/*
* MJPEG encoder
* Copyright (c) 2016 William Ma, Ted Ying, Jerry Jiang
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include "libavutil/avassert.h"
#include "libavutil/common.h"
#include "libavutil/error.h"
#include "libavutil/qsort.h"
#include "mjpegenc_huffman.h"
/**
* Comparison function for two PTables by prob
*
* @param a First PTable to compare
* @param b Second PTable to compare
* @return < 0 for less than, 0 for equals, > 0 for greater than
*/
static int compare_by_prob(const void *a, const void *b)
{
PTable a_val = *(PTable *) a;
PTable b_val = *(PTable *) b;
return a_val.prob - b_val.prob;
}
/**
* Comparison function for two HuffTables by length
*
* @param a First HuffTable to compare
* @param b Second HuffTable to compare
* @return < 0 for less than, 0 for equals, > 0 for greater than
*/
static int compare_by_length(const void *a, const void *b)
{
HuffTable a_val = *(HuffTable *) a;
HuffTable b_val = *(HuffTable *) b;
return a_val.length - b_val.length;
}
/**
* Computes the length of the Huffman encoding for each distinct input value.
* Uses package merge algorithm as follows:
* 1. start with an empty list, lets call it list(0), set i = 0
* 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
* 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
* 4. if there is more than 1 symbol left in the current list(i), then goto 3
* 5. i++
* 6. if i < 16 goto 2
* 7. select the n-1 elements in the last list with the lowest score (n = the number of symbols)
* 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
* Go to guru.multimedia.cx/small-tasks-for-ffmpeg/ for more details
*
* All probabilities should be positive integers. The output is sorted by code,
* not by length.
*
* @param prob_table input array of a PTable for each distinct input value
* @param distincts output array of a HuffTable that will be populated by this function
* @param size size of the prob_table array
* @param max_length max length of an encoding
*/
void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts, int size, int max_length)
{
PackageMergerList list_a, list_b, *to = &list_a, *from = &list_b, *temp;
int times, i, j, k;
int nbits[257] = {0};
int min;
av_assert0(max_length > 0);
to->nitems = 0;
from->nitems = 0;
to->item_idx[0] = 0;
from->item_idx[0] = 0;
AV_QSORT(prob_table, size, PTable, compare_by_prob);
for (times = 0; times <= max_length; times++) {
to->nitems = 0;
to->item_idx[0] = 0;
j = 0;
k = 0;
if (times < max_length) {
i = 0;
}
while (i < size || j + 1 < from->nitems) {
to->nitems++;
to->item_idx[to->nitems] = to->item_idx[to->nitems - 1];
if (i < size &&
(j + 1 >= from->nitems ||
prob_table[i].prob <
from->probability[j] + from->probability[j + 1])) {
to->items[to->item_idx[to->nitems]++] = prob_table[i].value;
to->probability[to->nitems - 1] = prob_table[i].prob;
i++;
} else {
for (k = from->item_idx[j]; k < from->item_idx[j + 2]; k++) {
to->items[to->item_idx[to->nitems]++] = from->items[k];
}
to->probability[to->nitems - 1] =
from->probability[j] + from->probability[j + 1];
j += 2;
}
}
temp = to;
to = from;
from = temp;
}
min = (size - 1 < from->nitems) ? size - 1 : from->nitems;
for (i = 0; i < from->item_idx[min]; i++) {
nbits[from->items[i]]++;
}
// we don't want to return the 256 bit count (it was just in here to prevent
// all 1s encoding)
j = 0;
for (i = 0; i < 256; i++) {
if (nbits[i] > 0) {
distincts[j].code = i;
distincts[j].length = nbits[i];
j++;
}
}
}
void ff_mjpeg_encode_huffman_init(MJpegEncHuffmanContext *s)
{
memset(s->val_count, 0, sizeof(s->val_count));
}
/**
* Produces a Huffman encoding with a given input
*
* @param s input to encode
* @param bits output array where the ith character represents how many input values have i length encoding
* @param val output array of input values sorted by their encoded length
* @param max_nval maximum number of distinct input values
*/
void ff_mjpeg_encode_huffman_close(MJpegEncHuffmanContext *s, uint8_t bits[17],
uint8_t val[], int max_nval)
{
int i, j;
int nval = 0;
PTable val_counts[257];
HuffTable distincts[256];
for (i = 0; i < 256; i++) {
if (s->val_count[i]) nval++;
}
av_assert0 (nval <= max_nval);
j = 0;
for (i = 0; i < 256; i++) {
if (s->val_count[i]) {
val_counts[j].value = i;
val_counts[j].prob = s->val_count[i];
j++;
}
}
val_counts[j].value = 256;
val_counts[j].prob = 0;
ff_mjpegenc_huffman_compute_bits(val_counts, distincts, nval + 1, 16);
AV_QSORT(distincts, nval, HuffTable, compare_by_length);
memset(bits, 0, sizeof(bits[0]) * 17);
for (i = 0; i < nval; i++) {
val[i] = distincts[i].code;
bits[distincts[i].length]++;
}
}