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/*
Copyright (c) 2010 The WebM project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE 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.
*/
#include "vp9/common/vp9_entropy.h"
#include "vp9/decoder/vp9_dsubexp.h"
static int inv_recenter_nonneg(int v, int m) {
if (v > 2 * m)
return v;
return v % 2 ? m - (v + 1) / 2 : m + v / 2;
}
static int decode_uniform(vp9_reader *r, int n) {
int v;
const int l = get_unsigned_bits(n);
const int m = (1 << l) - n;
if (!l)
return 0;
v = vp9_read_literal(r, l - 1);
return v < m ? v : (v << 1) - m + vp9_read_bit(r);
}
static int merge_index(int v, int n, int modulus) {
int max1 = (n - 1 - modulus / 2) / modulus + 1;
if (v < max1) {
v = v * modulus + modulus / 2;
} else {
int w;
v -= max1;
w = v;
v += (v + modulus - modulus / 2) / modulus;
while (v % modulus == modulus / 2 ||
w != v - (v + modulus - modulus / 2) / modulus) v++;
}
return v;
}
static int inv_remap_prob(int v, int m) {
const int n = 255;
v = merge_index(v, n - 1, MODULUS_PARAM);
m--;
if ((m << 1) <= n) {
return 1 + inv_recenter_nonneg(v + 1, m);
} else {
return n - inv_recenter_nonneg(v + 1, n - 1 - m);
}
}
static int decode_term_subexp(vp9_reader *r, int k, int num_syms) {
int i = 0, mk = 0, word;
while (1) {
const int b = i ? k + i - 1 : k;
const int a = 1 << b;
if (num_syms <= mk + 3 * a) {
word = decode_uniform(r, num_syms - mk) + mk;
break;
} else {
if (vp9_read_bit(r)) {
i++;
mk += a;
} else {
word = vp9_read_literal(r, b) + mk;
break;
}
}
}
return word;
}
void vp9_diff_update_prob(vp9_reader *r, vp9_prob* p) {
int delp = decode_term_subexp(r, SUBEXP_PARAM, 255);
*p = (vp9_prob)inv_remap_prob(delp, *p);
}
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