92 lines
3.1 KiB
C
92 lines
3.1 KiB
C
/*
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* jdhuff.c
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*
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* Copyright (C) 1991-1997, Thomas G. Lane.
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* This file is part of the Independent JPEG Group's software.
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* For conditions of distribution and use, see the accompanying README file.
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*
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* This file contains Huffman entropy decoding routines.
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*
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* Much of the complexity here has to do with supporting input suspension.
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* If the data source module demands suspension, we want to be able to back
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* up to the start of the current MCU. To do this, we copy state variables
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* into local working storage, and update them back to the permanent
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* storage only upon successful completion of an MCU.
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*/
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#define JPEG_INTERNALS
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#include "jinclude.h"
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#include "jpeglib.h"
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#include "jdhuff.h" /* Declarations shared with jdphuff.c */
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/*
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* Expanded entropy decoder object for Huffman decoding.
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*
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* The savable_state subrecord contains fields that change within an MCU,
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* but must not be updated permanently until we complete the MCU.
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*/
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typedef struct {
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int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
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} savable_state;
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/* This macro is to work around compilers with missing or broken
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* structure assignment. You'll need to fix this code if you have
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* such a compiler and you change MAX_COMPS_IN_SCAN.
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*/
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#ifndef NO_STRUCT_ASSIGN
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#define ASSIGN_STATE(dest,src) ((dest) = (src))
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#else
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#if MAX_COMPS_IN_SCAN == 4
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#define ASSIGN_STATE(dest,src) \
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((dest).last_dc_val[0] = (src).last_dc_val[0], \
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(dest).last_dc_val[1] = (src).last_dc_val[1], \
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(dest).last_dc_val[2] = (src).last_dc_val[2], \
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(dest).last_dc_val[3] = (src).last_dc_val[3])
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#endif
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#endif
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typedef struct {
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struct jpeg_entropy_decoder pub; /* public fields */
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/* These fields are loaded into local variables at start of each MCU.
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* In case of suspension, we exit WITHOUT updating them.
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*/
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bitread_perm_state bitstate; /* Bit buffer at start of MCU */
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savable_state saved; /* Other state at start of MCU */
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/* These fields are NOT loaded into local working state. */
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unsigned int restarts_to_go; /* MCUs left in this restart interval */
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/* Pointers to derived tables (these workspaces have image lifespan) */
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d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
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d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
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/* Precalculated info set up by start_pass for use in decode_mcu: */
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/* Pointers to derived tables to be used for each block within an MCU */
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d_derived_tbl * dc_cur_tbls[D_MAX_BLOCKS_IN_MCU];
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d_derived_tbl * ac_cur_tbls[D_MAX_BLOCKS_IN_MCU];
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/* Whether we care about the DC and AC coefficient values for each block */
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boolean dc_needed[D_MAX_BLOCKS_IN_MCU];
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boolean ac_needed[D_MAX_BLOCKS_IN_MCU];
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} huff_entropy_decoder;
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typedef huff_entropy_decoder * huff_entropy_ptr;
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GLOBAL(void)
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jpeg_reset_huff_decode (register j_decompress_ptr cinfo)
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{ register huff_entropy_ptr entropy = (huff_entropy_ptr)cinfo->entropy;
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register int ci = 0;
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/* Discard encoded input bits, up to the next Byte boundary */
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entropy->bitstate.bits_left &= ~7;
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/* Re-initialize DC predictions to 0 */
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do entropy->saved.last_dc_val[ci] = 0; while (++ci < cinfo->comps_in_scan);
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}
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