// Package hexutil decodes hex strings in the loose formats people paste.
package hexutil

import (
	"encoding/hex"
	"fmt"
	"strings"
)

// Decode decodes a hex string, tolerating common formatting:
// whitespace and newlines, ':' '-' ',' '.' separators, and "0x" / "\x" byte prefixes.
// So "45 00 00 3c", "45:00:00:3c", "0x45,0x00", "\x45\x00" and "4500003c" all work.
func Decode(s string) ([]byte, error) {
	s = strings.NewReplacer(`\x`, " ", `\X`, " ", "0x", " ", "0X", " ").Replace(s)

	digits := make([]byte, 0, len(s))
	for i := 0; i < len(s); i++ {
		c := s[i]
		switch {
		case isHexDigit(c):
			digits = append(digits, c)
		case c == ' ' || c == '\t' || c == '\n' || c == '\r' ||
			c == ':' || c == '-' || c == ',' || c == '.':
			// separator
		default:
			return nil, fmt.Errorf("invalid character %q in hex input", c)
		}
	}
	if len(digits)%2 != 0 {
		return nil, fmt.Errorf("odd number of hex digits (%d)", len(digits))
	}
	out := make([]byte, len(digits)/2)
	if _, err := hex.Decode(out, digits); err != nil {
		return nil, err
	}
	return out, nil
}

func isHexDigit(c byte) bool {
	return '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F'
}