com.google.uzaygezen.core.BigIntegerMath.java Source code

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Here is the source code for com.google.uzaygezen.core.BigIntegerMath.java

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/*
 * Copyright (C) 2012 Daniel Aioanei.
 *
 * Licensed 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.
 */

package com.google.uzaygezen.core;

import java.math.BigInteger;

import com.google.common.base.Preconditions;

/**
 * Math utilities.
 * 
 * @author Daniel Aioanei
 */
public class BigIntegerMath {

    private BigIntegerMath() {
    }

    public static byte[] nonnegativeBigIntegerToBigEndianByteArrayForBitSize(BigInteger s, int size) {
        // It will have at least one sign bit.
        byte[] b = s.toByteArray();
        Preconditions.checkArgument(b[0] >= 0, "%s is negative", s);
        int n = MathUtils.bitCountToByteCount(size);
        Preconditions.checkArgument(b.length <= n + 1, "%s has bits that are two high", s);
        int start;
        if (b.length == n + 1) {
            Preconditions.checkArgument(size == n << 3,
                    "A BigInteger's big endian length of %s cannot be copied into size=%s.", b.length, size);
            Preconditions.checkArgument(b[0] == 0, "The first byte is a sign bit, and it must be zero.");
            start = 1;
        } else {
            start = 0;
        }
        final byte[] array;
        if (b.length != n) {
            array = new byte[n];
            int len = b.length - start;
            System.arraycopy(b, start, array, n - len, len);
        } else {
            // Optimisation, but this branch is not otherwise necessary. 
            array = b;
        }
        return array;
    }
}