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brotli/enc/compress_fragment_two_pass.cc
Zoltan Szabadka b4c223cf27 Add missing headers.
2016-01-11 12:17:44 +01:00

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/* Copyright 2015 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/
// Function for fast encoding of an input fragment, independently from the input
// history. This function uses two-pass processing: in the first pass we save
// the found backward matches and literal bytes into a buffer, and in the
// second pass we emit them into the bit stream using prefix codes built based
// on the actual command and literal byte histograms.
#include "./compress_fragment_two_pass.h"
#include <algorithm>
#include "./brotli_bit_stream.h"
#include "./bit_cost.h"
#include "./entropy_encode.h"
#include "./fast_log.h"
#include "./find_match_length.h"
#include "./port.h"
#include "./write_bits.h"
namespace brotli {
// kHashMul32 multiplier has these properties:
// * The multiplier must be odd. Otherwise we may lose the highest bit.
// * No long streaks of 1s or 0s.
// * There is no effort to ensure that it is a prime, the oddity is enough
// for this use.
// * The number has been tuned heuristically against compression benchmarks.
static const uint32_t kHashMul32 = 0x1e35a7bd;
static inline uint32_t Hash(const uint8_t* p, size_t shift) {
const uint64_t h = (BROTLI_UNALIGNED_LOAD64(p) << 16) * kHashMul32;
return static_cast<uint32_t>(h >> shift);
}
static inline uint32_t HashBytesAtOffset(uint64_t v, int offset, size_t shift) {
assert(offset >= 0);
assert(offset <= 2);
const uint64_t h = ((v >> (8 * offset)) << 16) * kHashMul32;
return static_cast<uint32_t>(h >> shift);
}
static inline int IsMatch(const uint8_t* p1, const uint8_t* p2) {
return (BROTLI_UNALIGNED_LOAD32(p1) == BROTLI_UNALIGNED_LOAD32(p2) &&
p1[4] == p2[4] &&
p1[5] == p2[5]);
}
// Builds a command and distance prefix code (each 64 symbols) into "depth" and
// "bits" based on "histogram" and stores it into the bit stream.
static void BuildAndStoreCommandPrefixCode(
const uint32_t histogram[128],
uint8_t depth[128], uint16_t bits[128],
size_t* storage_ix, uint8_t* storage) {
CreateHuffmanTree(histogram, 64, 15, depth);
CreateHuffmanTree(&histogram[64], 64, 14, &depth[64]);
// We have to jump through a few hoopes here in order to compute
// the command bits because the symbols are in a different order than in
// the full alphabet. This looks complicated, but having the symbols
// in this order in the command bits saves a few branches in the Emit*
// functions.
uint8_t cmd_depth[64];
uint16_t cmd_bits[64];
memcpy(cmd_depth, depth + 24, 24);
memcpy(cmd_depth + 24, depth, 8);
memcpy(cmd_depth + 32, depth + 48, 8);
memcpy(cmd_depth + 40, depth + 8, 8);
memcpy(cmd_depth + 48, depth + 56, 8);
memcpy(cmd_depth + 56, depth + 16, 8);
ConvertBitDepthsToSymbols(cmd_depth, 64, cmd_bits);
memcpy(bits, cmd_bits + 24, 16);
memcpy(bits + 8, cmd_bits + 40, 16);
memcpy(bits + 16, cmd_bits + 56, 16);
memcpy(bits + 24, cmd_bits, 48);
memcpy(bits + 48, cmd_bits + 32, 16);
memcpy(bits + 56, cmd_bits + 48, 16);
ConvertBitDepthsToSymbols(&depth[64], 64, &bits[64]);
{
// Create the bit length array for the full command alphabet.
uint8_t cmd_depth[704] = { 0 };
memcpy(cmd_depth, depth + 24, 8);
memcpy(cmd_depth + 64, depth + 32, 8);
memcpy(cmd_depth + 128, depth + 40, 8);
memcpy(cmd_depth + 192, depth + 48, 8);
memcpy(cmd_depth + 384, depth + 56, 8);
for (size_t i = 0; i < 8; ++i) {
cmd_depth[128 + 8 * i] = depth[i];
cmd_depth[256 + 8 * i] = depth[8 + i];
cmd_depth[448 + 8 * i] = depth[16 + i];
}
StoreHuffmanTree(cmd_depth, 704, storage_ix, storage);
}
StoreHuffmanTree(&depth[64], 64, storage_ix, storage);
}
inline void EmitInsertLen(uint32_t insertlen, uint32_t** commands) {
if (insertlen < 6) {
**commands = insertlen;
} else if (insertlen < 130) {
insertlen -= 2;
const uint32_t nbits = Log2FloorNonZero(insertlen) - 1u;
const uint32_t prefix = insertlen >> nbits;
const uint32_t inscode = (nbits << 1) + prefix + 2;
const uint32_t extra = insertlen - (prefix << nbits);
**commands = inscode | (extra << 8);
} else if (insertlen < 2114) {
insertlen -= 66;
const uint32_t nbits = Log2FloorNonZero(insertlen);
const uint32_t code = nbits + 10;
const uint32_t extra = insertlen - (1 << nbits);
**commands = code | (extra << 8);
} else if (insertlen < 6210) {
const uint32_t extra = insertlen - 2114;
**commands = 21 | (extra << 8);
} else if (insertlen < 22594) {
const uint32_t extra = insertlen - 6210;
**commands = 22 | (extra << 8);
} else {
const uint32_t extra = insertlen - 22594;
**commands = 23 | (extra << 8);
}
++(*commands);
}
inline void EmitCopyLen(size_t copylen, uint32_t** commands) {
if (copylen < 10) {
**commands = static_cast<uint32_t>(copylen + 38);
} else if (copylen < 134) {
copylen -= 6;
const size_t nbits = Log2FloorNonZero(copylen) - 1;
const size_t prefix = copylen >> nbits;
const size_t code = (nbits << 1) + prefix + 44;
const size_t extra = copylen - (prefix << nbits);
**commands = static_cast<uint32_t>(code | (extra << 8));
} else if (copylen < 2118) {
copylen -= 70;
const size_t nbits = Log2FloorNonZero(copylen);
const size_t code = nbits + 52;
const size_t extra = copylen - (1 << nbits);
**commands = static_cast<uint32_t>(code | (extra << 8));
} else {
const size_t extra = copylen - 2118;
**commands = static_cast<uint32_t>(63 | (extra << 8));
}
++(*commands);
}
inline void EmitCopyLenLastDistance(size_t copylen, uint32_t** commands) {
if (copylen < 12) {
**commands = static_cast<uint32_t>(copylen + 20);
++(*commands);
} else if (copylen < 72) {
copylen -= 8;
const size_t nbits = Log2FloorNonZero(copylen) - 1;
const size_t prefix = copylen >> nbits;
const size_t code = (nbits << 1) + prefix + 28;
const size_t extra = copylen - (prefix << nbits);
**commands = static_cast<uint32_t>(code | (extra << 8));
++(*commands);
} else if (copylen < 136) {
copylen -= 8;
const size_t code = (copylen >> 5) + 54;
const size_t extra = copylen & 31;
**commands = static_cast<uint32_t>(code | (extra << 8));
++(*commands);
**commands = 64;
++(*commands);
} else if (copylen < 2120) {
copylen -= 72;
const size_t nbits = Log2FloorNonZero(copylen);
const size_t code = nbits + 52;
const size_t extra = copylen - (1 << nbits);
**commands = static_cast<uint32_t>(code | (extra << 8));
++(*commands);
**commands = 64;
++(*commands);
} else {
const size_t extra = copylen - 2120;
**commands = static_cast<uint32_t>(63 | (extra << 8));
++(*commands);
**commands = 64;
++(*commands);
}
}
inline void EmitDistance(uint32_t distance, uint32_t** commands) {
distance += 3;
uint32_t nbits = Log2FloorNonZero(distance) - 1;
const uint32_t prefix = (distance >> nbits) & 1;
const uint32_t offset = (2 + prefix) << nbits;
const uint32_t distcode = 2 * (nbits - 1) + prefix + 80;
uint32_t extra = distance - offset;
**commands = distcode | (extra << 8);
++(*commands);
}
// REQUIRES: len <= 1 << 20.
static void StoreMetaBlockHeader(
size_t len, bool is_uncompressed, size_t* storage_ix, uint8_t* storage) {
// ISLAST
WriteBits(1, 0, storage_ix, storage);
if (len <= (1U << 16)) {
// MNIBBLES is 4
WriteBits(2, 0, storage_ix, storage);
WriteBits(16, len - 1, storage_ix, storage);
} else {
// MNIBBLES is 5
WriteBits(2, 1, storage_ix, storage);
WriteBits(20, len - 1, storage_ix, storage);
}
// ISUNCOMPRESSED
WriteBits(1, is_uncompressed, storage_ix, storage);
}
void CreateCommands(const uint8_t* input, size_t block_size, size_t input_size,
const uint8_t* base_ip,
int* table, size_t table_size,
uint8_t** literals, uint32_t** commands) {
// "ip" is the input pointer.
const uint8_t* ip = input;
assert(table_size);
assert(table_size <= (1u << 31));
assert((table_size & (table_size - 1)) == 0); // table must be power of two
const size_t shift = 64u - Log2FloorNonZero(table_size);
assert(static_cast<size_t>(0xffffffffffffffffU >> shift) == table_size - 1);
const uint8_t* ip_end = input + block_size;
// "next_emit" is a pointer to the first byte that is not covered by a
// previous copy. Bytes between "next_emit" and the start of the next copy or
// the end of the input will be emitted as literal bytes.
const uint8_t* next_emit = input;
int last_distance = -1;
const size_t kInputMarginBytes = 16;
const size_t kMinMatchLen = 6;
if (PREDICT_TRUE(block_size >= kInputMarginBytes)) {
// For the last block, we need to keep a 16 bytes margin so that we can be
// sure that all distances are at most window size - 16.
// For all other blocks, we only need to keep a margin of 5 bytes so that
// we don't go over the block size with a copy.
const size_t len_limit = std::min(block_size - kMinMatchLen,
input_size - kInputMarginBytes);
const uint8_t* ip_limit = input + len_limit;
for (uint32_t next_hash = Hash(++ip, shift); ; ) {
assert(next_emit < ip);
// Step 1: Scan forward in the input looking for a 6-byte-long match.
// If we get close to exhausting the input then goto emit_remainder.
//
// Heuristic match skipping: If 32 bytes are scanned with no matches
// found, start looking only at every other byte. If 32 more bytes are
// scanned, look at every third byte, etc.. When a match is found,
// immediately go back to looking at every byte. This is a small loss
// (~5% performance, ~0.1% density) for compressible data due to more
// bookkeeping, but for non-compressible data (such as JPEG) it's a huge
// win since the compressor quickly "realizes" the data is incompressible
// and doesn't bother looking for matches everywhere.
//
// The "skip" variable keeps track of how many bytes there are since the
// last match; dividing it by 32 (ie. right-shifting by five) gives the
// number of bytes to move ahead for each iteration.
uint32_t skip = 32;
const uint8_t* next_ip = ip;
const uint8_t* candidate;
do {
ip = next_ip;
uint32_t hash = next_hash;
assert(hash == Hash(ip, shift));
uint32_t bytes_between_hash_lookups = skip++ >> 5;
next_ip = ip + bytes_between_hash_lookups;
if (PREDICT_FALSE(next_ip > ip_limit)) {
goto emit_remainder;
}
next_hash = Hash(next_ip, shift);
candidate = ip - last_distance;
if (IsMatch(ip, candidate)) {
if (PREDICT_TRUE(candidate < ip)) {
table[hash] = static_cast<int>(ip - base_ip);
break;
}
}
candidate = base_ip + table[hash];
assert(candidate >= base_ip);
assert(candidate < ip);
table[hash] = static_cast<int>(ip - base_ip);
} while (PREDICT_TRUE(!IsMatch(ip, candidate)));
// Step 2: Emit the found match together with the literal bytes from
// "next_emit", and then see if we can find a next macth immediately
// afterwards. Repeat until we find no match for the input
// without emitting some literal bytes.
uint64_t input_bytes;
{
// We have a 6-byte match at ip, and we need to emit bytes in
// [next_emit, ip).
const uint8_t* base = ip;
size_t matched = 6 + FindMatchLengthWithLimit(
candidate + 6, ip + 6, static_cast<size_t>(ip_end - ip) - 6);
ip += matched;
int distance = static_cast<int>(base - candidate); /* > 0 */
int insert = static_cast<int>(base - next_emit);
assert(0 == memcmp(base, candidate, matched));
EmitInsertLen(static_cast<uint32_t>(insert), commands);
memcpy(*literals, next_emit, static_cast<size_t>(insert));
*literals += insert;
if (distance == last_distance) {
**commands = 64;
++(*commands);
} else {
EmitDistance(static_cast<uint32_t>(distance), commands);
last_distance = distance;
}
EmitCopyLenLastDistance(matched, commands);
next_emit = ip;
if (PREDICT_FALSE(ip >= ip_limit)) {
goto emit_remainder;
}
// We could immediately start working at ip now, but to improve
// compression we first update "table" with the hashes of some positions
// within the last copy.
input_bytes = BROTLI_UNALIGNED_LOAD64(ip - 5);
uint32_t prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 5);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 4);
prev_hash = HashBytesAtOffset(input_bytes, 2, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 3);
input_bytes = BROTLI_UNALIGNED_LOAD64(ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 1);
uint32_t cur_hash = HashBytesAtOffset(input_bytes, 2, shift);
candidate = base_ip + table[cur_hash];
table[cur_hash] = static_cast<int>(ip - base_ip);
}
while (IsMatch(ip, candidate)) {
// We have a 6-byte match at ip, and no need to emit any
// literal bytes prior to ip.
const uint8_t* base = ip;
size_t matched = 6 + FindMatchLengthWithLimit(
candidate + 6, ip + 6, static_cast<size_t>(ip_end - ip) - 6);
ip += matched;
last_distance = static_cast<int>(base - candidate); /* > 0 */
assert(0 == memcmp(base, candidate, matched));
EmitCopyLen(matched, commands);
EmitDistance(static_cast<uint32_t>(last_distance), commands);
next_emit = ip;
if (PREDICT_FALSE(ip >= ip_limit)) {
goto emit_remainder;
}
// We could immediately start working at ip now, but to improve
// compression we first update "table" with the hashes of some positions
// within the last copy.
input_bytes = BROTLI_UNALIGNED_LOAD64(ip - 5);
uint32_t prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 5);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 4);
prev_hash = HashBytesAtOffset(input_bytes, 2, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 3);
input_bytes = BROTLI_UNALIGNED_LOAD64(ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 1);
uint32_t cur_hash = HashBytesAtOffset(input_bytes, 2, shift);
candidate = base_ip + table[cur_hash];
table[cur_hash] = static_cast<int>(ip - base_ip);
}
next_hash = Hash(++ip, shift);
}
}
emit_remainder:
assert(next_emit <= ip_end);
// Emit the remaining bytes as literals.
if (next_emit < ip_end) {
const uint32_t insert = static_cast<uint32_t>(ip_end - next_emit);
EmitInsertLen(insert, commands);
memcpy(*literals, next_emit, insert);
*literals += insert;
}
}
void StoreCommands(const uint8_t* literals, const size_t num_literals,
const uint32_t* commands, const size_t num_commands,
size_t* storage_ix, uint8_t* storage) {
uint8_t lit_depths[256] = { 0 };
uint16_t lit_bits[256] = { 0 };
uint32_t lit_histo[256] = { 0 };
for (size_t i = 0; i < num_literals; ++i) {
++lit_histo[literals[i]];
}
BuildAndStoreHuffmanTreeFast(lit_histo, num_literals,
/* max_bits = */ 8,
lit_depths, lit_bits,
storage_ix, storage);
uint8_t cmd_depths[128] = { 0 };
uint16_t cmd_bits[128] = { 0 };
uint32_t cmd_histo[128] = { 0 };
for (size_t i = 0; i < num_commands; ++i) {
++cmd_histo[commands[i] & 0xff];
}
cmd_histo[1] += 1;
cmd_histo[2] += 1;
cmd_histo[64] += 1;
cmd_histo[84] += 1;
BuildAndStoreCommandPrefixCode(cmd_histo, cmd_depths, cmd_bits,
storage_ix, storage);
static const uint32_t kNumExtraBits[128] = {
0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9, 10, 12, 14, 24,
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4,
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9, 10, 24,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16,
17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24,
};
static const uint32_t kInsertOffset[24] = {
0, 1, 2, 3, 4, 5, 6, 8, 10, 14, 18, 26, 34, 50, 66, 98, 130, 194, 322, 578,
1090, 2114, 6210, 22594,
};
for (size_t i = 0; i < num_commands; ++i) {
const uint32_t cmd = commands[i];
const uint32_t code = cmd & 0xff;
const uint32_t extra = cmd >> 8;
WriteBits(cmd_depths[code], cmd_bits[code], storage_ix, storage);
WriteBits(kNumExtraBits[code], extra, storage_ix, storage);
if (code < 24) {
const uint32_t insert = kInsertOffset[code] + extra;
for (uint32_t j = 0; j < insert; ++j) {
const uint8_t lit = *literals;
WriteBits(lit_depths[lit], lit_bits[lit], storage_ix, storage);
++literals;
}
}
}
}
bool ShouldCompress(const uint8_t* input, size_t input_size,
size_t num_literals) {
static const double kAcceptableLossForUncompressibleSpeedup = 0.02;
static const double kMaxRatioOfLiterals =
1.0 - kAcceptableLossForUncompressibleSpeedup;
if (num_literals < kMaxRatioOfLiterals * static_cast<double>(input_size)) {
return true;
}
uint32_t literal_histo[256] = { 0 };
static const uint32_t kSampleRate = 43;
static const double kMaxEntropy =
8 * (1.0 - kAcceptableLossForUncompressibleSpeedup);
const double max_total_bit_cost =
static_cast<double>(input_size) * kMaxEntropy / kSampleRate;
for (size_t i = 0; i < input_size; i += kSampleRate) {
++literal_histo[input[i]];
}
return BitsEntropy(literal_histo, 256) < max_total_bit_cost;
}
void BrotliCompressFragmentTwoPass(const uint8_t* input, size_t input_size,
bool is_last,
uint32_t* command_buf, uint8_t* literal_buf,
int* table, size_t table_size,
size_t* storage_ix, uint8_t* storage) {
// Save the start of the first block for position and distance computations.
const uint8_t* base_ip = input;
while (input_size > 0) {
size_t block_size = std::min(input_size, kCompressFragmentTwoPassBlockSize);
uint32_t* commands = command_buf;
uint8_t* literals = literal_buf;
CreateCommands(input, block_size, input_size, base_ip, table, table_size,
&literals, &commands);
const size_t num_literals = static_cast<size_t>(literals - literal_buf);
const size_t num_commands = static_cast<size_t>(commands - command_buf);
if (ShouldCompress(input, block_size, num_literals)) {
StoreMetaBlockHeader(block_size, 0, storage_ix, storage);
// No block splits, no contexts.
WriteBits(13, 0, storage_ix, storage);
StoreCommands(literal_buf, num_literals, command_buf, num_commands,
storage_ix, storage);
} else {
// Since we did not find many backward references and the entropy of
// the data is close to 8 bits, we can simply emit an uncompressed block.
// This makes compression speed of uncompressible data about 3x faster.
StoreMetaBlockHeader(block_size, 1, storage_ix, storage);
*storage_ix = (*storage_ix + 7u) & ~7u;
memcpy(&storage[*storage_ix >> 3], input, block_size);
*storage_ix += block_size << 3;
storage[*storage_ix >> 3] = 0;
}
input += block_size;
input_size -= block_size;
}
if (is_last) {
WriteBits(1, 1, storage_ix, storage); // islast
WriteBits(1, 1, storage_ix, storage); // isempty
*storage_ix = (*storage_ix + 7u) & ~7u;
}
}
} // namespace brotli
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