bitstream.c

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/*
 * FILE:    bitstream.c
 * PROGRAM: RAT
 * AUTHOR:  Orion Hodson
 *
 * Copyright (c) 2003 University of Southern California
 * Copyright (c) 1998-2001 University College London
 * All rights reserved.
 *
 * $Revision: 1.1 $
 * $Date: 2007/11/08 09:48:59 $
 */
 
#include "bitstream.h"
 
typedef struct s_bitstream {
        u_char *buf;            /* head of bitstream            */
        u_char *pos;            /* current byte in bitstream    */
        unsigned int remain;    /* bits remaining               */
        unsigned int len;       /* length of bitstream in bytes */
} bs;
 
int bs_create(bitstream_t ** ppb)
{
        bs *pb;
        pb = (bs *) malloc(sizeof(bs));
        if (pb) {
                memset(pb, 0, sizeof(bs));
                *ppb = pb;
                return TRUE;
        }
        return FALSE;
}
 
int bs_destroy(bitstream_t ** ppb)
{
        free(*ppb);
        return TRUE;
}
 
int bs_attach(bitstream_t * b, u_char * buf, int blen)
{
        b->buf = b->pos = buf;
        b->remain = 8;
        b->len = blen;
        memset(b->buf, 0, b->len);
        return TRUE;
}
 
int bs_put(bitstream_t * b, u_char bits, uint8_t nbits)
{
        assert(nbits != 0 && nbits <= 8);
 
        if (b->remain == 0) {
                b->pos++;
                b->remain = 8;
        }
 
        if (nbits > b->remain) {
                unsigned int over = nbits - b->remain;
                (*b->pos) |= (bits >> over);
                b->pos++;
                b->remain = 8 - over;
                (*b->pos) = (bits << b->remain);
        } else {
                (*b->pos) |= bits << (b->remain - nbits);
                b->remain -= nbits;
        }
 
        assert((unsigned int)(b->pos - b->buf) <= b->len);
        return TRUE;
}
 
u_char bs_get(bitstream_t * b, uint8_t nbits)
{
        u_char out;
 
        if (b->remain == 0) {
                b->pos++;
                b->remain = 8;
        }
 
        if (nbits > b->remain) {
                /* Get high bits */
                out = *b->pos;
                out <<= (8 - b->remain);
                out >>= (8 - nbits);
                b->pos++;
                b->remain += 8 - nbits;
                out |= (*b->pos) >> b->remain;
        } else {
                out = *b->pos;
                out <<= (8 - b->remain);
                out >>= (8 - nbits);
                b->remain -= nbits;
        }
 
        assert((unsigned int)(b->pos - b->buf) <= b->len);
        return out;
}
 
int bs_bytes_used(bitstream_t * b)
{
        unsigned int used = (unsigned int)(b->pos - b->buf);
        if (b->remain != 8) {
                used++;
        }
        return used;
}