kernel+ef: resolve readings, chain steps 1 and 2

Liturgical_day.citations has read "always empty until Plan 4" since Plan 3;
it is now filled. Rite.t gains a readings function, rite-supplied for the
same reason transfer_target is: what a day with no proper falls back to is a
rubric, not a universal. Calendar calls it and passes its own temporal
function as the callback the rite needs to reach another date.

Steps 1 and 2 only: the observed celebration's own proper, else the day's own
temporal slug. Nothing encodes "Lent has daily propers" -- the presence of an
entry is the discriminator.

4 files changed, 62 insertions, 1 deletions

diff --git a/lib/rites/rite_ef/lectionary_ef.ml b/lib/rites/rite_ef/lectionary_ef.ml
new file mode 100644
index 0000000..f9ce9ab
--- /dev/null
+++ b/lib/rites/rite_ef/lectionary_ef.ml
@@ -0,0 +1,40 @@
+open Colitur_kernel
+
+(* [data/ef/lectionary.sexp] is located relative to the BUILD TREE, not the
+   process's own cwd -- the same reasoning bin/main.ml's own [data_dir]
+   documents at length: cwd varies with how the binary that eventually links
+   this library is invoked (a user's shell for `dune exec colitur --`, a
+   dune cram test's own sandboxed temp directory, `_build/default/test/` for
+   `dune test`), and nothing in this project's build pins it to the
+   repository root. [Sys.executable_name] resolves through /proc/self/exe on
+   Linux, so it reports the executable's own canonical absolute path even
+   when launched through a symlink (dune's cram sandbox places exactly one;
+   confirmed working there already by bin/main.ml). Every executable that
+   links this library -- bin/main.exe, test/test_colitur.exe,
+   tools/*.exe -- sits exactly one directory below _build/default/, so
+   climbing up twice and back down into data/ef always finds the file,
+   regardless of the caller's own cwd. *)
+let data_path () =
+  Filename.dirname (Filename.dirname Sys.executable_name) ^ "/data/ef/lectionary.sexp"
+
+(* Loaded once, at module initialisation: the data is year-independent and
+   build_day is called 3 074 246 times over the full domain. *)
+let lectionary =
+  match Lectionary.load (data_path ()) with
+  | Ok l -> l
+  | Error e -> failwith ("lectionary_ef: " ^ e)
+
+(* Step 1: the observed celebration's own proper.
+   Step 2: the day's own temporal slug.
+
+   Nothing here encodes "Lent has daily propers": the presence of an entry is
+   the discriminator. Verified against lectio -- Lent 1 Monday returns its own
+   Ezech 34:11-16, while Advent, Christmas and post-Pentecost Mondays return
+   their Sunday's Mass. *)
+let readings ~observed ~temporal ~date:_ ~temporal_at:_ =
+  match observed.Celebration.citations with
+  | _ :: _ as cs -> cs
+  | [] -> (
+      match Lectionary.find lectionary temporal.Temporal.office.Celebration.slug with
+      | Some cs -> cs
+      | None -> [])
diff --git a/lib/rites/rite_ef/lectionary_ef.mli b/lib/rites/rite_ef/lectionary_ef.mli
new file mode 100644
index 0000000..fe9359e
--- /dev/null
+++ b/lib/rites/rite_ef/lectionary_ef.mli
@@ -0,0 +1,15 @@
+open Colitur_kernel
+
+(** The EF lectionary resolution chain. All rubric knowledge about what a day
+    with no proper falls back to lives here, not in the kernel.
+
+    Steps 1 and 2 only (Task 4): the observed celebration's own proper, else
+    the day's own temporal slug in the lectionary. A day matching neither
+    gets [] for now -- the ferial fallback to the preceding Sunday (Task 5)
+    and the Commons (Task 6) are not built here. *)
+val readings :
+  observed:Vocab_ef.rank Celebration.t ->
+  temporal:(Vocab_ef.season, Vocab_ef.rank) Temporal.t ->
+  date:Date.t ->
+  temporal_at:(Date.t -> (Vocab_ef.season, Vocab_ef.rank) Temporal.t) ->
+  Citation.t list
diff --git a/lib/rites/rite_ef/rite_ef.ml b/lib/rites/rite_ef/rite_ef.ml
index 7a29421..4cf52a6 100644
--- a/lib/rites/rite_ef/rite_ef.ml
+++ b/lib/rites/rite_ef/rite_ef.ml
@@ -7,6 +7,7 @@
 module Vocab_ef = Vocab_ef
 module Temporal_ef = Temporal_ef
 module Precedence_ef = Precedence_ef
+module Lectionary_ef = Lectionary_ef
 
 open Colitur_kernel
 
@@ -21,4 +22,5 @@ let context : (Vocab_ef.season, Vocab_ef.rank) Rite.t =
         disposition = Precedence_ef.disposition;
         admit = Precedence_ef.admit };
     season_runs = Vocab_ef.seasons;
-    transfer_target = Precedence_ef.transfer_target }
+    transfer_target = Precedence_ef.transfer_target;
+    readings = Lectionary_ef.readings }
diff --git a/lib/rites/rite_ef/rite_ef.mli b/lib/rites/rite_ef/rite_ef.mli
index e2b3e6d..710db1d 100644
--- a/lib/rites/rite_ef/rite_ef.mli
+++ b/lib/rites/rite_ef/rite_ef.mli
@@ -9,6 +9,7 @@
 module Vocab_ef = Vocab_ef
 module Temporal_ef = Temporal_ef
 module Precedence_ef = Precedence_ef
+module Lectionary_ef = Lectionary_ef
 
 (** The EF rite, bundled (design spec's [RITE] signature, realised as a
     {!Colitur_kernel.Rite.t} value rather than a functor -- see rite.mli):
@@ -24,6 +25,9 @@ module Precedence_ef = Precedence_ef
     - [transfer_target]: {!Precedence_ef.transfer_target}, RG 96 (see that
       value's own documentation for the termination and forward-progress
       argument {!Colitur_kernel.Rite.t.transfer_target}'s contract requires).
+    - [readings]: {!Lectionary_ef.readings} -- the observed celebration's own
+      proper, else the day's own temporal slug in data/ef/lectionary.sexp
+      (chain steps 1-2; the ferial fallback and the Commons are later work).
 
     Deliberately carries no [sanctoral]/[lectionary] fields the way the
     original design-doc sketch of [RITE] does: {!Colitur_kernel.Rite.t} (the