/*
 * Copyright (c) [2016] [ <ether.camp> ]
 * This file is part of the ethereumJ library.
 *
 * The ethereumJ library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * The ethereumJ library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with the ethereumJ library. If not, see <http://www.gnu.org/licenses/>.
 */
package org.ethereum.crypto.zksnark;

import java.math.BigInteger;

import static org.ethereum.crypto.zksnark.Params.R;
import static org.ethereum.crypto.zksnark.Params.TWIST_MUL_BY_P_X;
import static org.ethereum.crypto.zksnark.Params.TWIST_MUL_BY_P_Y;

/**
 * Implementation of specific cyclic subgroup of points belonging to {@link BN128Fp2} <br/>
 * Members of this subgroup are passed as a second param to pairing input {@link PairingCheck#addPair(BN128G1, BN128G2)} <br/>
 * <br/>
 *
 * The order of subgroup is {@link Params#R} <br/>
 * Generator of subgroup G = <br/>
 * (11559732032986387107991004021392285783925812861821192530917403151452391805634 * i + <br/>
 *  10857046999023057135944570762232829481370756359578518086990519993285655852781, <br/>
 *  4082367875863433681332203403145435568316851327593401208105741076214120093531 * i + <br/>
 *  8495653923123431417604973247489272438418190587263600148770280649306958101930) <br/>
 * <br/>
 *
 * @author Mikhail Kalinin
 * @since 31.08.2017
 */
public class BN128G2 extends BN128Fp2 {

    BN128G2(BN128<Fp2> p) {
        super(p.x, p.y, p.z);
    }

    BN128G2(Fp2 x, Fp2 y, Fp2 z) {
        super(x, y, z);
    }

    @Override
    public BN128G2 toAffine() {
        return new BN128G2(super.toAffine());
    }

    /**
     * Checks whether provided data are coordinates of a point belonging to subgroup,
     * if check has been passed it returns a point, otherwise returns null
     */
    public static BN128G2 create(byte[] a, byte[] b, byte[] c, byte[] d) {

        BN128<Fp2> p = BN128Fp2.create(a, b, c, d);

        // fails if point is invalid
        if (p == null) {
            return null;
        }

        // check whether point is a subgroup member
        if (!isGroupMember(p)) return null;

        return new BN128G2(p);
    }

    private static boolean isGroupMember(BN128<Fp2> p) {
        BN128<Fp2> left = p.mul(FR_NEG_ONE).add(p);
        return left.isZero(); // should satisfy condition: -1 * p + p == 0, where -1 belongs to F_r
    }
    static final BigInteger FR_NEG_ONE = BigInteger.ONE.negate().mod(R);

    BN128G2 mulByP() {

        Fp2 rx = TWIST_MUL_BY_P_X.mul(x.frobeniusMap(1));
        Fp2 ry = TWIST_MUL_BY_P_Y.mul(y.frobeniusMap(1));
        Fp2 rz = z.frobeniusMap(1);

        return new BN128G2(rx, ry, rz);
    }
}
