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Brotli format change: improved encoding of Huffman codes

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This change removes the redundant HCLEN, HLENINC and HLEN
fields from the encoding of the complex Huffman codes and
derives these from an invariant of the code length sequence.
Based on a patch by Robert Obryk.
This commit is contained in:
Zoltan Szabadka
2013-12-17 17:17:57 +01:00
parent 30625ba238
commit b89f3be40b
6 changed files with 109 additions and 145 deletions
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brotli/dec/decode.c
+25 -29
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@@ -147,11 +147,10 @@ static int ReadHuffmanCodeLengths(
BrotliBitReader* br) {
int ok = 0;
int symbol;
int max_symbol;
int decode_number_of_code_length_codes;
int prev_code_len = kDefaultCodeLength;
int repeat = 0;
int repeat_length = 0;
int space = 32768;
HuffmanTree tree;
if (!BrotliHuffmanTreeBuildImplicit(&tree, code_length_code_lengths,
@@ -165,28 +164,10 @@ static int ReadHuffmanCodeLengths(
printf("[ReadHuffmanCodeLengths] Unexpected end of input.\n");
return 0;
}
decode_number_of_code_length_codes = BrotliReadBits(br, 1);
BROTLI_LOG_UINT(decode_number_of_code_length_codes);
if (decode_number_of_code_length_codes) {
if (BrotliReadBits(br, 1)) {
max_symbol = 68 + BrotliReadBits(br, 7);
} else {
max_symbol = 4 + BrotliReadBits(br, 6);
}
if (max_symbol > num_symbols) {
printf("[ReadHuffmanCodeLengths] max_symbol > num_symbols (%d vs %d)\n",
max_symbol, num_symbols);
goto End;
}
} else {
max_symbol = num_symbols;
}
BROTLI_LOG_UINT(max_symbol);
symbol = 0;
while (symbol + repeat < num_symbols) {
while (symbol + repeat < num_symbols && space > 0) {
int code_len;
if (max_symbol-- == 0) break;
if (!BrotliReadMoreInput(br)) {
printf("[ReadHuffmanCodeLengths] Unexpected end of input.\n");
goto End;
@@ -206,17 +187,32 @@ static int ReadHuffmanCodeLengths(
}
if (code_len < kCodeLengthRepeatCode) {
code_lengths[symbol++] = code_len;
if (code_len != 0) prev_code_len = code_len;
if (code_len != 0) {
prev_code_len = code_len;
space -= 32768 >> code_len;
}
} else {
const int extra_bits = code_len - 14;
int i = repeat;
if (repeat > 0) {
repeat -= 2;
repeat <<= extra_bits;
}
repeat += BrotliReadBits(br, extra_bits) + 3;
repeat_length = (code_len == kCodeLengthRepeatCode ? prev_code_len : 0);
if (code_len == kCodeLengthRepeatCode) {
repeat_length = prev_code_len;
for (; i < repeat; ++i) {
space -= 32768 >> repeat_length;
}
} else {
repeat_length = 0;
}
}
}
if (space != 0) {
printf("[ReadHuffmanCodeLengths] space = %d\n", space);
goto End;
}
if (symbol + repeat > num_symbols) {
printf("[ReadHuffmanCodeLengths] symbol + repeat > num_symbols "
"(%d + %d vs %d)\n", symbol, repeat, num_symbols);
@@ -286,12 +282,9 @@ static int ReadHuffmanCode(int alphabet_size,
} else { /* Decode Huffman-coded code lengths. */
int i;
uint8_t code_length_code_lengths[CODE_LENGTH_CODES] = { 0 };
const int num_codes = BrotliReadBits(br, 4) + 3;
BROTLI_LOG_UINT(num_codes);
if (num_codes > CODE_LENGTH_CODES) {
return 0;
}
for (i = BrotliReadBits(br, 1) * 2; i < num_codes; ++i) {
int space = 32;
for (i = BrotliReadBits(br, 1) * 2;
i < CODE_LENGTH_CODES && space > 0; ++i) {
int code_len_idx = kCodeLengthCodeOrder[i];
int v = BrotliReadBits(br, 2);
if (v == 1) {
@@ -311,6 +304,9 @@ static int ReadHuffmanCode(int alphabet_size,
}
code_length_code_lengths[code_len_idx] = v;
BROTLI_LOG_ARRAY_INDEX(code_length_code_lengths, code_len_idx);
if (v != 0) {
space -= (32 >> v);
}
}
ok = ReadHuffmanCodeLengths(code_length_code_lengths, alphabet_size,
code_lengths, br);