package lists

import (
	"fmt"
	"strings"
	"unicode"
)

// EntryIssue describes a single invalid list entry.
type EntryIssue struct {
	Entry  string `json:"entry"`
	Reason string `json:"reason"`
}

// ValidateEntries checks each entry against Migadu-style address patterns.
// Empty input is valid. Entries are normalized before checking.
func ValidateEntries(entries []string) []EntryIssue {
	issues := make([]EntryIssue, 0)
	for _, value := range entries {
		raw := strings.TrimSpace(value)
		if raw == "" {
			continue
		}
		entry := normalizeEntry(raw)
		if reason := validateNormalizedEntry(entry); reason != "" {
			issues = append(issues, EntryIssue{Entry: raw, Reason: reason})
		}
	}
	return issues
}

// ValidateEntry returns an error reason for one entry, or "" if valid.
func ValidateEntry(value string) string {
	raw := strings.TrimSpace(value)
	if raw == "" {
		return "entry is empty"
	}
	return validateNormalizedEntry(normalizeEntry(raw))
}

func validateNormalizedEntry(entry string) string {
	if entry == "" {
		return "entry is empty"
	}
	if strings.ContainsAny(entry, " \t\r\n") {
		return "entry must not contain whitespace"
	}
	if strings.Count(entry, "@") != 1 {
		return "entry must look like local@domain (e.g. user@example.com or *@example.com)"
	}

	local, domain, ok := splitAddress(entry)
	if !ok {
		return "entry must look like local@domain"
	}
	if local == "" {
		return "local part before @ is empty"
	}
	if !validLocalPart(local) {
		return "invalid local part (use user, *@domain, or *prefix)"
	}
	if domain == "" {
		return "domain after @ is empty"
	}
	if !validDomainPart(domain) {
		return "invalid domain (use example.com or *.com / *.co.uk)"
	}
	return ""
}

func validLocalPart(local string) bool {
	if local == "*" {
		return true
	}
	// *prefix or normal local-part characters commonly used in email addresses.
	if strings.HasPrefix(local, "*") {
		rest := local[1:]
		if rest == "" {
			return true
		}
		return validLocalChars(rest)
	}
	return validLocalChars(local)
}

func validLocalChars(value string) bool {
	if value == "" {
		return false
	}
	for _, r := range value {
		if unicode.IsLetter(r) || unicode.IsDigit(r) {
			continue
		}
		switch r {
		case '.', '_', '+', '-', '%':
			continue
		default:
			return false
		}
	}
	return true
}

func validDomainPart(domain string) bool {
	if strings.HasPrefix(domain, "*.") {
		return validDNSSuffix(domain[2:])
	}
	return validDNSDomain(domain)
}

// validDNSSuffix allows "shop" or "ac.in" after a *. glob prefix.
func validDNSSuffix(suffix string) bool {
	if suffix == "" || len(suffix) > 253 {
		return false
	}
	if strings.HasPrefix(suffix, ".") || strings.HasSuffix(suffix, ".") {
		return false
	}
	labels := strings.Split(suffix, ".")
	if len(labels) < 1 {
		return false
	}
	for _, label := range labels {
		if !validDNSLabel(label) {
			return false
		}
	}
	tld := labels[len(labels)-1]
	for _, r := range tld {
		if !unicode.IsLetter(r) {
			return false
		}
	}
	return true
}

func validDNSDomain(domain string) bool {
	if domain == "" || len(domain) > 253 {
		return false
	}
	if strings.HasPrefix(domain, ".") || strings.HasSuffix(domain, ".") {
		return false
	}
	labels := strings.Split(domain, ".")
	if len(labels) < 2 {
		return false
	}
	for _, label := range labels {
		if !validDNSLabel(label) {
			return false
		}
	}
	// TLD should be alphabetic (allows multi-char like com, uk, info).
	tld := labels[len(labels)-1]
	for _, r := range tld {
		if !unicode.IsLetter(r) {
			return false
		}
	}
	return true
}

func validDNSLabel(label string) bool {
	if label == "" || len(label) > 63 {
		return false
	}
	if label[0] == '-' || label[len(label)-1] == '-' {
		return false
	}
	for i, r := range label {
		if unicode.IsLetter(r) || unicode.IsDigit(r) {
			continue
		}
		if r == '-' && i > 0 && i < len(label)-1 {
			continue
		}
		return false
	}
	return true
}

// FormatIssues returns a short multi-line summary for logs/UI.
func FormatIssues(issues []EntryIssue) string {
	if len(issues) == 0 {
		return ""
	}
	parts := make([]string, 0, len(issues))
	for _, issue := range issues {
		parts = append(parts, fmt.Sprintf("%s: %s", issue.Entry, issue.Reason))
	}
	return strings.Join(parts, "\n")
}