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