text: /*
   * jdhuff.c
   *
   * Copyright (C) 1991, 1992, 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.
   * These routines are invoked via the methods entropy_decode
   * and entropy_decode_init/term.
   */
  
  #include "jinclude.h"
  
  
  /* Static variables to avoid passing 'round extra parameters */
  
  static decompress_info_ptr dcinfo;
  
  static INT32 get_buffer;	/* current bit-extraction buffer */
  static int bits_left;		/* # of unused bits in it */
  static boolean printed_eod;	/* flag to suppress multiple end-of-data msgs */
  
  LOCAL void
  fix_huff_tbl (HUFF_TBL * htbl)
  /* Compute derived values for a Huffman table */
  {
    int p, i, l, si;
    char huffsize[257];
    UINT16 huffcode[257];
    UINT16 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->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++;
    }
  
    /* We don't bother to fill in the encoding tables ehufco[] and ehufsi[], */
    /* since they are not used for decoding. */
  
    /* Figure F.15: generate decoding tables */
  
    p = 0;
    for (l = 1; l <= 16; l++) {
      if (htbl->bits[l]) {
        htbl->valptr[l] = p;	/* huffval[] index of 1st sym of code len l */
        htbl->mincode[l] = huffcode[p]; /* minimum code of length l */
        p += htbl->bits[l];
        htbl->maxcode[l] = huffcode[p-1];	/* maximum code of length l */
      } else {
        htbl->maxcode[l] = -1;
      }
    }
    htbl->maxcode[17] = 0xFFFFFL;	/* ensures huff_DECODE terminates */
  }
  
  
  /*
   * Code for extracting the next N bits from the input stream.
   * (N never exceeds 15 for JPEG data.)
   * This needs to go as fast as possible!
   *
   * We read source bytes into get_buffer and dole out bits as needed.
   * If get_buffer already contains enough bits, they are fetched in-line
   * by the macros get_bits() and get_bit().  When there aren't enough bits,
   * fill_bit_buffer is called; it will attempt to fill get_buffer to the
   * "high water mark", then extract the desired number of bits.  The idea,
   * of course, is to minimize the function-call overhead cost of entering
   * fill_bit_buffer.
   * 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
   * relatively 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 fill_bit_buffer.
   */
  
  #ifdef SLOW_SHIFT_32
  #define MIN_GET_BITS  15	/* minimum allowable value */
  #else
  #define MIN_GET_BITS  25	/* max value for 32-bit get_buffer */
  #endif
  
  static const int bmask[16] =	/* bmask[n] is mask for n rightmost bits */
    { 0, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF,
      0x01FF, 0x03FF, 0x07FF, 0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF };
  
  
  LOCAL int
  fill_bit_buffer (int nbits)
  /* Load up the bit buffer and do get_bits(nbits) */
  {
    /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
    while (bits_left < MIN_GET_BITS) {
      register int c = JGETC(dcinfo);
      
      /* If it's 0xFF, check and discard stuffed zero byte */
      if (c == 0xFF) {
        int c2 = JGETC(dcinfo);
        if (c2 != 0) {
  	/* Oops, it's actually a marker indicating end of compressed data. */
  	/* Better put it back for use later */
  	JUNGETC(c2,dcinfo);
  	JUNGETC(c,dcinfo);
  	/* There should be enough bits still left in the data segment; */
  	/* if so, just break out of the while loop. */
  	if (bits_left >= nbits)
  	  break;
  	/* Uh-oh.  Report corrupted data to user and stuff zeroes into
  	 * the data stream, so we can produce some kind of image.
  	 * Note that this will be repeated for each byte demanded for the
  	 * rest of the segment; this is a bit slow but not unreasonably so.
  	 * The main thing is to avoid getting a zillion warnings, hence:
  	 */
  	if (! printed_eod) {
  	  WARNMS(dcinfo->emethods, "Corrupt JPEG data: premature end of data segment");
  	  printed_eod = TRUE;
  	}
  	c = 0;			/* insert a zero byte into bit buffer */
        }
      }
  
      /* OK, load c into get_buffer */
      get_buffer = (get_buffer << 8) | c;
      bits_left += 8;
    }
  
    /* Having filled get_buffer, extract desired bits (this simplifies macros) */
    bits_left -= nbits;
    return ((int) (get_buffer >> bits_left)) & bmask[nbits];
  }
  
  
  /* Macros to make things go at some speed! */
  /* NB: parameter to get_bits should be simple variable, not expression */
  
  #define get_bits(nbits) \
  	(bits_left >= (nbits) ? \
  	 ((int) (get_buffer >> (bits_left -= (nbits)))) & bmask[nbits] : \
  	 fill_bit_buffer(nbits))
  
  #define get_bit() \
  	(bits_left ? \
  	 ((int) (get_buffer >> (--bits_left))) & 1 : \
  	 fill_bit_buffer(1))
  
  
  /* Figure F.16: extract next coded symbol from input stream */
    
  INLINE
  LOCAL int
  huff_DECODE (HUFF_TBL * htbl)
  {
    register int l;
    register INT32 code;
    
    code = get_bit();
    l = 1;
    while (code > htbl->maxcode[l]) {
      code = (code << 1) | get_bit();
      l++;
    }
  
    /* With garbage input we may reach the sentinel value l = 17. */
  
    if (l > 16) {
      WARNMS(dcinfo->emethods, "Corrupt JPEG data: bad Huffman code");
      return 0;			/* fake a zero as the safest result */
    }
  
    return htbl->huffval[ htbl->valptr[l] + ((int) (code - htbl->mincode[l])) ];
  }
  
  
  /* Figure F.12: extend sign bit */
  
  #define huff_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))
  
  static const int extend_test[16] =   /* entry n is 2**(n-1) */
    { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
      0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
  
  static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
    { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
      ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
      ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
      ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };
  
  
  /*
   * Initialize for a Huffman-compressed scan.
   * This is invoked after reading the SOS marker.
   */
  
  METHODDEF void
  huff_decoder_init (decompress_info_ptr cinfo)
  {
    short ci;
    jpeg_component_info * compptr;
  
    /* Initialize static variables */
    dcinfo = cinfo;
    bits_left = 0;
    printed_eod = FALSE;
  
    for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
      compptr = cinfo->cur_comp_info[ci];
      /* Make sure requested tables are present */
      if (cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no] == NULL ||
  	cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no] == NULL)
        ERREXIT(cinfo->emethods, "Use of undefined Huffman table");
      /* 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(cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no]);
      fix_huff_tbl(cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no]);
      /* Initialize DC predictions to 0 */
      cinfo->last_dc_val[ci] = 0;
    }
  
    /* Initialize restart stuff */
    cinfo->restarts_to_go = cinfo->restart_interval;
    cinfo->next_restart_num = 0;
  }
  
  
  /*
   * Check for a restart marker & resynchronize decoder.
   */
  
  LOCAL void
  process_restart (decompress_info_ptr cinfo)
  {
    int c, nbytes;
    short ci;
  
    /* Throw away any unused bits remaining in bit buffer */
    nbytes = bits_left / 8;	/* count any full bytes loaded into buffer */
    bits_left = 0;
    printed_eod = FALSE;		/* next segment can get another warning */
  
    /* Scan for next JPEG marker */
    do {
      do {			/* skip any non-FF bytes */
        nbytes++;
        c = JGETC(cinfo);
      } while (c != 0xFF);
      do {			/* skip any duplicate FFs */
        /* we don't increment nbytes here since extra FFs are legal */
        c = JGETC(cinfo);
      } while (c == 0xFF);
    } while (c == 0);		/* repeat if it was a stuffed FF/00 */
  
    if (nbytes != 1)
      WARNMS2(cinfo->emethods,
  	    "Corrupt JPEG data: %d extraneous bytes before marker 0x%02x",
  	    nbytes-1, c);
  
    if (c != (RST0 + cinfo->next_restart_num)) {
      /* Uh-oh, the restart markers have been messed up too. */
      /* Let the file-format module try to figure out how to resync. */
      (*cinfo->methods->resync_to_restart) (cinfo, c);
    } else
      TRACEMS1(cinfo->emethods, 2, "RST%d", cinfo->next_restart_num);
  
    /* Re-initialize DC predictions to 0 */
    for (ci = 0; ci < cinfo->comps_in_scan; ci++)
      cinfo->last_dc_val[ci] = 0;
  
    /* Update restart state */
    cinfo->restarts_to_go = cinfo->restart_interval;
    cinfo->next_restart_num = (cinfo->next_restart_num + 1) & 7;
  }
  
  
  /* ZAG[i] is the natural-order position of the i'th element of zigzag order.
   * If the incoming data is corrupted, huff_decode_mcu could attempt to
   * reference values beyond the end of the array.  To avoid a wild store,
   * we put some extra zeroes after the real entries.
   */
  
  static const short ZAG[DCTSIZE2+16] = {
    0,  1,  8, 16,  9,  2,  3, 10,
   17, 24, 32, 25, 18, 11,  4,  5,
   12, 19, 26, 33, 40, 48, 41, 34,
   27, 20, 13,  6,  7, 14, 21, 28,
   35, 42, 49, 56, 57, 50, 43, 36,
   29, 22, 15, 23, 30, 37, 44, 51,
   58, 59, 52, 45, 38, 31, 39, 46,
   53, 60, 61, 54, 47, 55, 62, 63,
    0,  0,  0,  0,  0,  0,  0,  0, /* extra entries in case k>63 below */
    0,  0,  0,  0,  0,  0,  0,  0
  };
  
  
  /*
   * Decode and return one MCU's worth of Huffman-compressed coefficients.
   * This routine also handles quantization descaling and zigzag reordering
   * of coefficient values.
   *
   * The i'th block of the MCU is stored into the block pointed to by
   * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.
   * (Wholesale zeroing is usually a little faster than retail...)
   */
  
  METHODDEF void
  huff_decode_mcu (decompress_info_ptr cinfo, JBLOCKROW *MCU_data)
  {
    register int s, k, r;
    short blkn, ci;
    register JBLOCKROW block;
    register QUANT_TBL_PTR quanttbl;
    HUFF_TBL *dctbl;
    HUFF_TBL *actbl;
    jpeg_component_info * compptr;
  
    /* Account for restart interval, process restart marker if needed */
    if (cinfo->restart_interval) {
      if (cinfo->restarts_to_go == 0)
        process_restart(cinfo);
      cinfo->restarts_to_go--;
    }
  
    /* Outer loop handles each block in the MCU */
  
    for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
      block = MCU_data[blkn];
      ci = cinfo->MCU_membership[blkn];
      compptr = cinfo->cur_comp_info[ci];
      quanttbl = cinfo->quant_tbl_ptrs[compptr->quant_tbl_no];
      actbl = cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no];
      dctbl = cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no];
  
      /* Decode a single block's worth of coefficients */
  
      /* Section F.2.2.1: decode the DC coefficient difference */
      s = huff_DECODE(dctbl);
      if (s) {
        r = get_bits(s);
        s = huff_EXTEND(r, s);
      }
  
      /* Convert DC difference to actual value, update last_dc_val */
      s += cinfo->last_dc_val[ci];
      cinfo->last_dc_val[ci] = (JCOEF) s;
      /* Descale and output the DC coefficient (assumes ZAG[0] = 0) */
      (*block)[0] = (JCOEF) (((JCOEF) s) * quanttbl[0]);
      
      /* Section F.2.2.2: decode the AC coefficients */
      /* Since zero values are skipped, output area must be zeroed beforehand */
      for (k = 1; k < DCTSIZE2; k++) {
        r = huff_DECODE(actbl);
        
        s = r & 15;
        r = r >> 4;
        
        if (s) {
  	k += r;
  	r = get_bits(s);
  	s = huff_EXTEND(r, s);
  	/* Descale coefficient and output in natural (dezigzagged) order */
  	(*block)[ZAG[k]] = (JCOEF) (((JCOEF) s) * quanttbl[k]);
        } else {
  	if (r != 15)
  	  break;
  	k += 15;
        }
      }
    }
  }
  
  
  /*
   * Finish up at the end of a Huffman-compressed scan.
   */
  
  METHODDEF void
  huff_decoder_term (decompress_info_ptr cinfo)
  {
    /* No work needed */
  }
  
  
  /*
   * The method selection routine for Huffman entropy decoding.
   */
  
  GLOBAL void
  jseldhuffman (decompress_info_ptr cinfo)
  {
    if (! cinfo->arith_code) {
      cinfo->methods->entropy_decode_init = huff_decoder_init;
      cinfo->methods->entropy_decode = huff_decode_mcu;
      cinfo->methods->entropy_decode_term = huff_decoder_term;
    }
  }
  