package user

import (
	"strings"
)

// SanitizeForUsername coerces an arbitrary profile string into a candidate
// that matches usernameRE: lowercase ASCII alphanumerics, `_`, and `-`,
// length <= 30, with an alphanumeric first character. Returns "" when no
// usable handle can be derived.
func SanitizeForUsername(s string) string {
	s = strings.ToLower(strings.TrimSpace(s))
	if s == "" {
		return ""
	}

	var b strings.Builder
	b.Grow(len(s))
	prevSep := false
	for _, r := range s {
		switch {
		case (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9'):
			b.WriteRune(r)
			prevSep = false
		case r == '-' || r == '_':
			if b.Len() == 0 {
				continue
			}
			if prevSep {
				continue
			}
			b.WriteRune(r)
			prevSep = true
		default:
			if b.Len() == 0 {
				continue
			}
			if prevSep {
				continue
			}
			b.WriteRune('_')
			prevSep = true
		}
	}
	out := strings.TrimRight(b.String(), "_-")
	if len(out) > 30 {
		out = strings.TrimRight(out[:30], "_-")
	}
	return out
}

// nip05LocalPart returns the local part of a NIP-05 identifier (`local@domain`)
// when its domain matches `serviceDomain` (case-insensitive). The bare form
// `local` (no `@`) per NIP-05 implies `_@domain`, which we ignore for sync
// since we cannot prove the domain match.
func nip05LocalPart(nip05, serviceDomain string) string {
	nip05 = strings.TrimSpace(nip05)
	if nip05 == "" || serviceDomain == "" {
		return ""
	}
	at := strings.LastIndex(nip05, "@")
	if at <= 0 || at == len(nip05)-1 {
		return ""
	}
	local := nip05[:at]
	domain := nip05[at+1:]
	if !strings.EqualFold(domain, serviceDomain) {
		return ""
	}
	return local
}

// CandidateFromMetadata returns a sanitized, validated username derived from a
// kind:0 profile, or "" if no field yields a usable handle. Precedence:
//  1. NIP-05 local part when its domain matches serviceDomain.
//  2. `name` (NIP-01).
//  3. `display_name` / `displayName` (NIP-24).
//  4. Deprecated `username` field.
func CandidateFromMetadata(name, displayName, displayNameAlt, username, nip05, serviceDomain string, reserved []string) string {
	tryFields := []string{
		nip05LocalPart(nip05, serviceDomain),
		name,
		displayName,
		displayNameAlt,
		username,
	}
	for _, raw := range tryFields {
		if raw == "" {
			continue
		}
		c := SanitizeForUsername(raw)
		if c == "" {
			continue
		}
		if err := ValidateUsername(c, reserved); err != nil {
			continue
		}
		return c
	}
	return ""
}