Main.java Source code

Introduction

Here is the source code for Main.java

Source

//package com.java2s;
import java.math.BigInteger;

public class Main {
    private static final byte[] EMPTY_BYTES = new byte[0];

    /**
     * Computes the Window NAF (non-adjacent Form) of an integer.
     * @param width The width <code>w</code> of the Window NAF. The width is
     * defined as the minimal number <code>w</code>, such that for any
     * <code>w</code> consecutive digits in the resulting representation, at
     * most one is non-zero.
     * @param k The integer of which the Window NAF is computed.
     * @return The Window NAF of the given width, such that the following holds:
     * <code>k = &sum;<sub>i=0</sub><sup>l-1</sup> k<sub>i</sub>2<sup>i</sup>
     * </code>, where the <code>k<sub>i</sub></code> denote the elements of the
     * returned <code>byte[]</code>.
     */
    public static byte[] generateWindowNaf(int width, BigInteger k) {
        if (width == 2) {
            return generateNaf(k);
        }

        if (width < 2 || width > 8) {
            throw new IllegalArgumentException("'width' must be in the range [2, 8]");
        }
        if (k.signum() == 0) {
            return EMPTY_BYTES;
        }

        byte[] wnaf = new byte[k.bitLength() + 1];

        // 2^width and a mask and sign bit set accordingly
        int pow2 = 1 << width;
        int mask = pow2 - 1;
        int sign = pow2 >>> 1;

        boolean carry = false;
        int length = 0, pos = 0;

        while (pos <= k.bitLength()) {
            if (k.testBit(pos) == carry) {
                ++pos;
                continue;
            }

            k = k.shiftRight(pos);

            int digit = k.intValue() & mask;
            if (carry) {
                ++digit;
            }

            carry = (digit & sign) != 0;
            if (carry) {
                digit -= pow2;
            }

            length += (length > 0) ? pos - 1 : pos;
            wnaf[length++] = (byte) digit;
            pos = width;
        }

        // Reduce the WNAF array to its actual length
        if (wnaf.length > length) {
            wnaf = trim(wnaf, length);
        }

        return wnaf;
    }

    public static byte[] generateNaf(BigInteger k) {
        if (k.signum() == 0) {
            return EMPTY_BYTES;
        }

        BigInteger _3k = k.shiftLeft(1).add(k);

        int digits = _3k.bitLength() - 1;
        byte[] naf = new byte[digits];

        BigInteger diff = _3k.xor(k);

        for (int i = 1; i < digits; ++i) {
            if (diff.testBit(i)) {
                naf[i - 1] = (byte) (k.testBit(i) ? -1 : 1);
                ++i;
            }
        }

        naf[digits - 1] = 1;

        return naf;
    }

    private static byte[] trim(byte[] a, int length) {
        byte[] result = new byte[length];
        System.arraycopy(a, 0, result, 0, result.length);
        return result;
    }

    private static int[] trim(int[] a, int length) {
        int[] result = new int[length];
        System.arraycopy(a, 0, result, 0, result.length);
        return result;
    }
}