package inline

import (
	"errors"
)

const (
	hextable        = "0123456789abcdef"
	reverseHexTable = "" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\xff\xff\xff\xff\xff\xff" +
		"\xff\x0a\x0b\x0c\x0d\x0e\x0f\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\x0a\x0b\x0c\x0d\x0e\x0f\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
)

// EncodedLen returns the length of an encoding of n source bytes.
// Specifically, it returns n * 2.
func EncodedLen(n int) int { return n * 2 }

// Encode encodes src into [EncodedLen](len(src))
// bytes of dst. As a convenience, it returns the number
// of bytes written to dst, but this value is always [EncodedLen](len(src)).
// Encode implements hexadecimal encoding.
func Encode(dst, src []byte) int {
	j := 0
	for _, v := range src {
		dst[j] = hextable[v>>4]
		dst[j+1] = hextable[v&0x0f]
		j += 2
	}
	return len(src) * 2
}

// ErrLength reports an attempt to decode an odd-length input
// using [Decode] or [DecodeString].
// The stream-based Decoder returns [io.ErrUnexpectedEOF] instead of ErrLength.
var ErrLength = errors.New("encoding/hex: odd length hex string")

// InvalidByteError values describe errors resulting from an invalid byte in a hex string.
type InvalidByteError byte

func (e InvalidByteError) Error() string {
	return "encoding/hex: invalid byte"
}

// DecodedLen returns the length of a decoding of x source bytes.
// Specifically, it returns x / 2.
func DecodedLen(x int) int { return x / 2 }

// Decode decodes src into [DecodedLen](len(src)) bytes,
// returning the actual number of bytes written to dst.
//
// Decode expects that src contains only hexadecimal
// characters and that src has even length.
// If the input is malformed, Decode returns the number
// of bytes decoded before the error.
func Decode(dst, src []byte) (int, error) {
	i, j := 0, 1
	for ; j < len(src); j += 2 {
		p := src[j-1]
		q := src[j]

		a := reverseHexTable[p]
		b := reverseHexTable[q]
		if a > 0x0f {
			return i, InvalidByteError(p)
		}
		if b > 0x0f {
			return i, InvalidByteError(q)
		}
		dst[i] = (a << 4) | b
		i++
	}
	if len(src)%2 == 1 {
		// Check for invalid char before reporting bad length,
		// since the invalid char (if present) is an earlier problem.
		if reverseHexTable[src[j-1]] > 0x0f {
			return i, InvalidByteError(src[j-1])
		}
		return i, ErrLength
	}
	return i, nil
}