kernel(lectionary): slug-keyed reading citations

Data only, the same shape and discipline as Layer: slug-canonical,
duplicates rejected at construction naming the offending slug, sexp
round-trips. Which slug a day falls back to is a rubric and belongs to the
rite module, so nothing here knows about ferias or Sundays.

2 files changed, 54 insertions, 0 deletions

diff --git a/lib/kernel/lectionary.ml b/lib/kernel/lectionary.ml
new file mode 100644
index 0000000..1da3d1e
--- /dev/null
+++ b/lib/kernel/lectionary.ml
@@ -0,0 +1,33 @@
+open Sexplib0.Sexp_conv
+
+type entry = Slug.t * Citation.t list [@@deriving sexp]
+type t = entry list [@@deriving sexp]
+
+let empty = []
+
+let of_entries es =
+  let sorted = List.stable_sort (fun (a, _) (b, _) -> Slug.compare a b) es in
+  let rec dup = function
+    | (a, _) :: ((b, _) :: _ as rest) ->
+        if Slug.equal a b then Some a else dup rest
+    | _ -> None
+  in
+  match dup sorted with
+  | Some s ->
+      Error (Printf.sprintf "lectionary: duplicate slug %S" (Slug.to_string s))
+  | None -> Ok sorted
+
+let find t s = List.assoc_opt s t
+let mem t s = List.mem_assoc s t
+let entries t = t
+
+let load path =
+  match Sexplib.Sexp.load_sexp path with
+  | exception Sexplib.Sexp.Parse_error e ->
+      Error (Printf.sprintf "lectionary: %s: %s" path e.err_msg)
+  | exception Sys_error e -> Error (Printf.sprintf "lectionary: %s" e)
+  | sexp -> (
+      match t_of_sexp sexp with
+      | exception Sexplib0.Sexp_conv_error.Of_sexp_error (exn, _) ->
+          Error (Printf.sprintf "lectionary: %s: %s" path (Printexc.to_string exn))
+      | parsed -> of_entries parsed)
diff --git a/lib/kernel/lectionary.mli b/lib/kernel/lectionary.mli
new file mode 100644
index 0000000..1595377
--- /dev/null
+++ b/lib/kernel/lectionary.mli
@@ -0,0 +1,21 @@
+(** Reading citations keyed by liturgical slug (parent spec §2: "keyed by
+    liturgical slug + cycle -> citation set"). Data only: which slug a given
+    day falls back to is a rubric, and lives in the rite module.
+
+    Same discipline as {!Layer}: slug-canonical, validated at construction,
+    sexp round-trips. *)
+type t [@@deriving sexp]
+
+val empty : t
+
+(** Canonically sorts by slug. [Error] on a duplicate slug, naming it --
+    a lectionary with two answers for one day is not a lectionary. *)
+val of_entries : (Slug.t * Citation.t list) list -> (t, string) result
+
+val find : t -> Slug.t -> Citation.t list option
+val mem : t -> Slug.t -> bool
+val entries : t -> (Slug.t * Citation.t list) list
+
+(** Loads from a sexp file. Parse and validation failures come back as
+    [Error], never as an exception. *)
+val load : string -> (t, string) result