// Package slug turns titles into URL-safe identifiers.
package slug

import (
	"fmt"
	"regexp"
	"strings"
	"unicode"

	"golang.org/x/text/runes"
	"golang.org/x/text/transform"
	"golang.org/x/text/unicode/norm"
)

var (
	nonAlnum   = regexp.MustCompile(`[^a-z0-9]+`)
	validRe    = regexp.MustCompile(`^[a-z0-9](?:-?[a-z0-9])*$`)
	stripMarks = transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
)

// greek maps Greek letters to Latin so Greek titles get readable slugs.
var greek = strings.NewReplacer(
	"α", "a", "β", "v", "γ", "g", "δ", "d", "ε", "e", "ζ", "z", "η", "i", "θ", "th",
	"ι", "i", "κ", "k", "λ", "l", "μ", "m", "ν", "n", "ξ", "x", "ο", "o", "π", "p",
	"ρ", "r", "σ", "s", "ς", "s", "τ", "t", "υ", "y", "φ", "f", "χ", "ch", "ψ", "ps", "ω", "o",
)

// Make returns a slug for s; falls back to "untitled" when nothing usable remains.
func Make(s string) string {
	if t, _, err := transform.String(stripMarks, s); err == nil {
		s = t
	}
	s = greek.Replace(strings.ToLower(s))
	s = nonAlnum.ReplaceAllString(s, "-")
	s = strings.Trim(s, "-")
	if len(s) > 80 {
		s = strings.Trim(s[:80], "-")
	}
	if s == "" {
		return "untitled"
	}
	return s
}

// WithSuffix returns "base-n" (or base when n < 2), used to resolve collisions.
func WithSuffix(base string, n int) string {
	if n < 2 {
		return base
	}
	return fmt.Sprintf("%s-%d", base, n)
}

// Valid reports whether s is an acceptable slug as typed by a user.
func Valid(s string) bool { return validRe.MatchString(s) && len(s) <= 80 }