huffman codes may be up to 16 bit long (1fa97645) · Commits · derf / zlib-deflate-nostdlib

README.md

+0 −6

Original line number	Diff line number	Diff line
		@@ -95,12 +95,6 @@ following exceptions.

		The code almost complies with RFC 1951, with the following exceptions.

		* zlib-deflate-nostdlib assumes that Huffman codes are limited to a length
		of 12 bits and that there are no more than 255 codes per length. This appears
		to be a reasonable assumption for embedded devices, whose decompression
		abilities are limited by the amount of RAM anyways. I have not yet determined
		whether longer Huffman codes can appear in practice or not, and if so, under
		which conditions.
		* zlib-deflate-nostdlib does not yet support compressed items consisting of
		more than one deflate block. I intend to fix this.

src/inflate.c

+16 −19

Original line number	Diff line number	Diff line
		@@ -88,36 +88,33 @@ uint8_t deflate_hc_lengths[19];

		/*
		* Code lengths of the literal/length and distance alphabets.
		* up to 286 literal/length codes + up to 32 distance codes.
		*/
		uint8_t deflate_lld_lengths[318];

		/*
		* Assumptions:
		* * huffman code length is limited to 11 bits
		* * there are no more than 255 huffman codes with the same length
		*
		* Rationale: longer huffman codes might appear when handling large data
		* sets. We don't do that; instead, we expect the uncompressed source to
		* be no more than a few kB of data.
		*/

		/*
		* Bit length counts and next code entries for Literal/Length alphabet.
		* Combined with the code lengths in deflate_lld_lengths, these make up the
		* Literal/Length alphabet. See the algorithm in RFC 1951 section 3.2.2 for
		* details.
		*
		* In deflate, these variables are also used for the huffman alphabet in
		* dynamic huffman blocks.
		* Assumption: There are no more than 255 huffman codes with the same length.
		* As the largest alphabet (the literal/length alphabet) contains just 288
		* codes in total, this should be reasonable.
		*
		* These variables are also used for the huffman alphabet in dynamic huffman
		* blocks.
		*/
		uint8_t deflate_bl_count_ll[12];
		uint16_t deflate_next_code_ll[12];
		uint8_t deflate_bl_count_ll[16];
		uint16_t deflate_next_code_ll[16];

		/*
		* Bit length counts and next code entries for Distance alphabet.
		* Bit length counts and next code entries for Distance alphabet. Note that,
		* even though there are just 30 different distance codes, individual
		* codes may be up to 16 bits long.
		*/
		uint8_t deflate_bl_count_d[12];
		uint16_t deflate_next_code_d[12];
		uint8_t deflate_bl_count_d[16];
		uint16_t deflate_next_code_d[16];

		static uint16_t deflate_rev_word(uint16_t word, uint8_t bits)
		{
		@@ -168,7 +165,7 @@ static void deflate_build_alphabet(uint8_t * lengths, uint16_t size,
		uint16_t i;
		uint16_t code = 0;
		uint16_t max_len = 0;
		for (i = 0; i < 12; i++) {
		for (i = 0; i < 16; i++) {
		bl_count[i] = 0;
		}

		@@ -191,7 +188,7 @@ static uint16_t deflate_huff(uint8_t * lengths, uint16_t size,
		uint8_t * bl_count, uint16_t * next_code)
		{
		uint16_t next_word = deflate_get_word();
		for (uint8_t num_bits = 1; num_bits < 12; num_bits++) {
		for (uint8_t num_bits = 1; num_bits < 16; num_bits++) {
		uint16_t next_bits = deflate_rev_word(next_word, num_bits);
		if (bl_count[num_bits] && next_bits >= next_code[num_bits]
		&& next_bits < next_code[num_bits] + bl_count[num_bits]) {

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