// Package period understands the Compass lesson label:
//
//	[period] - [subject code] - [room code] - [teacher initials]
//
// for example "1 - 7SCF - H14LAB - KNI". It is a leaf package so the parser,
// the brief and the calendar all agree on what a routine class period is
// (AGENTS.md §4 FR-16/FR-17, §7).
package period

import (
	"regexp"
	"strings"
)

// labelRe matches the period part of a lesson label: "5", "C" or
// "RLE GROUP D". A school event's first part ("End of Year Senior Music
// Concert") does not match, which is how a lesson is told from an event.
var labelRe = regexp.MustCompile(`^(\d{1,2}|[A-Z]|RLE GROUP [A-Z])$`)

// IsPeriodLabel reports whether s is a period slot rather than an event name.
func IsPeriodLabel(s string) bool {
	return labelRe.MatchString(strings.TrimSpace(s))
}

// Split breaks a lesson title into its components. ok is false when the first
// part is not a period label, so the caller renders the title unchanged.
//
// A lesson may have fewer than four parts when Compass omits the room or the
// teacher ("1 - 11HLEGB5 -"), so missing components come back as "".
func Split(title string) (period, subject, room, teacher string, ok bool) {
	parts := SplitDash(title)
	if len(parts) < 2 || !IsPeriodLabel(parts[0]) {
		return "", "", "", "", false
	}
	period = parts[0]
	subject = parts[1]
	if len(parts) >= 3 {
		room = parts[2]
	}
	if len(parts) >= 4 {
		teacher = parts[3]
	}
	return period, subject, room, teacher, true
}

// SplitDash splits on " - " and trims each part.
func SplitDash(s string) []string {
	raw := strings.Split(s, " - ")
	out := make([]string, 0, len(raw))
	for _, p := range raw {
		out = append(out, strings.TrimSpace(p))
	}
	return out
}