2 files changed, 81 insertions, 0 deletions
diff --git a/lib/kernel/layer.ml b/lib/kernel/layer.ml
new file mode 100644
index 0000000..bf71206
--- /dev/null
+++ b/lib/kernel/layer.ml
@@ -0,0 +1,56 @@
+open Sexplib0.Sexp_conv
+
+(* One calendar layer: a rite's base sanctoral, or any data file with the same
+   shape. Entries are kept sorted by slug as the canonical form, so equal layers
+   serialise identically and merge results do not depend on input order. *)
+type 'r entry = { date : Date_spec.t; cel : 'r Celebration.t } [@@deriving sexp]
+type 'r t = { id : string; name : string; entries : 'r entry list } [@@deriving sexp]
+
+let by_slug a b = Slug.compare a.cel.Celebration.slug b.cel.Celebration.slug
+let canonical entries = List.sort by_slug entries
+let empty ~id ~name = { id; name; entries = [] }
+let of_entries ~id ~name entries = { id; name; entries = canonical entries }
+let find t slug = List.find_opt (fun e -> Slug.equal e.cel.Celebration.slug slug) t.entries
+let mem t slug = find t slug <> None
+
+let remove t slug =
+  { t with entries = List.filter (fun e -> not (Slug.equal e.cel.Celebration.slug slug)) t.entries }
+
+let set t entry =
+  let t = remove t entry.cel.Celebration.slug in
+  { t with entries = canonical (entry :: t.entries) }
+
+(* Dates are year-independent, so the index is built once per layer rather than
+   once per year -- a full 1583..9999 sweep would otherwise rescan the entry
+   list for every day. *)
+type 'r by_date = (int * int, 'r entry list) Hashtbl.t
+
+let key = function Date_spec.Fixed { month; day } -> (month, day)
+
+let index_by_date t =
+  let tbl : 'r by_date = Hashtbl.create 512 in
+  List.iter
+    (fun e ->
+      let k = key e.date in
+      Hashtbl.replace tbl k (e :: (try Hashtbl.find tbl k with Not_found -> [])))
+    t.entries;
+  (* restore canonical order within each date bucket *)
+  Hashtbl.iter (fun k v -> Hashtbl.replace tbl k (canonical v)) tbl;
+  tbl
+
+let on_date tbl ~month ~day =
+  try Hashtbl.find tbl (month, day) with Not_found -> []
+
+let load rank_of_sexp path =
+  match Sexplib.Sexp.load_sexp path with
+  | exception Sys_error msg -> Error msg
+  (* This sexplib version raises [Failure] for some malformed inputs (e.g. an
+     unterminated list or string) rather than [Sexplib.Sexp.Parse_error], so a
+     catch-all here -- placed last among the exception branches -- is what
+     actually keeps every parse failure inside [Error] instead of escaping. *)
+  | exception exn -> Error (Printf.sprintf "%s: %s" path (Printexc.to_string exn))
+  | sexp -> (
+      match t_of_sexp rank_of_sexp sexp with
+      | t -> Ok { t with entries = canonical t.entries }
+      | exception Sexplib0.Sexp_conv_error.Of_sexp_error (exn, _) ->
+          Error (Printf.sprintf "%s: %s" path (Printexc.to_string exn)))
diff --git a/lib/kernel/layer.mli b/lib/kernel/layer.mli
new file mode 100644
index 0000000..3092353
--- /dev/null
+++ b/lib/kernel/layer.mli
@@ -0,0 +1,25 @@
+(** One calendar layer: a rite's base sanctoral, or a data file of the same
+    shape. Entries are canonically sorted by slug. *)
+type 'r entry = { date : Date_spec.t; cel : 'r Celebration.t } [@@deriving sexp]
+type 'r t = { id : string; name : string; entries : 'r entry list } [@@deriving sexp]
+
+val empty : id:string -> name:string -> 'r t
+val of_entries : id:string -> name:string -> 'r entry list -> 'r t
+val find : 'r t -> Slug.t -> 'r entry option
+val mem : 'r t -> Slug.t -> bool
+
+(** Replaces any entry with the same slug. *)
+val set : 'r t -> 'r entry -> 'r t
+
+val remove : 'r t -> Slug.t -> 'r t
+
+(** Date index. Built once per layer, not per year: [Date_spec] dates are
+    year-independent. *)
+type 'r by_date
+
+val index_by_date : 'r t -> 'r by_date
+val on_date : 'r by_date -> month:int -> day:int -> 'r entry list
+
+(** Loads a layer from a sexp file. Parse and validation failures come back as
+    [Error], never as an exception. *)
+val load : (Sexplib0.Sexp.t -> 'r) -> string -> ('r t, string) result