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Filename : RAND.CPP

//----------------------------------------------------------------------
// IMPLEMENTATION FILE (rand.cpp)
// This module exports facilities for pseudorandom number generation.
// Machine dependency: long ints must be at least 32 bits (4 bytes).
//----------------------------------------------------------------------
#include "rand.h"

static long currentSeed; // Updated as a global variable

const long MULTIPLIER = 16807;
const long MAX = 2147483647;
const long QUOT = 127773; // Quotient of MAX / MULTIPLIER
const long REM = 2836; // Remainder of MAX / MULTIPLIER

void SetSeed( /* in */ long initSeed )
//..................................................................
// PRE: initSeed >= 1
// POST: 1 <= currentSeed < MAX
// NOTE: This routine MUST be called prior to NextRand()
//..................................................................
{
initSeed = initSeed % MAX;
currentSeed = (initSeed > 0) ? initSeed : 1;
}

float NextRand()
//..................................................................
// PRE: 1 <= currentSeed < MAX
// POST: currentSeed == (currentSeed * MULTIPLIER) modulo MAX
// && FCTVAL == currentSeed / MAX
// NOTE: This is a prime modulus multiplicative linear congruential
// generator that uses the global variable currentSeed
//..................................................................
{
long temp = MULTIPLIER*(currentSeed%QUOT) - REM*(currentSeed/QUOT);

currentSeed = (temp > 0) ? temp : temp + MAX;
return float(currentSeed) / float(MAX);
}