// Copyright 2020 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum 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 go-ethereum 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 go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package bls12381

import (
	"encoding/hex"
	"errors"
	"math/big"
)

// fromHex returns the bytes represented by the hexadecimal string s.
// s may be prefixed with "0x".
func fromHex(s string) []byte {
	if has0xPrefix(s) {
		s = s[2:]
	}
	if len(s)%2 == 1 {
		s = "0" + s
	}
	h, err := hex.DecodeString(s)
	if err != nil {
		panic(err)
	}
	return h
}

// has0xPrefix validates str begins with '0x' or '0X'.
func has0xPrefix(str string) bool {
	return len(str) >= 2 && str[0] == '0' && (str[1] == 'x' || str[1] == 'X')
}

func bigFromHex(s string) *big.Int {
	return new(big.Int).SetBytes(fromHex(s))
}

// decodeFieldElement expects 64 byte input with zero top 16 bytes,
// returns lower 48 bytes.
func decodeFieldElement(in []byte) ([]byte, error) {
	if len(in) != 64 {
		return nil, errors.New("invalid field element length")
	}
	// check top bytes
	for i := 0; i < 16; i++ {
		if in[i] != byte(0x00) {
			return nil, errors.New("invalid field element top bytes")
		}
	}
	out := make([]byte, 48)
	copy(out[:], in[16:])
	return out, nil
}