package calendar

import (
	"sort"
	"strings"
	"time"
)

// Compute returns the liturgical day for date under the given Selection, with
// the ordered layer stack supplying celebrations. It is pure and total: any
// valid Gregorian date yields a LiturgicalDay. Selection.Form "old" computes the
// Extraordinary Form (1962); anything else computes the Ordinary Form.
func Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
	date = date.UTC().Truncate(24 * time.Hour)
	if sel.Form == "old" {
		return computeEF(date, sel, layers)
	}
	return computeOF(date, sel, layers)
}

// TemporalSlug returns the slug of date's TEMPORAL day (the season's Sunday or
// feria), ignoring any sanctoral celebration that outranks it. The EF's
// green-season weekday-repeats-the-Sunday rule uses this so a Monday still finds
// its Sunday Mass even when that Sunday was displaced by a feast (e.g. the
// Purification on Feb 2).
func TemporalSlug(date time.Time, sel Selection) string {
	date = date.UTC().Truncate(24 * time.Hour)
	if sel.Form == "old" {
		return temporalEF(date).Cel.Slug
	}
	return temporal(date, sel).Cel.Slug
}

// computeEF computes the Extraordinary Form (1962) day.
func computeEF(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
	td := temporalEF(date)
	merged := mergeLayers(layers)
	year := date.Year()
	// occupiedByRank reports whether some OTHER fixed-date sanctoral
	// celebration whose rank passes `allowed` resolves onto d this year.
	// transferIfImpededEF uses this at two different thresholds: class 1
	// only, to decide whether a candidate is impeded in the first place (a
	// class-2 occupant never impedes a class-1 feast -- class 1 always beats
	// class 2 outright, no tie exists); and class 1 OR 2, for RG 96's "next
	// day that is not I or II class" once a transfer is already under way
	// (e.g. the Visitation, 2 July, blocking the Precious Blood's transfer
	// off 1 July in 2011).
	occupiedByRank := func(d time.Time, exceptSlug string, allowed func(Rank) bool) bool {
		for slug2, rc2 := range merged {
			if slug2 == exceptSlug {
				continue
			}
			cel2 := buildCelebration(slug2, rc2)
			if !allowed(cel2.Rank) {
				continue
			}
			if when2, ok := celebrationDate(cel2, year, sel); ok && sameDay(when2, d) {
				return true
			}
		}
		return false
	}
	isClass1 := func(r Rank) bool { return r == RankClass1 }
	isClass1Or2 := func(r Rank) bool { return r == RankClass1 || r == RankClass2 }
	occ1 := func(d time.Time, except string) bool { return occupiedByRank(d, except, isClass1) }
	occ1Or2 := func(d time.Time, except string) bool { return occupiedByRank(d, except, isClass1Or2) }
	// RG 97/98: the year's impeded I-class transfers are resolved as a set,
	// not one at a time, so two feasts impeded by the same early Easter cannot
	// both claim the same free day and lose one of themselves.
	//
	// efTransferPlan is pure in (year, merged content, sel) and identical for
	// every day of the year it is asked about -- computeEF runs once PER DAY,
	// so a multi-day view (mobile.Days's week, a month view) was rebuilding
	// it from scratch on every single one. efTransferPlanCached memoises it;
	// see transfer_plan_cache.go for the cache key and why each of its three
	// parts is load-bearing.
	plan := efTransferPlanCached(year, merged, sel, occ1, occ1Or2)
	cands := []candidate{{Cel: td.Cel, Temporal: true, Season: td.Season, Sunday: td.Sunday}}
	for slug, rc := range merged {
		cel := buildCelebration(slug, rc)
		when, ok := celebrationDate(cel, year, sel)
		if !ok {
			continue
		}
		// The Holy Family propers' own rubric: on a 13 January that IS the
		// Sunday after Epiphany the Baptism is omitted "sine commemoratione",
		// so the candidate is dropped rather than ranked. See efBaptismOmitted.
		if efBaptismOmitted(date) && strings.Contains(cel.Slug, "commemoration-of-the-baptism") {
			continue
		}
		effective, planned := plan[cel.Slug]
		if !planned {
			effective = transferIfImpededEF(cel, when,
				func(d time.Time) bool { return occ1(d, cel.Slug) },
				func(d time.Time) bool { return occ1Or2(d, cel.Slug) })
		}
		if sameDay(effective, date) {
			cands = append(cands, candidate{Cel: cel, Temporal: false, Season: td.Season})
		}
	}
	obs, others := pickEF(cands)
	day := LiturgicalDay{
		Date: date, Season: td.Season, Week: td.Week, Weekday: date.Weekday(),
		Observed: obs.Cel, Colour: colourOf(obs, td), ObservedBand: precedenceEF(obs),
	}
	for _, o := range others {
		if o.Temporal {
			continue
		}
		day.Others = append(day.Others, o.Cel)
	}
	return day
}

// computeOF computes the Ordinary Form day.
func computeOF(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
	td := temporal(date, sel)
	merged := mergeLayers(layers)

	cands := []candidate{{
		Cel: td.Cel, Temporal: true, Privileged: td.Privileged,
		PrivFeria: td.PrivFeria, Sunday: td.Sunday, Season: td.Season,
	}}
	for slug, rc := range merged {
		cel := buildCelebration(slug, rc)
		when, ok := celebrationDate(cel, date.Year(), sel)
		if !ok {
			continue
		}
		// On band-2 privileged days only solemnities survive (and they transfer);
		// on Sundays (band 6) only feasts and solemnities compete.
		if td.Privileged && ofRankOrder(cel.Rank) < ofRankOrder(RankSolemnity) {
			continue
		}
		if td.Sunday && ofRankOrder(cel.Rank) < ofRankOrder(RankFeast) {
			continue
		}
		effective := transferIfImpeded(cel, when, sel)
		if sameDay(effective, date) {
			cands = append(cands, candidate{Cel: cel, Temporal: false, Season: td.Season})
		}
	}
	// Movable Marian memorials that have no fixed date (so they cannot live in the
	// sanctoral file): Mary, Mother of the Church, and the Immaculate Heart.
	for _, mm := range movableMemorialsOF(date.Year()) {
		if sameDay(mm.when, date) {
			cands = append(cands, candidate{Cel: mm.cel, Temporal: false, Season: td.Season})
		}
	}

	obs, others := pick(cands)
	day := LiturgicalDay{
		Date: date, Season: td.Season, Week: td.Week, Weekday: date.Weekday(),
		Observed: obs.Cel, Colour: colourOf(obs, td), ObservedBand: precedence(obs),
		SundayCycle:  sundayCycle(liturgicalYearStart(date)),
		WeekdayCycle: weekdayCycle(liturgicalYearStart(date).Year() + 1),
	}
	for _, o := range others {
		if o.Temporal {
			continue // a superseded ferial/Sunday is not a commemoration
		}
		day.Others = append(day.Others, o.Cel)
	}
	return day
}

func colourOf(obs candidate, td temporalDay) Colour {
	if obs.Cel.Colour != "" {
		return obs.Cel.Colour
	}
	return td.Colour
}

// celebrationDate resolves a sanctoral celebration's date for the year.
func celebrationDate(cel Celebration, year int, sel Selection) (time.Time, bool) {
	return resolveDate(cel.Date, year, Easter(year))
}

// movableCel is a celebration whose date is computed (relative to Easter), not
// read from a fixed-date layer.
type movableCel struct {
	when time.Time
	cel  Celebration
}

// movableMemorialsOF returns the Ordinary Form's two movable Marian memorials,
// which have no fixed calendar date: Mary, Mother of the Church (the Monday after
// Pentecost) and the Immaculate Heart of Mary (the Saturday after the Sacred
// Heart, i.e. easter+69).
func movableMemorialsOF(y int) []movableCel {
	e := Easter(y)
	mk := func(off int, slug, en, pl string, first, psalm, gospel string) movableCel {
		return movableCel{when: e.AddDate(0, 0, off), cel: Celebration{
			Slug: slug, Rank: RankMemorial, Class: ClassBVM, Colour: White, Layer: "universal",
			Name: map[string]string{"en": en, "pl": pl},
			// Proper readings (these memorials have no fixed-date entry to key from).
			Masses: []Mass{{Readings: []Reading{
				{Part: "first", Citation: first}, {Part: "psalm", Citation: psalm}, {Part: "gospel", Citation: gospel},
			}}},
		}}
	}
	return []movableCel{
		mk(50, "mary-mother-of-the-church", "Mary, Mother of the Church", "Najświętszej Maryi Panny, Matki Kościoła",
			"Genesis 3:9-15,20", "Psalms 87:1-2,3+5,6-7", "John 19:25-34"),
		mk(69, "the-immaculate-heart-of-the-blessed-virgin-mary", "The Immaculate Heart of the Blessed Virgin Mary", "Niepokalanego Serca Najświętszej Maryi Panny",
			"Isaiah 61:9-11", "1 Samuel 2:1,4-5,6-7,8abcd", "Luke 2:41-51"),
	}
}

// transferIfImpeded moves a solemnity off a band-1/2 temporal day to the next
// free day. Non-solemnities are never transferred (they yield instead).
//
// Two impeded solemnities are ANTICIPATED (moved earlier) rather than deferred,
// matching the General Roman Calendar's published transfers:
//   - St Joseph (Mar 19) falling in Holy Week goes to the Saturday before Palm
//     Sunday. (The Annunciation, by contrast, defers past the octave via the
//     generic forward walk below -- to the Monday after the 2nd Sunday of Easter.)
//   - The Nativity of St John the Baptist (Jun 24), when the day is a solemnity
//     of the Lord (the Sacred Heart or Corpus Christi falling on Jun 24), goes
//     to Jun 23.
func transferIfImpeded(cel Celebration, when time.Time, sel Selection) time.Time {
	if cel.Rank != RankSolemnity {
		return when
	}
	if strings.Contains(cel.Slug, "joseph-husband") {
		if palm := Easter(when.Year()).AddDate(0, 0, -7); !when.Before(palm) {
			return palm.AddDate(0, 0, -1) // Holy Week -> Saturday before Palm Sunday
		}
	}
	if strings.Contains(cel.Slug, "john-the-baptist") {
		if td := temporal(when, sel); td.Cel.Rank == RankSolemnity && td.Class == ClassLord && !td.Sunday {
			return when.AddDate(0, 0, -1) // Sacred Heart / Corpus Christi on Jun 24 -> Jun 23
		}
	}
	day := when
	for i := 0; i < 30; i++ {
		b := temporal(day, sel)
		band := precedence(candidate{
			Cel: b.Cel, Temporal: true, Privileged: b.Privileged,
			PrivFeria: b.PrivFeria, Sunday: b.Sunday, Season: b.Season,
		})
		if band <= 2 { // impeded by Triduum / a privileged day: push forward
			day = day.AddDate(0, 0, 1)
			continue
		}
		return day
	}
	return day
}

// efTransferPlan resolves ALL of a year's impeded I-class transfers together,
// which RG 97/98 require and which resolving them one at a time cannot do.
//
// RG 96(a) is applied first and is decisive for the only collision the
// universal calendar actually produces: the Annunciation, when transferred
// after Easter, has a SEDES PROPRIA -- "festum Annuntiationis B. Mariae Virg.,
// quando est transferendum post Pascha, transfertur, tamquam in sedem propriam,
// in feriam II post dominicam in albis". A proper seat is fixed by law, so it
// does not queue and no tie-break decides it: it claims the Monday after Low
// Sunday and everything else walks around it.
//
// The remainder then transfer in RG 98's order -- "servetur ordo quo in tabella
// praecedentiae inscribuntur; in paritate autem Officium prius impeditum
// praecedit" (table order; at equal position the office impeded FIRST goes
// first) -- each claiming its target so the next walks past it.
//
// Worked example, 2035 (Easter 25 March, Low Sunday 1 April): the Annunciation
// takes its proper seat on 2 April; St Joseph, impeded on the 19th, walks past
// Holy Week, the Easter octave and that claimed Monday to 3 April. Before this,
// St Joseph was observed on no day of 2008, 2035 or 2046 at all.
func efTransferPlan(year int, merged map[string]RawCelebration, sel Selection,
	occupiedByClass1, occupiedByClass1Or2 func(time.Time, string) bool) map[string]time.Time {

	type pending struct {
		slug string
		cel  Celebration
		when time.Time // the date it is impeded ON
		band int       // RG 91 table position, lower = higher dignity
	}
	var queue []pending
	plan := map[string]time.Time{}
	claimed := map[string]bool{}
	claim := func(d time.Time) { claimed[d.Format("2006-01-02")] = true }

	easter := Easter(year)
	lowSundayMonday := easter.AddDate(0, 0, 8) // feria II post dominicam in albis

	for slug, rc := range merged {
		cel := buildCelebration(slug, rc)
		if cel.Rank != RankClass1 {
			continue
		}
		when, ok := celebrationDate(cel, year, sel)
		if !ok {
			continue
		}
		st := temporalEF(when)
		tCand := candidate{Cel: st.Cel, Temporal: true, Season: st.Season, Sunday: st.Sunday}
		sCand := candidate{Cel: cel, Temporal: false}
		if precedenceEF(tCand) >= precedenceEF(sCand) && !occupiedByClass1(when, cel.Slug) {
			continue // not impeded; stays put
		}
		// RG 96(a): a proper seat, claimed before anything queues. Guarded to
		// a transfer that is genuinely "post Pascha" -- 25 March inside Holy
		// Week or the Easter octave -- not to any impediment whatever.
		if strings.Contains(cel.Slug, "annunciation") &&
			!when.Before(easter.AddDate(0, 0, -7)) && !when.After(easter.AddDate(0, 0, 7)) {
			plan[cel.Slug] = lowSundayMonday
			claim(lowSundayMonday)
			continue
		}
		queue = append(queue, pending{slug: cel.Slug, cel: cel, when: when, band: precedenceEF(sCand)})
	}

	// RG 98: table order, then the office impeded first.
	sort.Slice(queue, func(i, j int) bool {
		if queue[i].band != queue[j].band {
			return queue[i].band < queue[j].band
		}
		if !queue[i].when.Equal(queue[j].when) {
			return queue[i].when.Before(queue[j].when)
		}
		return queue[i].slug < queue[j].slug
	})

	for _, p := range queue {
		day := p.when.AddDate(0, 0, 1)
		for i := 0; i < 60; i++ {
			b := temporalEF(day)
			isHighClass := b.Cel.Rank == RankClass1 || b.Cel.Rank == RankClass2
			if isHighClass || occupiedByClass1Or2(day, p.cel.Slug) || claimed[day.Format("2006-01-02")] {
				day = day.AddDate(0, 0, 1)
				continue
			}
			break
		}
		plan[p.slug] = day
		claim(day)
	}
	return plan
}

// transferIfImpededEF moves an EF sanctoral feast off a date it cannot be kept
// on. Two rules cover the 1962 cases: All Souls, when Nov 2 is a Sunday, is
// transferred to the Monday (Nov 3); and a I class feast genuinely impeded is
// pushed forward to the next day that is itself neither I nor II class (RG
// 96: "the next following day that is not I or II class"). Lower-rank feasts
// are only commemorated, never transferred (RG 95).
//
// occupiedByClass1 and occupiedByClass1Or2 are deliberately different
// thresholds, not the same check reused twice:
//   - a candidate is impeded IN THE FIRST PLACE only by a genuine class-1
//     collision -- the temporal office of `when` is itself I class (a Sunday
//     of Advent/Lent/Passiontide, Holy Week, a named I-class feast), or
//     another FIXED I-class sanctoral feast already sits on `when` (RG
//     97/98). A II-class occupant, temporal or sanctoral, never impedes a
//     I-class feast at all -- I class always outranks II class outright, no
//     tie-break is even reached -- so using the wider I-OR-II threshold here
//     would (and, before this fix, did: e.g. All Saints, 1 Nov, was wrongly
//     bumped to 3 Nov merely for landing on an ordinary II-class Sunday it
//     would have won outright) send an unimpeded I-class feast on an
//     unnecessary walk.
//   - once a transfer is under way, the DESTINATION must avoid landing on
//     ANY I- or II-class day (RG 96's own wider "not I or II class" text) --
//     e.g. the Visitation, 2 July, blocks the Precious Blood's transfer off 1
//     July in 2011, and 3 July's ordinary Sunday blocks it a second time.
//
// KNOWN GAP, not fixed here (recorded per RG 95, so the next person does not
// have to rediscover it from scratch): this function resolves ONE
// candidate's own transfer walk in isolation. It has no way to notice that a
// SECOND, separately-transferred I-class candidate has landed on the exact
// same destination day. Concretely: St Joseph (19 March) and the
// Annunciation (25 March) are both impeded in an early-Easter year (2008,
// 2035, 2046) and, resolved independently, both walk to the Monday after Low
// Sunday -- where one of them then loses pickEF and vanishes from the year
// entirely. RG 95 grants the right of translation "solummodo festis I
// classis", so both genuinely have it and both must land SOMEWHERE.
//
// That collision is no longer settled here: efTransferPlan resolves the year's
// impeded I-class feasts together, as RG 97/98 require, and hands each one its
// target. This function keeps the single-candidate walk and All Souls only.
func transferIfImpededEF(cel Celebration, when time.Time, occupiedByClass1, occupiedByClass1Or2 func(time.Time) bool) time.Time {
	if when.Month() == time.November && when.Day() == 2 && when.Weekday() == time.Sunday &&
		strings.Contains(cel.Slug, "all-souls") {
		return when.AddDate(0, 0, 1)
	}
	if cel.Rank != RankClass1 {
		return when
	}
	// Is cel actually impeded on `when` at all? Compare its OWN precedence
	// (via precedenceEF, the same function pickEF uses to decide the day)
	// against the temporal office's, rather than testing the temporal
	// office's class in isolation -- a plain "is the temporal day I class"
	// test wrongly impedes a candidate that would in fact WIN a tie against
	// it: RG 91 entry 4 (the Immaculate Conception, the Assumption) sits
	// ABOVE entry 6 (Sundays), so the Immaculate Conception is not impeded
	// by the Advent Sunday it may fall on at all, unlike an ordinary I-class
	// feast (entry 11, e.g. St Joseph), which is. occupiedByClass1 still
	// covers the separate case of a competing FIXED I-class SANCTORAL
	// feast, which this temporal-only comparison cannot see.
	startTemporal := temporalEF(when)
	tCand := candidate{Cel: startTemporal.Cel, Temporal: true, Season: startTemporal.Season, Sunday: startTemporal.Sunday}
	sCand := candidate{Cel: cel, Temporal: false}
	startImpeded := precedenceEF(tCand) < precedenceEF(sCand) || occupiedByClass1(when)
	if !startImpeded {
		return when
	}
	day := when.AddDate(0, 0, 1)
	for i := 0; i < 30; i++ {
		b := temporalEF(day)
		// A direct CLASS test, not precedenceEF's tie-break band: band
		// encodes which of two EQUAL-class candidates wins a tie (e.g. a
		// Sunday beats an equal-class feast, but a privileged Ember/late-
		// Advent FERIA of the same class yields to one, defect 2) -- an
		// unrelated question from RG 96's own "is this day itself I or II
		// class" test. Reusing band here undercounted: a day within the
		// Octave of the Nativity (26-31 Dec) is genuinely II class (RG 67),
		// but as an ordinary, non-Sunday II-class temporal candidate its OWN
		// band yields (5, not <=3) -- so band<=3 alone let an impeded I-class
		// feast wrongly land there. isHighClass tests the class directly.
		isHighClass := b.Cel.Rank == RankClass1 || b.Cel.Rank == RankClass2
		if isHighClass || occupiedByClass1Or2(day) {
			day = day.AddDate(0, 0, 1) // still I- or II-class; keep walking
			continue
		}
		return day
	}
	return day
}

// buildCelebration types a RawCelebration.
func buildCelebration(slug string, rc RawCelebration) Celebration {
	c := Celebration{Slug: strings.TrimSpace(slug), Name: map[string]string{}, Layer: rc.Fields["layer"]}
	for k, v := range rc.Fields {
		switch {
		case k == "rank":
			c.Rank = ParseRank(v)
		case k == "class":
			c.Class = ParseClass(v)
		case k == "colour":
			c.Colour = ParseColour(v)
		case k == "date":
			c.Date = DateSpec(v)
		case strings.HasPrefix(k, "name."):
			c.Name[strings.TrimPrefix(k, "name.")] = v
		}
	}
	if c.Rank == "" {
		c.Rank = RankFerial
	}
	if m := massFrom(rc.Fields); len(m.Readings) > 0 {
		c.Masses = append(c.Masses, m)
	}
	var variants []string
	for v := range rc.Variants {
		variants = append(variants, v)
	}
	sort.Strings(variants)
	for _, v := range variants {
		mm := massFrom(rc.Variants[v])
		mm.Variant = v
		c.Masses = append(c.Masses, mm)
	}
	return c
}

// BuildCelebrationForTest exposes buildCelebration for cross-package data tests.
func BuildCelebrationForTest(slug string, rc RawCelebration) Celebration {
	return buildCelebration(slug, rc)
}

var readingParts = []string{"first", "psalm", "second", "acclamation", "gospel"}

func massFrom(fields map[string]string) Mass {
	var m Mass
	for _, part := range readingParts {
		if cit := fields["reading."+part]; cit != "" {
			m.Readings = append(m.Readings, Reading{Part: part, Citation: cit})
		}
	}
	return m
}

// liturgicalYearStart returns the First Sunday of Advent that opened the
// liturgical year containing date.
func liturgicalYearStart(date time.Time) time.Time {
	a := adventStart(date.Year())
	if date.Before(a) {
		return adventStart(date.Year() - 1)
	}
	return a
}

// sundayCycle: the liturgical year's civil year (Advent year + 1) mod 3 →
// C(0), A(1), B(2).
func sundayCycle(start time.Time) string {
	switch (start.Year() + 1) % 3 {
	case 0:
		return "C"
	case 1:
		return "A"
	default:
		return "B"
	}
}

// weekdayCycle: odd liturgical-year civil year → I, even → II.
func weekdayCycle(year int) string {
	if year%2 == 1 {
		return "I"
	}
	return "II"
}