JPEGView/Independent JPEG Group/jdhuff.c

/*
 * jdhuff.c
 *
 * Copyright (C) 1991-1994, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains Huffman entropy decoding routines.
 *
 * Much of the complexity here has to do with supporting input suspension.
 * If the data source module demands suspension, we want to be able to back
 * up to the start of the current MCU.  To do this, we copy state variables
 * into local working storage, and update them back to the permanent JPEG
 * objects only upon successful completion of an MCU.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"


/* Expanded entropy decoder object for Huffman decoding.
 *
 * The savable_state subrecord contains fields that change within an MCU,
 * but must not be updated permanently until we complete the MCU.
 */

typedef struct {
  INT32 get_buffer;		/* current bit-extraction buffer */
  int bits_left;		/* # of unused bits in it */
  int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
} savable_state;

typedef struct {
  struct jpeg_entropy_decoder pub; /* public fields */

  savable_state saved;		/* Bit buffer & DC state at start of MCU */

  /* These fields are NOT loaded into local working state. */
  unsigned int restarts_to_go;	/* MCUs left in this restart interval */
  boolean printed_eod;		/* flag to suppress extra end-of-data msgs */
} huff_entropy_decoder;

typedef huff_entropy_decoder * huff_entropy_ptr;

/* Working state while scanning an MCU.
 * This struct contains all the fields that are needed by subroutines.
 */

typedef struct {
  int unread_marker;		/* nonzero if we have hit a marker */
  const JOCTET * next_input_byte; /* => next byte to read from source */
  size_t bytes_in_buffer;	/* # of bytes remaining in source buffer */
  savable_state cur;		/* Current bit buffer & DC state */
  j_decompress_ptr cinfo;	/* fill_bit_buffer needs access to this */
} working_state;


/* Forward declarations */
LOCAL void fix_huff_tbl JPP((JDHUFF_TBL * htbl));


/*
 * Initialize for a Huffman-compressed scan.
 */

METHODDEF void
start_pass (j_decompress_ptr cinfo)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  int ci, dctbl, actbl;
  jpeg_component_info * compptr;

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
    compptr = cinfo->cur_comp_info[ci];
    dctbl = compptr->dc_tbl_no;
    actbl = compptr->ac_tbl_no;
    /* Make sure requested tables are present */
    if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS ||
	cinfo->dc_huff_tbl_ptrs[dctbl] == NULL)
      ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl);
    if (actbl < 0 || actbl >= NUM_HUFF_TBLS ||
	cinfo->ac_huff_tbl_ptrs[actbl] == NULL)
      ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl);
    /* Compute derived values for Huffman tables */
    /* We may do this more than once for same table, but it's not a big deal */
    fix_huff_tbl((JDHUFF_TBL *) cinfo->dc_huff_tbl_ptrs[dctbl]);
    fix_huff_tbl((JDHUFF_TBL *) cinfo->ac_huff_tbl_ptrs[actbl]);
    /* Initialize DC predictions to 0 */
    entropy->saved.last_dc_val[ci] = 0;
  }

  /* Initialize private state variables */
  entropy->saved.bits_left = 0;
  entropy->printed_eod = FALSE;

  /* Initialize restart counter */
  entropy->restarts_to_go = cinfo->restart_interval;
}


LOCAL void
fix_huff_tbl (JDHUFF_TBL * htbl)
/* Compute the derived values for a Huffman table */
{
  int p, i, l, si;
  int lookbits, ctr;
  char huffsize[257];
  unsigned int huffcode[257];
  unsigned int code;
  
  /* Figure C.1: make table of Huffman code length for each symbol */
  /* Note that this is in code-length order. */

  p = 0;
  for (l = 1; l <= 16; l++) {
    for (i = 1; i <= (int) htbl->pub.bits[l]; i++)
      huffsize[p++] = (char) l;
  }
  huffsize[p] = 0;
  
  /* Figure C.2: generate the codes themselves */
  /* Note that this is in code-length order. */
  
  code = 0;
  si = huffsize[0];
  p = 0;
  while (huffsize[p]) {
    while (((int) huffsize[p]) == si) {
      huffcode[p++] = code;
      code++;
    }
    code