Yushin Cho | 77bba8d | 2016-11-04 16:36:56 -0700 | [diff] [blame] | 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 | */ |
Nathan E. Egge | 1078dee | 2016-03-06 10:59:29 -0500 | [diff] [blame] | 11 | |
Yaowu Xu | 931bc2a | 2016-10-14 13:53:51 -0700 | [diff] [blame] | 12 | #include "aom_dsp/entcode.h" |
Nathan E. Egge | 1078dee | 2016-03-06 10:59:29 -0500 | [diff] [blame] | 13 | |
Nathan E. Egge | 1078dee | 2016-03-06 10:59:29 -0500 | [diff] [blame] | 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 | uint32_t od_ec_tell_frac(uint32_t nbits_total, uint32_t rng) { |
| 24 | uint32_t nbits; |
| 25 | int l; |
| 26 | 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 | nbits = nbits_total << OD_BITRES; |
| 40 | l = 0; |
| 41 | for (i = OD_BITRES; i-- > 0;) { |
| 42 | int b; |
| 43 | rng = rng * rng >> 15; |
| 44 | b = (int)(rng >> 16); |
| 45 | l = l << 1 | b; |
| 46 | rng >>= b; |
| 47 | } |
| 48 | return nbits - l; |
| 49 | } |