// Copyright 2016 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 hexutil implements hex encoding with 0x prefix.
This encoding is used by the Ethereum RPC API to transport binary data in JSON payloads.

# Encoding Rules

All hex data must have prefix "0x".

For byte slices, the hex data must be of even length. An empty byte slice
encodes as "0x".

Integers are encoded using the least amount of digits (no leading zero digits). Their
encoding may be of uneven length. The number zero encodes as "0x0".
*/
package hexutil

import (
	"encoding/hex"
	"fmt"
	"math/big"
	"strconv"
)

const uintBits = 32 << (uint64(^uint(0)) >> 63)

// Errors
var (
	ErrEmptyString   = &decError{"empty hex string"}
	ErrSyntax        = &decError{"invalid hex string"}
	ErrMissingPrefix = &decError{"hex string without 0x prefix"}
	ErrOddLength     = &decError{"hex string of odd length"}
	ErrEmptyNumber   = &decError{"hex string \"0x\""}
	ErrLeadingZero   = &decError{"hex number with leading zero digits"}
	ErrUint64Range   = &decError{"hex number > 64 bits"}
	ErrUintRange     = &decError{fmt.Sprintf("hex number > %d bits", uintBits)}
	ErrBig256Range   = &decError{"hex number > 256 bits"}
)

type decError struct{ msg string }

func (err decError) Error() string { return err.msg }

// Decode decodes a hex string with 0x prefix.
func Decode(input string) ([]byte, error) {
	if len(input) == 0 {
		return nil, ErrEmptyString
	}
	if !has0xPrefix(input) {
		return nil, ErrMissingPrefix
	}
	b, err := hex.DecodeString(input[2:])
	if err != nil {
		err = mapError(err)
	}
	return b, err
}

// MustDecode decodes a hex string with 0x prefix. It panics for invalid input.
func MustDecode(input string) []byte {
	dec, err := Decode(input)
	if err != nil {
		panic(err)
	}
	return dec
}

// Encode encodes b as a hex string with 0x prefix.
func Encode(b []byte) string {
	enc := make([]byte, len(b)*2+2)
	copy(enc, "0x")
	hex.Encode(enc[2:], b)
	return string(enc)
}

// DecodeUint64 decodes a hex string with 0x prefix as a quantity.
func DecodeUint64(input string) (uint64, error) {
	raw, err := checkNumber(input)
	if err != nil {
		return 0, err
	}
	dec, err := strconv.ParseUint(raw, 16, 64)
	if err != nil {
		err = mapError(err)
	}
	return dec, err
}

// MustDecodeUint64 decodes a hex string with 0x prefix as a quantity.
// It panics for invalid input.
func MustDecodeUint64(input string) uint64 {
	dec, err := DecodeUint64(input)
	if err != nil {
		panic(err)
	}
	return dec
}

// EncodeUint64 encodes i as a hex string with 0x prefix.
func EncodeUint64(i uint64) string {
	enc := make([]byte, 2, 10)
	copy(enc, "0x")
	return string(strconv.AppendUint(enc, i, 16))
}

var bigWordNibbles int

func init() {
	// This is a weird way to compute the number of nibbles required for big.Word.
	// The usual way would be to use constant arithmetic but go vet can't handle that.
	b, _ := new(big.Int).SetString("FFFFFFFFFF", 16)
	switch len(b.Bits()) {
	case 1:
		bigWordNibbles = 16
	case 2:
		bigWordNibbles = 8
	default:
		panic("weird big.Word size")
	}
}

// DecodeBig decodes a hex string with 0x prefix as a quantity.
// Numbers larger than 256 bits are not accepted.
func DecodeBig(input string) (*big.Int, error) {
	raw, err := checkNumber(input)
	if err != nil {
		return nil, err
	}
	if len(raw) > 64 {
		return nil, ErrBig256Range
	}
	words := make([]big.Word, len(raw)/bigWordNibbles+1)
	end := len(raw)
	for i := range words {
		start := end - bigWordNibbles
		if start < 0 {
			start = 0
		}
		for ri := start; ri < end; ri++ {
			nib := decodeNibble(raw[ri])
			if nib == badNibble {
				return nil, ErrSyntax
			}
			words[i] *= 16
			words[i] += big.Word(nib)
		}
		end = start
	}
	dec := new(big.Int).SetBits(words)
	return dec, nil
}

// MustDecodeBig decodes a hex string with 0x prefix as a quantity.
// It panics for invalid input.
func MustDecodeBig(input string) *big.Int {
	dec, err := DecodeBig(input)
	if err != nil {
		panic(err)
	}
	return dec
}

// EncodeBig encodes bigint as a hex string with 0x prefix.
// The sign of the integer is ignored.
func EncodeBig(bigint *big.Int) string {
	nbits := bigint.BitLen()
	if nbits == 0 {
		return "0x0"
	}
	return fmt.Sprintf("%#x", bigint)
}

func has0xPrefix(input string) bool {
	return len(input) >= 2 && input[0] == '0' && (input[1] == 'x' || input[1] == 'X')
}

func checkNumber(input string) (raw string, err error) {
	if len(input) == 0 {
		return "", ErrEmptyString
	}
	if !has0xPrefix(input) {
		return "", ErrMissingPrefix
	}
	input = input[2:]
	if len(input) == 0 {
		return "", ErrEmptyNumber
	}
	if len(input) > 1 && input[0] == '0' {
		return "", ErrLeadingZero
	}
	return input, nil
}

const badNibble = ^uint64(0)

func decodeNibble(in byte) uint64 {
	switch {
	case in >= '0' && in <= '9':
		return uint64(in - '0')
	case in >= 'A' && in <= 'F':
		return uint64(in - 'A' + 10)
	case in >= 'a' && in <= 'f':
		return uint64(in - 'a' + 10)
	default:
		return badNibble
	}
}

func mapError(err error) error {
	if err, ok := err.(*strconv.NumError); ok {
		switch err.Err {
		case strconv.ErrRange:
			return ErrUint64Range
		case strconv.ErrSyntax:
			return ErrSyntax
		}
	}
	if _, ok := err.(hex.InvalidByteError); ok {
		return ErrSyntax
	}
	if err == hex.ErrLength {
		return ErrOddLength
	}
	return err
}
