kernel(date): proleptic Gregorian date, validated make + arithmetic

Hinnant civil<->days rep (1970-epoch rata die); make validates month/day and
the 1583..9999 domain; of_rata/add_days are total arithmetic. Weekday, compare.
Tested: known weekdays, leap boundaries, rejects; qcheck round-trip / add-inverse
/ weekday-cycle properties over random in-range dates.

1 files changed, 62 insertions, 0 deletions

diff --git a/test/test_date.ml b/test/test_date.ml
new file mode 100644
index 0000000..2458659
--- /dev/null
+++ b/test/test_date.ml
@@ -0,0 +1,62 @@
+module D = Colitur_kernel.Date
+
+let mk y m d =
+  match D.make ~year:y ~month:m ~day:d with
+  | Ok t -> t
+  | Error e -> Alcotest.failf "make %04d-%02d-%02d: %s" y m d e
+
+let wd_str = function
+  | D.Sun -> "Sun" | D.Mon -> "Mon" | D.Tue -> "Tue" | D.Wed -> "Wed"
+  | D.Thu -> "Thu" | D.Fri -> "Fri" | D.Sat -> "Sat"
+
+let ymd d = (D.year d, D.month d, D.day d)
+
+let test_weekday () =
+  Alcotest.(check string) "1970-01-01 is Thursday" "Thu" (wd_str (D.weekday (mk 1970 1 1)));
+  Alcotest.(check string) "2026-07-31 is Friday" "Fri" (wd_str (D.weekday (mk 2026 7 31)))
+
+let test_roundtrip () =
+  let d = mk 2026 7 31 in
+  Alcotest.(check int) "of_rata(to_rata d) = d" 0 (D.compare (D.of_rata (D.to_rata d)) d)
+
+let test_add_days () =
+  Alcotest.(check (triple int int int)) "2024-02-28 +1 = 2024-02-29"
+    (2024, 2, 29) (ymd (D.add_days (mk 2024 2 28) 1));
+  Alcotest.(check (triple int int int)) "2023-02-28 +1 = 2023-03-01"
+    (2023, 3, 1) (ymd (D.add_days (mk 2023 2 28) 1))
+
+let test_make_reject () =
+  Alcotest.(check bool) "reject 2026-02-30" true (Result.is_error (D.make ~year:2026 ~month:2 ~day:30));
+  Alcotest.(check bool) "reject year 1000" true (Result.is_error (D.make ~year:1000 ~month:1 ~day:1))
+
+(* ---- properties (year-independent, the confidence-to-9999 core) ---- *)
+
+let arb_date =
+  let open QCheck in
+  map
+    (fun (y, off) ->
+      let jan1 = match D.make ~year:y ~month:1 ~day:1 with Ok t -> t | Error e -> failwith e in
+      D.of_rata (D.to_rata jan1 + off))
+    (pair (int_range 1583 9999) (int_range 0 364))
+
+let prop_roundtrip =
+  QCheck.Test.make ~name:"of_rata . to_rata = id" arb_date
+    (fun d -> D.compare (D.of_rata (D.to_rata d)) d = 0)
+
+let prop_add_inverse =
+  QCheck.Test.make ~name:"add_days n then -n = id"
+    QCheck.(pair arb_date (int_range (-4000) 4000))
+    (fun (d, n) -> D.compare (D.add_days (D.add_days d n) (-n)) d = 0)
+
+let prop_weekday_cycle =
+  QCheck.Test.make ~name:"weekday(add_days d 7) = weekday d" arb_date
+    (fun d -> D.weekday (D.add_days d 7) = D.weekday d)
+
+let suite =
+  ( "Date",
+    [ Alcotest.test_case "weekday" `Quick test_weekday;
+      Alcotest.test_case "roundtrip" `Quick test_roundtrip;
+      Alcotest.test_case "add_days boundaries" `Quick test_add_days;
+      Alcotest.test_case "make rejects invalid" `Quick test_make_reject ]
+    @ List.map QCheck_alcotest.to_alcotest
+        [ prop_roundtrip; prop_add_inverse; prop_weekday_cycle ] )