/* 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 one-pass processing: when we find a backward
// match, we immediately emit the corresponding command and literal codes to
// the bit stream.
//
// Adapted from the CompressFragment() function in
// https://github.com/google/snappy/blob/master/snappy.cc
#include "./compress_fragment.h"
#include <algorithm>
#include <cstring>
#include "./brotli_bit_stream.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) << 24) * 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 <= 3);
const uint64_t h = ((v >> (8 * offset)) << 24) * 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]);
}
// Builds a literal prefix code into "depths" and "bits" based on the statistics
// of the "input" string and stores it into the bit stream.
// Note that the prefix code here is built from the pre-LZ77 input, therefore
// we can only approximate the statistics of the actual literal stream.
// Moreover, for long inputs we build a histogram from a sample of the input
// and thus have to assign a non-zero depth for each literal.
void BuildAndStoreLiteralPrefixCode(const uint8_t* input,
const size_t input_size,
uint8_t depths[256],
uint16_t bits[256],
size_t* storage_ix,
uint8_t* storage) {
uint32_t histogram[256] = { 0 };
size_t histogram_total;
if (input_size < (1 << 15)) {
for (size_t i = 0; i < input_size; ++i) {
++histogram[input[i]];
}
histogram_total = input_size;
for (size_t i = 0; i < 256; ++i) {
// We weigh the first 11 samples with weight 3 to account for the
// balancing effect of the LZ77 phase on the histogram.
const uint32_t adjust = 2 * std::min(histogram[i], 11u);
histogram[i] += adjust;
histogram_total += adjust;
}
} else {
static const size_t kSampleRate = 29;
for (size_t i = 0; i < input_size; i += kSampleRate) {
++histogram[input[i]];
}
histogram_total = (input_size + kSampleRate - 1) / kSampleRate;
for (size_t i = 0; i < 256; ++i) {
// We add 1 to each population count to avoid 0 bit depths (since this is
// only a sample and we don't know if the symbol appears or not), and we
// weigh the first 11 samples with weight 3 to account for the balancing
// effect of the LZ77 phase on the histogram (more frequent symbols are
// more likely to be in backward references instead as literals).
const uint32_t adjust = 1 + 2 * std::min(histogram[i], 11u);
histogram[i] += adjust;
histogram_total += adjust;
}
}
BuildAndStoreHuffmanTreeFast(histogram, histogram_total,
/* max_bits = */ 8,
depths, bits, storage_ix, storage);
}
// Builds a command and distance prefix code (each 64 symbols) into "depth" and
// "bits" based on "histogram" and stores it into the bit stream.
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);
memcpy(cmd_depth + 24, depth + 40, 8);
memcpy(cmd_depth + 32, depth + 24, 8);
memcpy(cmd_depth + 40, depth + 48, 8);
memcpy(cmd_depth + 48, depth + 32, 8);
memcpy(cmd_depth + 56, depth + 56, 8);
ConvertBitDepthsToSymbols(cmd_depth, 64, cmd_bits);
memcpy(bits, cmd_bits, 48);
memcpy(bits + 24, cmd_bits + 32, 16);
memcpy(bits + 32, cmd_bits + 48, 16);
memcpy(bits + 40, cmd_bits + 24, 16);
memcpy(bits + 48, cmd_bits + 40, 16);
memcpy(bits + 56, cmd_bits + 56, 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, 8);
memcpy(cmd_depth + 64, depth + 8, 8);
memcpy(cmd_depth + 128, depth + 16, 8);
memcpy(cmd_depth + 192, depth + 24, 8);
memcpy(cmd_depth + 384, depth + 32, 8);
for (size_t i = 0; i < 8; ++i) {
cmd_depth[128 + 8 * i] = depth[40 + i];
cmd_depth[256 + 8 * i] = depth[48 + i];
cmd_depth[448 + 8 * i] = depth[56 + i];
}
StoreHuffmanTree(cmd_depth, 704, storage_ix, storage);
}
StoreHuffmanTree(&depth[64], 64, storage_ix, storage);
}
// REQUIRES: insertlen < 6210
inline void EmitInsertLen(size_t insertlen,
const uint8_t depth[128],
const uint16_t bits[128],
uint32_t histo[128],
size_t* storage_ix,
uint8_t* storage) {
if (insertlen < 6) {
const size_t code = insertlen + 40;
WriteBits(depth[code], bits[code], storage_ix, storage);
++histo[code];
} else if (insertlen < 130) {
insertlen -= 2;
const uint32_t nbits = Log2FloorNonZero(insertlen) - 1u;
const size_t prefix = insertlen >> nbits;
const size_t inscode = (nbits << 1) + prefix + 42;
WriteBits(depth[inscode], bits[inscode], storage_ix, storage);
WriteBits(nbits, insertlen - (prefix << nbits), storage_ix, storage);
++histo[inscode];
} else if (insertlen < 2114) {
insertlen -= 66;
const uint32_t nbits = Log2FloorNonZero(insertlen);
const size_t code = nbits + 50;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(nbits, insertlen - (1 << nbits), storage_ix, storage);
++histo[code];
} else {
WriteBits(depth[61], bits[61], storage_ix, storage);
WriteBits(12, insertlen - 2114, storage_ix, storage);
++histo[21];
}
}
inline void EmitLongInsertLen(size_t insertlen,
const uint8_t depth[128],
const uint16_t bits[128],
uint32_t histo[128],
size_t* storage_ix,
uint8_t* storage) {
if (insertlen < 22594) {
WriteBits(depth[62], bits[62], storage_ix, storage);
WriteBits(14, insertlen - 6210, storage_ix, storage);
++histo[22];
} else {
WriteBits(depth[63], bits[63], storage_ix, storage);
WriteBits(24, insertlen - 22594, storage_ix, storage);
++histo[23];
}
}
inline void EmitCopyLen(size_t copylen,
const uint8_t depth[128],
const uint16_t bits[128],
uint32_t histo[128],
size_t* storage_ix,
uint8_t* storage) {
if (copylen < 10) {
WriteBits(depth[copylen + 14], bits[copylen + 14], storage_ix, storage);
++histo[copylen + 14];
} else if (copylen < 134) {
copylen -= 6;
const uint32_t nbits = Log2FloorNonZero(copylen) - 1u;
const size_t prefix = copylen >> nbits;
const size_t code = (nbits << 1) + prefix + 20;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(nbits, copylen - (prefix << nbits), storage_ix, storage);
++histo[code];
} else if (copylen < 2118) {
copylen -= 70;
const uint32_t nbits = Log2FloorNonZero(copylen);
const size_t code = nbits + 28;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(nbits, copylen - (1 << nbits), storage_ix, storage);
++histo[code];
} else {
WriteBits(depth[39], bits[39], storage_ix, storage);
WriteBits(24, copylen - 2118, storage_ix, storage);
++histo[47];
}
}
inline void EmitCopyLenLastDistance(size_t copylen,
const uint8_t depth[128],
const uint16_t bits[128],
uint32_t histo[128],
size_t* storage_ix,
uint8_t* storage) {
if (copylen < 12) {
WriteBits(depth[copylen - 4], bits[copylen - 4], storage_ix, storage);
++histo[copylen - 4];
} else if (copylen < 72) {
copylen -= 8;
const uint32_t nbits = Log2FloorNonZero(copylen) - 1;
const size_t prefix = copylen >> nbits;
const size_t code = (nbits << 1) + prefix + 4;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(nbits, copylen - (prefix << nbits), storage_ix, storage);
++histo[code];
} else if (copylen < 136) {
copylen -= 8;
const size_t code = (copylen >> 5) + 30;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(5, copylen & 31, storage_ix, storage);
WriteBits(depth[64], bits[64], storage_ix, storage);
++histo[code];
++histo[64];
} else if (copylen < 2120) {
copylen -= 72;
const uint32_t nbits = Log2FloorNonZero(copylen);
const size_t code = nbits + 28;
WriteBits(depth[code], bits[code], storage_ix, storage);
WriteBits(nbits, copylen - (1 << nbits), storage_ix, storage);
WriteBits(depth[64], bits[64], storage_ix, storage);
++histo[code];
++histo[64];
} else {
WriteBits(depth[39], bits[39], storage_ix, storage);
WriteBits(24, copylen - 2120, storage_ix, storage);
WriteBits(depth[64], bits[64], storage_ix, storage);
++histo[47];
++histo[64];
}
}
inline void EmitDistance(size_t distance,
const uint8_t depth[128],
const uint16_t bits[128],
uint32_t histo[128],
size_t* storage_ix, uint8_t* storage) {
distance += 3;
const uint32_t nbits = Log2FloorNonZero(distance) - 1u;
const size_t prefix = (distance >> nbits) & 1;
const size_t offset = (2 + prefix) << nbits;
const size_t distcode = 2 * (nbits - 1) + prefix + 80;
WriteBits(depth[distcode], bits[distcode], storage_ix, storage);
WriteBits(nbits, distance - offset, storage_ix, storage);
++histo[distcode];
}
inline void EmitLiterals(const uint8_t* input, const size_t len,
const uint8_t depth[256], const uint16_t bits[256],
size_t* storage_ix, uint8_t* storage) {
for (size_t j = 0; j < len; j++) {
const uint8_t lit = input[j];
WriteBits(depth[lit], bits[lit], storage_ix, storage);
}
}
// 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 UpdateBits(size_t n_bits,
uint32_t bits,
size_t pos,
uint8_t *array) {
while (n_bits > 0) {
size_t byte_pos = pos >> 3;
size_t n_unchanged_bits = pos & 7;
size_t n_changed_bits = std::min(n_bits, 8 - n_unchanged_bits);
size_t total_bits = n_unchanged_bits + n_changed_bits;
uint32_t mask = (~((1 << total_bits) - 1)) | ((1 << n_unchanged_bits) - 1);
uint32_t unchanged_bits = array[byte_pos] & mask;
uint32_t changed_bits = bits & ((1 << n_changed_bits) - 1);
array[byte_pos] =
static_cast<uint8_t>((changed_bits << n_unchanged_bits) |
unchanged_bits);
n_bits -= n_changed_bits;
bits >>= n_changed_bits;
pos += n_changed_bits;
}
}
void RewindBitPosition(const size_t new_storage_ix,
size_t* storage_ix, uint8_t* storage) {
const size_t bitpos = new_storage_ix & 7;
const size_t mask = (1u << bitpos) - 1;
storage[new_storage_ix >> 3] &= static_cast<uint8_t>(mask);
*storage_ix = new_storage_ix;
}
bool ShouldMergeBlock(const uint8_t* data, size_t len, const uint8_t* depths) {
size_t histo[256] = { 0 };
static const size_t kSampleRate = 43;
for (size_t i = 0; i < len; i += kSampleRate) {
++histo[data[i]];
}
const size_t total = (len + kSampleRate - 1) / kSampleRate;
double r = (FastLog2(total) + 0.5) * static_cast<double>(total) + 200;
for (size_t i = 0; i < 256; ++i) {
r -= static_cast<double>(histo[i]) * (depths[i] + FastLog2(histo[i]));
}
return r >= 0.0;
}
inline bool ShouldUseUncompressedMode(const uint8_t* metablock_start,
const uint8_t* next_emit,
const size_t insertlen,
const uint8_t literal_depths[256]) {
const size_t compressed = static_cast<size_t>(next_emit - metablock_start);
if (compressed * 50 > insertlen) {
return false;
}
static const double kAcceptableLossForUncompressibleSpeedup = 0.02;
static const double kMinEntropy =
8 * (1.0 - kAcceptableLossForUncompressibleSpeedup);
uint32_t sum = 0;
for (int i = 0; i < 256; ++i) {
const uint32_t n = literal_depths[i];
sum += n << (15 - n);
}
return sum > static_cast<uint32_t>((1 << 15) * kMinEntropy);
}
void EmitUncompressedMetaBlock(const uint8_t* begin, const uint8_t* end,
const size_t storage_ix_start,
size_t* storage_ix, uint8_t* storage) {
const size_t len = static_cast<size_t>(end - begin);
RewindBitPosition(storage_ix_start, storage_ix, storage);
StoreMetaBlockHeader(len, 1, storage_ix, storage);
*storage_ix = (*storage_ix + 7u) & ~7u;
memcpy(&storage[*storage_ix >> 3], begin, len);
*storage_ix += len << 3;
storage[*storage_ix >> 3] = 0;
}
void BrotliCompressFragmentFast(const uint8_t* input, size_t input_size,
bool is_last,
int* table, size_t table_size,
uint8_t cmd_depth[128], uint16_t cmd_bits[128],
size_t* cmd_code_numbits, uint8_t* cmd_code,
size_t* storage_ix, uint8_t* storage) {
if (input_size == 0) {
assert(is_last);
WriteBits(1, 1, storage_ix, storage); // islast
WriteBits(1, 1, storage_ix, storage); // isempty
*storage_ix = (*storage_ix + 7u) & ~7u;
return;
}
// "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;
// Save the start of the first block for position and distance computations.
const uint8_t* base_ip = input;
static const size_t kFirstBlockSize = 3 << 15;
static const size_t kMergeBlockSize = 1 << 16;
const uint8_t* metablock_start = input;
size_t block_size = std::min(input_size, kFirstBlockSize);
size_t total_block_size = block_size;
// Save the bit position of the MLEN field of the meta-block header, so that
// we can update it later if we decide to extend this meta-block.
size_t mlen_storage_ix = *storage_ix + 3;
StoreMetaBlockHeader(block_size, 0, storage_ix, storage);
// No block splits, no contexts.
WriteBits(13, 0, storage_ix, storage);
uint8_t lit_depth[256] = { 0 };
uint16_t lit_bits[256] = { 0 };
BuildAndStoreLiteralPrefixCode(input, block_size, lit_depth, lit_bits,
storage_ix, storage);
// Store the pre-compressed command and distance prefix codes.
for (size_t i = 0; i + 7 < *cmd_code_numbits; i += 8) {
WriteBits(8, cmd_code[i >> 3], storage_ix, storage);
}
WriteBits(*cmd_code_numbits & 7, cmd_code[*cmd_code_numbits >> 3],
storage_ix, storage);
emit_commands:
// Initialize the command and distance histograms. We will gather
// statistics of command and distance codes during the processing
// of this block and use it to update the command and distance
// prefix codes for the next block.
uint32_t cmd_histo[128] = {
0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 0, 0, 0, 0,
};
// "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;
int last_distance = -1;
const size_t kInputMarginBytes = 16;
const size_t kMinMatchLen = 5;
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 5-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" to the bit stream, 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 5-byte match at ip, and we need to emit bytes in
// [next_emit, ip).
const uint8_t* base = ip;
size_t matched = 5 + FindMatchLengthWithLimit(
candidate + 5, ip + 5, static_cast<size_t>(ip_end - ip) - 5);
ip += matched;
int distance = static_cast<int>(base - candidate); /* > 0 */
size_t insert = static_cast<size_t>(base - next_emit);
assert(0 == memcmp(base, candidate, matched));
if (PREDICT_TRUE(insert < 6210)) {
EmitInsertLen(insert, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
} else if (ShouldUseUncompressedMode(metablock_start, next_emit, insert,
lit_depth)) {
EmitUncompressedMetaBlock(metablock_start, base, mlen_storage_ix - 3,
storage_ix, storage);
input_size -= static_cast<size_t>(base - input);
input = base;
next_emit = input;
goto next_block;
} else {
EmitLongInsertLen(insert, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
}
EmitLiterals(next_emit, insert, lit_depth, lit_bits,
storage_ix, storage);
if (distance == last_distance) {
WriteBits(cmd_depth[64], cmd_bits[64], storage_ix, storage);
++cmd_histo[64];
} else {
EmitDistance(static_cast<size_t>(distance), cmd_depth, cmd_bits,
cmd_histo, storage_ix, storage);
last_distance = distance;
}
EmitCopyLenLastDistance(matched, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
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 - 3);
uint32_t prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 3);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 2, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 1);
uint32_t cur_hash = HashBytesAtOffset(input_bytes, 3, shift);
candidate = base_ip + table[cur_hash];
table[cur_hash] = static_cast<int>(ip - base_ip);
}
while (IsMatch(ip, candidate)) {
// We have a 5-byte match at ip, and no need to emit any literal bytes
// prior to ip.
const uint8_t* base = ip;
size_t matched = 5 + FindMatchLengthWithLimit(
candidate + 5, ip + 5, static_cast<size_t>(ip_end - ip) - 5);
ip += matched;
last_distance = static_cast<int>(base - candidate); /* > 0 */
assert(0 == memcmp(base, candidate, matched));
EmitCopyLen(matched, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
EmitDistance(static_cast<size_t>(last_distance), cmd_depth, cmd_bits,
cmd_histo, storage_ix, storage);
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 - 3);
uint32_t prev_hash = HashBytesAtOffset(input_bytes, 0, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 3);
prev_hash = HashBytesAtOffset(input_bytes, 1, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 2);
prev_hash = HashBytesAtOffset(input_bytes, 2, shift);
table[prev_hash] = static_cast<int>(ip - base_ip - 1);
uint32_t cur_hash = HashBytesAtOffset(input_bytes, 3, 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);
input += block_size;
input_size -= block_size;
block_size = std::min(input_size, kMergeBlockSize);
// Decide if we want to continue this meta-block instead of emitting the
// last insert-only command.
if (input_size > 0 &&
total_block_size + block_size <= (1 << 20) &&
ShouldMergeBlock(input, block_size, lit_depth)) {
assert(total_block_size > (1 << 16));
// Update the size of the current meta-block and continue emitting commands.
// We can do this because the current size and the new size both have 5
// nibbles.
total_block_size += block_size;
UpdateBits(20, static_cast<uint32_t>(total_block_size - 1),
mlen_storage_ix, storage);
goto emit_commands;
}
// Emit the remaining bytes as literals.
if (next_emit < ip_end) {
const size_t insert = static_cast<size_t>(ip_end - next_emit);
if (PREDICT_TRUE(insert < 6210)) {
EmitInsertLen(insert, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
EmitLiterals(next_emit, insert, lit_depth, lit_bits, storage_ix, storage);
} else if (ShouldUseUncompressedMode(metablock_start, next_emit, insert,
lit_depth)) {
EmitUncompressedMetaBlock(metablock_start, ip_end, mlen_storage_ix - 3,
storage_ix, storage);
} else {
EmitLongInsertLen(insert, cmd_depth, cmd_bits, cmd_histo,
storage_ix, storage);
EmitLiterals(next_emit, insert, lit_depth, lit_bits,
storage_ix, storage);
}
}
next_emit = ip_end;
next_block:
// If we have more data, write a new meta-block header and prefix codes and
// then continue emitting commands.
if (input_size > 0) {
metablock_start = input;
block_size = std::min(input_size, kFirstBlockSize);
total_block_size = block_size;
// Save the bit position of the MLEN field of the meta-block header, so that
// we can update it later if we decide to extend this meta-block.
mlen_storage_ix = *storage_ix + 3;
StoreMetaBlockHeader(block_size, 0, storage_ix, storage);
// No block splits, no contexts.
WriteBits(13, 0, storage_ix, storage);
memset(lit_depth, 0, sizeof(lit_depth));
memset(lit_bits, 0, sizeof(lit_bits));
BuildAndStoreLiteralPrefixCode(input, block_size, lit_depth, lit_bits,
storage_ix, storage);
BuildAndStoreCommandPrefixCode(cmd_histo, cmd_depth, cmd_bits,
storage_ix, storage);
goto emit_commands;
}
if (is_last) {
WriteBits(1, 1, storage_ix, storage); // islast
WriteBits(1, 1, storage_ix, storage); // isempty
*storage_ix = (*storage_ix + 7u) & ~7u;
} else {
// If this is not the last block, update the command and distance prefix
// codes for the next block and store the compressed forms.
cmd_code[0] = 0;
*cmd_code_numbits = 0;
BuildAndStoreCommandPrefixCode(cmd_histo, cmd_depth, cmd_bits,
cmd_code_numbits, cmd_code);
}
}
} // namespace brotli
