src/liblzma/rangecoder/range_decoder.h

   1 ///////////////////////////////////////////////////////////////////////////////
   2 //
   3 /// \file       range_decoder.h
   4 /// \brief      Range 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_DECODER_H
  15 #define LZMA_RANGE_DECODER_H
  16
  17 #include "range_common.h"
  18
  19
  20 typedef struct {
  21         uint32_t range;
  22         uint32_t code;
  23         uint32_t init_bytes_left;
  24 } lzma_range_decoder;
  25
  26
  27 /// Reads the first five bytes to initialize the range decoder.
  28 static inline lzma_ret
  29 rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
  30                 size_t *restrict in_pos, size_t in_size)
  31 {
  32         while (rc->init_bytes_left > 0) {
  33                 if (*in_pos == in_size)
  34                         return LZMA_OK;
  35
  36                 // The first byte is always 0x00. It could have been omitted
  37                 // in LZMA2 but it wasn't, so one byte is wasted in every
  38                 // LZMA2 chunk.
  39                 if (rc->init_bytes_left == 5 && in[*in_pos] != 0x00)
  40                         return LZMA_DATA_ERROR;
  41
  42                 rc->code = (rc->code << 8) | in[*in_pos];
  43                 ++*in_pos;
  44                 --rc->init_bytes_left;
  45         }
  46
  47         return LZMA_STREAM_END;
  48 }
  49
  50
  51 /// Makes local copies of range decoder and *in_pos variables. Doing this
  52 /// improves speed significantly. The range decoder macros expect also
  53 /// variables `in' and `in_size' to be defined.
  54 #define rc_to_local(range_decoder, in_pos) \
  55         lzma_range_decoder rc = range_decoder; \
  56         size_t rc_in_pos = (in_pos); \
  57         uint32_t rc_bound
  58
  59
  60 /// Stores the local copes back to the range decoder structure.
  61 #define rc_from_local(range_decoder, in_pos) \
  62 do { \
  63         range_decoder = rc; \
  64         in_pos = rc_in_pos; \
  65 } while (0)
  66
  67
  68 /// Resets the range decoder structure.
  69 #define rc_reset(range_decoder) \
  70 do { \
  71         (range_decoder).range = UINT32_MAX; \
  72         (range_decoder).code = 0; \
  73         (range_decoder).init_bytes_left = 5; \
  74 } while (0)
  75
  76
  77 /// When decoding has been properly finished, rc.code is always zero unless
  78 /// the input stream is corrupt. So checking this can catch some corrupt
  79 /// files especially if they don't have any other integrity check.
  80 #define rc_is_finished(range_decoder) \
  81         ((range_decoder).code == 0)
  82
  83
  84 /// Read the next input byte if needed. If more input is needed but there is
  85 /// no more input available, "goto out" is used to jump out of the main
  86 /// decoder loop.
  87 #define rc_normalize(seq) \
  88 do { \
  89         if (rc.range < RC_TOP_VALUE) { \
  90                 if (unlikely(rc_in_pos == in_size)) { \
  91                         coder->sequence = seq; \
  92                         goto out; \
  93                 } \
  94                 rc.range <<= RC_SHIFT_BITS; \
  95                 rc.code = (rc.code << RC_SHIFT_BITS) | in[rc_in_pos++]; \
  96         } \
  97 } while (0)
  98
  99
 100 /// Start decoding a bit. This must be used together with rc_update_0()
 101 /// and rc_update_1():
 102 ///
 103 ///     rc_if_0(prob, seq) {
 104 ///         rc_update_0(prob);
 105 ///         // Do something
 106 ///     } else {
 107 ///         rc_update_1(prob);
 108 ///         // Do something else
 109 ///     }
 110 ///
 111 #define rc_if_0(prob, seq) \
 112         rc_normalize(seq); \
 113         rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * (prob); \
 114         if (rc.code < rc_bound)
 115
 116
 117 /// Update the range decoder state and the used probability variable to
 118 /// match a decoded bit of 0.
 119 #define rc_update_0(prob) \
 120 do { \
 121         rc.range = rc_bound; \
 122         prob += (RC_BIT_MODEL_TOTAL - (prob)) >> RC_MOVE_BITS; \
 123 } while (0)
 124
 125
 126 /// Update the range decoder state and the used probability variable to
 127 /// match a decoded bit of 1.
 128 #define rc_update_1(prob) \
 129 do { \
 130         rc.range -= rc_bound; \
 131         rc.code -= rc_bound; \
 132         prob -= (prob) >> RC_MOVE_BITS; \
 133 } while (0)
 134
 135
 136 /// Decodes one bit and runs action0 or action1 depending on the decoded bit.
 137 /// This macro is used as the last step in bittree reverse decoders since
 138 /// those don't use "symbol" for anything else than indexing the probability
 139 /// arrays.
 140 #define rc_bit_last(prob, action0, action1, seq) \
 141 do { \
 142         rc_if_0(prob, seq) { \
 143                 rc_update_0(prob); \
 144                 action0; \
 145         } else { \
 146                 rc_update_1(prob); \
 147                 action1; \
 148         } \
 149 } while (0)
 150
 151
 152 /// Decodes one bit, updates "symbol", and runs action0 or action1 depending
 153 /// on the decoded bit.
 154 #define rc_bit(prob, action0, action1, seq) \
 155         rc_bit_last(prob, \
 156                 symbol <<= 1; action0, \
 157                 symbol = (symbol << 1) + 1; action1, \
 158                 seq);
 159
 160
 161 /// Like rc_bit() but add "case seq:" as a prefix. This makes the unrolled
 162 /// loops more readable because the code isn't littered with "case"
 163 /// statements. On the other hand this also makes it less readable, since
 164 /// spotting the places where the decoder loop may be restarted is less
 165 /// obvious.
 166 #define rc_bit_case(prob, action0, action1, seq) \
 167         case seq: rc_bit(prob, action0, action1, seq)
 168
 169
 170 /// Decode a bit without using a probability.
 171 #define rc_direct(dest, seq) \
 172 do { \
 173         rc_normalize(seq); \
 174         rc.range >>= 1; \
 175         rc.code -= rc.range; \
 176         rc_bound = UINT32_C(0) - (rc.code >> 31); \
 177         rc.code += rc.range & rc_bound; \
 178         dest = (dest << 1) + (rc_bound + 1); \
 179 } while (0)
 180
 181
 182 // NOTE: No macros are provided for bittree decoding. It seems to be simpler
 183 // to just write them open in the code.
 184
 185 #endif