bit.c

/*
 * libmad - MPEG audio decoder library
 * Copyright (C) 2000-2003 Underbit Technologies, Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * $Id: bit.c,v 1.1 2003/08/31 18:59:46 gabest Exp $
 */

# ifdef HAVE_CONFIG_H
#  include "config.h"
# endif

# include "global.h"

# ifdef HAVE_LIMITS_H
#  include <limits.h>
# else
#  define CHAR_BIT  8
# endif

# include "bit.h"

/*
 * This is the lookup table for computing the CRC-check word.
 * As described in section 2.4.3.1 and depicted in Figure A.9
 * of ISO/IEC 11172-3, the generator polynomial is:
 *
 * G(X) = X^16 + X^15 + X^2 + 1
 */
static
unsigned short const crc_table[256] = {
  0x0000, 0x8005, 0x800f, 0x000a, 0x801b, 0x001e, 0x0014, 0x8011,
  0x8033, 0x0036, 0x003c, 0x8039, 0x0028, 0x802d, 0x8027, 0x0022,
  0x8063, 0x0066, 0x006c, 0x8069, 0x0078, 0x807d, 0x8077, 0x0072,
  0x0050, 0x8055, 0x805f, 0x005a, 0x804b, 0x004e, 0x0044, 0x8041,
  0x80c3, 0x00c6, 0x00cc, 0x80c9, 0x00d8, 0x80dd, 0x80d7, 0x00d2,
  0x00f0, 0x80f5, 0x80ff, 0x00fa, 0x80eb, 0x00ee, 0x00e4, 0x80e1,
  0x00a0, 0x80a5, 0x80af, 0x00aa, 0x80bb, 0x00be, 0x00b4, 0x80b1,
  0x8093, 0x0096, 0x009c, 0x8099, 0x0088, 0x808d, 0x8087, 0x0082,

  0x8183, 0x0186, 0x018c, 0x8189, 0x0198, 0x819d, 0x8197, 0x0192,
  0x01b0, 0x81b5, 0x81bf, 0x01ba, 0x81ab, 0x01ae, 0x01a4, 0x81a1,
  0x01e0, 0x81e5, 0x81ef, 0x01ea, 0x81fb, 0x01fe, 0x01f4, 0x81f1,
  0x81d3, 0x01d6, 0x01dc, 0x81d9, 0x01c8, 0x81cd, 0x81c7, 0x01c2,
  0x0140, 0x8145, 0x814f, 0x014a, 0x815b, 0x015e, 0x0154, 0x8151,
  0x8173, 0x0176, 0x017c, 0x8179, 0x0168, 0x816d, 0x8167, 0x0162,
  0x8123, 0x0126, 0x012c, 0x8129, 0x0138, 0x813d, 0x8137, 0x0132,
  0x0110, 0x8115, 0x811f, 0x011a, 0x810b, 0x010e, 0x0104, 0x8101,

  0x8303, 0x0306, 0x030c, 0x8309, 0x0318, 0x831d, 0x8317, 0x0312,
  0x0330, 0x8335, 0x833f, 0x033a, 0x832b, 0x032e, 0x0324, 0x8321,
  0x0360, 0x8365, 0x836f, 0x036a, 0x837b, 0x037e, 0x0374, 0x8371,
  0x8353, 0x0356, 0x035c, 0x8359, 0x0348, 0x834d, 0x8347, 0x0342,
  0x03c0, 0x83c5, 0x83cf, 0x03ca, 0x83db, 0x03de, 0x03d4, 0x83d1,
  0x83f3, 0x03f6, 0x03fc, 0x83f9, 0x03e8, 0x83ed, 0x83e7, 0x03e2,
  0x83a3, 0x03a6, 0x03ac, 0x83a9, 0x03b8, 0x83bd, 0x83b7, 0x03b2,
  0x0390, 0x8395, 0x839f, 0x039a, 0x838b, 0x038e, 0x0384, 0x8381,

  0x0280, 0x8285, 0x828f, 0x028a, 0x829b, 0x029e, 0x0294, 0x8291,
  0x82b3, 0x02b6, 0x02bc, 0x82b9, 0x02a8, 0x82ad, 0x82a7, 0x02a2,
  0x82e3, 0x02e6, 0x02ec, 0x82e9, 0x02f8, 0x82fd, 0x82f7, 0x02f2,
  0x02d0, 0x82d5, 0x82df, 0x02da, 0x82cb, 0x02ce, 0x02c4, 0x82c1,
  0x8243, 0x0246, 0x024c, 0x8249, 0x0258, 0x825d, 0x8257, 0x0252,
  0x0270, 0x8275, 0x827f, 0x027a, 0x826b, 0x026e, 0x0264, 0x8261,
  0x0220, 0x8225, 0x822f, 0x022a, 0x823b, 0x023e, 0x0234, 0x8231,
  0x8213, 0x0216, 0x021c, 0x8219, 0x0208, 0x820d, 0x8207, 0x0202
};

# define CRC_POLY  0x8005

/*
 * NAME:        bit->init()
 * DESCRIPTION: initialize bit pointer struct
 */
void mad_bit_init(struct mad_bitptr *bitptr, unsigned char const *byte)
{
  bitptr->byte  = byte;
  bitptr->cache = 0;
  bitptr->left  = CHAR_BIT;
}

/*
 * NAME:        bit->length()
 * DESCRIPTION: return number of bits between start and end points
 */
unsigned int mad_bit_length(struct mad_bitptr const *begin,
                            struct mad_bitptr const *end)
{
  return begin->left +
    CHAR_BIT * (end->byte - (begin->byte + 1)) + (CHAR_BIT - end->left);
}

/*
 * NAME:        bit->nextbyte()
 * DESCRIPTION: return pointer to next unprocessed byte
 */
unsigned char const *mad_bit_nextbyte(struct mad_bitptr const *bitptr)
{
  return bitptr->left == CHAR_BIT ? bitptr->byte : bitptr->byte + 1;
}

/*
 * NAME:        bit->skip()
 * DESCRIPTION: advance bit pointer
 */
void mad_bit_skip(struct mad_bitptr *bitptr, unsigned int len)
{
  bitptr->byte += len / CHAR_BIT;
  bitptr->left -= len % CHAR_BIT;

  if (bitptr->left > CHAR_BIT) {
    bitptr->byte++;
    bitptr->left += CHAR_BIT;
  }

  if (bitptr->left < CHAR_BIT)
    bitptr->cache = *bitptr->byte;
}

/*
 * NAME:        bit->read()
 * DESCRIPTION: read an arbitrary number of bits and return their UIMSBF value
 */
unsigned long mad_bit_read(struct mad_bitptr *bitptr, unsigned int len)
{
  register unsigned long value;

  if (bitptr->left == CHAR_BIT)
    bitptr->cache = *bitptr->byte;

  if (len < bitptr->left) {
    value = (bitptr->cache & ((1 << bitptr->left) - 1)) >>
      (bitptr->left - len);
    bitptr->left -= len;

    return value;
  }

  /* remaining bits in current byte */

  value = bitptr->cache & ((1 << bitptr->left) - 1);
  len  -= bitptr->left;

  bitptr->byte++;
  bitptr->left = CHAR_BIT;

  /* more bytes */

  while (len >= CHAR_BIT) {
    value = (value << CHAR_BIT) | *bitptr->byte++;
    len  -= CHAR_BIT;
  }

  if (len > 0) {
    bitptr->cache = *bitptr->byte;

    value = (value << len) | (bitptr->cache >> (CHAR_BIT - len));
    bitptr->left -= len;
  }

  return value;
}

# if 0
/*
 * NAME:        bit->write()
 * DESCRIPTION: write an arbitrary number of bits
 */
void mad_bit_write(struct mad_bitptr *bitptr, unsigned int len,
                   unsigned long value)
{
  unsigned char *ptr;

  ptr = (unsigned char *) bitptr->byte;

  /* ... */
}
# endif

/*
 * NAME:        bit->crc()
 * DESCRIPTION: compute CRC-check word
 */
unsigned short mad_bit_crc(struct mad_bitptr bitptr, unsigned int len,
                           unsigned short init)
{
  register unsigned int crc;

  for (crc = init; len >= 32; len -= 32) {
    register unsigned long data;

    data = mad_bit_read(&bitptr, 32);

    crc = (crc << 8) ^ crc_table[((crc >> 8) ^ (data >> 24)) & 0xff];
    crc = (crc << 8) ^ crc_table[((crc >> 8) ^ (data >> 16)) & 0xff];
    crc = (crc << 8) ^ crc_table[((crc >> 8) ^ (data >>  8)) & 0xff];
    crc = (crc << 8) ^ crc_table[((crc >> 8) ^ (data >>  0)) & 0xff];
  }

  switch (len / 8) {
  case 3: crc = (crc << 8) ^
            crc_table[((crc >> 8) ^ mad_bit_read(&bitptr, 8)) & 0xff];
  case 2: crc = (crc << 8) ^
            crc_table[((crc >> 8) ^ mad_bit_read(&bitptr, 8)) & 0xff];
  case 1: crc = (crc << 8) ^
            crc_table[((crc >> 8) ^ mad_bit_read(&bitptr, 8)) & 0xff];

  len %= 8;

  case 0: break;
  }

  while (len--) {
    register unsigned int msb;

    msb = mad_bit_read(&bitptr, 1) ^ (crc >> 15);

    crc <<= 1;
    if (msb & 1)
      crc ^= CRC_POLY;
  }

  return crc & 0xffff;
}

---