// Package idgen generates stable, opaque, URL-safe identifiers for domain
// entities (users, folders, artists, albums, songs, ...).
//
// IDs are random and assigned once, then persisted. They are never derived
// from filesystem position or recomputed on rescan: the scanner looks up
// existing rows by natural key (e.g. a song's path within its music
// folder) and only calls NewID for rows it has never seen before, so IDs
// stay stable across restarts and normal rescans.
package idgen

import (
	"crypto/rand"
	"encoding/base32"
	"strings"
)

// enc is unpadded base32 using Crockford's alphabet-like lowercase set,
// which is URL-safe and avoids ambiguous characters better than base64url
// for anything a human might need to read back (log lines, debugging).
var enc = base32.NewEncoding("0123456789abcdefghjkmnpqrstvwxyz").WithPadding(base32.NoPadding)

// New returns a new opaque ID with the given short type prefix, e.g.
// New("us") -> "us_1a2b3c4d5e6f7g8h9j0k1m2n".
func New(prefix string) string {
	b := make([]byte, 16)
	if _, err := rand.Read(b); err != nil {
		panic(err) // crypto/rand failing means the system is broken
	}
	id := strings.ToLower(enc.EncodeToString(b))
	if prefix == "" {
		return id
	}
	return prefix + "_" + id
}