package calendar

import (
	"strconv"
	"strings"
	"time"
)

// Extraordinary Form (1962) season constants. Advent/Christmas/Lent/Easter are
// shared with the OF; these are EF-specific.
const (
	Septuagesima       Season = "septuagesima"
	Passiontide        Season = "passiontide"
	TimeAfterEpiphany  Season = "time-after-epiphany"
	TimeAfterPentecost Season = "time-after-pentecost"
)

// efChristTheKing is the last Sunday of October (the 1962 date — unlike the OF's
// last Sunday before Advent).
func efChristTheKing(year int) time.Time {
	oct31 := time.Date(year, 10, 31, 0, 0, 0, 0, time.UTC)
	return oct31.AddDate(0, 0, -int(oct31.Weekday())) // Sunday on/before Oct 31
}

// efColour is the default (Sunday/ferial) colour of an EF season.
func efColour(s Season) Colour {
	switch s {
	case Advent, Septuagesima, Lent, Passiontide:
		return Violet
	case Christmas, Easter_:
		return White
	default: // TimeAfterEpiphany, TimeAfterPentecost
		return Green
	}
}

// efSeason returns the 1962 season for date, by chronological date boundaries.
// efSlugSeason is the season used to NAME slugs and NUMBER weeks. It is not
// always the liturgical season: 6-13 January is liturgically Christmas Time
// (RG 72, see efSeason) while its days are still named and numbered from
// Epiphany, which is what the lectionary keys and every downstream consumer
// expect. Splitting the two is the whole point -- before this, one function
// served both and the wrong boundary in one was the wrong boundary in both.
func efSlugSeason(date time.Time, y int, easter time.Time) Season {
	adventThis := adventStart(y)
	christmasThis := time.Date(y, 12, 25, 0, 0, 0, 0, time.UTC)
	jan6 := time.Date(y, 1, 6, 0, 0, 0, 0, time.UTC)
	septStart := easter.AddDate(0, 0, -63) // Septuagesima Sunday
	ashWed := easter.AddDate(0, 0, -46)
	passionSun := easter.AddDate(0, 0, -14)
	paschalEnd := easter.AddDate(0, 0, 55) // Saturday after Pentecost; Trinity = +56

	switch {
	case !date.Before(adventThis) && date.Before(christmasThis):
		return Advent
	case !date.Before(christmasThis): // Dec 25..31
		return Christmas
	case date.Before(jan6): // Jan 1..5
		return Christmas
	case date.Before(septStart): // Epiphany .. before Septuagesima
		return TimeAfterEpiphany
	case date.Before(ashWed):
		return Septuagesima
	case date.Before(passionSun):
		return Lent
	case date.Before(easter):
		return Passiontide
	case !date.After(paschalEnd):
		return Easter_
	default:
		return TimeAfterPentecost
	}
}

func efCel(season Season, slug string, col Colour, rank Rank, week int) temporalDay {
	return temporalDay{
		Season: season, Colour: col, Week: week, Rank: rank,
		Sunday: false,
		Cel:    Celebration{Slug: slug, Rank: rank, Colour: col, Layer: "temporal"},
	}
}

// efSunday is efCel plus the Sunday flag, for the temporal candidates that
// are actual Sundays -- precedenceEF's tie-break needs to tell a Sunday (RG
// 91 entry 15, wins its tie) apart from an equal-class privileged FERIA
// (entry 18, the Ember/late-Advent days, which yields).
func efSunday(season Season, slug string, col Colour, rank Rank, week int) temporalDay {
	day := efCel(season, slug, col, rank, week)
	day.Sunday = true
	return day
}

// temporalEF computes the 1962 temporal identity of date. Season is exact
// (oracle-validated); ranks/weeks are best-effort for display.
// efSeason is the LITURGICAL season, which is what the colour follows.
//
// It differs from efSlugSeason on exactly one window. RG 72: "Tempus
// natalicium decurrit a I Vesperis Nativitatis Domini usque ad diem 13
// ianuarii INCLUSIVE" -- Christmas Time runs to 13 January, so 6-13 January is
// the Epiphany SECTION OF Christmas Time, not the start of Time after
// Epiphany. RG 119(a) backs the same boundary from the colour side, white
// "usque ad expletum tempus Epiphaniae". Those eight days were coming out
// green because this distinction did not exist.
//
// The slug season is deliberately left alone there: the days keep their
// Epiphany names and week numbers, which is what the lectionary keys and the
// generated downstream tables are built on. colitur, which models the two
// axes separately from the start, does the same -- season christmastide,
// slug ef-time-after-epiphany-sunday-1.
func efSeason(date time.Time, y int, easter time.Time) Season {
	if s := efSlugSeason(date, y, easter); s == TimeAfterEpiphany &&
		date.Month() == time.January && date.Day() <= 13 {
		return Christmas
	}
	return efSlugSeason(date, y, easter)
}

func temporalEF(date time.Time) temporalDay {
	date = date.UTC().Truncate(24 * time.Hour)
	y := date.Year()
	easter := Easter(y)
	slugSeason := efSlugSeason(date, y, easter)
	season := efSeason(date, y, easter)
	col := efColour(season)
	// The Octave of Pentecost (Whit Monday through the Ember Saturday, easter+50
	// to +55) is red, not the white of the rest of Paschaltide.
	if !date.Before(easter.AddDate(0, 0, 50)) && !date.After(easter.AddDate(0, 0, 55)) {
		col = Red
	}
	// Holy Thursday (the Mass of Chrism and the Mass in Cena Domini) is white,
	// a whole-Mass exception to Passiontide's violet -- RG 128(b)'s own named
	// exception list, and RG 122 stating the same fact affirmatively in the
	// White section.
	if sameDay(date, easter.AddDate(0, 0, -3)) {
		col = White
	}
	// Good Friday is BLACK. RG 128(b)'s exception list carries it in the same
	// sentence as Holy Thursday's -- "...Actione liturgica feria VI in Passione
	// et Morte Domini usque ad Communionem exclusive..." excepts the day from
	// Passiontide's violet -- and RG 132 assigns black to it. Adopted
	// 2026-08-18 from the sibling project colitur, which closed the same gap
	// against the Missal; this comment previously called it "a separate,
	// unmodelled gap" and it no longer is.
	//
	// The rubric is per-ACTION ("usque ad Communionem exclusive", violet
	// returns for the Communion rite) and this model carries one colour per
	// day, so black is the day's principal colour -- the same acknowledged
	// limit the Palm Sunday blessing already has. missalemeum's own colour set
	// for the day is "bv", black first, so even the upstream this file is
	// generated against agrees on the ordering.
	if sameDay(date, easter.AddDate(0, 0, -2)) {
		col = Black
	}
	sun := date.Weekday() == time.Sunday

	// Major feasts of the Lord (I class): nice titles + colour.
	switch {
	case date.Month() == time.December && date.Day() == 25:
		return efCel(Christmas, "ef-nativity", White, RankClass1, 0)
	case date.Month() == time.January && date.Day() == 1:
		return efCel(Christmas, "ef-circumcision", White, RankClass1, 0)
	case date.Month() == time.January && date.Day() == 6:
		// Christmas, not TimeAfterEpiphany: RG 72 puts Epiphany INSIDE Christmas
		// Time, which runs to 13 January. The slug is unchanged.
		return efCel(Christmas, "ef-epiphany", White, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, -46)):
		return efCel(Lent, "ef-ash-wednesday", Violet, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, -14)):
		return efCel(Passiontide, "ef-passion-sunday", Violet, RankClass1, 1)
	case sameDay(date, easter.AddDate(0, 0, -7)):
		return efCel(Passiontide, "ef-palm-sunday", Violet, RankClass1, 2)
	case sameDay(date, easter):
		return efCel(Easter_, "ef-easter-sunday", White, RankClass1, 1)
	case sameDay(date, easter.AddDate(0, 0, 7)):
		return efCel(Easter_, "ef-low-sunday", White, RankClass1, 2)
	case sameDay(date, easter.AddDate(0, 0, 39)):
		return efCel(Easter_, "ef-ascension", White, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, 49)):
		return efCel(Easter_, "ef-pentecost", Red, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, 56)):
		return efCel(TimeAfterPentecost, "ef-trinity", White, RankClass1, 1)
	case sameDay(date, easter.AddDate(0, 0, 60)):
		return efCel(TimeAfterPentecost, "ef-corpus-christi", White, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, 68)):
		return efCel(TimeAfterPentecost, "ef-sacred-heart", White, RankClass1, 0)
	case sameDay(date, efChristTheKing(y)):
		return efCel(TimeAfterPentecost, "ef-christ-the-king", White, RankClass1, 0)
	case sameDay(date, easter.AddDate(0, 0, 38)): // Wednesday: Vigil of the Ascension (II class)
		return efCel(Easter_, "ef-ascension-vigil", White, RankClass2, 0)
	case sameDay(date, easter.AddDate(0, 0, 48)): // Saturday: Vigil of Pentecost / Whitsun Eve (I class)
		return efCel(Easter_, "ef-pentecost-vigil", Red, RankClass1, 0)
	}

	// Sundays vs ferias of the current season.
	if sun {
		week := efWeek(date, slugSeason, y, easter)
		rank := RankClass2
		sundayColour := col
		if season == Advent || season == Lent {
			// RG 11-12 / RG 91 entry 6: every Sunday of Advent and every Sunday
			// of Lent is I class -- not only Advent I, the sole case previously
			// handled (Passiontide's own two Sundays, Passion and Palm, are
			// already I class via the named-feast switch above; Low Sunday
			// likewise).
			rank = RankClass1
			switch {
			case season == Advent && week == 3:
				sundayColour = Rose // Gaudete -- RG 131, that Sunday's Office/Mass only
			case season == Lent && week == 4:
				sundayColour = Rose // Laetare -- RG 131, that Sunday's Office/Mass only
			}
		}
		// Resumed Sundays after Pentecost (1960 Rubrics): when Easter is early
		// there are more than 24 Sundays after Pentecost. The LAST Sunday before
		// Advent always keeps the 24th (Last) Mass; the surplus Sundays between the
		// 23rd and the last resume the Sundays after Epiphany that Septuagesima cut
		// short -- the highest-numbered ones, so the 6th sits just before the Last.
		if season == TimeAfterPentecost {
			lastSun := adventStart(y).AddDate(0, 0, -7)
			if sameDay(date, lastSun) {
				return efSunday(TimeAfterPentecost, "ef-time-after-pentecost-sunday-24", col, rank, 24)
			}
			if week > 23 {
				p := daysBetween(easter.AddDate(0, 0, 49), lastSun) / 7 // total Sundays after Pentecost
				e := week - p + 7                                       // resumed Sunday after Epiphany
				// Season stays time-after-pentecost (calendrical); the Epiphany
				// slug only routes the readings to the resumed Mass.
				return efSunday(TimeAfterPentecost, "ef-time-after-epiphany-sunday-"+strconv.Itoa(e), Green, rank, e)
			}
		}
		// Season reported liturgically, slug named from the slug season -- the
		// same split the resumed-Sunday branch just above already makes for
		// its own reason ("Season stays time-after-pentecost; the Epiphany
		// slug only routes the readings").
		return efSunday(season, "ef-"+string(slugSeason)+"-sunday-"+strconv.Itoa(week), sundayColour, rank, week)
	}
	// The days between Ash Wednesday and the 1st Sunday of Lent have their own
	// proper Masses (not part of a numbered Lenten week).
	if season == Lent && date.Before(easter.AddDate(0, 0, -42)) {
		return efCel(Lent, "ef-lent-after-ashes-"+weekdayLower(date), Violet, RankClass3, 0)
	}
	week := efWeek(date, slugSeason, y, easter)
	rank := RankClass4 // per-annum feria
	if season == Advent || season == Lent || season == Passiontide {
		// III class ferias (1960 rubrics): Advent to Dec 16, and Lent/Passiontide.
		// These outrank III class feasts (the saint is only commemorated). The
		// Septuagesima season's ferias are IV class per annum, so a III class
		// saint (e.g. the Conversion of St Paul, Jan 25) displaces them.
		rank = RankClass3
	}
	if season == Advent && date.Month() == time.December && date.Day() >= 17 && date.Day() <= 23 {
		// RG 91 entry 18: the late-Advent ferias (17-23 Dec) are II class
		// privileged ferias, not the ordinary III class of the rest of Advent.
		rank = RankClass2
	}
	if season == Christmas && date.Month() == time.December && date.Day() >= 26 {
		// RG 67/68: "Dies infra octavam sunt II classis" -- days within the
		// Octave of the Nativity (26-31 Dec) are II class; the octave day
		// itself (1 Jan) is I class, already handled above as a named feast.
		rank = RankClass2
	}
	if efPrivilegedFeria(date, easter) {
		// The days of Holy Week and the privileged octaves of Easter and
		// Pentecost are I class; no saint's feast is admitted (the feast is
		// transferred or, in the octaves, omitted). Ranking them class-1 makes
		// the temporal office win, matching the 1962 occurrence rules.
		rank = RankClass1
	}
	// The Ember Days of September, Advent and Lent (Wednesday, Friday, Saturday
	// after the anchor Sunday) are II class privileged ferias in violet; they
	// displace a III class saint (only commemorated) -- RG 91 entry 18. The
	// Whitsun Ember days fall inside the I class Pentecost octave handled above.
	if es, ok := efEmberSlug(date, y, easter); ok {
		return efCel(season, es, Violet, RankClass2, week)
	}
	// Unique ferial slug (season-week-weekday) so proper ferias (Lent, Advent,
	// Holy Week, Ember days) can key their own readings; green-season ferias
	// simply have no lectionary entry and fall back to the Sunday.
	return efCel(season, "ef-"+string(slugSeason)+"-"+strconv.Itoa(week)+"-"+weekdayLower(date), col, rank, week)
}

// thirdSundayOfSeptember returns the third Sunday in September of year y.
func thirdSundayOfSeptember(y int) time.Time {
	d := time.Date(y, time.September, 1, 0, 0, 0, 0, time.UTC)
	for d.Weekday() != time.Sunday {
		d = d.AddDate(0, 0, 1)
	}
	return d.AddDate(0, 0, 14)
}

// efEmberSlug returns the Ember-day slug for date, if it is an Ember Wednesday,
// Friday or Saturday of September, Advent or Lent (the day after the anchor
// Sunday + 3/5/6: the third Sunday of September, Advent III, or Lent I --
// MR1962, "De anno et eius partibus", Quatuor Tempora). Returns ("", false)
// otherwise. The Whitsun Ember days are not listed here: they fall inside the
// I class Pentecost octave, handled separately by efPrivilegedFeria.
func efEmberSlug(date time.Time, y int, easter time.Time) (string, bool) {
	for _, e := range []struct {
		anchor time.Time
		season string
	}{
		{thirdSundayOfSeptember(y), "september"},
		{adventStart(y).AddDate(0, 0, 14), "advent"}, // 3rd Sunday of Advent
		{easter.AddDate(0, 0, -42), "lent"},          // 1st Sunday of Lent
	} {
		switch {
		case sameDay(date, e.anchor.AddDate(0, 0, 3)):
			return "ef-" + e.season + "-ember-wed", true
		case sameDay(date, e.anchor.AddDate(0, 0, 5)):
			return "ef-" + e.season + "-ember-fri", true
		case sameDay(date, e.anchor.AddDate(0, 0, 6)):
			return "ef-" + e.season + "-ember-sat", true
		}
	}
	return "", false
}

func weekdayLower(d time.Time) string {
	return strings.ToLower(d.Weekday().String())
}

// efPrivilegedFeria reports whether date is an I class temporal weekday that
// impedes saints' feasts: the ferias of Holy Week (the six days before Easter)
// and the weekdays within the privileged octaves of Easter (Easter Mon–Sat) and
// Pentecost (Whit Mon–Sat). Easter/Pentecost Sundays and Low Sunday are already
// handled as named I class feasts above; this covers only their octave weekdays.
func efPrivilegedFeria(date, easter time.Time) bool {
	days := int(date.Sub(easter) / (24 * time.Hour))
	switch {
	case days >= -6 && days <= -1: // Monday..Saturday of Holy Week (incl. Triduum)
		return true
	case days >= 1 && days <= 6: // Easter octave weekdays
		return true
	case days >= 50 && days <= 55: // Pentecost octave weekdays (Pentecost = easter+49)
		return true
	}
	return false
}

// efWeek is a best-effort week-within-season number for display (not
// oracle-checked). Time after Pentecost counts Sundays from Pentecost.
func efWeek(date time.Time, season Season, y int, easter time.Time) int {
	switch season {
	case Advent:
		return daysBetween(adventStart(y), date)/7 + 1
	case TimeAfterPentecost:
		return daysBetween(easter.AddDate(0, 0, 49), date) / 7 // Sundays after Pentecost
	case Lent:
		return daysBetween(easter.AddDate(0, 0, -42), date)/7 + 1 // from 1st Sunday of Lent
	case TimeAfterEpiphany:
		jan6 := time.Date(y, 1, 6, 0, 0, 0, 0, time.UTC)
		return daysBetween(jan6, date)/7 + 1
	case Septuagesima:
		return daysBetween(easter.AddDate(0, 0, -63), date)/7 + 1
	case Easter_:
		return daysBetween(easter, date)/7 + 1
	default:
		return 0
	}
}