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Add a UTF-8 decoder

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Victor Zverovich 2019-12-15 07:27:09 -08:00
parent 9e450911fa
commit 0012917f69
1 changed files with 57 additions and 0 deletions
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57
include/fmt/format-inl.h
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@ -1240,6 +1240,63 @@ template <> struct formatter<internal::bigint> {
}
};
// A public domain branchless UTF-8 decoder:
// https://github.com/skeeto/branchless-utf8
/* 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};
auto s = reinterpret_cast<unsigned char*>(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.
*/
unsigned char* 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;
}
#if FMT_USE_WINDOWS_H
FMT_FUNC internal::utf8_to_utf16::utf8_to_utf16(string_view s) {