package calendar

import "sort"

// efRankOrder orders EF (1960) ranks: class-1 highest, then class-2..4,
// commemoration, ferial lowest.
func efRankOrder(r Rank) int {
	switch r {
	case RankClass1:
		return 5
	case RankClass2:
		return 4
	case RankClass3:
		return 3
	case RankClass4:
		return 2
	case RankCommemoration:
		return 1
	default: // ferial / unset
		return 0
	}
}

// precedenceEF ranks an EF candidate for occurrence (lower = higher precedence):
// by class first, then a tie-break at equal class (RG 91's Table of
// Precedence, read entry-by-entry within each class).
//
// The tie-break is asymmetric by season AND, at class 2, by whether the
// temporal day is a Sunday or a privileged FERIA:
//   - III/IV class: an ORDINARY feria (per annum, Advent to 16 Dec,
//     Septuagesima) yields to an equal-class feast (entries 24/25 above the
//     feria) -- the feast is celebrated, the feria commemorated (e.g. St
//     Francis Xavier, Dec 3). The ferias of Lent and Passiontide are
//     privileged (entry 22, ABOVE entry 24): they outrank an equal-class
//     feast, which is only commemorated.
//   - II class: an ordinary SUNDAY wins its tie (entry 15, above entry 16's
//     feasts) -- but a II-class privileged FERIA (the Ember days, the
//     late-Advent 17-23 Dec ferias, entry 18) sits BELOW entry 16 and yields
//     to an equal-class feast, the opposite of a Sunday's own tie. This is
//     the same table shape as III class's Advent/per-annum ferias, just
//     inverted for which II-class temporal days count as "privileged".
//   - I class: Sundays (entry 6) and I-class ferias (entry 7, Ash
//     Wednesday/Holy Week) both sit above entry 11's feasts, so they always
//     win -- there is no I-class "ordinary feria yields" case at all.
//
// The *2 class spacing means the ±1 tie-break never crosses a class boundary.
func precedenceEF(c candidate) int {
	p := (6 - efRankOrder(c.Cel.Rank)) * 2 // class-1 -> 2, ferial -> 12
	if c.Temporal {
		yieldsToFeast := false
		switch c.Cel.Rank {
		case RankClass3, RankClass4:
			yieldsToFeast = c.Season != Lent && c.Season != Passiontide
		case RankClass2:
			// A II-class Sunday (entry 15) wins; a II-class privileged feria --
			// Ember days, the late-Advent 17-23 Dec ferias (entry 18) -- yields
			// to an equal-class feast (entry 16), e.g. St Matthew (21 Sep)
			// beating the September Ember Wednesday, or St Thomas (21 Dec)
			// beating an Advent late feria.
			yieldsToFeast = !c.Sunday
		}
		if yieldsToFeast {
			p++
		} else {
			p-- // Sundays, I-class ferias, and penitential (Lent/Passiontide) ferias win their ties
		}
	} else if c.Cel.Class == ClassLord && c.Cel.Rank == RankClass2 {
		// A II class feast of the Lord takes the place of a II class Sunday it
		// falls on (unlike a saint's feast, which is only commemorated). Give it
		// the edge over the Sunday's tie-break bonus.
		p -= 2
	}
	return p
}

// pickEF returns the observed EF celebration and the commemorations, ordered
// deterministically by precedence then slug.
func pickEF(cands []candidate) (candidate, []candidate) {
	sorted := make([]candidate, len(cands))
	copy(sorted, cands)
	sort.SliceStable(sorted, func(i, j int) bool {
		pi, pj := precedenceEF(sorted[i]), precedenceEF(sorted[j])
		if pi != pj {
			return pi < pj
		}
		return sorted[i].Cel.Slug < sorted[j].Cel.Slug
	})
	return sorted[0], sorted[1:]
}