bit_reader.c 2.2 KB

12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576
  1. /* Copyright 2013 Google Inc. All Rights Reserved.
  2. Distributed under MIT license.
  3. See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
  4. */
  5. /* Bit reading helpers */
  6. #include "./bit_reader.h"
  7. #include "../common/platform.h"
  8. #include <brotli/types.h>
  9. #if defined(__cplusplus) || defined(c_plusplus)
  10. extern "C" {
  11. #endif
  12. const uint32_t kBrotliBitMask[33] = { 0x00000000,
  13. 0x00000001, 0x00000003, 0x00000007, 0x0000000F,
  14. 0x0000001F, 0x0000003F, 0x0000007F, 0x000000FF,
  15. 0x000001FF, 0x000003FF, 0x000007FF, 0x00000FFF,
  16. 0x00001FFF, 0x00003FFF, 0x00007FFF, 0x0000FFFF,
  17. 0x0001FFFF, 0x0003FFFF, 0x0007FFFF, 0x000FFFFF,
  18. 0x001FFFFF, 0x003FFFFF, 0x007FFFFF, 0x00FFFFFF,
  19. 0x01FFFFFF, 0x03FFFFFF, 0x07FFFFFF, 0x0FFFFFFF,
  20. 0x1FFFFFFF, 0x3FFFFFFF, 0x7FFFFFFF, 0xFFFFFFFF
  21. };
  22. void BrotliInitBitReader(BrotliBitReader* const br) {
  23. br->val_ = 0;
  24. br->bit_pos_ = sizeof(br->val_) << 3;
  25. }
  26. BROTLI_BOOL BrotliWarmupBitReader(BrotliBitReader* const br) {
  27. size_t aligned_read_mask = (sizeof(br->val_) >> 1) - 1;
  28. /* Fixing alignment after unaligned BrotliFillWindow would result accumulator
  29. overflow. If unalignment is caused by BrotliSafeReadBits, then there is
  30. enough space in accumulator to fix alignment. */
  31. if (!BROTLI_ALIGNED_READ) {
  32. aligned_read_mask = 0;
  33. }
  34. if (BrotliGetAvailableBits(br) == 0) {
  35. if (!BrotliPullByte(br)) {
  36. return BROTLI_FALSE;
  37. }
  38. }
  39. while ((((size_t)br->next_in) & aligned_read_mask) != 0) {
  40. if (!BrotliPullByte(br)) {
  41. /* If we consumed all the input, we don't care about the alignment. */
  42. return BROTLI_TRUE;
  43. }
  44. }
  45. return BROTLI_TRUE;
  46. }
  47. BROTLI_BOOL BrotliSafeReadBits32Slow(BrotliBitReader* const br,
  48. uint32_t n_bits, uint32_t* val) {
  49. uint32_t low_val;
  50. uint32_t high_val;
  51. BrotliBitReaderState memento;
  52. BROTLI_DCHECK(n_bits <= 32);
  53. BROTLI_DCHECK(n_bits > 24);
  54. BrotliBitReaderSaveState(br, &memento);
  55. if (!BrotliSafeReadBits(br, 16, &low_val) ||
  56. !BrotliSafeReadBits(br, n_bits - 16, &high_val)) {
  57. BrotliBitReaderRestoreState(br, &memento);
  58. return BROTLI_FALSE;
  59. }
  60. *val = low_val | (high_val << 16);
  61. return BROTLI_TRUE;
  62. }
  63. #if defined(__cplusplus) || defined(c_plusplus)
  64. } /* extern "C" */
  65. #endif