diff options
Diffstat (limited to 'src/str/ft_strlen.c')
| -rw-r--r-- | src/str/ft_strlen.c | 78 |
1 files changed, 55 insertions, 23 deletions
diff --git a/src/str/ft_strlen.c b/src/str/ft_strlen.c index 0d593e1..999763e 100644 --- a/src/str/ft_strlen.c +++ b/src/str/ft_strlen.c @@ -6,36 +6,68 @@ /* By: cacharle <marvin@42.fr> +#+ +:+ +#+ */ /* +#+#+#+#+#+ +#+ */ /* Created: 2019/10/07 10:32:48 by cacharle #+# #+# */ -/* Updated: 2020/01/17 11:13:43 by cacharle ### ########.fr */ +/* Updated: 2020/04/01 21:45:38 by charles ### ########.fr */ /* */ /* ************************************************************************** */ #include "libft.h" +#include <stdint.h> + +/* +** Determining if one byte of a long word is 0 +** +** ~((((lw & 0x7F7F7F7F) + 0x7F7F7F7F) | lw) | 0x7F7F7F7F) +** +** where `lw` is a long word +** +** 0x7F -> 0b 0111 1111 +** +** null_high = lw & 0x7F7F7F7F // will set the high bit of each byte to 0 +** overflow = null_high + 0x7F7F7F7F // addition will overflow the high bit +** is one of the other bits was 1. +** +** oring = overflow | lw // the high bit of a byte is set +** iff any bit in the byte was set +** ones = oring | 0x7F7F7F7F // the high bits and ones everywhere else +** has_no_zero_byte = ~ones // the ones become zeros, if no high bit +** was set, there was no zero +** +** (lw - 0x01010101) & ~lw & 0x80808080 +** +** overflow = lw - 0x01010101 // overflow the high bit if one was +** 0 or > 0x80 (0b 1000 0000) 0 || >0x80 +** no_high_bit = ~lw & 0x80808080 // high bit set if the high bit was +** 0 (i.e < 0x80) 0 || <0x80 +** has_zero = overflow & no_high_bit // (0 || >0x80) && (0 || <0x80) -> 0 && 0 +** +** +** libc's strlen only filter out < 0x80 bytes by omitting the ~lw & 0x80808080 +** part because most string only contain ascii characters. +** +** sources: +** - https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord +** - https://stackoverflow.com/questions/20021066 +*/ + +#define HIMAGIC 0x8080808080808080L +#define LOMAGIC 0x0101010101010101L size_t ft_strlen(const char *s) { - unsigned long int *ptr; - const char *cpy; + uint64_t *ptr; + const char *cpy; + const char *found; - ptr = (unsigned long int*)s; + cpy = s; + while (((uint64_t)cpy & 0b111) != 0) + if (*cpy++ == '\0') + return (cpy - s - 1); + ptr = (uint64_t*)cpy; while (TRUE) - { - cpy = (const char*)ptr++; - if (cpy[0] == '\0') - return (cpy - s); - if (cpy[1] == '\0') - return (cpy + 1 - s); - if (cpy[2] == '\0') - return (cpy + 2 - s); - if (cpy[3] == '\0') - return (cpy + 3 - s); - if (cpy[4] == '\0') - return (cpy + 4 - s); - if (cpy[5] == '\0') - return (cpy + 5 - s); - if (cpy[6] == '\0') - return (cpy + 6 - s); - if (cpy[7] == '\0') - return (cpy + 7 - s); - } + if (((*ptr++ - LOMAGIC) & HIMAGIC) != 0) + { + cpy = (const char*)(ptr - 1); + if ((found = ft_memchr(cpy, '\0', sizeof(uint64_t))) != NULL) + return (found - s); + } } |