eff that old crap!

typedef struct bit_context
{
    unsigned char *p;
    size_t offset;
} bit_context;

int advance_bytewise(bit_context *ctx)
{
    if (++ctx->offset % 8 != 0)
        return 1;

    ctx->offset = 0;
    ctx->p++;
    return 0;
}

int copy_bits(bit_context *dst, bit_context *src, size_t n)
{
    unsigned char shunt;        /* holds next src bit */

    /* prepare to loop */
    shunt = *src->p;

    while (n-- > 0)
    {
        unsigned char sb;       /* source byte */

        sb = shunt;             /* use saved values */
        shunt = *src->p;        /* save current value */

        /* make dst bit the same as source bit. */
      if ((sb & (0x80 >> src->offset)) == 0)
            *dst->p = *dst->p & ~(0x80 >> dst->offset);
        else
            *dst->p = *dst->p | 0x80 >> dst->offset;

        advance_bytewise(dst);  /* move along */

        /* be careful to keep the backup bits in sync when
         * the source byte changes. */
        if (advance_bytewise(src) == 0)
            shunt = *src->p;
    }

    return 0;
}