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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 and up.
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package org.bouncycastle.pqc.math.ntru.polynomial;

import org.bouncycastle.util.Arrays;

/**
 * A polynomial class that combines two coefficients into one long value for
 * faster multiplication in 64 bit environments.

 * Coefficients can be between 0 and 2047 and are stored in pairs in the bits 0..10 and 24..34 of a long number.
 */
public class LongPolynomial2
{
    private long[] coeffs;   // each representing two coefficients in the original IntegerPolynomial
    private int numCoeffs;

    /**
     * Constructs a LongPolynomial2 from a IntegerPolynomial. The two polynomials are independent of each other.
     *
     * @param p the original polynomial. Coefficients must be between 0 and 2047.
     */
    public LongPolynomial2(IntegerPolynomial p)
    {
        numCoeffs = p.coeffs.length;
        coeffs = new long[(numCoeffs + 1) / 2];
        int idx = 0;
        for (int pIdx = 0; pIdx < numCoeffs; )
        {
            int c0 = p.coeffs[pIdx++];
            while (c0 < 0)
            {
                c0 += 2048;
            }
            long c1 = pIdx < numCoeffs ? p.coeffs[pIdx++] : 0;
            while (c1 < 0)
            {
                c1 += 2048;
            }
            coeffs[idx] = c0 + (c1 << 24);
            idx++;
        }
    }

    private LongPolynomial2(long[] coeffs)
    {
        this.coeffs = coeffs;
    }

    private LongPolynomial2(int N)
    {
        coeffs = new long[N];
    }

    /**
     * Multiplies the polynomial with another, taking the indices mod N and the values mod 2048.
     */
    public LongPolynomial2 mult(LongPolynomial2 poly2)
    {
        int N = coeffs.length;
        if (poly2.coeffs.length != N || numCoeffs != poly2.numCoeffs)
        {
            throw new IllegalArgumentException("Number of coefficients must be the same");
        }

        LongPolynomial2 c = multRecursive(poly2);

        if (c.coeffs.length > N)
        {
            if (numCoeffs % 2 == 0)
            {
                for (int k = N; k < c.coeffs.length; k++)
                {
                    c.coeffs[k - N] = (c.coeffs[k - N] + c.coeffs[k]) & 0x7FF0007FFL;
                }
                c.coeffs = Arrays.copyOf(c.coeffs, N);
            }
            else
            {
                for (int k = N; k < c.coeffs.length; k++)
                {
                    c.coeffs[k - N] = c.coeffs[k - N] + (c.coeffs[k - 1] >> 24);
                    c.coeffs[k - N] = c.coeffs[k - N] + ((c.coeffs[k] & 2047) << 24);
                    c.coeffs[k - N] &= 0x7FF0007FFL;
                }
                c.coeffs = Arrays.copyOf(c.coeffs, N);
                c.coeffs[c.coeffs.length - 1] &= 2047;
            }
        }

        c = new LongPolynomial2(c.coeffs);
        c.numCoeffs = numCoeffs;
        return c;
    }

    public IntegerPolynomial toIntegerPolynomial()
    {
        int[] intCoeffs = new int[numCoeffs];
        int uIdx = 0;
        for (int i = 0; i < coeffs.length; i++)
        {
            intCoeffs[uIdx++] = (int)(coeffs[i] & 2047);
            if (uIdx < numCoeffs)
            {
                intCoeffs[uIdx++] = (int)((coeffs[i] >> 24) & 2047);
            }
        }
        return new IntegerPolynomial(intCoeffs);
    }

    /**
     * Karazuba multiplication
     */
    private LongPolynomial2 multRecursive(LongPolynomial2 poly2)
    {
        long[] a = coeffs;
        long[] b = poly2.coeffs;

        int n = poly2.coeffs.length;
        if (n <= 32)
        {
            int cn = 2 * n;
            LongPolynomial2 c = new LongPolynomial2(new long[cn]);
            for (int k = 0; k < cn; k++)
            {
                for (int i = Math.max(0, k - n + 1); i <= Math.min(k, n - 1); i++)
                {
                    long c0 = a[k - i] * b[i];
                    long cu = c0 & 0x7FF000000L + (c0 & 2047);
                    long co = (c0 >>> 48) & 2047;

                    c.coeffs[k] = (c.coeffs[k] + cu) & 0x7FF0007FFL;
                    c.coeffs[k + 1] = (c.coeffs[k + 1] + co) & 0x7FF0007FFL;
                }
            }
            return c;
        }
        else
        {
            int n1 = n / 2;

            LongPolynomial2 a1 = new LongPolynomial2(Arrays.copyOf(a, n1));
            LongPolynomial2 a2 = new LongPolynomial2(Arrays.copyOfRange(a, n1, n));
            LongPolynomial2 b1 = new LongPolynomial2(Arrays.copyOf(b, n1));
            LongPolynomial2 b2 = new LongPolynomial2(Arrays.copyOfRange(b, n1, n));

            LongPolynomial2 A = (LongPolynomial2)a1.clone();
            A.add(a2);
            LongPolynomial2 B = (LongPolynomial2)b1.clone();
            B.add(b2);

            LongPolynomial2 c1 = a1.multRecursive(b1);
            LongPolynomial2 c2 = a2.multRecursive(b2);
            LongPolynomial2 c3 = A.multRecursive(B);
            c3.sub(c1);
            c3.sub(c2);

            LongPolynomial2 c = new LongPolynomial2(2 * n);
            for (int i = 0; i < c1.coeffs.length; i++)
            {
                c.coeffs[i] = c1.coeffs[i] & 0x7FF0007FFL;
            }
            for (int i = 0; i < c3.coeffs.length; i++)
            {
                c.coeffs[n1 + i] = (c.coeffs[n1 + i] + c3.coeffs[i]) & 0x7FF0007FFL;
            }
            for (int i = 0; i < c2.coeffs.length; i++)
            {
                c.coeffs[2 * n1 + i] = (c.coeffs[2 * n1 + i] + c2.coeffs[i]) & 0x7FF0007FFL;
            }
            return c;
        }
    }

    /**
     * Adds another polynomial which can have a different number of coefficients.
     *
     * @param b another polynomial
     */
    private void add(LongPolynomial2 b)
    {
        if (b.coeffs.length > coeffs.length)
        {
            coeffs = Arrays.copyOf(coeffs, b.coeffs.length);
        }
        for (int i = 0; i < b.coeffs.length; i++)
        {
            coeffs[i] = (coeffs[i] + b.coeffs[i]) & 0x7FF0007FFL;
        }
    }

    /**
     * Subtracts another polynomial which can have a different number of coefficients.
     *
     * @param b another polynomial
     */
    private void sub(LongPolynomial2 b)
    {
        if (b.coeffs.length > coeffs.length)
        {
            coeffs = Arrays.copyOf(coeffs, b.coeffs.length);
        }
        for (int i = 0; i < b.coeffs.length; i++)
        {
            coeffs[i] = (0x0800000800000L + coeffs[i] - b.coeffs[i]) & 0x7FF0007FFL;
        }
    }

    /**
     * Subtracts another polynomial which must have the same number of coefficients,
     * and applies an AND mask to the upper and lower halves of each coefficients.
     *
     * @param b    another polynomial
     * @param mask a bit mask less than 2048 to apply to each 11-bit coefficient
     */
    public void subAnd(LongPolynomial2 b, int mask)
    {
        long longMask = (((long)mask) << 24) + mask;
        for (int i = 0; i < b.coeffs.length; i++)
        {
            coeffs[i] = (0x0800000800000L + coeffs[i] - b.coeffs[i]) & longMask;
        }
    }

    /**
     * Multiplies this polynomial by 2 and applies an AND mask to the upper and
     * lower halves of each coefficients.
     *
     * @param mask a bit mask less than 2048 to apply to each 11-bit coefficient
     */
    public void mult2And(int mask)
    {
        long longMask = (((long)mask) << 24) + mask;
        for (int i = 0; i < coeffs.length; i++)
        {
            coeffs[i] = (coeffs[i] << 1) & longMask;
        }
    }

    public Object clone()
    {
        LongPolynomial2 p = new LongPolynomial2(coeffs.clone());
        p.numCoeffs = numCoeffs;
        return p;
    }

    public boolean equals(Object obj)
    {
        if (obj instanceof LongPolynomial2)
        {
            return Arrays.areEqual(coeffs, ((LongPolynomial2)obj).coeffs);
        }
        else
        {
            return false;
        }
    }
}