BaseTools/Source/C/BrotliCompress/enc/write_bits.h

   1 /* Copyright 2010 Google Inc. All Rights Reserved.
   2
   3    Distributed under MIT license.
   4    See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
   5 */
   6
   7 /* Write bits into a byte array. */
   8
   9 #ifndef BROTLI_ENC_WRITE_BITS_H_
  10 #define BROTLI_ENC_WRITE_BITS_H_
  11
  12 #include "../common/platform.h"
  13 #include <brotli/types.h>
  14
  15 #if defined(__cplusplus) || defined(c_plusplus)
  16 extern "C" {
  17 #endif
  18
  19 /*#define BIT_WRITER_DEBUG */
  20
  21 /* This function writes bits into bytes in increasing addresses, and within
  22    a byte least-significant-bit first.
  23
  24    The function can write up to 56 bits in one go with WriteBits
  25    Example: let's assume that 3 bits (Rs below) have been written already:
  26
  27    BYTE-0     BYTE+1       BYTE+2
  28
  29    0000 0RRR    0000 0000    0000 0000
  30
  31    Now, we could write 5 or less bits in MSB by just sifting by 3
  32    and OR'ing to BYTE-0.
  33
  34    For n bits, we take the last 5 bits, OR that with high bits in BYTE-0,
  35    and locate the rest in BYTE+1, BYTE+2, etc. */
  36 static BROTLI_INLINE void BrotliWriteBits(size_t n_bits,
  37                                           uint64_t bits,
  38                                           size_t* BROTLI_RESTRICT pos,
  39                                           uint8_t* BROTLI_RESTRICT array) {
  40 #if defined(BROTLI_LITTLE_ENDIAN)
  41   /* This branch of the code can write up to 56 bits at a time,
  42      7 bits are lost by being perhaps already in *p and at least
  43      1 bit is needed to initialize the bit-stream ahead (i.e. if 7
  44      bits are in *p and we write 57 bits, then the next write will
  45      access a byte that was never initialized). */
  46   uint8_t* p = &array[*pos >> 3];
  47   uint64_t v = (uint64_t)(*p);  /* Zero-extend 8 to 64 bits. */
  48   BROTLI_LOG(("WriteBits  %2d  0x%08x%08x  %10d\n", (int)n_bits,
  49       (uint32_t)(bits >> 32), (uint32_t)(bits & 0xFFFFFFFF),
  50       (int)*pos));
  51   BROTLI_DCHECK((bits >> n_bits) == 0);
  52   BROTLI_DCHECK(n_bits <= 56);
  53   v |= bits << (*pos & 7);
  54   BROTLI_UNALIGNED_STORE64LE(p, v);  /* Set some bits. */
  55   *pos += n_bits;
  56 #else
  57   /* implicit & 0xFF is assumed for uint8_t arithmetics */
  58   uint8_t* array_pos = &array[*pos >> 3];
  59   const size_t bits_reserved_in_first_byte = (*pos & 7);
  60   size_t bits_left_to_write;
  61   bits <<= bits_reserved_in_first_byte;
  62   *array_pos++ |= (uint8_t)bits;
  63   for (bits_left_to_write = n_bits + bits_reserved_in_first_byte;
  64        bits_left_to_write >= 9;
  65        bits_left_to_write -= 8) {
  66     bits >>= 8;
  67     *array_pos++ = (uint8_t)bits;
  68   }
  69   *array_pos = 0;
  70   *pos += n_bits;
  71 #endif
  72 }
  73
  74 static BROTLI_INLINE void BrotliWriteBitsPrepareStorage(
  75     size_t pos, uint8_t* array) {
  76   BROTLI_LOG(("WriteBitsPrepareStorage            %10d\n", (int)pos));
  77   BROTLI_DCHECK((pos & 7) == 0);
  78   array[pos >> 3] = 0;
  79 }
  80
  81 #if defined(__cplusplus) || defined(c_plusplus)
  82 }  /* extern "C" */
  83 #endif
  84
  85 #endif  /* BROTLI_ENC_WRITE_BITS_H_ */