erand48.c

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/*-------------------------------------------------------------------------
 *
 * erand48.c
 *
 * This file supplies pg_erand48(), pg_lrand48(), and pg_srand48(), which
 * are just like erand48(), lrand48(), and srand48() except that we use
 * our own implementation rather than the one provided by the operating
 * system.  We used to test for an operating system version rather than
 * unconditionally using our own, but (1) some versions of Cygwin have a
 * buggy erand48() that always returns zero and (2) as of 2011, glibc's
 * erand48() is strangely coded to be almost-but-not-quite thread-safe,
 * which doesn't matter for the backend but is important for pgbench.
 *
 *
 * Copyright (c) 1993 Martin Birgmeier
 * All rights reserved.
 *
 * You may redistribute unmodified or modified versions of this source
 * code provided that the above copyright notice and this and the
 * following conditions are retained.
 *
 * This software is provided ``as is'', and comes with no warranties
 * of any kind. I shall in no event be liable for anything that happens
 * to anyone/anything when using this software.
 *
 * IDENTIFICATION
 *    src/port/erand48.c
 *
 *-------------------------------------------------------------------------
 */

#include "c.h"

#include <math.h>

#define RAND48_SEED_0   (0x330e)
#define RAND48_SEED_1   (0xabcd)
#define RAND48_SEED_2   (0x1234)
#define RAND48_MULT_0   (0xe66d)
#define RAND48_MULT_1   (0xdeec)
#define RAND48_MULT_2   (0x0005)
#define RAND48_ADD      (0x000b)

static unsigned short _rand48_seed[3] = {
    RAND48_SEED_0,
    RAND48_SEED_1,
    RAND48_SEED_2
};
static unsigned short _rand48_mult[3] = {
    RAND48_MULT_0,
    RAND48_MULT_1,
    RAND48_MULT_2
};
static unsigned short _rand48_add = RAND48_ADD;


static void
_dorand48(unsigned short xseed[3])
{
    unsigned long accu;
    unsigned short temp[2];

    accu = (unsigned long) _rand48_mult[0] * (unsigned long) xseed[0] +
        (unsigned long) _rand48_add;
    temp[0] = (unsigned short) accu;    /* lower 16 bits */
    accu >>= sizeof(unsigned short) * 8;
    accu += (unsigned long) _rand48_mult[0] * (unsigned long) xseed[1] +
        (unsigned long) _rand48_mult[1] * (unsigned long) xseed[0];
    temp[1] = (unsigned short) accu;    /* middle 16 bits */
    accu >>= sizeof(unsigned short) * 8;
    accu += _rand48_mult[0] * xseed[2] + _rand48_mult[1] * xseed[1] + _rand48_mult[2] * xseed[0];
    xseed[0] = temp[0];
    xseed[1] = temp[1];
    xseed[2] = (unsigned short) accu;
}


double
pg_erand48(unsigned short xseed[3])
{
    _dorand48(xseed);
    return ldexp((double) xseed[0], -48) +
        ldexp((double) xseed[1], -32) +
        ldexp((double) xseed[2], -16);
}

long
pg_lrand48(void)
{
    _dorand48(_rand48_seed);
    return ((long) _rand48_seed[2] << 15) + ((long) _rand48_seed[1] >> 1);
}

long
pg_jrand48(unsigned short xseed[3])
{
    _dorand48(xseed);
    return ((long) xseed[2] << 16) + ((long) xseed[1]);
}

void
pg_srand48(long seed)
{
    _rand48_seed[0] = RAND48_SEED_0;
    _rand48_seed[1] = (unsigned short) seed;
    _rand48_seed[2] = (unsigned short) (seed >> 16);
    _rand48_mult[0] = RAND48_MULT_0;
    _rand48_mult[1] = RAND48_MULT_1;
    _rand48_mult[2] = RAND48_MULT_2;
    _rand48_add = RAND48_ADD;
}