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/* This file is part of libmspack.
* (C) 2003-2010 Stuart Caie.
*
* libmspack is free software; you can redistribute it and/or modify it under
* the terms of the GNU Lesser General Public License (LGPL) version 2.1
*
* For further details, see the file COPYING.LIB distributed with libmspack
*/
#ifndef MSPACK_READBITS_H
#define MSPACK_READBITS_H 1
/* this header defines macros that read data streams by
* the individual bits
*
* INIT_BITS initialises bitstream state in state structure
* STORE_BITS stores bitstream state in state structure
* RESTORE_BITS restores bitstream state from state structure
* ENSURE_BITS(n) ensure there are at least N bits in the bit buffer
* READ_BITS(var,n) takes N bits from the buffer and puts them in var
* PEEK_BITS(n) extracts without removing N bits from the bit buffer
* REMOVE_BITS(n) removes N bits from the bit buffer
*
* READ_BITS simply calls ENSURE_BITS, PEEK_BITS and REMOVE_BITS,
* which means it's limited to reading the number of bits you can
* ensure at any one time. It also fails if asked to read zero bits.
* If you need to read zero bits, or more bits than can be ensured in
* one go, use READ_MANY_BITS instead.
*
* These macros have variable names baked into them, so to use them
* you have to define some macros:
* - BITS_TYPE: the type name of your state structure
* - BITS_VAR: the variable that points to your state structure
* - define BITS_ORDER_MSB if bits are read from the MSB, or
* define BITS_ORDER_LSB if bits are read from the LSB
* - READ_BYTES: some code that reads more data into the bit buffer,
* it should use READ_IF_NEEDED (calls read_input if the byte buffer
* is empty), then INJECT_BITS(data,n) to put data from the byte
* buffer into the bit buffer.
*
* You also need to define some variables and structure members:
* - unsigned char *i_ptr; // current position in the byte buffer
* - unsigned char *i_end; // end of the byte buffer
* - unsigned int bit_buffer; // the bit buffer itself
* - unsigned int bits_left; // number of bits remaining
*
* If you use read_input() and READ_IF_NEEDED, they also expect these
* structure members:
* - struct mspack_system *sys; // to access sys->read()
* - unsigned int error; // to record/return read errors
* - unsigned char input_end; // to mark reaching the EOF
* - unsigned char *inbuf; // the input byte buffer
* - unsigned int inbuf_size; // the size of the input byte buffer
*
* Your READ_BYTES implementation should read data from *i_ptr and
* put them in the bit buffer. READ_IF_NEEDED will call read_input()
* if i_ptr reaches i_end, and will fill up inbuf and set i_ptr to
* the start of inbuf and i_end to the end of inbuf.
*
* If you're reading in MSB order, the routines work by using the area
* beyond the MSB and the LSB of the bit buffer as a free source of
* zeroes when shifting. This avoids having to mask any bits. So we
* have to know the bit width of the bit buffer variable. We use
* <limits.h> and CHAR_BIT to find the size of the bit buffer in bits.
*
* If you are reading in LSB order, bits need to be masked. Normally
* this is done by computing the mask: N bits are masked by the value
* (1<<N)-1). However, you can define BITS_LSB_TABLE to use a lookup
* table instead of computing this. This adds two new macros,
* PEEK_BITS_T and READ_BITS_T which work the same way as PEEK_BITS
* and READ_BITS, except they use this lookup table. This is useful if
* you need to look up a number of bits that are only known at
* runtime, so the bit mask can't be turned into a constant by the
* compiler.
* The bit buffer datatype should be at least 32 bits wide: it must be
* possible to ENSURE_BITS(17), so it must be possible to add 16 new bits
* to the bit buffer when the bit buffer already has 1 to 15 bits left.
*/
#ifndef BITS_VAR
# error "define BITS_VAR as the state structure poiner variable name"
#endif
#ifndef BITS_TYPE
# error "define BITS_TYPE as the state structure type"
#endif
#if defined(BITS_ORDER_MSB) && defined(BITS_ORDER_LSB)
# error "you must define either BITS_ORDER_MSB or BITS_ORDER_LSB"
#else
# if !(defined(BITS_ORDER_MSB) || defined(BITS_ORDER_LSB))
# error "you must define BITS_ORDER_MSB or BITS_ORDER_LSB"
# endif
#endif
#if HAVE_LIMITS_H
# include <limits.h>
#endif
#ifndef CHAR_BIT
# define CHAR_BIT (8)
#endif
#define BITBUF_WIDTH (sizeof(bit_buffer) * CHAR_BIT)
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#define INIT_BITS do { \
BITS_VAR->i_ptr = &BITS_VAR->inbuf[0]; \
BITS_VAR->i_end = &BITS_VAR->inbuf[0]; \
BITS_VAR->bit_buffer = 0; \
BITS_VAR->bits_left = 0; \
BITS_VAR->input_end = 0; \ |
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} while (0)
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#define STORE_BITS do { \
BITS_VAR->i_ptr = i_ptr; \
BITS_VAR->i_end = i_end; \
BITS_VAR->bit_buffer = bit_buffer; \
BITS_VAR->bits_left = bits_left; \ |
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} while (0)
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#define RESTORE_BITS do { \
i_ptr = BITS_VAR->i_ptr; \
i_end = BITS_VAR->i_end; \
bit_buffer = BITS_VAR->bit_buffer; \
bits_left = BITS_VAR->bits_left; \ |
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} while (0)
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#define ENSURE_BITS(nbits) do { \
while (bits_left < (nbits)) READ_BYTES; \ |
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} while (0)
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#define READ_BITS(val, nbits) do { \
ENSURE_BITS(nbits); \
(val) = PEEK_BITS(nbits); \
REMOVE_BITS(nbits); \ |
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} while (0)
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#define READ_MANY_BITS(val, bits) do { \
unsigned char needed = (bits), bitrun; \
(val) = 0; \
while (needed > 0) { \
if (bits_left <= (BITBUF_WIDTH - 16)) READ_BYTES; \
bitrun = (bits_left < needed) ? bits_left : needed; \
(val) = ((val) << bitrun) | PEEK_BITS(bitrun); \
REMOVE_BITS(bitrun); \
needed -= bitrun; \
} \ |
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} while (0)
#ifdef BITS_ORDER_MSB
# define PEEK_BITS(nbits) (bit_buffer >> (BITBUF_WIDTH - (nbits)))
# define REMOVE_BITS(nbits) ((bit_buffer <<= (nbits)), (bits_left -= (nbits)))
# define INJECT_BITS(bitdata,nbits) ((bit_buffer |= \
(bitdata) << (BITBUF_WIDTH - (nbits) - bits_left)), (bits_left += (nbits)))
#else /* BITS_ORDER_LSB */
# define PEEK_BITS(nbits) (bit_buffer & ((1 << (nbits))-1))
# define REMOVE_BITS(nbits) ((bit_buffer >>= (nbits)), (bits_left -= (nbits)))
# define INJECT_BITS(bitdata,nbits) ((bit_buffer |= \
(bitdata) << bits_left), (bits_left += (nbits)))
#endif
#ifdef BITS_LSB_TABLE
/* lsb_bit_mask[n] = (1 << n) - 1 */
static const unsigned short lsb_bit_mask[17] = {
0x0000, 0x0001, 0x0003, 0x0007, 0x000f, 0x001f, 0x003f, 0x007f, 0x00ff,
0x01ff, 0x03ff, 0x07ff, 0x0fff, 0x1fff, 0x3fff, 0x7fff, 0xffff
};
# define PEEK_BITS_T(nbits) (bit_buffer & lsb_bit_mask[(nbits)]) |
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# define READ_BITS_T(val, nbits) do { \
ENSURE_BITS(nbits); \
(val) = PEEK_BITS_T(nbits); \
REMOVE_BITS(nbits); \ |
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} while (0)
#endif
#ifndef BITS_NO_READ_INPUT |
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# define READ_IF_NEEDED do { \
if (i_ptr >= i_end) { \
if (read_input(BITS_VAR)) \
return BITS_VAR->error; \
i_ptr = BITS_VAR->i_ptr; \
i_end = BITS_VAR->i_end; \
} \ |
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} while (0)
static int read_input(BITS_TYPE *p) {
int read = p->sys->read(p->input, &p->inbuf[0], (int)p->inbuf_size);
if (read < 0) return p->error = MSPACK_ERR_READ;
/* we might overrun the input stream by asking for bits we don't use,
* so fake 2 more bytes at the end of input */
if (read == 0) { |
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if (p->input_end) {
D(("out of input bytes"))
return p->error = MSPACK_ERR_READ;
}
else {
read = 2;
p->inbuf[0] = p->inbuf[1] = 0;
p->input_end = 1;
} |
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}
/* update i_ptr and i_end */
p->i_ptr = &p->inbuf[0];
p->i_end = &p->inbuf[read];
return MSPACK_ERR_OK;
}
#endif
#endif |