OXYGENHUB
◆ main enhanced-x16-emulator / src / utf8.h
2.2 KiB · Objective-C · last changed chore(): initial commit 25 days ago source blame history raw
9120e874 Andy McCall 1 
/* Branchless UTF-8 decoder
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 *
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 * This is free and unencumbered software released into the public domain.
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 */
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#ifndef UTF8_H
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#define UTF8_H
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#include <stdint.h>
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/* Decode the next character, C, from BUF, reporting errors in E.
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 *
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 * Since this is a branchless decoder, four bytes will be read from the
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 * buffer regardless of the actual length of the next character. This
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 * means the buffer _must_ have at least three bytes of zero padding
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 * following the end of the data stream.
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 *
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 * Errors are reported in E, which will be non-zero if the parsed
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 * character was somehow invalid: invalid byte sequence, non-canonical
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 * encoding, or a surrogate half.
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 *
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 * The function returns a pointer to the next character. When an error
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 * occurs, this pointer will be a guess that depends on the particular
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 * error, but it will always advance at least one byte.
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 */
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static void *
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utf8_decode(void *buf, uint32_t *c, int *e)
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{
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	static const char lengths[] = {
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		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
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		0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 2, 3, 3, 4, 0
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	};
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	static const int masks[] = {0x00, 0x7f, 0x1f, 0x0f, 0x07};
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	static const uint32_t mins[] = {4194304, 0, 128, 2048, 65536};
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	static const int shiftc[] = {0, 18, 12, 6, 0};
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	static const int shifte[] = {0, 6, 4, 2, 0};
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	unsigned char *s = buf;
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	int len = lengths[s[0] >> 3];
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	/* Compute the pointer to the next character early so that the next
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	 * iteration can start working on the next character. Neither Clang
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	 * nor GCC figure out this reordering on their own.
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	 */
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	unsigned char *next = s + len + !len;
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	/* Assume a four-byte character and load four bytes. Unused bits are
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	 * shifted out.
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	 */
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	*c  = (uint32_t)(s[0] & masks[len]) << 18;
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	*c |= (uint32_t)(s[1] & 0x3f) << 12;
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	*c |= (uint32_t)(s[2] & 0x3f) <<  6;
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	*c |= (uint32_t)(s[3] & 0x3f) <<  0;
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	*c >>= shiftc[len];
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	/* Accumulate the various error conditions. */
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	*e  = (*c < mins[len]) << 6; // non-canonical encoding
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	*e |= ((*c >> 11) == 0x1b) << 7;  // surrogate half?
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	*e |= (*c > 0x10FFFF) << 8;  // out of range?
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	*e |= (s[1] & 0xc0) >> 2;
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	*e |= (s[2] & 0xc0) >> 4;
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	*e |= (s[3]       ) >> 6;
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	*e ^= 0x2a; // top two bits of each tail byte correct?
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	*e >>= shifte[len];
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	return next;
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}
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#endif