/src/aom/aom_dsp/entcode.c
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1 | | /* |
2 | | * Copyright (c) 2001-2016, Alliance for Open Media. All rights reserved. |
3 | | * |
4 | | * This source code is subject to the terms of the BSD 2 Clause License and |
5 | | * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License |
6 | | * was not distributed with this source code in the LICENSE file, you can |
7 | | * obtain it at www.aomedia.org/license/software. If the Alliance for Open |
8 | | * Media Patent License 1.0 was not distributed with this source code in the |
9 | | * PATENTS file, you can obtain it at www.aomedia.org/license/patent. |
10 | | */ |
11 | | |
12 | | #include "aom_dsp/entcode.h" |
13 | | |
14 | | /*Given the current total integer number of bits used and the current value of |
15 | | rng, computes the fraction number of bits used to OD_BITRES precision. |
16 | | This is used by od_ec_enc_tell_frac() and od_ec_dec_tell_frac(). |
17 | | nbits_total: The number of whole bits currently used, i.e., the value |
18 | | returned by od_ec_enc_tell() or od_ec_dec_tell(). |
19 | | rng: The current value of rng from either the encoder or decoder state. |
20 | | Return: The number of bits scaled by 2**OD_BITRES. |
21 | | This will always be slightly larger than the exact value (e.g., all |
22 | | rounding error is in the positive direction).*/ |
23 | 0 | uint32_t od_ec_tell_frac(uint32_t nbits_total, uint32_t rng) { |
24 | 0 | uint32_t nbits; |
25 | 0 | int l; |
26 | 0 | int i; |
27 | | /*To handle the non-integral number of bits still left in the encoder/decoder |
28 | | state, we compute the worst-case number of bits of val that must be |
29 | | encoded to ensure that the value is inside the range for any possible |
30 | | subsequent bits. |
31 | | The computation here is independent of val itself (the decoder does not |
32 | | even track that value), even though the real number of bits used after |
33 | | od_ec_enc_done() may be 1 smaller if rng is a power of two and the |
34 | | corresponding trailing bits of val are all zeros. |
35 | | If we did try to track that special case, then coding a value with a |
36 | | probability of 1/(1 << n) might sometimes appear to use more than n bits. |
37 | | This may help explain the surprising result that a newly initialized |
38 | | encoder or decoder claims to have used 1 bit.*/ |
39 | 0 | nbits = nbits_total << OD_BITRES; |
40 | 0 | l = 0; |
41 | 0 | for (i = OD_BITRES; i-- > 0;) { |
42 | 0 | int b; |
43 | 0 | rng = rng * rng >> 15; |
44 | 0 | b = (int)(rng >> 16); |
45 | 0 | l = l << 1 | b; |
46 | 0 | rng >>= b; |
47 | 0 | } |
48 | 0 | return nbits - l; |
49 | 0 | } |