(* RG 91's Table of Precedence (docs/research/rules-register.md §4). Each
   branch below is one of the table's 28 entries, checked in the table's own
   numeric order -- lower wins, and because occasional entries are true
   exceptions to a later, broader one (RG 91 entry 18's Ember days are an
   exception carved out of entry 22's Lent ferias; entry 21/26's vigils are
   an exception carved out of the generic Class2/Class3 sanctoral-feast
   entries that would otherwise also match), checking in table order and
   returning on the first match is what makes the exception actually win
   without a separate exclusion for every later entry it pre-empts.

   Two kinds of evidence decide an entry:
   - The temporal cycle's own office (Nativity, a Sunday, a feria, a vigil of
     the Lord) is identified structurally, from the context's date/season/
     weekday and the day's Easter offset -- never from its slug, which is
     just a label. [origin = Temporal] gates every such entry so a sanctoral
     candidate that happens to share a date (Immaculate Conception can never
     coincide with the movable cycle, but nothing stops a future rite bug
     from producing one) cannot be mistaken for the office itself.
   - A sanctoral feast's entry (11-13 I class, 14/16/19/20 II class, 23/24
     III class) is decided by its [rank] plus, per the brief's structural
     insight, its {!Celebration.t}.layer: a celebration whose layer is not
     the universal base is an overlay, hence "proper" or "indult" rather
     than the universal entry (see precedence_ef.mli). [origin = Sanctoral]
     gates these for the same reason: temporal-origin celebrations carry the
     literal layer id "temporal" (rite_ef/temporal_ef.ml's [build]), which is
     not [universal_layer] either, and would otherwise be misread as
     "proper" by the layer test alone.

   Vigils (21, 26) are the one shape neither of those two kinds fully
   describes on their own: a II/III-class vigil can be temporal-origin (the
   Ascension Vigil, produced by temporal_ef today) or sanctoral-origin (a
   saint's vigil, not yet loaded by any task), so its entry cannot be gated
   on [origin] at all. Nothing in the day's other fields marks "this is a
   vigil, not an ordinary office of the same rank" either, so this reads it
   off the temporal cycle's own slug convention (a "-vigil" suffix -- see
   [named] in temporal_ef.ml) rather than guessing a new one. *)

open Colitur_kernel

(* Not an RG citation -- RG 91 ranks proper and indult feasts, it does not
   encode how a computer tells them apart. See precedence_ef.mli. *)
let universal_layer = "ef-universal"
let indult_prefix = "indult:"
let unclassified = max_int

let is_indult layer = String.starts_with ~prefix:indult_prefix layer
let is_universal layer = String.equal layer universal_layer

(* Ember days are identified by the temporal cycle's own slug convention
   (rite_ef/temporal_ef.ml's [ember]: "ef-<set>-ember-<day>"), not
   re-derived here: the September anchor in particular is one of the more
   contested dates in the 1962 calendar (temporal_ef.ml's own comment on
   [third_sunday_of_september]), and re-deriving it a second time would only
   create a second place for that same uncertainty to drift. Only the
   Advent, Lent and September sets are listed: RG 91 entry 18 names exactly
   those three; the Whitsun (Pentecost) set is I class and falls inside the
   Pentecost octave, entry 10, matched below before this is ever reached. *)
let is_ember_18 slug =
  String.starts_with ~prefix:"ef-advent-ember-" slug
  || String.starts_with ~prefix:"ef-lent-ember-" slug
  || String.starts_with ~prefix:"ef-september-ember-" slug

let band (ctx : Vocab_ef.season Precedence.context) (c : Vocab_ef.rank Precedence.candidate) :
    int =
  let cel = c.Precedence.cel in
  let rank = cel.Celebration.rank in
  let subject = cel.Celebration.subject in
  let layer = cel.Celebration.layer in
  let slug = Slug.to_string cel.Celebration.slug in
  let is_temporal = c.Precedence.origin = Precedence.Temporal in
  let is_vigil = String.ends_with ~suffix:"-vigil" slug in
  let date = ctx.Precedence.date in
  let season = ctx.Precedence.season in
  let weekday = ctx.Precedence.weekday in
  let is_sunday = weekday = Date.Sun in
  let m = Date.month date and d = Date.day date in
  (* Easter offset, the same convention as temporal_ef.ml's [days_between
     easter d]: 0 is Easter itself, negative before, positive after. *)
  let off = Date.to_rata date - Date.to_rata (Computus.gregorian_easter (Date.year date)) in
  let open Vocab_ef in
  (* 1: Nativity, Easter Sunday, Pentecost Sunday (I class w/ octave). *)
  if is_temporal && rank = Class1 && ((m = 12 && d = 25) || off = 0 || off = 49) then 1
    (* 2: Sacred Triduum (Thu-Sat of Holy Week). *)
  else if is_temporal && rank = Class1 && off >= -3 && off <= -1 then 2
    (* 3: Epiphany, Ascension, Holy Trinity, Corpus Christi, Sacred Heart,
       Christ the King. *)
  else if is_temporal && rank = Class1
          && ((m = 1 && d = 6) (* Epiphany *)
             || off = 39 (* Ascension *) || off = 56 (* Trinity *)
             || off = 60 (* Corpus Christi *) || off = 68 (* Sacred Heart *)
             || Date.compare date (Temporal_ef.christ_the_king (Date.year date)) = 0)
  then 3
    (* 4: Immaculate Conception, Assumption BVM. *)
  else if (not is_temporal) && rank = Class1 && ((m = 12 && d = 8) || (m = 8 && d = 15)) then 4
    (* 5: Vigil & Octave day of the Nativity. *)
  else if is_temporal && rank = Class1 && ((m = 12 && d = 24) || (m = 1 && d = 1)) then 5
    (* 6: Sundays of Advent, Lent, Passiontide, and Low Sunday. *)
  else if is_temporal && rank = Class1 && is_sunday
          && (season = Advent || season = Lent || season = Passiontide || off = 7)
  then 6
    (* 7: I-class ferias not above -- Ash Wednesday; Mon/Tue/Wed of Holy Week.
       Thu-Sat of Holy Week are the Triduum, entry 2 above, not this entry. *)
  else if is_temporal && rank = Class1 && (off = -46 || (off >= -6 && off <= -4)) then 7
    (* 8: All Souls. *)
  else if (not is_temporal) && rank = Class1 && m = 11 && d = 2 then 8
    (* 9: Vigil of Pentecost. *)
  else if is_temporal && rank = Class1 && off = 48 then 9
    (* 10: Days within the Octaves of Easter and Pentecost. *)
  else if is_temporal && rank = Class1 && ((off >= 1 && off <= 6) || (off >= 50 && off <= 55))
  then 10
    (* 11: I-class feasts of the universal Church not above. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class1 && is_universal layer then 11
    (* 12: Proper I-class feasts. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class1 && not (is_indult layer) then 12
    (* 13: Indult I-class feasts. By elimination once 11 and 12 have failed:
       not the universal layer (11), and marked as an indult overlay (12's
       "not indult" test having just failed). *)
  else if (not is_temporal) && (not is_vigil) && rank = Class1 then 13
    (* 14: Feasts of the Lord, II class. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class2 && is_universal layer
          && subject = Subject.Lord
  then 14
    (* 15: Sundays, II class (every Sunday not already named at 6). *)
  else if is_temporal && rank = Class2 && is_sunday then 15
    (* 16: II-class feasts of the universal Church, not of the Lord. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class2 && is_universal layer then 16
    (* 17: Days within the Octave of the Nativity (26-28 Dec are Stephen,
       John, the Innocents -- sanctoral, not this entry). *)
  else if is_temporal && rank = Class2 && m = 12 && (d = 29 || d = 30 || d = 31) then 17
    (* 18: II-class ferias -- Advent 17-23 Dec; Ember days of Advent, Lent,
       September. *)
  else if is_temporal && rank = Class2
          && ((season = Advent && m = 12 && d >= 17 && d <= 23) || is_ember_18 slug)
  then 18
    (* 19: Proper II-class feasts. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class2 && not (is_indult layer) then 19
    (* 20: Indult II-class feasts. By elimination, as at 13. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class2 then 20
    (* 21: II-class vigils (Ascension, Assumption, John Baptist, Peter & Paul
       -- can be temporal- or sanctoral-origin, see the file comment above). *)
  else if rank = Class2 && is_vigil then 21
    (* 22: Ferias of Lent and Passiontide (Thursday after Ash Wednesday to the
       Saturday before Palm Sunday), except the Ember days (18 above). *)
  else if is_temporal && rank = Class3 && (season = Lent || season = Passiontide) then 22
    (* 23: III-class feasts in particular calendars. Unlike 11/12 and 14/16
       above, the universal entry (24) is the HIGHER number here -- RG 91's
       own table ranks a particular-calendar III-class feast ahead of a
       universal one, the reverse of the I/II-class ordering. Transcribed as
       the register states it, not "corrected" into the other classes'
       pattern. RG 91 has no indult sub-rank at III class, so every non-base
       layer lands here, not split further. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class3 && not (is_universal layer) then 23
    (* 24: III-class feasts in the universal calendar. *)
  else if (not is_temporal) && (not is_vigil) && rank = Class3 then 24
    (* 25: Ferias of Advent to 16 Dec, except the Ember days (18 above). *)
  else if is_temporal && rank = Class3 && season = Advent then 25
    (* 26: III-class vigils (St Lawrence). *)
  else if rank = Class3 && is_vigil then 26
    (* 27: Office of the BVM on Saturday -- every otherwise-unoccupied IV-class
       Saturday, per the historical default that fills it; ordinary Mass
       propers still make Rogation Mon/Tue/Wed proper without changing the
       Office (RG 88, see temporal_ef.ml's [temporal]), so those never carry
       this entry unless they happen to fall on the Saturday itself. Excludes
       vigils for the same reason 11-13/14/16/19/20/23/24 do: RG 91 has no
       IV-class vigil at all (its own vigil list, register lines 381-384,
       stops at III class), so one would be an anomaly, not this entry. *)
  else if is_temporal && (not is_vigil) && rank = Class4 && weekday = Date.Sat then 27
    (* 28: IV-class ferias -- the unqualified catch-all (temporal_ef.ml's own
       comment on [ferial_rank] cites the same primary text, "Feriae IV
       classis"). Excludes vigils for the same reason as 27 above: a IV-class
       "feria" that is also a vigil is not a feria RG 91 describes. *)
  else if (not is_vigil) && rank = Class4 then 28
  else unclassified