
+}