meerkat-java/destributed-key-generation/src/main/java/ShamirSecretSharing/Polynomial.java

package ShamirSecretSharing;

import org.bouncycastle.util.Arrays;
import org.factcenter.qilin.primitives.concrete.ECGroup;

import java.math.BigInteger;

/**
 * Created by Tzlil on 1/27/2016.
 */
public class Polynomial {

    private static final Polynomial ZERO = new Polynomial(new BigInteger[]{BigInteger.ZERO});
    private static final Polynomial ONE = new Polynomial(new BigInteger[]{BigInteger.ONE});
    private final int degree;
    private final BigInteger[] coefficients;

    public Polynomial(BigInteger[] coefficients) {
        this.degree = coefficients.length - 1;
        this.coefficients = coefficients;
    }

    public Polynomial(Polynomial polynomial) {
        this.degree = polynomial.getDegree();
        this.coefficients = polynomial.getCoefficients();
    }

    public BigInteger image(BigInteger x){
        BigInteger result = BigInteger.ZERO;
        BigInteger power = BigInteger.ONE;
        for(int i = 0 ; i <= degree ; i++){
            result.add(coefficients[i].multiply(power));
            power.multiply(x);
        }
        return result;
    }

    public static Polynomial interpolation(Point[] points){
        Polynomial[] factors = new Polynomial[points.length];
        for (int i = 0 ; i < factors.length ; i++){
            factors[i] = new Polynomial(new BigInteger[]{BigInteger.ONE,points[i].x}); // X - Xi
        }
        Polynomial result = ZERO;
        BigInteger constant;
        Polynomial product;
        for (int i = 0 ; i < points.length; i++){
            constant = points[i].y;
            product = ONE;
            for (int j = 0 ; j < points.length; j ++){
                if(i != j ) {
                    constant = constant.divide(points[i].x.subtract(points[j].x));
                    product = product.mul(factors[j]);
                }
            }
            result.add(product.mul(constant));
        }
        return result;
    }

    public Polynomial add(Polynomial other){
        Polynomial bigger,smaller;
        if(this.degree < other.degree){
            bigger = other;
            smaller = this;
        }else{
            bigger = this;
            smaller = other;
        }
        BigInteger[] coefficients = bigger.getCoefficients();

        for (int i = 0; i <= smaller.degree ; i++){
            coefficients[i] = smaller.coefficients[i].add(bigger.coefficients[i]);
        }
        return  new Polynomial(coefficients);
    }

    public Polynomial mul(BigInteger constant){

        BigInteger[] coefficients = this.getCoefficients();

        for (int i = 0; i <= this.degree ; i++){
            coefficients[i] = constant.multiply(coefficients[i]);
        }
        return  new Polynomial(coefficients);
    }

    public Polynomial mul(Polynomial other){

        BigInteger[] coefficients = new BigInteger[this.degree + other.degree + 1];
        java.util.Arrays.fill(coefficients,BigInteger.ZERO);

        for (int i = 0; i <= this.degree ; i++){
            for (int j = 0; j <= other.degree; j++){
                coefficients[i+j] = coefficients[i+j].add(this.coefficients[i].multiply(other.coefficients[j]));
            }
        }
        return  new Polynomial(coefficients);
    }


    public BigInteger[] getCoefficients() {
        return Arrays.clone(coefficients);
    }
    public int getDegree() {
        return degree;
    }


    public static class Point{
        public final BigInteger x;
        public final BigInteger y;

        public Point(BigInteger x, BigInteger y) {
            this.x = x;
            this.y = y;
        }
    }

}