package calendar

import (
	"strconv"
	"time"
)

// temporalDay is the temporal (season/movable) identity of a date, before any
// sanctoral celebration is considered. Privileged marks band 1-2 days (Sundays
// of Advent/Lent/Easter, Holy Week, Triduum, Easter octave, the great
// solemnities); PrivFeria marks band-9 privileged ferias (Advent 17-24,
// Lenten weekdays); Sunday marks an Ordinary/Christmas Sunday (band 6).
type temporalDay struct {
	Season     Season
	Week       int
	Colour     Colour
	Cel        Celebration
	Rank       Rank
	Class      Class
	Privileged bool
	PrivFeria  bool
	Sunday     bool
}

// adventStart returns the First Sunday of Advent that opens the liturgical year
// containing Christmas of the given calendar year. The 4th Sunday of Advent is
// the Sunday on or before Christmas Eve (Dec 24) — anchoring on Dec 24, not
// Dec 25, keeps Christmas Day itself from being counted as an Advent Sunday
// (which loses a week in years when Dec 25 is a Sunday). Advent I is 3 Sundays
// earlier.
func adventStart(year int) time.Time {
	eve := time.Date(year, 12, 24, 0, 0, 0, 0, time.UTC)
	sun := eve.AddDate(0, 0, -int(eve.Weekday())) // 4th Sunday of Advent
	return sun.AddDate(0, 0, -21)                 // 1st Sunday of Advent
}

func sameDay(a, b time.Time) bool {
	return a.Year() == b.Year() && a.YearDay() == b.YearDay()
}

// daysBetween is the whole-day count from..to (both UTC midnight).
func daysBetween(from, to time.Time) int {
	return int(to.Sub(from).Hours()/24 + 0.5)
}

func weekdayAbbr(wd time.Weekday) string {
	return [...]string{"sun", "mon", "tue", "wed", "thu", "fri", "sat"}[wd]
}

// epiphany is fixed Jan 6, or (sel.Epiphany=="sunday") the Sunday between Jan 2 and Jan 8.
func epiphany(year int, sel Selection) time.Time {
	if sel.Epiphany == "sunday" {
		return nextWeekday(time.Date(year, 1, 2, 0, 0, 0, 0, time.UTC), time.Sunday)
	}
	return time.Date(year, 1, 6, 0, 0, 0, 0, time.UTC)
}

// baptismOfLord is the Sunday after Epiphany; when Epiphany (transferred to a
// Sunday) is Jan 7 or 8, Baptism is the next day.
func baptismOfLord(year int, sel Selection) time.Time {
	epi := epiphany(year, sel)
	if sel.Epiphany == "sunday" && (epi.Day() == 7 || epi.Day() == 8) {
		return epi.AddDate(0, 0, 1)
	}
	return nextWeekday(epi.AddDate(0, 0, 1), time.Sunday)
}

// holyFamily is the Sunday within the Christmas octave (Dec 26-31), or Dec 30
// when Christmas itself is a Sunday.
func holyFamily(year int) time.Time {
	xmas := time.Date(year, 12, 25, 0, 0, 0, 0, time.UTC)
	if xmas.Weekday() == time.Sunday {
		return time.Date(year, 12, 30, 0, 0, 0, 0, time.UTC)
	}
	return nextWeekday(xmas.AddDate(0, 0, 1), time.Sunday)
}

// ascension: Thursday (easter+39) or, where transferred, the 7th Sunday of Easter (easter+42).
func ascension(y int, sel Selection) time.Time {
	e := Easter(y)
	if sel.Ascension == "sunday" {
		return e.AddDate(0, 0, 42)
	}
	return e.AddDate(0, 0, 39)
}

// corpusChristi: Thursday (easter+60) or, where transferred, Sunday (easter+63).
func corpusChristi(y int, sel Selection) time.Time {
	e := Easter(y)
	if sel.CorpusChristi == "sunday" {
		return e.AddDate(0, 0, 63)
	}
	return e.AddDate(0, 0, 60)
}

// --- small constructors ---

func solemn(season Season, slug string, col Colour, privileged bool) temporalDay {
	return temporalDay{Season: season, Colour: col, Rank: RankSolemnity, Class: ClassLord, Privileged: privileged,
		Cel: Celebration{Slug: slug, Rank: RankSolemnity, Class: ClassLord, Colour: col, Layer: "temporal"}}
}

func feastOfLord(season Season, slug string, col Colour) temporalDay {
	return temporalDay{Season: season, Colour: col, Rank: RankFeast, Class: ClassLord,
		Cel: Celebration{Slug: slug, Rank: RankFeast, Class: ClassLord, Colour: col, Layer: "temporal"}}
}

func sundayDay(season Season, slug string, col Colour, week int) temporalDay {
	priv := season == Advent || season == Lent || season == Easter_
	td := temporalDay{Season: season, Colour: col, Week: week, Rank: RankSolemnity, Privileged: priv, Sunday: !priv,
		Cel: Celebration{Slug: slug, Rank: RankSolemnity, Colour: col, Layer: "temporal"}}
	return td
}

func ferialDay(season Season, slug string, col Colour, week int, privFeria bool) temporalDay {
	return temporalDay{Season: season, Colour: col, Week: week, Rank: RankFerial, PrivFeria: privFeria,
		Cel: Celebration{Slug: slug, Rank: RankFerial, Colour: col, Layer: "temporal"}}
}

// privFerialDay builds a band-2 privileged weekday (Ash Wednesday, Holy Week
// Monday-Wednesday): a ferial no sanctoral feast or memorial can displace (only
// a solemnity may, and solemnities transfer off it -- Table of Liturgical Days
// #2). Distinct from ferialDay's PrivFeria (band 9) Lenten/Advent weekdays.
func privFerialDay(season Season, slug string, col Colour, week int) temporalDay {
	return temporalDay{Season: season, Colour: col, Week: week, Rank: RankFerial, Privileged: true,
		Cel: Celebration{Slug: slug, Rank: RankFerial, Colour: col, Layer: "temporal"}}
}

// temporal computes the temporal identity of date under the given Selection.
func temporal(date time.Time, sel Selection) temporalDay {
	date = date.UTC().Truncate(24 * time.Hour)
	y := date.Year()
	easter := Easter(y)
	ashWed := easter.AddDate(0, 0, -46)
	palmSunday := easter.AddDate(0, 0, -7)
	holyThu := easter.AddDate(0, 0, -3)
	pentecost := easter.AddDate(0, 0, 49)
	adventThis := adventStart(y)
	christmasThis := time.Date(y, 12, 25, 0, 0, 0, 0, time.UTC)
	baptismThis := baptismOfLord(y, sel)
	christTheKing := adventThis.AddDate(0, 0, -7)
	wd := date.Weekday()
	sun := wd == time.Sunday

	// 1. Movable / dated solemnities and feasts (highest identity first).
	switch {
	case sameDay(date, easter):
		return solemn(Easter_, "easter-sunday", White, true)
	case date.After(easter) && !date.After(easter.AddDate(0, 0, 7)):
		// Octave of Easter (Mon..Sun); easter+7 = 2nd Sunday of Easter.
		td := solemn(Easter_, "easter-octave-"+weekdayAbbr(wd), White, true)
		td.Week = 1
		return td
	case sameDay(date, ascension(y, sel)):
		return solemn(Easter_, "ascension", White, true)
	case sameDay(date, pentecost):
		return solemn(Easter_, "pentecost", Red, true)
	case sameDay(date, easter.AddDate(0, 0, 56)):
		return solemn(Ordinary, "trinity-sunday", White, false)
	case sameDay(date, corpusChristi(y, sel)):
		return solemn(Ordinary, "corpus-christi", White, false)
	case sameDay(date, easter.AddDate(0, 0, 68)):
		return solemn(Ordinary, "sacred-heart", White, false)
	case sameDay(date, christTheKing):
		return solemn(Ordinary, "christ-the-king", White, false)
	case sameDay(date, christmasThis):
		return solemn(Christmas, "christmas", White, true)
	case sameDay(date, epiphany(y, sel)):
		return solemn(Christmas, "epiphany", White, true)
	case sameDay(date, baptismThis):
		return feastOfLord(Christmas, "baptism-of-the-lord", White)
	case sameDay(date, holyFamily(y)):
		return feastOfLord(Christmas, "holy-family", White)
	}

	// 2. Seasons by span (chronological within the calendar year).
	switch {
	case !date.Before(holyThu) && date.Before(easter): // Triduum: Thu, Fri, Sat
		col := White
		if wd == time.Friday {
			col = Red
		} else if wd == time.Saturday {
			col = Violet
		}
		td := solemn(Triduum, "triduum-"+weekdayAbbr(wd), col, true)
		return td

	case !date.Before(palmSunday) && date.Before(holyThu): // Palm Sunday + Holy Week Mon-Wed
		if sun {
			return sundayDay(Lent, "palm-sunday", Red, 6)
		}
		// Holy Week Mon-Wed are band-2 privileged: no feast/memorial is kept.
		return privFerialDay(Lent, "holy-week-"+weekdayAbbr(wd), Violet, 6)

	case !date.Before(ashWed) && date.Before(palmSunday): // Lent
		if sameDay(date, ashWed) {
			// Ash Wednesday is band-2 privileged (suppresses any coinciding feast).
			return privFerialDay(Lent, "lent-after-ashes-wed", Violet, 0)
		}
		if date.Before(ashWed.AddDate(0, 0, 4)) {
			// Thursday-Saturday after Ash Wednesday (before Lent I): band-9 privileged.
			return ferialDay(Lent, "lent-after-ashes-"+weekdayAbbr(wd), Violet, 0, true)
		}
		lent1 := nextWeekday(ashWed, time.Sunday)
		week := daysBetween(lent1, date)/7 + 1
		col := Violet
		if week == 4 {
			col = Rose // Laetare
		}
		if sun {
			return sundayDay(Lent, "lent-sunday-"+itoa(week), col, week)
		}
		return ferialDay(Lent, "lent-"+itoa(week)+"-"+weekdayAbbr(wd), Violet, week, true)

	case !date.Before(easter) && !date.After(pentecost): // Easter season
		week := daysBetween(easter, date)/7 + 1
		if sun {
			return sundayDay(Easter_, "easter-sunday-"+itoa(week), White, week)
		}
		return ferialDay(Easter_, "easter-"+itoa(week)+"-"+weekdayAbbr(wd), White, week, false)

	case !date.Before(adventThis) && date.Before(christmasThis): // Advent
		week := daysBetween(adventThis, date)/7 + 1
		col := Violet
		if week == 3 {
			col = Rose // Gaudete
		}
		privFeria := date.Month() == time.December && date.Day() >= 17
		if sun {
			return sundayDay(Advent, "advent-sunday-"+itoa(week), col, week)
		}
		// From Dec 17 the weekday readings are proper to the DATE (the "O Antiphons"
		// days), not the Advent week, so key them by date rather than by week-day.
		if privFeria && date.Day() <= 24 {
			return ferialDay(Advent, "advent-dec-"+itoa(date.Day()), Violet, week, true)
		}
		return ferialDay(Advent, "advent-"+itoa(week)+"-"+weekdayAbbr(wd), Violet, week, privFeria)

	case !date.Before(christmasThis): // Dec 25..Dec 31 — Christmas octave
		// The octave-day ferials (Dec 29-31) have readings proper to the date;
		// Dec 25-28 are the Nativity and its feasts (Stephen/John/Innocents).
		if date.Day() >= 29 {
			return ferialDay(Christmas, "christmas-dec-"+itoa(date.Day()), White, 0, false)
		}
		return ferialDay(Christmas, "christmas-octave-"+weekdayAbbr(wd), White, 0, false)

	case !date.After(baptismThis): // Jan 1..Baptism — Christmas season
		if sun {
			return sundayDay(Christmas, "christmas-sunday-"+weekdayAbbr(wd), White, 0)
		}
		// Before Epiphany the weekdays are proper to the date (Jan 2-7); after
		// Epiphany they are keyed by weekday (the days before the Baptism).
		if date.Before(epiphany(y, sel)) {
			return ferialDay(Christmas, "christmas-jan-"+itoa(date.Day()), White, 0, false)
		}
		return ferialDay(Christmas, "christmas-after-epiphany-"+weekdayAbbr(wd), White, 0, false)

	default: // Ordinary Time (span 1: after Baptism..before Lent; span 2: after Pentecost..before Advent)
		var week int
		if date.Before(ashWed) {
			week = daysBetween(baptismThis, date)/7 + 1
		} else {
			// Span 2 resumes after Pentecost; Sundays are numbered backward from
			// Christ the King (the 34th and last Sunday). A weekday takes the number
			// of its preceding Sunday, so count from that Sunday, not from the
			// weekday itself (which would round toward Christ the King and land a
			// week too high — the OT ferial off-by-one).
			precedingSunday := date.AddDate(0, 0, -int(date.Weekday()))
			week = 34 - daysBetween(precedingSunday, christTheKing)/7
		}
		if week < 1 {
			week = 1
		}
		if sun {
			return sundayDay(Ordinary, "ordinary-sunday-"+itoa(week), Green, week)
		}
		return ferialDay(Ordinary, "ordinary-"+itoa(week)+"-"+weekdayAbbr(wd), Green, week, false)
	}
}

func itoa(n int) string { return strconv.Itoa(n) }