Source code
Revision control
Copy as Markdown
Other Tools
// Copyright (c) the JPEG XL Project Authors. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
#include "lib/jxl/enc_huffman_tree.h"
// Suppress any -Wdeprecated-declarations warning that might be emitted by
// GCC or Clang by std::stable_sort in C++17 or later mode
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
#elif defined(__GNUC__)
#pragma GCC push_options
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
#endif
#include <algorithm>
#ifdef __clang__
#pragma clang diagnostic pop
#elif defined(__GNUC__)
#pragma GCC pop_options
#endif
#include <limits>
#include <vector>
#include "lib/jxl/base/status.h"
namespace jxl {
void SetDepth(const HuffmanTree& p, HuffmanTree* pool, uint8_t* depth,
uint8_t level) {
if (p.index_left >= 0) {
++level;
SetDepth(pool[p.index_left], pool, depth, level);
SetDepth(pool[p.index_right_or_value], pool, depth, level);
} else {
depth[p.index_right_or_value] = level;
}
}
// Sort the root nodes, least popular first.
static JXL_INLINE bool Compare(const HuffmanTree& v0, const HuffmanTree& v1) {
return v0.total_count < v1.total_count;
}
// This function will create a Huffman tree.
//
// The catch here is that the tree cannot be arbitrarily deep.
// Brotli specifies a maximum depth of 15 bits for "code trees"
// and 7 bits for "code length code trees."
//
// count_limit is the value that is to be faked as the minimum value
// and this minimum value is raised until the tree matches the
// maximum length requirement.
//
// This algorithm is not of excellent performance for very long data blocks,
// especially when population counts are longer than 2**tree_limit, but
// we are not planning to use this with extremely long blocks.
//
void CreateHuffmanTree(const uint32_t* data, const size_t length,
const int tree_limit, uint8_t* depth) {
// For block sizes below 64 kB, we never need to do a second iteration
// of this loop. Probably all of our block sizes will be smaller than
// that, so this loop is mostly of academic interest. If we actually
// would need this, we would be better off with the Katajainen algorithm.
for (uint32_t count_limit = 1;; count_limit *= 2) {
std::vector<HuffmanTree> tree;
tree.reserve(2 * length + 1);
for (size_t i = length; i != 0;) {
--i;
if (data[i]) {
const uint32_t count = std::max(data[i], count_limit - 1);
tree.emplace_back(count, -1, static_cast<int16_t>(i));
}
}
const size_t n = tree.size();
if (n == 1) {
// Fake value; will be fixed on upper level.
depth[tree[0].index_right_or_value] = 1;
break;
}
std::stable_sort(tree.begin(), tree.end(), Compare);
// The nodes are:
// [0, n): the sorted leaf nodes that we start with.
// [n]: we add a sentinel here.
// [n + 1, 2n): new parent nodes are added here, starting from
// (n+1). These are naturally in ascending order.
// [2n]: we add a sentinel at the end as well.
// There will be (2n+1) elements at the end.
const HuffmanTree sentinel(std::numeric_limits<uint32_t>::max(), -1, -1);
tree.push_back(sentinel);
tree.push_back(sentinel);
size_t i = 0; // Points to the next leaf node.
size_t j = n + 1; // Points to the next non-leaf node.
for (size_t k = n - 1; k != 0; --k) {
size_t left;
size_t right;
if (tree[i].total_count <= tree[j].total_count) {
left = i;
++i;
} else {
left = j;
++j;
}
if (tree[i].total_count <= tree[j].total_count) {
right = i;
++i;
} else {
right = j;
++j;
}
// The sentinel node becomes the parent node.
size_t j_end = tree.size() - 1;
tree[j_end].total_count =
tree[left].total_count + tree[right].total_count;
tree[j_end].index_left = static_cast<int16_t>(left);
tree[j_end].index_right_or_value = static_cast<int16_t>(right);
// Add back the last sentinel node.
tree.push_back(sentinel);
}
JXL_DASSERT(tree.size() == 2 * n + 1);
SetDepth(tree[2 * n - 1], tree.data(), depth, 0);
// We need to pack the Huffman tree in tree_limit bits.
// If this was not successful, add fake entities to the lowest values
// and retry.
if (*std::max_element(&depth[0], &depth[length]) <= tree_limit) {
break;
}
}
}
void Reverse(uint8_t* v, size_t start, size_t end) {
--end;
while (start < end) {
uint8_t tmp = v[start];
v[start] = v[end];
v[end] = tmp;
++start;
--end;
}
}
void WriteHuffmanTreeRepetitions(const uint8_t previous_value,
const uint8_t value, size_t repetitions,
size_t* tree_size, uint8_t* tree,
uint8_t* extra_bits_data) {
JXL_DASSERT(repetitions > 0);
if (previous_value != value) {
tree[*tree_size] = value;
extra_bits_data[*tree_size] = 0;
++(*tree_size);
--repetitions;
}
if (repetitions == 7) {
tree[*tree_size] = value;
extra_bits_data[*tree_size] = 0;
++(*tree_size);
--repetitions;
}
if (repetitions < 3) {
for (size_t i = 0; i < repetitions; ++i) {
tree[*tree_size] = value;
extra_bits_data[*tree_size] = 0;
++(*tree_size);
}
} else {
repetitions -= 3;
size_t start = *tree_size;
while (true) {
tree[*tree_size] = 16;
extra_bits_data[*tree_size] = repetitions & 0x3;
++(*tree_size);
repetitions >>= 2;
if (repetitions == 0) {
break;
}
--repetitions;
}
Reverse(tree, start, *tree_size);
Reverse(extra_bits_data, start, *tree_size);
}
}
void WriteHuffmanTreeRepetitionsZeros(size_t repetitions, size_t* tree_size,
uint8_t* tree, uint8_t* extra_bits_data) {
if (repetitions == 11) {
tree[*tree_size] = 0;
extra_bits_data[*tree_size] = 0;
++(*tree_size);
--repetitions;
}
if (repetitions < 3) {
for (size_t i = 0; i < repetitions; ++i) {
tree[*tree_size] = 0;
extra_bits_data[*tree_size] = 0;
++(*tree_size);
}
} else {
repetitions -= 3;
size_t start = *tree_size;
while (true) {
tree[*tree_size] = 17;
extra_bits_data[*tree_size] = repetitions & 0x7;
++(*tree_size);
repetitions >>= 3;
if (repetitions == 0) {
break;
}
--repetitions;
}
Reverse(tree, start, *tree_size);
Reverse(extra_bits_data, start, *tree_size);
}
}
static void DecideOverRleUse(const uint8_t* depth, const size_t length,
bool* use_rle_for_non_zero,
bool* use_rle_for_zero) {
size_t total_reps_zero = 0;
size_t total_reps_non_zero = 0;
size_t count_reps_zero = 1;
size_t count_reps_non_zero = 1;
for (size_t i = 0; i < length;) {
const uint8_t value = depth[i];
size_t reps = 1;
for (size_t k = i + 1; k < length && depth[k] == value; ++k) {
++reps;
}
if (reps >= 3 && value == 0) {
total_reps_zero += reps;
++count_reps_zero;
}
if (reps >= 4 && value != 0) {
total_reps_non_zero += reps;
++count_reps_non_zero;
}
i += reps;
}
*use_rle_for_non_zero = total_reps_non_zero > count_reps_non_zero * 2;
*use_rle_for_zero = total_reps_zero > count_reps_zero * 2;
}
void WriteHuffmanTree(const uint8_t* depth, size_t length, size_t* tree_size,
uint8_t* tree, uint8_t* extra_bits_data) {
uint8_t previous_value = 8;
// Throw away trailing zeros.
size_t new_length = length;
for (size_t i = 0; i < length; ++i) {
if (depth[length - i - 1] == 0) {
--new_length;
} else {
break;
}
}
// First gather statistics on if it is a good idea to do rle.
bool use_rle_for_non_zero = false;
bool use_rle_for_zero = false;
if (length > 50) {
// Find rle coding for longer codes.
// Shorter codes seem not to benefit from rle.
DecideOverRleUse(depth, new_length, &use_rle_for_non_zero,
&use_rle_for_zero);
}
// Actual rle coding.
for (size_t i = 0; i < new_length;) {
const uint8_t value = depth[i];
size_t reps = 1;
if ((value != 0 && use_rle_for_non_zero) ||
(value == 0 && use_rle_for_zero)) {
for (size_t k = i + 1; k < new_length && depth[k] == value; ++k) {
++reps;
}
}
if (value == 0) {
WriteHuffmanTreeRepetitionsZeros(reps, tree_size, tree, extra_bits_data);
} else {
WriteHuffmanTreeRepetitions(previous_value, value, reps, tree_size, tree,
extra_bits_data);
previous_value = value;
}
i += reps;
}
}
namespace {
uint16_t ReverseBits(int num_bits, uint16_t bits) {
static const size_t kLut[16] = {// Pre-reversed 4-bit values.
0x0, 0x8, 0x4, 0xc, 0x2, 0xa, 0x6, 0xe,
0x1, 0x9, 0x5, 0xd, 0x3, 0xb, 0x7, 0xf};
size_t retval = kLut[bits & 0xf];
for (int i = 4; i < num_bits; i += 4) {
retval <<= 4;
bits = static_cast<uint16_t>(bits >> 4);
retval |= kLut[bits & 0xf];
}
retval >>= (-num_bits & 0x3);
return static_cast<uint16_t>(retval);
}
} // namespace
void ConvertBitDepthsToSymbols(const uint8_t* depth, size_t len,
uint16_t* bits) {
// In Brotli, all bit depths are [1..15]
// 0 bit depth means that the symbol does not exist.
const int kMaxBits = 16; // 0..15 are values for bits
uint16_t bl_count[kMaxBits] = {0};
{
for (size_t i = 0; i < len; ++i) {
++bl_count[depth[i]];
}
bl_count[0] = 0;
}
uint16_t next_code[kMaxBits];
next_code[0] = 0;
{
int code = 0;
for (size_t i = 1; i < kMaxBits; ++i) {
code = (code + bl_count[i - 1]) << 1;
next_code[i] = static_cast<uint16_t>(code);
}
}
for (size_t i = 0; i < len; ++i) {
if (depth[i]) {
bits[i] = ReverseBits(depth[i], next_code[depth[i]]++);
}
}
}
} // namespace jxl