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+// Copyright 2012 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+// Utilities for building and looking up Huffman trees.
+//
+// Author: Urvang Joshi (urvang@google.com)
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include "src/utils/huffman_utils.h"
+#include "src/utils/utils.h"
+#include "src/webp/format_constants.h"
+
+// Huffman data read via DecodeImageStream is represented in two (red and green)
+// bytes.
+#define MAX_HTREE_GROUPS 0x10000
+
+HTreeGroup* VP8LHtreeGroupsNew(int num_htree_groups) {
+ HTreeGroup* const htree_groups =
+ (HTreeGroup*)WebPSafeMalloc(num_htree_groups, sizeof(*htree_groups));
+ if (htree_groups == NULL) {
+ return NULL;
+ }
+ assert(num_htree_groups <= MAX_HTREE_GROUPS);
+ return htree_groups;
+}
+
+void VP8LHtreeGroupsFree(HTreeGroup* const htree_groups) {
+ if (htree_groups != NULL) {
+ WebPSafeFree(htree_groups);
+ }
+}
+
+// Returns reverse(reverse(key, len) + 1, len), where reverse(key, len) is the
+// bit-wise reversal of the len least significant bits of key.
+static WEBP_INLINE uint32_t GetNextKey(uint32_t key, int len) {
+ uint32_t step = 1 << (len - 1);
+ while (key & step) {
+ step >>= 1;
+ }
+ return step ? (key & (step - 1)) + step : key;
+}
+
+// Stores code in table[0], table[step], table[2*step], ..., table[end].
+// Assumes that end is an integer multiple of step.
+static WEBP_INLINE void ReplicateValue(HuffmanCode* table,
+ int step, int end,
+ HuffmanCode code) {
+ assert(end % step == 0);
+ do {
+ end -= step;
+ table[end] = code;
+ } while (end > 0);
+}
+
+// Returns the table width of the next 2nd level table. count is the histogram
+// of bit lengths for the remaining symbols, len is the code length of the next
+// processed symbol
+static WEBP_INLINE int NextTableBitSize(const int* const count,
+ int len, int root_bits) {
+ int left = 1 << (len - root_bits);
+ while (len < MAX_ALLOWED_CODE_LENGTH) {
+ left -= count[len];
+ if (left <= 0) break;
+ ++len;
+ left <<= 1;
+ }
+ return len - root_bits;
+}
+
+// sorted[code_lengths_size] is a pre-allocated array for sorting symbols
+// by code length.
+static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
+ const int code_lengths[], int code_lengths_size,
+ uint16_t sorted[]) {
+ HuffmanCode* table = root_table; // next available space in table
+ int total_size = 1 << root_bits; // total size root table + 2nd level table
+ int len; // current code length
+ int symbol; // symbol index in original or sorted table
+ // number of codes of each length:
+ int count[MAX_ALLOWED_CODE_LENGTH + 1] = { 0 };
+ // offsets in sorted table for each length:
+ int offset[MAX_ALLOWED_CODE_LENGTH + 1];
+
+ assert(code_lengths_size != 0);
+ assert(code_lengths != NULL);
+ assert(root_table != NULL);
+ assert(root_bits > 0);
+
+ // Build histogram of code lengths.
+ for (symbol = 0; symbol < code_lengths_size; ++symbol) {
+ if (code_lengths[symbol] > MAX_ALLOWED_CODE_LENGTH) {
+ return 0;
+ }
+ ++count[code_lengths[symbol]];
+ }
+
+ // Error, all code lengths are zeros.
+ if (count[0] == code_lengths_size) {
+ return 0;
+ }
+
+ // Generate offsets into sorted symbol table by code length.
+ offset[1] = 0;
+ for (len = 1; len < MAX_ALLOWED_CODE_LENGTH; ++len) {
+ if (count[len] > (1 << len)) {
+ return 0;
+ }
+ offset[len + 1] = offset[len] + count[len];
+ }
+
+ // Sort symbols by length, by symbol order within each length.
+ for (symbol = 0; symbol < code_lengths_size; ++symbol) {
+ const int symbol_code_length = code_lengths[symbol];
+ if (code_lengths[symbol] > 0) {
+ sorted[offset[symbol_code_length]++] = symbol;
+ }
+ }
+
+ // Special case code with only one value.
+ if (offset[MAX_ALLOWED_CODE_LENGTH] == 1) {
+ HuffmanCode code;
+ code.bits = 0;
+ code.value = (uint16_t)sorted[0];
+ ReplicateValue(table, 1, total_size, code);
+ return total_size;
+ }
+
+ {
+ int step; // step size to replicate values in current table
+ uint32_t low = -1; // low bits for current root entry
+ uint32_t mask = total_size - 1; // mask for low bits
+ uint32_t key = 0; // reversed prefix code
+ int num_nodes = 1; // number of Huffman tree nodes
+ int num_open = 1; // number of open branches in current tree level
+ int table_bits = root_bits; // key length of current table
+ int table_size = 1 << table_bits; // size of current table
+ symbol = 0;
+ // Fill in root table.
+ for (len = 1, step = 2; len <= root_bits; ++len, step <<= 1) {
+ num_open <<= 1;
+ num_nodes += num_open;
+ num_open -= count[len];
+ if (num_open < 0) {
+ return 0;
+ }
+ for (; count[len] > 0; --count[len]) {
+ HuffmanCode code;
+ code.bits = (uint8_t)len;
+ code.value = (uint16_t)sorted[symbol++];
+ ReplicateValue(&table[key], step, table_size, code);
+ key = GetNextKey(key, len);
+ }
+ }
+
+ // Fill in 2nd level tables and add pointers to root table.
+ for (len = root_bits + 1, step = 2; len <= MAX_ALLOWED_CODE_LENGTH;
+ ++len, step <<= 1) {
+ num_open <<= 1;
+ num_nodes += num_open;
+ num_open -= count[len];
+ if (num_open < 0) {
+ return 0;
+ }
+ for (; count[len] > 0; --count[len]) {
+ HuffmanCode code;
+ if ((key & mask) != low) {
+ table += table_size;
+ table_bits = NextTableBitSize(count, len, root_bits);
+ table_size = 1 << table_bits;
+ total_size += table_size;
+ low = key & mask;
+ root_table[low].bits = (uint8_t)(table_bits + root_bits);
+ root_table[low].value = (uint16_t)((table - root_table) - low);
+ }
+ code.bits = (uint8_t)(len - root_bits);
+ code.value = (uint16_t)sorted[symbol++];
+ ReplicateValue(&table[key >> root_bits], step, table_size, code);
+ key = GetNextKey(key, len);
+ }
+ }
+
+ // Check if tree is full.
+ if (num_nodes != 2 * offset[MAX_ALLOWED_CODE_LENGTH] - 1) {
+ return 0;
+ }
+ }
+
+ return total_size;
+}
+
+// Maximum code_lengths_size is 2328 (reached for 11-bit color_cache_bits).
+// More commonly, the value is around ~280.
+#define MAX_CODE_LENGTHS_SIZE \
+ ((1 << MAX_CACHE_BITS) + NUM_LITERAL_CODES + NUM_LENGTH_CODES)
+// Cut-off value for switching between heap and stack allocation.
+#define SORTED_SIZE_CUTOFF 512
+int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
+ const int code_lengths[], int code_lengths_size) {
+ int total_size;
+ assert(code_lengths_size <= MAX_CODE_LENGTHS_SIZE);
+ if (code_lengths_size <= SORTED_SIZE_CUTOFF) {
+ // use local stack-allocated array.
+ uint16_t sorted[SORTED_SIZE_CUTOFF];
+ total_size = BuildHuffmanTable(root_table, root_bits,
+ code_lengths, code_lengths_size, sorted);
+ } else { // rare case. Use heap allocation.
+ uint16_t* const sorted =
+ (uint16_t*)WebPSafeMalloc(code_lengths_size, sizeof(*sorted));
+ if (sorted == NULL) return 0;
+ total_size = BuildHuffmanTable(root_table, root_bits,
+ code_lengths, code_lengths_size, sorted);
+ WebPSafeFree(sorted);
+ }
+ return total_size;
+}
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