// Package dateformat translates a small, user-friendly token pattern (e.g.
// "DD.MM.YYYY HH:mm") into a Go time layout ("02.01.2006 15:04"). It is a
// neutral package imported by both internal/api and internal/tenantstore so
// they can share one validator without importing each other (which would be a
// circular dependency).
//
// Admins may enter an arbitrary token string; anything that is not a known
// token is preserved verbatim as literal text in the resulting layout.
package dateformat

import (
	"fmt"
	"regexp"
	"unicode/utf8"
)

// MaxPatternLen bounds the token pattern length, mirroring the scan-title
// prefix limit so an oversized value can never be persisted.
const MaxPatternLen = 40

// tokenLayout maps each supported token to its Go reference-time fragment.
// Order matters at match time (longest first) so e.g. "YYYY" is not consumed
// as "YY"+"YY"; the regex alternation below encodes that ordering explicitly.
var tokenLayout = map[string]string{
	"AM/PM": "PM",
	"YYYY":  "2006",
	"YY":    "06",
	"MM":    "01",
	"DD":    "02",
	"HH":    "15",
	"hh":    "03",
	"mm":    "04",
	"ss":    "05",
	"PM":    "PM",
}

// tokenRE matches known tokens, longest alternatives first so a greedy leftmost
// match never splits a long token into shorter ones.
var tokenRE = regexp.MustCompile(`AM/PM|YYYY|YY|MM|DD|HH|hh|mm|ss|PM`)

// Translate converts a token pattern into a Go time layout. It fails when the
// pattern is empty, too long, or contains no recognised token (a pattern of
// pure literal text would make time.Format return that literal unchanged,
// which is never what the admin intends).
func Translate(pattern string) (goLayout string, err error) {
	if pattern == "" {
		return "", fmt.Errorf("Format darf nicht leer sein")
	}
	if utf8.RuneCountInString(pattern) > MaxPatternLen {
		return "", fmt.Errorf("Format zu lang (max %d Zeichen)", MaxPatternLen)
	}
	matched := false
	layout := tokenRE.ReplaceAllStringFunc(pattern, func(tok string) string {
		matched = true
		return tokenLayout[tok]
	})
	if !matched {
		return "", fmt.Errorf("Format muss mindestens einen Datums-/Zeit-Platzhalter enthalten")
	}
	return layout, nil
}