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FFmpeg/libavcodec/aac/aacdec_ac.c

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/*
* AAC definitions and structures
* Copyright (c) 2024 Lynne
*
* 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 "libavcodec/aactab.h"
#include "aacdec_ac.h"
uint32_t ff_aac_ac_map_process(AACArithState *state, int reset, int N)
{
float ratio;
if (reset) {
memset(state->last, 0, sizeof(state->last));
state->last_len = N;
} else if (state->last_len != N) {
int i;
uint8_t last[512 /* 2048 / 4 */];
memcpy(last, state->last, sizeof(last));
ratio = state->last_len / (float)N;
for (i = 0; i < N/2; i++) {
int k = (int)(i * ratio);
state->last[i] = last[k];
}
for (; i < FF_ARRAY_ELEMS(state->last); i++)
state->last[i] = 0;
state->last_len = N;
}
state->cur[3] = 0;
state->cur[2] = 0;
state->cur[1] = 0;
state->cur[0] = 1;
state->state_pre = state->last[0] << 12;
return state->last[0] << 12;
}
uint32_t ff_aac_ac_get_context(AACArithState *state, uint32_t c, int i, int N)
{
c = state->state_pre >> 8;
c = c + (state->last[i + 1] << 8);
c = (c << 4);
c += state->cur[1];
state->state_pre = c;
if (i > 3 &&
((state->cur[3] + state->cur[2] + state->cur[1]) < 5))
return c + 0x10000;
return c;
}
uint32_t ff_aac_ac_get_pk(uint32_t c)
{
int i_min = -1;
int i, j;
int i_max = FF_ARRAY_ELEMS(ff_aac_ac_lookup_m) - 1;
while ((i_max - i_min) > 1) {
i = i_min + ((i_max - i_min) / 2);
j = ff_aac_ac_hash_m[i];
if (c < (j >> 8))
i_max = i;
else if (c > (j >> 8))
i_min = i;
else
return (j & 0xFF);
}
return ff_aac_ac_lookup_m[i_max];
}
void ff_aac_ac_update_context(AACArithState *state, int idx,
uint16_t a, uint16_t b)
{
state->cur[0] = a + b + 1;
if (state->cur[0] > 0xF)
state->cur[0] = 0xF;
state->cur[3] = state->cur[2];
state->cur[2] = state->cur[1];
state->cur[1] = state->cur[0];
state->last[idx] = state->cur[0];
}
/* Initialize AC */
void ff_aac_ac_init(AACArith *ac, GetBitContext *gb)
{
ac->low = 0;
ac->high = UINT16_MAX;
ac->val = get_bits(gb, 16);
}
uint16_t ff_aac_ac_decode(AACArith *ac, GetBitContext *gb,
const uint16_t *cdf, uint16_t cdf_len)
{
int val = ac->val;
int low = ac->low;
int high = ac->high;
int sym;
int rng = high - low + 1;
int c = ((((int)(val - low + 1)) << 14) - ((int)1));
const uint16_t *p = cdf - 1;
/* One for each possible CDF length in the spec */
switch (cdf_len) {
case 2:
if ((p[1] * rng) > c)
p += 1;
break;
case 4:
if ((p[2] * rng) > c)
p += 2;
if ((p[1] * rng) > c)
p += 1;
break;
case 17:
/* First check if the current probability is even met at all */
if ((p[1] * rng) <= c)
break;
p += 1;
for (int i = 8; i >= 1; i >>= 1)
if ((p[i] * rng) > c)
p += i;
break;
case 27:
if ((p[16] * rng) > c)
p += 16;
if ((p[8] * rng) > c)
p += 8;
if (p != (cdf - 1 + 24))
if ((p[4] * rng) > c)
p += 4;
if ((p[2] * rng) > c)
p += 2;
if (p != (cdf - 1 + 24 + 2))
if ((p[1] * rng) > c)
p += 1;
break;
default:
/* This should never happen */
av_assert2(0);
}
sym = (int)((ptrdiff_t)(p - cdf)) + 1;
if (sym)
high = low + ((rng * cdf[sym - 1]) >> 14) - 1;
low += (rng * cdf[sym]) >> 14;
/* This loop could be done faster */
while (1) {
if (high < 32768) {
;
} else if (low >= 32768) {
val -= 32768;
low -= 32768;
high -= 32768;
} else if (low >= 16384 && high < 49152) {
val -= 16384;
low -= 16384;
high -= 16384;
} else {
break;
}
low += low;
high += high + 1;
val = (val << 1) | get_bits1(gb);
};
ac->low = low;
ac->high = high;
ac->val = val;
return sym;
}
void ff_aac_ac_finish(AACArithState *state, int offset, int N)
{
int i;
for (i = offset; i < N/2; i++)
state->last[i] = 1;
for (; i < FF_ARRAY_ELEMS(state->last); i++)
state->last[i] = 0;
}