diff options
Diffstat (limited to 'git2d/utf8.h')
| -rw-r--r-- | git2d/utf8.h | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/git2d/utf8.h b/git2d/utf8.h new file mode 100644 index 0000000..894cbd5 --- /dev/null +++ b/git2d/utf8.h @@ -0,0 +1,74 @@ +/*- + * SPDX-License-Identifier: Unlicense + * SPDX-FileContributor: Chris Wellons <wellons@nullprogram.com> + * + * From: https://nullprogram.com/blog/2017/10/06/ + */ + +#ifndef UTF8_H +#define UTF8_H + +#include <stdint.h> + +/* + * Decode the next character, C, from BUF, reporting errors in E. + * + * Since this is a branchless decoder, four bytes will be read from the + * buffer regardless of the actual length of the next character. This + * means the buffer _must_ have at least three bytes of zero padding + * following the end of the data stream. + * + * Errors are reported in E, which will be non-zero if the parsed + * character was somehow invalid: invalid byte sequence, non-canonical + * encoding, or a surrogate half. + * + * The function returns a pointer to the next character. When an error + * occurs, this pointer will be a guess that depends on the particular + * error, but it will always advance at least one byte. + */ +static void * +utf8_decode(void *buf, uint32_t *c, int *e) +{ + static const char lengths[] = { + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 2, 3, 3, 4, 0 + }; + static const int masks[] = {0x00, 0x7f, 0x1f, 0x0f, 0x07}; + static const uint32_t mins[] = {4194304, 0, 128, 2048, 65536}; + static const int shiftc[] = {0, 18, 12, 6, 0}; + static const int shifte[] = {0, 6, 4, 2, 0}; + + uint8_t *s = buf; + int len = lengths[s[0] >> 3]; + + /* + * Compute the pointer to the next character early so that the next + * iteration can start working on the next character. Neither Clang + * nor GCC figure out this reordering on their own. + */ + uint8_t *next = s + len + !len; + + /* + * Assume a four-byte character and load four bytes. Unused bits are + * shifted out. + */ + *c = (uint32_t)(s[0] & masks[len]) << 18; + *c |= (uint32_t)(s[1] & 0x3f) << 12; + *c |= (uint32_t)(s[2] & 0x3f) << 6; + *c |= (uint32_t)(s[3] & 0x3f) << 0; + *c >>= shiftc[len]; + + /* Accumulate the various error conditions. */ + *e = (*c < mins[len]) << 6; /* non-canonical encoding */ + *e |= ((*c >> 11) == 0x1b) << 7; /* surrogate half? */ + *e |= (*c > 0x10FFFF) << 8; /* out of range? */ + *e |= (s[1] & 0xc0) >> 2; + *e |= (s[2] & 0xc0) >> 4; + *e |= (s[3] ) >> 6; + *e ^= 0x2a; /* top two bits of each tail byte correct? */ + *e >>= shifte[len]; + + return next; +} + +#endif |