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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8.

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package org.bouncycastle.math.ec;

import java.math.BigInteger;

import org.bouncycastle.util.Integers;

/**
 * Class implementing the WNAF (Window Non-Adjacent Form) multiplication
 * algorithm.
 */
public class WNafL2RMultiplier extends AbstractECMultiplier
{
    /**
     * Multiplies this by an integer k using the
     * Window NAF method.
     * @param k The integer by which this is multiplied.
     * @return A new ECPoint which equals this
     * multiplied by k.
     */
    protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
    {
        int minWidth = WNafUtil.getWindowSize(k.bitLength());

        WNafPreCompInfo info = WNafUtil.precompute(p, minWidth, true);
        ECPoint[] preComp = info.getPreComp();
        ECPoint[] preCompNeg = info.getPreCompNeg();
        int width = info.getWidth();

        int[] wnaf = WNafUtil.generateCompactWindowNaf(width, k);

        ECPoint R = p.getCurve().getInfinity();

        int i = wnaf.length;

        /*
         * NOTE: We try to optimize the first window using the precomputed points to substitute an
         * addition for 2 or more doublings.
         */
        if (i > 1)
        {
            int wi = wnaf[--i];
            int digit = wi >> 16, zeroes = wi & 0xFFFF;

            int n = Math.abs(digit);
            ECPoint[] table = digit < 0 ? preCompNeg : preComp;

            // Optimization can only be used for values in the lower half of the table
            if ((n << 2) < (1 << width))
            {
                int highest = 32 - Integers.numberOfLeadingZeros(n);

                // TODO Get addition/doubling cost ratio from curve and compare to 'scale' to see if worth substituting?
                int scale = width - highest;
                int lowBits =  n ^ (1 << (highest - 1));

                int i1 = ((1 << (width - 1)) - 1);
                int i2 = (lowBits << scale) + 1;
                R = table[i1 >>> 1].add(table[i2 >>> 1]);

                zeroes -= scale;

//              System.out.println("Optimized: 2^" + scale + " * " + n + " = " + i1 + " + " + i2);
            }
            else
            {
                R = table[n >>> 1];
            }

            R = R.timesPow2(zeroes);
        }

        while (i > 0)
        {
            int wi = wnaf[--i];
            int digit = wi >> 16, zeroes = wi & 0xFFFF;

            int n = Math.abs(digit);
            ECPoint[] table = digit < 0 ? preCompNeg : preComp;
            ECPoint r = table[n >>> 1];

            R = R.twicePlus(r);
            R = R.timesPow2(zeroes);
        }

        return R;
    }
}