1/*
2 * MJPEG encoder
3 * Copyright (c) 2016 William Ma, Ted Ying, Jerry Jiang
4 *
5 * This file is part of FFmpeg.
6 *
7 * FFmpeg is free software; you can redistribute it and/or
8 * modify it under the terms of the GNU Lesser General Public
9 * License as published by the Free Software Foundation; either
10 * version 2.1 of the License, or (at your option) any later version.
11 *
12 * FFmpeg is distributed in the hope that it will be useful,
13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
15 * Lesser General Public License for more details.
16 *
17 * You should have received a copy of the GNU Lesser General Public
18 * License along with FFmpeg; if not, write to the Free Software
19 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20 */
21
22#include <string.h>
23#include <stdint.h>
24#include "libavutil/avassert.h"
25#include "libavutil/qsort.h"
26#include "mjpegenc_huffman.h"
27
28/**
29 * Comparison function for two PTables by prob
30 *
31 * @param a First PTable to compare
32 * @param b Second PTable to compare
33 * @return < 0 for less than, 0 for equals, > 0 for greater than
34 */
35static int compare_by_prob(const void *a, const void *b)
36{
37    PTable a_val = *(PTable *) a;
38    PTable b_val = *(PTable *) b;
39    return a_val.prob - b_val.prob;
40}
41
42/**
43 * Comparison function for two HuffTables by length
44 *
45 * @param a First HuffTable to compare
46 * @param b Second HuffTable to compare
47 * @return < 0 for less than, 0 for equals, > 0 for greater than
48 */
49static int compare_by_length(const void *a, const void *b)
50{
51    HuffTable a_val = *(HuffTable *) a;
52    HuffTable b_val = *(HuffTable *) b;
53    return a_val.length - b_val.length;
54}
55
56/**
57 * Computes the length of the Huffman encoding for each distinct input value.
58 * Uses package merge algorithm as follows:
59 * 1. start with an empty list, lets call it list(0), set i = 0
60 * 2. add 1 entry to list(i) for each symbol we have and give each a score equal to the probability of the respective symbol
61 * 3. merge the 2 symbols of least score and put them in list(i+1), and remove them from list(i). The new score will be the sum of the 2 scores
62 * 4. if there is more than 1 symbol left in the current list(i), then goto 3
63 * 5. i++
64 * 6. if i < 16 goto 2
65 * 7. select the n-1 elements in the last list with the lowest score (n = the number of symbols)
66 * 8. the length of the huffman code for symbol s will be equal to the number of times the symbol occurs in the select elements
67 * Go to guru.multimedia.cx/small-tasks-for-ffmpeg/ for more details
68 *
69 * All probabilities should be positive integers. The output is sorted by code,
70 * not by length.
71 *
72 * @param prob_table input array of a PTable for each distinct input value
73 * @param distincts  output array of a HuffTable that will be populated by this function
74 * @param size       size of the prob_table array
75 * @param max_length max length of an encoding
76 */
77void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts, int size, int max_length)
78{
79    PackageMergerList list_a, list_b, *to = &list_a, *from = &list_b, *temp;
80
81    int times, i, j, k;
82
83    int nbits[257] = {0};
84
85    int min;
86
87    av_assert0(max_length > 0);
88
89    to->nitems = 0;
90    from->nitems = 0;
91    to->item_idx[0] = 0;
92    from->item_idx[0] = 0;
93    AV_QSORT(prob_table, size, PTable, compare_by_prob);
94
95    for (times = 0; times <= max_length; times++) {
96        to->nitems = 0;
97        to->item_idx[0] = 0;
98
99        j = 0;
100        k = 0;
101
102        if (times < max_length) {
103            i = 0;
104        }
105        while (i < size || j + 1 < from->nitems) {
106            to->nitems++;
107            to->item_idx[to->nitems] = to->item_idx[to->nitems - 1];
108            if (i < size &&
109                (j + 1 >= from->nitems ||
110                 prob_table[i].prob <
111                     from->probability[j] + from->probability[j + 1])) {
112                to->items[to->item_idx[to->nitems]++] = prob_table[i].value;
113                to->probability[to->nitems - 1] = prob_table[i].prob;
114                i++;
115            } else {
116                for (k = from->item_idx[j]; k < from->item_idx[j + 2]; k++) {
117                    to->items[to->item_idx[to->nitems]++] = from->items[k];
118                }
119                to->probability[to->nitems - 1] =
120                    from->probability[j] + from->probability[j + 1];
121                j += 2;
122            }
123        }
124        temp = to;
125        to = from;
126        from = temp;
127    }
128
129    min = (size - 1 < from->nitems) ? size - 1 : from->nitems;
130    for (i = 0; i < from->item_idx[min]; i++) {
131        nbits[from->items[i]]++;
132    }
133    // we don't want to return the 256 bit count (it was just in here to prevent
134    // all 1s encoding)
135    j = 0;
136    for (i = 0; i < 256; i++) {
137        if (nbits[i] > 0) {
138            distincts[j].code = i;
139            distincts[j].length = nbits[i];
140            j++;
141        }
142    }
143}
144
145void ff_mjpeg_encode_huffman_init(MJpegEncHuffmanContext *s)
146{
147    memset(s->val_count, 0, sizeof(s->val_count));
148}
149
150/**
151 * Produces a Huffman encoding with a given input
152 *
153 * @param s         input to encode
154 * @param bits      output array where the ith character represents how many input values have i length encoding
155 * @param val       output array of input values sorted by their encoded length
156 * @param max_nval  maximum number of distinct input values
157 */
158void ff_mjpeg_encode_huffman_close(MJpegEncHuffmanContext *s, uint8_t bits[17],
159                                   uint8_t val[], int max_nval)
160{
161    int i, j;
162    int nval = 0;
163    PTable val_counts[257];
164    HuffTable distincts[256];
165
166    for (i = 0; i < 256; i++) {
167        if (s->val_count[i]) nval++;
168    }
169    av_assert0 (nval <= max_nval);
170
171    j = 0;
172    for (i = 0; i < 256; i++) {
173        if (s->val_count[i]) {
174            val_counts[j].value = i;
175            val_counts[j].prob = s->val_count[i];
176            j++;
177        }
178    }
179    val_counts[j].value = 256;
180    val_counts[j].prob = 0;
181    ff_mjpegenc_huffman_compute_bits(val_counts, distincts, nval + 1, 16);
182    AV_QSORT(distincts, nval, HuffTable, compare_by_length);
183
184    memset(bits, 0, sizeof(bits[0]) * 17);
185    for (i = 0; i < nval; i++) {
186        val[i] = distincts[i].code;
187        bits[distincts[i].length]++;
188    }
189}
190