/*
 * We use a pseudorandom sequence generator function in two places:
 * when generating comfort noise (GSM 06.12 section 6.1), and when
 * randomizing grid position parameters as part of speech muting
 * during error handling (GSM 06.11 section 6).
 *
 * Our PRNG is a copy of the pseudonoise() function from ETSI EFR code,
 * where it is used for similar purposes.
 */

#include <stdint.h>
#include "tw_gsmfr.h"
#include "pp_internal.h"

uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *st, uint16_t no_bits)
{
    uint16_t noise_bits, Sn, i;

    noise_bits = 0;
    for (i = 0; i < no_bits; i++)
    {
        /* State n == 31 */
        if ((st->cn_random_lfsr & 0x00000001L) != 0)
        {
            Sn = 1;
        }
        else
        {
            Sn = 0;
        }

        /* State n == 3 */
        if ((st->cn_random_lfsr & 0x10000000L) != 0)
        {
            Sn = Sn ^ 1;
        }
        else
        {
            Sn = Sn ^ 0;
        }

        noise_bits = noise_bits << 1;
        noise_bits = noise_bits | st->cn_random_lfsr & 1;

        st->cn_random_lfsr >>= 1;
        if (Sn & 1)
        {
            st->cn_random_lfsr |= 0x40000000L;
        }
    }

    return noise_bits;
}