package sm2

import (
	"encoding/asn1"
	"math/big"
)

func Decompress(a []byte) *PublicKey {
	var aa, xx, xx3 sm2P256FieldElement

	P256Sm2()
	x := new(big.Int).SetBytes(a[1:])
	curve := sm2P256
	sm2P256FromBig(&xx, x)
	sm2P256Square(&xx3, &xx)       // x3 = x ^ 2
	sm2P256Mul(&xx3, &xx3, &xx)    // x3 = x ^ 2 * x
	sm2P256Mul(&aa, &curve.a, &xx) // a = a * x
	sm2P256Add(&xx3, &xx3, &aa)
	sm2P256Add(&xx3, &xx3, &curve.b)

	y2 := sm2P256ToBig(&xx3)
	y := new(big.Int).ModSqrt(y2, sm2P256.P)
	if getLastBit(y)+2 != uint(a[0]) {
		y.Sub(sm2P256.P, y)
	}
	return &PublicKey{
		Curve: P256Sm2(),
		X:     x,
		Y:     y,
	}
}

func Compress(a *PublicKey) []byte {
	buf := []byte{}
	yp := getLastBit(a.Y)
	buf = append(buf, a.X.Bytes()...)
	if n := len(a.X.Bytes()); n < 32 {
		buf = append(zeroByteSlice()[:(32-n)], buf...)
	}
	buf = append([]byte{byte(yp + 2)}, buf...)
	return buf
}



func SignDigitToSignData(r, s *big.Int) ([]byte, error) {
	return asn1.Marshal(sm2Signature{r, s})
}

func SignDataToSignDigit(sign []byte) (*big.Int, *big.Int, error) {
	var sm2Sign sm2Signature

	_, err := asn1.Unmarshal(sign, &sm2Sign)
	if err != nil {
		return nil, nil, err
	}
	return sm2Sign.R, sm2Sign.S, nil
}