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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/enc/encode.cc
+15 -35
View File
@@ -122,12 +122,20 @@ void StoreHuffmanTreeOfHuffmanTreeToBitMask(
};
// Throw away trailing zeros:
int codes_to_store = kCodeLengthCodes;
for (; codes_to_store > 3; --codes_to_store) {
for (; codes_to_store > 0; --codes_to_store) {
if (code_length_bitdepth[kStorageOrder[codes_to_store - 1]] != 0) {
break;
}
}
WriteBits(4, codes_to_store - 3, storage_ix, storage);
int num_codes = 0;
for (int i = 0; i < codes_to_store; ++i) {
if (code_length_bitdepth[kStorageOrder[i]] != 0) {
++num_codes;
}
}
if (num_codes == 1) {
codes_to_store = kCodeLengthCodes;
}
const int skip_two_first =
code_length_bitdepth[kStorageOrder[0]] == 0 &&
code_length_bitdepth[kStorageOrder[1]] == 0;
@@ -229,40 +237,8 @@ void StoreHuffmanCode(const EntropyCode<kSize>& code, int alphabet_size,
EntropyCode<kCodeLengthCodes> huffman_tree_entropy;
BuildEntropyCode(huffman_tree_histogram, 5, kCodeLengthCodes,
&huffman_tree_entropy);
Histogram<kCodeLengthCodes> trimmed_histogram = huffman_tree_histogram;
uint8_t* last_code = &huffman_tree[huffman_tree_size - 1];
while (*last_code == 0 || *last_code >= 17) {
trimmed_histogram.Remove(*last_code--);
}
int trimmed_size = trimmed_histogram.total_count_;
bool write_length = false;
if (trimmed_size >= 4 && trimmed_size <= 195 &&
trimmed_size < huffman_tree_size) {
EntropyCode<kCodeLengthCodes> trimmed_entropy;
BuildEntropyCode(trimmed_histogram, 5, kCodeLengthCodes, &trimmed_entropy);
int huffman_bit_cost = HuffmanTreeBitCost(huffman_tree_histogram,
huffman_tree_entropy);
int trimmed_bit_cost = HuffmanTreeBitCost(trimmed_histogram,
trimmed_entropy);;
trimmed_bit_cost += (trimmed_size < 68 ? 7 : 8);
if (trimmed_bit_cost < huffman_bit_cost) {
write_length = true;
huffman_tree_size = trimmed_size;
huffman_tree_entropy = trimmed_entropy;
}
}
StoreHuffmanTreeOfHuffmanTreeToBitMask(
&huffman_tree_entropy.depth_[0], storage_ix, storage);
WriteBits(1, write_length, storage_ix, storage);
if (write_length) {
WriteBits(1, huffman_tree_size >= 68, storage_ix, storage);
if (huffman_tree_size < 68) {
WriteBits(6, huffman_tree_size - 4, storage_ix, storage);
} else {
WriteBits(7, huffman_tree_size - 68, storage_ix, storage);
}
}
StoreHuffmanTreeToBitMask(&huffman_tree[0], &huffman_tree_extra_bits[0],
huffman_tree_size, huffman_tree_entropy,
storage_ix, storage);
@@ -322,9 +298,13 @@ void ComputeDistanceShortCodes(std::vector<Command>* cmds,
int cur_dist = (*cmds)[i].copy_distance_;
if (cur_dist == 0) break;
int dist_code = cur_dist + 16;
int limits[16] = { 0, 4, 10, 11,
6, 6, 11, 11,
11, 11, 11, 11,
12, 12, 12, 12 };
for (int k = 0; k < 16; ++k) {
// Only accept more popular choices.
if (cur_dist < 11 && ((k >= 2 && k < 4) || k >= 6)) {
if (cur_dist < limits[k]) {
// Typically unpopular ranges, don't replace a short distance
// with them.
continue;