// GoGOST -- Pure Go GOST cryptographic functions library
// Copyright (C) 2015-2021 Sergey Matveev <stargrave@stargrave.org>
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, version 3 of the License.
//
// This program 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 General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package gost3410

import (
	"errors"
	"math/big"
)

var (
	zero    *big.Int = big.NewInt(0)
	bigInt1 *big.Int = big.NewInt(1)
	bigInt2 *big.Int = big.NewInt(2)
	bigInt3 *big.Int = big.NewInt(3)
	bigInt4 *big.Int = big.NewInt(4)
)

type Curve struct {
	Name string // Just simple identifier

	P *big.Int // Characteristic of the underlying prime field
	Q *big.Int // Elliptic curve subgroup order

	Co *big.Int // Cofactor

	// Equation coefficients of the elliptic curve in canonical form
	A *big.Int
	B *big.Int

	// Equation coefficients of the elliptic curve in twisted Edwards form
	E *big.Int
	D *big.Int

	// Basic point X and Y coordinates
	X *big.Int
	Y *big.Int

	// Temporary variable for the add method
	t  *big.Int
	tx *big.Int
	ty *big.Int

	// Cached s/t parameters for Edwards curve points conversion
	edS *big.Int
	edT *big.Int
}

func NewCurve(p, q, a, b, x, y, e, d, co *big.Int) (*Curve, error) {
	c := Curve{
		Name: "unknown",
		P:    p,
		Q:    q,
		A:    a,
		B:    b,
		X:    x,
		Y:    y,
		t:    big.NewInt(0),
		tx:   big.NewInt(0),
		ty:   big.NewInt(0),
	}
	r1 := big.NewInt(0)
	r2 := big.NewInt(0)
	r1.Mul(c.Y, c.Y)
	r1.Mod(r1, c.P)
	r2.Mul(c.X, c.X)
	r2.Add(r2, c.A)
	r2.Mul(r2, c.X)
	r2.Add(r2, c.B)
	r2.Mod(r2, c.P)
	c.pos(r2)
	if r1.Cmp(r2) != 0 {
		return nil, errors.New("gogost/gost3410: invalid curve parameters")
	}
	if e != nil && d != nil {
		c.E = e
		c.D = d
	}
	if co == nil {
		c.Co = bigInt1
	} else {
		c.Co = co
	}
	return &c, nil
}

func (c *Curve) PointSize() int {
	return PointSize(c.P)
}

func (c *Curve) pos(v *big.Int) {
	if v.Cmp(zero) < 0 {
		v.Add(v, c.P)
	}
}

func (c *Curve) add(p1x, p1y, p2x, p2y *big.Int) {
	if p1x.Cmp(p2x) == 0 && p1y.Cmp(p2y) == 0 {
		// double
		c.t.Mul(p1x, p1x)
		c.t.Mul(c.t, bigInt3)
		c.t.Add(c.t, c.A)
		c.tx.Mul(bigInt2, p1y)
		c.tx.ModInverse(c.tx, c.P)
		c.t.Mul(c.t, c.tx)
		c.t.Mod(c.t, c.P)
	} else {
		c.tx.Sub(p2x, p1x)
		c.tx.Mod(c.tx, c.P)
		c.pos(c.tx)
		c.ty.Sub(p2y, p1y)
		c.ty.Mod(c.ty, c.P)
		c.pos(c.ty)
		c.t.ModInverse(c.tx, c.P)
		c.t.Mul(c.t, c.ty)
		c.t.Mod(c.t, c.P)
	}
	c.tx.Mul(c.t, c.t)
	c.tx.Sub(c.tx, p1x)
	c.tx.Sub(c.tx, p2x)
	c.tx.Mod(c.tx, c.P)
	c.pos(c.tx)
	c.ty.Sub(p1x, c.tx)
	c.ty.Mul(c.ty, c.t)
	c.ty.Sub(c.ty, p1y)
	c.ty.Mod(c.ty, c.P)
	c.pos(c.ty)
	p1x.Set(c.tx)
	p1y.Set(c.ty)
}

func (c *Curve) Exp(degree, xS, yS *big.Int) (*big.Int, *big.Int, error) {
	if degree.Cmp(zero) == 0 {
		return nil, nil, errors.New("gogost/gost3410: zero degree value")
	}
	dg := big.NewInt(0).Sub(degree, bigInt1)
	tx := big.NewInt(0).Set(xS)
	ty := big.NewInt(0).Set(yS)
	cx := big.NewInt(0).Set(xS)
	cy := big.NewInt(0).Set(yS)
	for dg.Cmp(zero) != 0 {
		if dg.Bit(0) == 1 {
			c.add(tx, ty, cx, cy)
		}
		dg.Rsh(dg, 1)
		c.add(cx, cy, cx, cy)
	}
	return tx, ty, nil
}