libgrapheme

utf8.c (6131B)

---

 /* See LICENSE file for copyright and license details. */
 #include <stddef.h>
 #include <stdint.h>
 
 #include "../grapheme.h"
 #include "util.h"
 
 #define BETWEEN(c, l, u) ((c) >= (l) && (c) <= (u))
 
 /* lookup-table for the types of sequence first bytes */
 static const struct {
 	uint_least8_t lower;  /* lower bound of sequence first byte */
 	uint_least8_t upper;  /* upper bound of sequence first byte */
 	uint_least32_t mincp; /* smallest non-overlong encoded codepoint */
 	uint_least32_t maxcp; /* largest encodable codepoint */
 			      /*
 	                       * implicit: table-offset represents the number of following
 	                       * bytes of the form 10xxxxxx (6 bits capacity each)
 	                       */
 } lut[] = {
 	[0] = {
 		/* 0xxxxxxx */
 		.lower = 0x00, /* 00000000 */
 		.upper = 0x7F, /* 01111111 */
 		.mincp = (uint_least32_t)0,
 		.maxcp = ((uint_least32_t)1 << 7) - 1, /* 7 bits capacity */
 	},
 	[1] = {
 		/* 110xxxxx */
 		.lower = 0xC0, /* 11000000 */
 		.upper = 0xDF, /* 11011111 */
 		.mincp = (uint_least32_t)1 << 7,
 		.maxcp = ((uint_least32_t)1 << 11) - 1, /* 5+6=11 bits capacity */
 	},
 	[2] = {
 		/* 1110xxxx */
 		.lower = 0xE0, /* 11100000 */
 		.upper = 0xEF, /* 11101111 */
 		.mincp = (uint_least32_t)1 << 11,
 		.maxcp = ((uint_least32_t)1 << 16) - 1, /* 4+6+6=16 bits capacity */
 	},
 	[3] = {
 		/* 11110xxx */
 		.lower = 0xF0, /* 11110000 */
 		.upper = 0xF7, /* 11110111 */
 		.mincp = (uint_least32_t)1 << 16,
 		.maxcp = ((uint_least32_t)1 << 21) - 1, /* 3+6+6+6=21 bits capacity */
 	},
 };
 
 size_t
 grapheme_decode_utf8(const char *str, size_t len, uint_least32_t *cp)
 {
 	size_t off, i;
 	uint_least32_t tmp;
 
 	if (cp == NULL) {
 		/*
 		 * instead of checking every time if cp is NULL within
 		 * the decoder, simply point it at a dummy variable here.
 		 */
 		cp = &tmp;
 	}
 
 	if (str == NULL || len == 0) {
 		/* a sequence must be at least 1 byte long */
 		*cp = GRAPHEME_INVALID_CODEPOINT;
 		return 0;
 	}
 
 	/* identify sequence type with the first byte */
 	for (off = 0; off < LEN(lut); off++) {
 		if (BETWEEN(((const unsigned char *)str)[0], lut[off].lower,
 		            lut[off].upper)) {
 			/*
 			 * first byte is within the bounds; fill
 			 * p with the the first bits contained in
 			 * the first byte (by subtracting the high bits)
 			 */
 			*cp = ((const unsigned char *)str)[0] - lut[off].lower;
 			break;
 		}
 	}
 	if (off == LEN(lut)) {
 		/*
 		 * first byte does not match a sequence type;
 		 * set cp as invalid and return 1 byte processed
 		 *
 		 * this also includes the cases where bits higher than
 		 * the 8th are set on systems with CHAR_BIT > 8
 		 */
 		*cp = GRAPHEME_INVALID_CODEPOINT;
 		return 1;
 	}
 	if (1 + off > len) {
 		/*
 		 * input is not long enough, set cp as invalid
 		 */
 		*cp = GRAPHEME_INVALID_CODEPOINT;
 
 		/*
 		 * count the following continuation bytes, but nothing
 		 * else in case we have a "rogue" case where e.g. such a
 		 * sequence starter occurs right before a NUL-byte.
 		 */
 		for (i = 0; 1 + i < len; i++) {
 			if (!BETWEEN(((const unsigned char *)str)[1 + i], 0x80,
 			             0xBF)) {
 				break;
 			}
 		}
 
 		/*
 		 * if the continuation bytes do not continue until
 		 * the end, return the incomplete sequence length.
 		 * Otherwise return the number of bytes we actually
 		 * expected, which is larger than n.
 		 */
 		return ((1 + i) < len) ? (1 + i) : (1 + off);
 	}
 
 	/*
 	 * process 'off' following bytes, each of the form 10xxxxxx
 	 * (i.e. between 0x80 (10000000) and 0xBF (10111111))
 	 */
 	for (i = 1; i <= off; i++) {
 		if (!BETWEEN(((const unsigned char *)str)[i], 0x80, 0xBF)) {
 			/*
 			 * byte does not match format; return
 			 * number of bytes processed excluding the
 			 * unexpected character as recommended since
 			 * Unicode 6 (chapter 3)
 			 *
 			 * this also includes the cases where bits
 			 * higher than the 8th are set on systems
 			 * with CHAR_BIT > 8
 			 */
 			*cp = GRAPHEME_INVALID_CODEPOINT;
 			return 1 + (i - 1);
 		}
 		/*
 		 * shift codepoint by 6 bits and add the 6 stored bits
 		 * in s[i] to it using the bitmask 0x3F (00111111)
 		 */
 		*cp = (*cp << 6) | (((const unsigned char *)str)[i] & 0x3F);
 	}
 
 	if (*cp < lut[off].mincp ||
 	    BETWEEN(*cp, UINT32_C(0xD800), UINT32_C(0xDFFF)) ||
 	    *cp > UINT32_C(0x10FFFF)) {
 		/*
 		 * codepoint is overlong encoded in the sequence, is a
 		 * high or low UTF-16 surrogate half (0xD800..0xDFFF) or
 		 * not representable in UTF-16 (>0x10FFFF) (RFC-3629
 		 * specifies the latter two conditions)
 		 */
 		*cp = GRAPHEME_INVALID_CODEPOINT;
 	}
 
 	return 1 + off;
 }
 
 size_t
 grapheme_encode_utf8(uint_least32_t cp, char *str, size_t len)
 {
 	size_t off, i;
 
 	if (BETWEEN(cp, UINT32_C(0xD800), UINT32_C(0xDFFF)) ||
 	    cp > UINT32_C(0x10FFFF)) {
 		/*
 		 * codepoint is a high or low UTF-16 surrogate half
 		 * (0xD800..0xDFFF) or not representable in UTF-16
 		 * (>0x10FFFF), which RFC-3629 deems invalid for UTF-8.
 		 */
 		cp = GRAPHEME_INVALID_CODEPOINT;
 	}
 
 	/* determine necessary sequence type */
 	for (off = 0; off < LEN(lut); off++) {
 		if (cp <= lut[off].maxcp) {
 			break;
 		}
 	}
 	if (1 + off > len || str == NULL || len == 0) {
 		/*
 		 * specified buffer is too small to store sequence or
 		 * the caller just wanted to know how many bytes the
 		 * codepoint needs by passing a NULL-buffer.
 		 */
 		return 1 + off;
 	}
 
 	/* build sequence by filling cp-bits into each byte */
 
 	/*
 	 * lut[off].lower is the bit-format for the first byte and
 	 * the bits to fill into it are determined by shifting the
 	 * cp 6 times the number of following bytes, as each
 	 * following byte stores 6 bits, yielding the wanted bits.
 	 *
 	 * We do not overwrite the mask because we guaranteed earlier
 	 * that there are no bits higher than the mask allows.
 	 */
 	((unsigned char *)str)[0] =
 		lut[off].lower | (uint_least8_t)(cp >> (6 * off));
 
 	for (i = 1; i <= off; i++) {
 		/*
 		 * the bit-format for following bytes is 10000000 (0x80)
 		 * and it each stores 6 bits in the 6 low bits that we
 		 * extract from the properly-shifted value using the
 		 * mask 00111111 (0x3F)
 		 */
 		((unsigned char *)str)[i] =
 			0x80 | ((cp >> (6 * (off - i))) & 0x3F);
 	}
 
 	return 1 + off;
 }