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