| /* |
| * Copyright (c) 2021, Alliance for Open Media. All rights reserved |
| * |
| * This source code is subject to the terms of the BSD 3-Clause Clear License |
| * and the Alliance for Open Media Patent License 1.0. If the BSD 3-Clause Clear |
| * License was not distributed with this source code in the LICENSE file, you |
| * can obtain it at aomedia.org/license/software-license/bsd-3-c-c/. If the |
| * Alliance for Open Media Patent License 1.0 was not distributed with this |
| * source code in the PATENTS file, you can obtain it at |
| * aomedia.org/license/patent-license/. |
| */ |
| |
| #include "aom_dsp/entcode.h" |
| |
| /*Given the current total integer number of bits used and the current value of |
| rng, computes the fraction number of bits used to OD_BITRES precision. |
| This is used by od_ec_enc_tell_frac() and od_ec_dec_tell_frac(). |
| nbits_total: The number of whole bits currently used, i.e., the value |
| returned by od_ec_enc_tell() or od_ec_dec_tell(). |
| rng: The current value of rng from either the encoder or decoder state. |
| Return: The number of bits scaled by 2**OD_BITRES. |
| This will always be slightly larger than the exact value (e.g., all |
| rounding error is in the positive direction).*/ |
| uint32_t od_ec_tell_frac(uint32_t nbits_total, uint32_t rng) { |
| uint32_t nbits; |
| int l; |
| int i; |
| /*To handle the non-integral number of bits still left in the encoder/decoder |
| state, we compute the worst-case number of bits of val that must be |
| encoded to ensure that the value is inside the range for any possible |
| subsequent bits. |
| The computation here is independent of val itself (the decoder does not |
| even track that value), even though the real number of bits used after |
| od_ec_enc_done() may be 1 smaller if rng is a power of two and the |
| corresponding trailing bits of val are all zeros. |
| If we did try to track that special case, then coding a value with a |
| probability of 1/(1 << n) might sometimes appear to use more than n bits. |
| This may help explain the surprising result that a newly initialized |
| encoder or decoder claims to have used 1 bit.*/ |
| nbits = nbits_total << OD_BITRES; |
| l = 0; |
| for (i = OD_BITRES; i-- > 0;) { |
| int b; |
| rng = rng * rng >> 15; |
| b = (int)(rng >> 16); |
| l = l << 1 | b; |
| rng >>= b; |
| } |
| return nbits - l; |
| } |