lfsr.c

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/*
 * Copyright (C) 2004, 2005, 2007  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1999-2002  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

/* $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);
}

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