package dnstrie

import (
	"strings"
	"unicode"
)

// isValidDomainName validates if the given string is a valid DNS hostname.
// A valid hostname consists of a series of labels separated by dots.
// Each label must:
// - Be between 1 and 63 characters long.
// - Consist only of ASCII letters ('a'-'z', 'A'-'Z'), digits ('0'-'9'), and hyphens ('-').
// - Not start or end with a hyphen.
// The total length of the hostname (including dots) must not exceed 253 characters.
func isValidDomainName(host string) bool {
	// 1. Check total length. The maximum length of a full hostname is 253 characters.
	if len(host) > 253 {
		return false
	}

	// An empty string is not a valid hostname.
	if host == "" {
		return false
	}

	// 2. Handle optional trailing dot for FQDNs.
	// If the hostname ends with a dot, we remove it for validation purposes.
	if strings.HasSuffix(host, ".") {
		host = host[:len(host)-1]
	}

	// After removing a potential trailing dot, the string might be empty.
	if host == "" {
		return false
	}

	// 3. Split the hostname into labels.
	labels := strings.Split(host, ".")

	// 4. Validate each label.
	for _, label := range labels {
		// a. Check label length (1 to 63 characters).
		if len(label) < 1 || len(label) > 63 {
			return false
		}

		// b. Check if label starts or ends with a hyphen.
		if strings.HasPrefix(label, "-") || strings.HasSuffix(label, "-") {
			return false
		}

		// c. Check for allowed characters in the label.
		for _, char := range label {
			if !unicode.IsLetter(char) && !unicode.IsDigit(char) && char != '-' {
				return false
			}
		}
	}

	// If all checks pass, the hostname is valid.
	return true
}