lib/isc/lfsr.c

/*
 * Copyright (C) 1999-2002, 2004, 2005, 2007, 2016  Internet Systems Consortium, Inc. ("ISC")
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

/* $Id: lfsr.c,v 1.20 2007/06/19 23:47:17 tbox Exp $ */

/*! \file */

#include <config.h>

#include <stddef.h>
#include <stdlib.h>

#include <isc/assertions.h>
#include <isc/lfsr.h>
#include <isc/util.h>

#define VALID_LFSR(x)   (x != NULL)

void
isc_lfsr_init(isc_lfsr_t *lfsr, isc_uint32_t state, unsigned int bits,
          isc_uint32_t tap, unsigned int count,
          isc_lfsrreseed_t reseed, void *arg)
{
    REQUIRE(VALID_LFSR(lfsr));
    REQUIRE(8 <= bits && bits <= 32);
    REQUIRE(tap != 0);

    lfsr->state = state;
    lfsr->bits = bits;
    lfsr->tap = tap;
    lfsr->count = count;
    lfsr->reseed = reseed;
    lfsr->arg = arg;

    if (count == 0 && reseed != NULL)
        reseed(lfsr, arg);
    if (lfsr->state == 0)
        lfsr->state = 0xffffffffU >> (32 - lfsr->bits);
}

/*!
 * Return the next state of the lfsr.
 */
static inline isc_uint32_t
lfsr_generate(isc_lfsr_t *lfsr)
{

    /*
     * If the previous state is zero, we must fill it with something
     * here, or we will begin to generate an extremely predictable output.
     *
     * First, give the reseed function a crack at it.  If the state is
     * still 0, set it to all ones.
     */
    if (lfsr->state == 0) {
        if (lfsr->reseed != NULL)
            lfsr->reseed(lfsr, lfsr->arg);
        if (lfsr->state == 0)
            lfsr->state = 0xffffffffU >> (32 - lfsr->bits);
    }

    if (lfsr->state & 0x01) {
        lfsr->state = (lfsr->state >> 1) ^ lfsr->tap;
        return (1);
    } else {
        lfsr->state >>= 1;
        return (0);
    }
}

void
isc_lfsr_generate(isc_lfsr_t *lfsr, void *data, unsigned int count)
{
    unsigned char *p;
    unsigned int bit;
    unsigned int byte;

    REQUIRE(VALID_LFSR(lfsr));
    REQUIRE(data != NULL);
    REQUIRE(count > 0);

    p = data;
    byte = count;

    while (byte--) {
        *p = 0;
        for (bit = 0; bit < 7; bit++) {
            *p |= lfsr_generate(lfsr);
            *p <<= 1;
        }
        *p |= lfsr_generate(lfsr);
        p++;
    }

    if (lfsr->count != 0 && lfsr->reseed != NULL) {
        if (lfsr->count <= count * 8)
            lfsr->reseed(lfsr, lfsr->arg);
        else
            lfsr->count -= (count * 8);
    }
}

static inline isc_uint32_t
lfsr_skipgenerate(isc_lfsr_t *lfsr, unsigned int skip)
{
    while (skip--)
        (void)lfsr_generate(lfsr);

    (void)lfsr_generate(lfsr);

    return (lfsr->state);
}

/*
 * Skip "skip" states in "lfsr".
 */
void
isc_lfsr_skip(isc_lfsr_t *lfsr, unsigned int skip)
{
    REQUIRE(VALID_LFSR(lfsr));

    while (skip--)
        (void)lfsr_generate(lfsr);
}

/*
 * Skip states in lfsr1 and lfsr2 using the other's current state.
 * Return the final state of lfsr1 ^ lfsr2.
 */
isc_uint32_t
isc_lfsr_generate32(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2)
{
    isc_uint32_t state1, state2;
    isc_uint32_t skip1, skip2;

    REQUIRE(VALID_LFSR(lfsr1));
    REQUIRE(VALID_LFSR(lfsr2));

    skip1 = lfsr1->state & 0x01;
    skip2 = lfsr2->state & 0x01;

    /* cross-skip. */
    state1 = lfsr_skipgenerate(lfsr1, skip2);
    state2 = lfsr_skipgenerate(lfsr2, skip1);

    return (state1 ^ state2);
}