aomedia / av1-xbox-one / eb966e45d9fd4561743909aeef007fc368d2e228 / . / libav1 / aom_dsp / entcode.c

/* | |

* Copyright (c) 2001-2016, Alliance for Open Media. All rights reserved | |

* | |

* This source code is subject to the terms of the BSD 2 Clause License and | |

* the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License | |

* was not distributed with this source code in the LICENSE file, you can | |

* obtain it at www.aomedia.org/license/software. 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 www.aomedia.org/license/patent. | |

*/ | |

#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; | |

} |