package hexutility

import (
	"encoding/hex"
	"fmt"
)

const (
	badNibble = ^uint64(0)
)

// UnmarshalFixedText decodes the input as a string with 0x prefix. The length of out
// determines the required input length. This function is commonly used to implement the
// UnmarshalText method for fixed-size types.
func UnmarshalFixedText(typname string, input, out []byte) error {
	raw, err := checkText(input, true)
	if err != nil {
		return err
	}
	if len(raw)/2 != len(out) {
		return fmt.Errorf("hex string has length %d, want %d for %s", len(raw), len(out)*2, typname)
	}
	// Pre-verify syntax before modifying out.
	for _, b := range raw {
		if decodeNibble(b) == badNibble {
			return ErrSyntax
		}
	}
	_, err = hex.Decode(out, raw)
	return err
}

func checkText(input []byte, wantPrefix bool) ([]byte, error) {
	if len(input) == 0 {
		return nil, nil // empty strings are allowed
	}
	if bytesHave0xPrefix(input) {
		input = input[2:]
	} else if wantPrefix {
		return nil, ErrMissingPrefix
	}
	if len(input)%2 != 0 {
		return nil, ErrOddLength
	}
	return input, nil
}

func bytesHave0xPrefix(input []byte) bool {
	return len(input) >= 2 && input[0] == '0' && (input[1] == 'x' || input[1] == 'X')
}

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
	}
}