Java gcd gcd(long a, long b)

Description

Return the greatest common divisor of a and b, consistently with BigInteger#gcd(BigInteger) .

License

Apache License

Declaration

public static long gcd(long a, long b) 

Method Source Code

//package com.java2s;
/*// ww  w .  ja v a 2s . c o m
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

public class Main {
    /** Return the greatest common divisor of <code>a</code> and <code>b</code>,
     *  consistently with {@link BigInteger#gcd(BigInteger)}.
     *  <p><b>NOTE</b>: A greatest common divisor must be positive, but
     *  <code>2^64</code> cannot be expressed as a long although it
     *  is the GCD of {@link Long#MIN_VALUE} and <code>0</code> and the GCD of
     *  {@link Long#MIN_VALUE} and {@link Long#MIN_VALUE}. So in these 2 cases,
     *  and only them, this method will return {@link Long#MIN_VALUE}. */
    // see http://en.wikipedia.org/wiki/Binary_GCD_algorithm#Iterative_version_in_C.2B.2B_using_ctz_.28count_trailing_zeros.29
    public static long gcd(long a, long b) {
        a = Math.abs(a);
        b = Math.abs(b);
        if (a == 0) {
            return b;
        } else if (b == 0) {
            return a;
        }
        final int commonTrailingZeros = Long.numberOfTrailingZeros(a | b);
        a >>>= Long.numberOfTrailingZeros(a);
        while (true) {
            b >>>= Long.numberOfTrailingZeros(b);
            if (a == b) {
                break;
            } else if (a > b || a == Long.MIN_VALUE) { // MIN_VALUE is treated as 2^64
                final long tmp = a;
                a = b;
                b = tmp;
            }
            if (a == 1) {
                break;
            }
            b -= a;
        }
        return a << commonTrailingZeros;
    }
}

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