Line data    Source code

       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             : #ifdef HAVE_CONFIG_H
      13             : #include "./config.h"
      14             : #endif
      15             : 
      16             : #include "aom_dsp/entcode.h"
      17             : 
      18             : /*Given the current total integer number of bits used and the current value of
      19             :    rng, computes the fraction number of bits used to OD_BITRES precision.
      20             :   This is used by od_ec_enc_tell_frac() and od_ec_dec_tell_frac().
      21             :   nbits_total: The number of whole bits currently used, i.e., the value
      22             :                 returned by od_ec_enc_tell() or od_ec_dec_tell().
      23             :   rng: The current value of rng from either the encoder or decoder state.
      24             :   Return: The number of bits scaled by 2**OD_BITRES.
      25             :           This will always be slightly larger than the exact value (e.g., all
      26             :            rounding error is in the positive direction).*/
      27           0 : uint32_t od_ec_tell_frac(uint32_t nbits_total, uint32_t rng) {
      28             :   uint32_t nbits;
      29             :   int l;
      30             :   int i;
      31             :   /*To handle the non-integral number of bits still left in the encoder/decoder
      32             :      state, we compute the worst-case number of bits of val that must be
      33             :      encoded to ensure that the value is inside the range for any possible
      34             :      subsequent bits.
      35             :     The computation here is independent of val itself (the decoder does not
      36             :      even track that value), even though the real number of bits used after
      37             :      od_ec_enc_done() may be 1 smaller if rng is a power of two and the
      38             :      corresponding trailing bits of val are all zeros.
      39             :     If we did try to track that special case, then coding a value with a
      40             :      probability of 1/(1 << n) might sometimes appear to use more than n bits.
      41             :     This may help explain the surprising result that a newly initialized
      42             :      encoder or decoder claims to have used 1 bit.*/
      43           0 :   nbits = nbits_total << OD_BITRES;
      44           0 :   l = 0;
      45           0 :   for (i = OD_BITRES; i-- > 0;) {
      46             :     int b;
      47           0 :     rng = rng * rng >> 15;
      48           0 :     b = (int)(rng >> 16);
      49           0 :     l = l << 1 | b;
      50           0 :     rng >>= b;
      51             :   }
      52           0 :   return nbits - l;
      53             : }