src/liblzma/rangecoder/range_common.h

   1 ///////////////////////////////////////////////////////////////////////////////
   2 //
   3 /// \file       range_common.h
   4 /// \brief      Common things for range encoder and decoder
   5 ///
   6 //  Authors:    Igor Pavlov
   7 //              Lasse Collin
   8 //
   9 //  This file has been put into the public domain.
  10 //  You can do whatever you want with this file.
  11 //
  12 ///////////////////////////////////////////////////////////////////////////////
  13
  14 #ifndef LZMA_RANGE_COMMON_H
  15 #define LZMA_RANGE_COMMON_H
  16
  17 #include "common.h"
  18
  19
  20 ///////////////
  21 // Constants //
  22 ///////////////
  23
  24 #define RC_SHIFT_BITS 8
  25 #define RC_TOP_BITS 24
  26 #define RC_TOP_VALUE (UINT32_C(1) << RC_TOP_BITS)
  27 #define RC_BIT_MODEL_TOTAL_BITS 11
  28 #define RC_BIT_MODEL_TOTAL (UINT32_C(1) << RC_BIT_MODEL_TOTAL_BITS)
  29 #define RC_MOVE_BITS 5
  30
  31
  32 ////////////
  33 // Macros //
  34 ////////////
  35
  36 // Resets the probability so that both 0 and 1 have probability of 50 %
  37 #define bit_reset(prob) \
  38         prob = RC_BIT_MODEL_TOTAL >> 1
  39
  40 // This does the same for a complete bit tree.
  41 // (A tree represented as an array.)
  42 #define bittree_reset(probs, bit_levels) \
  43         for (uint32_t bt_i = 0; bt_i < (1 << (bit_levels)); ++bt_i) \
  44                 bit_reset((probs)[bt_i])
  45
  46
  47 //////////////////////
  48 // Type definitions //
  49 //////////////////////
  50
  51 /// \brief      Type of probabilities used with range coder
  52 ///
  53 /// This needs to be at least 12-bit integer, so uint16_t is a logical choice.
  54 /// However, on some architecture and compiler combinations, a bigger type
  55 /// may give better speed, because the probability variables are accessed
  56 /// a lot. On the other hand, bigger probability type increases cache
  57 /// footprint, since there are 2 to 14 thousand probability variables in
  58 /// LZMA (assuming the limit of lc + lp <= 4; with lc + lp <= 12 there
  59 /// would be about 1.5 million variables).
  60 ///
  61 /// With malicious files, the initialization speed of the LZMA decoder can
  62 /// become important. In that case, smaller probability variables mean that
  63 /// there is less bytes to write to RAM, which makes initialization faster.
  64 /// With big probability type, the initialization can become so slow that it
  65 /// can be a problem e.g. for email servers doing virus scanning.
  66 ///
  67 /// I will be sticking to uint16_t unless some specific architectures
  68 /// are *much* faster (20-50 %) with uint32_t.
  69 typedef uint16_t probability;
  70
  71 #endif