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102 lines
3.3 KiB
C
102 lines
3.3 KiB
C
/*
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Copyright 2011 Google Inc. All Rights Reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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Author: lode.vandevenne@gmail.com (Lode Vandevenne)
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Author: jyrki.alakuijala@gmail.com (Jyrki Alakuijala)
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*/
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#include "tree.h"
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#include <assert.h>
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#include <math.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "katajainen.h"
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#include "util.h"
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void ZopfliLengthsToSymbols(const unsigned* lengths, size_t n, unsigned maxbits,
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unsigned* symbols) {
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size_t* bl_count = (size_t*)malloc(sizeof(size_t) * (maxbits + 1));
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size_t* next_code = (size_t*)malloc(sizeof(size_t) * (maxbits + 1));
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unsigned bits, i;
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unsigned code;
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for (i = 0; i < n; i++) {
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symbols[i] = 0;
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}
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/* 1) Count the number of codes for each code length. Let bl_count[N] be the
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number of codes of length N, N >= 1. */
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for (bits = 0; bits <= maxbits; bits++) {
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bl_count[bits] = 0;
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}
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for (i = 0; i < n; i++) {
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assert(lengths[i] <= maxbits);
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bl_count[lengths[i]]++;
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}
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/* 2) Find the numerical value of the smallest code for each code length. */
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code = 0;
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bl_count[0] = 0;
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for (bits = 1; bits <= maxbits; bits++) {
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code = (code + bl_count[bits-1]) << 1;
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next_code[bits] = code;
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}
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/* 3) Assign numerical values to all codes, using consecutive values for all
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codes of the same length with the base values determined at step 2. */
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for (i = 0; i < n; i++) {
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unsigned len = lengths[i];
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if (len != 0) {
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symbols[i] = next_code[len];
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next_code[len]++;
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}
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}
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free(bl_count);
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free(next_code);
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}
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void ZopfliCalculateEntropy(const size_t* count, size_t n, double* bitlengths) {
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static const double kInvLog2 = 1.4426950408889; /* 1.0 / log(2.0) */
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unsigned sum = 0;
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unsigned i;
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double log2sum;
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for (i = 0; i < n; ++i) {
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sum += count[i];
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}
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log2sum = (sum == 0 ? log(n) : log(sum)) * kInvLog2;
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for (i = 0; i < n; ++i) {
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/* When the count of the symbol is 0, but its cost is requested anyway, it
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means the symbol will appear at least once anyway, so give it the cost as if
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its count is 1.*/
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if (count[i] == 0) bitlengths[i] = log2sum;
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else bitlengths[i] = log2sum - log(count[i]) * kInvLog2;
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/* Depending on compiler and architecture, the above subtraction of two
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floating point numbers may give a negative result very close to zero
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instead of zero (e.g. -5.973954e-17 with gcc 4.1.2 on Ubuntu 11.4). Clamp
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it to zero. These floating point imprecisions do not affect the cost model
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significantly so this is ok. */
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if (bitlengths[i] < 0 && bitlengths[i] > -1e-5) bitlengths[i] = 0;
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assert(bitlengths[i] >= 0);
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}
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}
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void ZopfliCalculateBitLengths(const size_t* count, size_t n, int maxbits,
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unsigned* bitlengths) {
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int error = ZopfliLengthLimitedCodeLengths(count, n, maxbits, bitlengths);
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(void) error;
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assert(!error);
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}
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