feat(naming): the config file

Owns precedence and provenance and nothing else, and never reads the
filesystem, so it is as testable as the language table.

resolve returns the value AND its source, because a setting that silently
comes from a file the user forgot about is worse than no setting at all --
config --show can then say where each effective value came from.

overlay accumulates rather than last-wins: a user has more than one.

An unknown key is reported, never fatal. A config written for a newer
colitur must still work on an older one, but silently dropping a line the
user wrote is how a typo becomes invisible.

1 files changed, 43 insertions, 0 deletions

diff --git a/lib/naming/config.ml b/lib/naming/config.ml
new file mode 100644
index 0000000..ce1b433
--- /dev/null
+++ b/lib/naming/config.ml
@@ -0,0 +1,43 @@
+module OI = Colitur_kernel.Overlay_ini
+
+type t = {
+  lang : string option;
+  overlays : string list;
+  template : string option;
+  format : string option;
+  unknown_keys : string list;
+}
+
+let empty = { lang = None; overlays = []; template = None; format = None; unknown_keys = [] }
+
+let lang t = t.lang
+let overlays t = t.overlays
+let template t = t.template
+let format t = t.format
+let unknown_keys t = t.unknown_keys
+
+let of_string text =
+  match OI.parse_sections text with
+  | Error e -> Error e
+  | Ok sections -> (
+      match List.find_opt (fun (s : OI.section) -> s.OI.name = "defaults") sections with
+      | None -> Ok empty
+      | Some s ->
+          let acc =
+            List.fold_left
+              (fun acc (k, v) ->
+                match k with
+                | "lang" -> { acc with lang = Some v }
+                | "template" -> { acc with template = Some v }
+                | "format" -> { acc with format = Some v }
+                (* accumulates: a user has more than one overlay *)
+                | "overlay" -> { acc with overlays = acc.overlays @ [ v ] }
+                | other -> { acc with unknown_keys = acc.unknown_keys @ [ other ] })
+              empty s.OI.fields
+          in
+          Ok acc)
+
+let resolve ~flag ~config ~default =
+  match flag with
+  | Some v -> (v, "flag")
+  | None -> ( match config with Some v -> (v, "config") | None -> (default, "default"))