Read bits from stream

typedef struct readBitsContext {
    const muchar_t *buff;
    muint32_t bitpos;
    muint32_t nb;
} readBitsContext_t;

static muint32_t readBits(readBitsContext_t *ctx, muint32_t nbits)
{
    const muchar_t *p = ctx->buff + ctx->bitpos/8;
    muint32_t pos = ctx->bitpos % 8;
    mint32_t bits2go = nbits;
    muint64_t res = 0;
    const muchar_t mask[8] = {0xff, 0x7f, 0x3f, 0x1f, 0x0f, 0x07, 0x03, 0x01};
    if(pos + nbits > ctx->nb * 8) {
        ctx->bitpos = ctx->nb*8;
        ELOG(log_domain,"Trying to read past end of buffer, ignoring");
        goto done;
    }
    while (bits2go > 0) {
        res = (res << 8) | (*p++ & mask[pos]);
        bits2go -= (8 - pos);
        pos = 0;
    }
    bits2go = -bits2go;
    res >>= bits2go;

    ctx->bitpos += nbits;
done:
    return (muint32_t) res;
}