intmath.cc

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/*
 * Copyright (c) 2001, 2003-2005 The Regents of The University of Michigan
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met: redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer;
 * redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution;
 * neither the name of the copyright holders nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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 *
 * Authors: Nathan Binkert
 *          Steve Reinhardt
 */

#include "base/intmath.hh"

int
prevPrime(int n)
{
    int decr;

    // If the number is even, let's start with the previous odd number.
    if (!(n & 1))
        --n;

    // Lets test for divisibility by 3.  Then we will be able to easily
    // avoid numbers that are divisible by 3 in the future.
    decr = n % 3;
    if (decr == 0) {
        n -= 2;
        decr = 2;
    }
    else if (decr == 1)
        decr = 4;

    for (;;) {
        if (isPrime(n))
            return n;
        n -= decr;
        // Toggle between 2 and 4 to prevent trying numbers that are known
        // to be divisible by 3.
        decr = 6 - decr;
    }
}