jdhuff.h 9.3 KB

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  1. /*
  2. * jdhuff.h
  3. *
  4. * This file was part of the Independent JPEG Group's software:
  5. * Copyright (C) 1991-1997, Thomas G. Lane.
  6. * libjpeg-turbo Modifications:
  7. * Copyright (C) 2010-2011, 2015-2016, D. R. Commander.
  8. * For conditions of distribution and use, see the accompanying README.ijg
  9. * file.
  10. *
  11. * This file contains declarations for Huffman entropy decoding routines
  12. * that are shared between the sequential decoder (jdhuff.c) and the
  13. * progressive decoder (jdphuff.c). No other modules need to see these.
  14. */
  15. #include "jconfigint.h"
  16. /* Derived data constructed for each Huffman table */
  17. #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */
  18. typedef struct {
  19. /* Basic tables: (element [0] of each array is unused) */
  20. JLONG maxcode[18]; /* largest code of length k (-1 if none) */
  21. /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
  22. JLONG valoffset[18]; /* huffval[] offset for codes of length k */
  23. /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
  24. * the smallest code of length k; so given a code of length k, the
  25. * corresponding symbol is huffval[code + valoffset[k]]
  26. */
  27. /* Link to public Huffman table (needed only in jpeg_huff_decode) */
  28. JHUFF_TBL *pub;
  29. /* Lookahead table: indexed by the next HUFF_LOOKAHEAD bits of
  30. * the input data stream. If the next Huffman code is no more
  31. * than HUFF_LOOKAHEAD bits long, we can obtain its length and
  32. * the corresponding symbol directly from this tables.
  33. *
  34. * The lower 8 bits of each table entry contain the number of
  35. * bits in the corresponding Huffman code, or HUFF_LOOKAHEAD + 1
  36. * if too long. The next 8 bits of each entry contain the
  37. * symbol.
  38. */
  39. int lookup[1 << HUFF_LOOKAHEAD];
  40. } d_derived_tbl;
  41. /* Expand a Huffman table definition into the derived format */
  42. EXTERN(void) jpeg_make_d_derived_tbl(j_decompress_ptr cinfo, boolean isDC,
  43. int tblno, d_derived_tbl **pdtbl);
  44. /*
  45. * Fetching the next N bits from the input stream is a time-critical operation
  46. * for the Huffman decoders. We implement it with a combination of inline
  47. * macros and out-of-line subroutines. Note that N (the number of bits
  48. * demanded at one time) never exceeds 15 for JPEG use.
  49. *
  50. * We read source bytes into get_buffer and dole out bits as needed.
  51. * If get_buffer already contains enough bits, they are fetched in-line
  52. * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough
  53. * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
  54. * as full as possible (not just to the number of bits needed; this
  55. * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
  56. * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
  57. * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
  58. * at least the requested number of bits --- dummy zeroes are inserted if
  59. * necessary.
  60. */
  61. #if !defined(_WIN32) && !defined(SIZEOF_SIZE_T)
  62. #error Cannot determine word size
  63. #endif
  64. #if SIZEOF_SIZE_T == 8 || defined(_WIN64)
  65. typedef size_t bit_buf_type; /* type of bit-extraction buffer */
  66. #define BIT_BUF_SIZE 64 /* size of buffer in bits */
  67. #else
  68. typedef unsigned long bit_buf_type; /* type of bit-extraction buffer */
  69. #define BIT_BUF_SIZE 32 /* size of buffer in bits */
  70. #endif
  71. /* If long is > 32 bits on your machine, and shifting/masking longs is
  72. * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
  73. * appropriately should be a win. Unfortunately we can't define the size
  74. * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
  75. * because not all machines measure sizeof in 8-bit bytes.
  76. */
  77. typedef struct { /* Bitreading state saved across MCUs */
  78. bit_buf_type get_buffer; /* current bit-extraction buffer */
  79. int bits_left; /* # of unused bits in it */
  80. } bitread_perm_state;
  81. typedef struct { /* Bitreading working state within an MCU */
  82. /* Current data source location */
  83. /* We need a copy, rather than munging the original, in case of suspension */
  84. const JOCTET *next_input_byte; /* => next byte to read from source */
  85. size_t bytes_in_buffer; /* # of bytes remaining in source buffer */
  86. /* Bit input buffer --- note these values are kept in register variables,
  87. * not in this struct, inside the inner loops.
  88. */
  89. bit_buf_type get_buffer; /* current bit-extraction buffer */
  90. int bits_left; /* # of unused bits in it */
  91. /* Pointer needed by jpeg_fill_bit_buffer. */
  92. j_decompress_ptr cinfo; /* back link to decompress master record */
  93. } bitread_working_state;
  94. /* Macros to declare and load/save bitread local variables. */
  95. #define BITREAD_STATE_VARS \
  96. register bit_buf_type get_buffer; \
  97. register int bits_left; \
  98. bitread_working_state br_state
  99. #define BITREAD_LOAD_STATE(cinfop, permstate) \
  100. br_state.cinfo = cinfop; \
  101. br_state.next_input_byte = cinfop->src->next_input_byte; \
  102. br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
  103. get_buffer = permstate.get_buffer; \
  104. bits_left = permstate.bits_left;
  105. #define BITREAD_SAVE_STATE(cinfop, permstate) \
  106. cinfop->src->next_input_byte = br_state.next_input_byte; \
  107. cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
  108. permstate.get_buffer = get_buffer; \
  109. permstate.bits_left = bits_left
  110. /*
  111. * These macros provide the in-line portion of bit fetching.
  112. * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
  113. * before using GET_BITS, PEEK_BITS, or DROP_BITS.
  114. * The variables get_buffer and bits_left are assumed to be locals,
  115. * but the state struct might not be (jpeg_huff_decode needs this).
  116. * CHECK_BIT_BUFFER(state, n, action);
  117. * Ensure there are N bits in get_buffer; if suspend, take action.
  118. * val = GET_BITS(n);
  119. * Fetch next N bits.
  120. * val = PEEK_BITS(n);
  121. * Fetch next N bits without removing them from the buffer.
  122. * DROP_BITS(n);
  123. * Discard next N bits.
  124. * The value N should be a simple variable, not an expression, because it
  125. * is evaluated multiple times.
  126. */
  127. #define CHECK_BIT_BUFFER(state, nbits, action) { \
  128. if (bits_left < (nbits)) { \
  129. if (!jpeg_fill_bit_buffer(&(state), get_buffer, bits_left, nbits)) \
  130. { action; } \
  131. get_buffer = (state).get_buffer; bits_left = (state).bits_left; \
  132. } \
  133. }
  134. #define GET_BITS(nbits) \
  135. (((int)(get_buffer >> (bits_left -= (nbits)))) & ((1 << (nbits)) - 1))
  136. #define PEEK_BITS(nbits) \
  137. (((int)(get_buffer >> (bits_left - (nbits)))) & ((1 << (nbits)) - 1))
  138. #define DROP_BITS(nbits) \
  139. (bits_left -= (nbits))
  140. /* Load up the bit buffer to a depth of at least nbits */
  141. EXTERN(boolean) jpeg_fill_bit_buffer(bitread_working_state *state,
  142. register bit_buf_type get_buffer,
  143. register int bits_left, int nbits);
  144. /*
  145. * Code for extracting next Huffman-coded symbol from input bit stream.
  146. * Again, this is time-critical and we make the main paths be macros.
  147. *
  148. * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
  149. * without looping. Usually, more than 95% of the Huffman codes will be 8
  150. * or fewer bits long. The few overlength codes are handled with a loop,
  151. * which need not be inline code.
  152. *
  153. * Notes about the HUFF_DECODE macro:
  154. * 1. Near the end of the data segment, we may fail to get enough bits
  155. * for a lookahead. In that case, we do it the hard way.
  156. * 2. If the lookahead table contains no entry, the next code must be
  157. * more than HUFF_LOOKAHEAD bits long.
  158. * 3. jpeg_huff_decode returns -1 if forced to suspend.
  159. */
  160. #define HUFF_DECODE(result, state, htbl, failaction, slowlabel) { \
  161. register int nb, look; \
  162. if (bits_left < HUFF_LOOKAHEAD) { \
  163. if (!jpeg_fill_bit_buffer(&state, get_buffer, bits_left, 0)) \
  164. { failaction; } \
  165. get_buffer = state.get_buffer; bits_left = state.bits_left; \
  166. if (bits_left < HUFF_LOOKAHEAD) { \
  167. nb = 1; goto slowlabel; \
  168. } \
  169. } \
  170. look = PEEK_BITS(HUFF_LOOKAHEAD); \
  171. if ((nb = (htbl->lookup[look] >> HUFF_LOOKAHEAD)) <= HUFF_LOOKAHEAD) { \
  172. DROP_BITS(nb); \
  173. result = htbl->lookup[look] & ((1 << HUFF_LOOKAHEAD) - 1); \
  174. } else { \
  175. slowlabel: \
  176. if ((result = \
  177. jpeg_huff_decode(&state, get_buffer, bits_left, htbl, nb)) < 0) \
  178. { failaction; } \
  179. get_buffer = state.get_buffer; bits_left = state.bits_left; \
  180. } \
  181. }
  182. #define HUFF_DECODE_FAST(s, nb, htbl) \
  183. FILL_BIT_BUFFER_FAST; \
  184. s = PEEK_BITS(HUFF_LOOKAHEAD); \
  185. s = htbl->lookup[s]; \
  186. nb = s >> HUFF_LOOKAHEAD; \
  187. /* Pre-execute the common case of nb <= HUFF_LOOKAHEAD */ \
  188. DROP_BITS(nb); \
  189. s = s & ((1 << HUFF_LOOKAHEAD) - 1); \
  190. if (nb > HUFF_LOOKAHEAD) { \
  191. /* Equivalent of jpeg_huff_decode() */ \
  192. /* Don't use GET_BITS() here because we don't want to modify bits_left */ \
  193. s = (get_buffer >> bits_left) & ((1 << (nb)) - 1); \
  194. while (s > htbl->maxcode[nb]) { \
  195. s <<= 1; \
  196. s |= GET_BITS(1); \
  197. nb++; \
  198. } \
  199. s = htbl->pub->huffval[(int)(s + htbl->valoffset[nb]) & 0xFF]; \
  200. }
  201. /* Out-of-line case for Huffman code fetching */
  202. EXTERN(int) jpeg_huff_decode(bitread_working_state *state,
  203. register bit_buf_type get_buffer,
  204. register int bits_left, d_derived_tbl *htbl,
  205. int min_bits);