jdhuff.c

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/*
 * jdhuff.c
 *
 * Copyright (C) 1991-1995, 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
 * storage only upon successful completion of an MCU.
 */

#include "layers.h"

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"
#include "jdhuff.h"		/* Declarations shared with jdphuff.c */


/*
 * 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 {
  int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
} savable_state;

/* This macro is to work around compilers with missing or broken
 * structure assignment.  You'll need to fix this code if you have
 * such a compiler and you change MAX_COMPS_IN_SCAN.
 */

#ifndef NO_STRUCT_ASSIGN
#define ASSIGN_STATE(dest,src)  ((dest) = (src))
#else
#if MAX_COMPS_IN_SCAN == 4
#define ASSIGN_STATE(dest,src)  \
	((dest).last_dc_val[0] = (src).last_dc_val[0], \
	 (dest).last_dc_val[1] = (src).last_dc_val[1], \
	 (dest).last_dc_val[2] = (src).last_dc_val[2], \
	 (dest).last_dc_val[3] = (src).last_dc_val[3])
#endif
#endif


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

  /* These fields are loaded into local variables at start of each MCU.
   * In case of suspension, we exit WITHOUT updating them.
   */
  bitread_perm_state bitstate;	/* Bit buffer at start of MCU */
  savable_state saved;		/* Other 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 */

  /* Pointers to derived tables (these workspaces have image lifespan) */
  d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
} huff_entropy_decoder;

typedef huff_entropy_decoder * huff_entropy_ptr;


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

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

  /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
   * This ought to be an error condition, but we make it a warning because
   * there are some baseline files out there with all zeroes in these bytes.
   */
  if (cinfo->Ss != 0 || cinfo->Se != DCTSIZE2-1 ||
      cinfo->Ah != 0 || cinfo->Al != 0)
    WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);

  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 a table, but it's not expensive */
    jpeg_make_d_derived_tbl(cinfo, cinfo->dc_huff_tbl_ptrs[dctbl],
			    & entropy->dc_derived_tbls[dctbl]);
    jpeg_make_d_derived_tbl(cinfo, cinfo->ac_huff_tbl_ptrs[actbl],
			    & entropy->ac_derived_tbls[actbl]);
    /* Initialize DC predictions to 0 */
    entropy->saved.last_dc_val[ci] = 0;
  }

  /* Initialize bitread state variables */
  entropy->bitstate.bits_left = 0;
  entropy->bitstate.get_buffer = 0; /* unnecessary, but keeps Purify quiet */
  entropy->bitstate.printed_eod = FALSE;

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


/*
 * Compute the derived values for a Huffman table.
 * Note this is also used by jdphuff.c.
 */

GLOBAL void
jpeg_make_d_derived_tbl (j_decompress_ptr cinfo, JHUFF_TBL * htbl,
			 d_derived_tbl ** pdtbl)
{
  d_derived_tbl *dtbl;
  int p, i, l, si;
  int lookbits, ctr;
  char huffsize[257];
  unsigned int huffcode[257];
  unsigned int code;

  /* Allocate a workspace if we haven't already done so. */
  if (*pdtbl == NULL)
    *pdtbl = (d_derived_tbl *)
      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				  SIZEOF(d_derived_tbl));
  dtbl = *pdtbl;
  dtbl->pub = htbl;		/* fill in back link */
  
  /* 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->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 <<= 1;
    si++;
  }

  /* Figure F.15: generate decoding tables for bit-sequential decoding */

  p = 0;
  for (l = 1; l <= 16; l++) {
    if (htbl->bits[l]) {
      dtbl->valptr[l] = p; /* huffval[] index of 1st symbol of code length l */
      dtbl->mincode[l] = huffcode[p]; /* minimum code of length l */
      p += htbl->bits[l];
      dtbl->maxcode[l] = huffcode[p-1]; /* maximum code of length l */
    } else {
      dtbl->maxcode[l] = -1;	/* -1 if no codes of this length */
    }
  }
  dtbl->maxcode[17] = 0xFFFFFL; /* ensures jpeg_huff_decode terminates */

  /* Compute lookahead tables to speed up decoding.
   * First we set all the table entries to 0, indicating "too long";
   * then we iterate through the Huffman codes that are short enough and
   * fill in all the entries that correspond to bit sequences starting
   * with that code.
   */

  MEMZERO(dtbl->look_nbits, SIZEOF(dtbl->look_nbits));

  p = 0;
  for (l = 1; l <= HUFF_LOOKAHEAD; l++) {
    for (i = 1; i <= (int) htbl->bits[l]; i++, p++) {
      /* l = current code's length, p = its index in huffcode[] & huffval[]. */
      /* Generate left-justified code followed by all possible bit sequences */
      lookbits = huffcode[p] << (HUFF_LOOKAHEAD-l);
      for (ctr = 1 << (HUFF_LOOKAHEAD-l); ctr > 0; ctr--) {
	dtbl->look_nbits[lookbits] = l;
	dtbl->look_sym[lookbits] = htbl->huffval[p];
	lookbits++;
      }
    }
  }
}


/*
 * Out-of-line code for bit fetching (shared with jdphuff.c).
 * See jdhuff.h for info about usage.
 * Note: current values of get_buffer and bits_left are passed as parameters,
 * but are returned in the corresponding fields of the state struct.
 *
 * On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width
 * of get_buffer to be used.  (On machines with wider words, an even larger
 * buffer could be used.)  However, on some machines 32-bit shifts are
 * quite slow and take time proportional to the number of places shifted.
 * (This is true with most PC compilers, for instance.)  In this case it may
 * be a win to set MIN_GET_BITS to the minimum value of 15.  This reduces the
 * average shift distance at the cost of more calls to jpeg_fill_bit_buffer.
 */

#ifdef SLOW_SHIFT_32
#define MIN_GET_BITS  15	/* minimum allowable value */
#else
#define MIN_GET_BITS  (BIT_BUF_SIZE-7)
#endif

OVERLAY_SEGMENT("code_jeg")

GLOBAL boolean
jpeg_fill_bit_buffer (bitread_working_state * state,
		      register bit_buf_type get_buffer, register int bits_left,
		      int nbits)
/* Load up the bit buffer to a depth of at least nbits */
{
  /* Copy heavily used state fields into locals (hopefully registers) */
  register const JOCTET * next_input_byte = state->next_input_byte;
  register size_t bytes_in_buffer = state->bytes_in_buffer;
  register int c;

  /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
  /* (It is assumed that no request will be for more than that many bits.) */

  while (bits_left < MIN_GET_BITS) {
    /* Attempt to read a byte */
    if (state->unread_marker != 0)
      goto no_more_data;	/* can't advance past a marker */

    if (bytes_in_buffer == 0) {
      if (! (*state->cinfo->src->fill_input_buffer) (state->cinfo))
	return FALSE;
      next_input_byte = state->cinfo->src->next_input_byte;
      bytes_in_buffer = state->cinfo->src->bytes_in_buffer;
    }
    bytes_in_buffer--;
    c = GETJOCTET(*next_input_byte++);

    /* If it's 0xFF, check and discard stuffed zero byte */
    if (c == 0xFF) {
      do {
	if (bytes_in_buffer == 0) {
	  if (! (*state->cinfo->src->fill_input_buffer) (state->cinfo))
	    return FALSE;
	  next_input_byte = state->cinfo->src->next_input_byte;
	  bytes_in_buffer = state->cinfo->src->bytes_in_buffer;
	}
	bytes_in_buffer--;
	c = GETJOCTET(*next_input_byte++);
      } while (c == 0xFF);

      if (c == 0) {
	/* Found FF/00, which represents an FF data byte */
	c = 0xFF;
      } else {
	/* Oops, it's actually a marker indicating end of compressed data. */
	/* Better put it back for use later */
	state->unread_marker = c;

      no_more_data:
	/* There should be enough bits still left in the data segment; */
	/* if so, just break out of the outer while loop. */
	if (bits_left >= nbits)
	  break;