From 95d50b25c990e8c945ce2507b16ff3c8b039d286 Mon Sep 17 00:00:00 2001
From: Rutger Broekhoff
Date: Mon, 25 Aug 2025 19:48:19 +0200
Subject: OCaml

---
 lib/iban.ml | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 lib/iban.ml

(limited to 'lib/iban.ml')

diff --git a/lib/iban.ml b/lib/iban.ml
new file mode 100644
index 0000000..6e47e9d
--- /dev/null
+++ b/lib/iban.ml
@@ -0,0 +1,87 @@
+open Core
+open Option.Let_syntax
+
+type t = string
+
+(* Modulo-97 arithmetic. Prevents us from having to use Zarith here. *)
+module M97 : sig
+  type t
+
+  val of_int : int -> t
+  val lt : t -> t -> bool
+  val equal : t -> t -> bool
+  val ( * ) : t -> t -> t
+  val ( + ) : t -> t -> t
+  val ( ~$ ) : int -> t
+end = struct
+  type t = int
+
+  let of_int x = x % 97
+  let equal = Int.( = )
+  let lt = Int.( < )
+  let ( * ) x y = x * y % 97
+  let ( + ) x y = (x + y) % 97
+  let ( ~$ ) = of_int
+end
+
+let m97_of_alnum c =
+  let v = Char.to_int c in
+  if Char.is_digit c then Some (M97.of_int (v - Char.to_int '0'))
+  else if Char.is_alpha c then
+    if Char.is_lowercase c then Some (M97.of_int (v - Char.to_int 'a' + 10))
+    else Some (M97.of_int (v - Char.to_int 'A' + 10))
+  else None
+
+let m97_of_digit c =
+  match m97_of_alnum c with Some v when M97.(lt v ~$10) -> Some v | _ -> None
+
+let m97_of_alpha c =
+  match m97_of_alnum c with
+  | Some v when not M97.(lt v ~$10) -> Some v
+  | _ -> None
+
+let string_fold_option ~(init : 'a) ~(f : 'a -> char -> 'a option) s =
+  let rec go i (acc : 'a) : 'a option =
+    if i >= String.length s then Some acc
+    else Option.(f acc (String.unsafe_get s i) >>= go (i + 1))
+  in
+  go 0 init
+
+let m97_of_iban s =
+  string_fold_option s ~init:`In_country1 ~f:(fun st c ->
+      match st with
+      | `In_country1 ->
+          let%map co1 = m97_of_alpha c in
+          `In_country2 co1
+      | `In_country2 co1 ->
+          let%map co2 = m97_of_alpha c in
+          `In_check1 M97.((co1 * ~$100) + co2)
+      | `In_check1 co ->
+          let%map ch1 = m97_of_digit c in
+          `In_check2 (co, ch1)
+      | `In_check2 (co, ch1) ->
+          let%map ch2 = m97_of_digit c in
+          `In_bban M97.(co, (ch1 * ~$10) + ch2, ~$0)
+      | `In_bban (co, ch, bban) ->
+          let%map v = m97_of_alnum c in
+          let bban' =
+            M97.(if lt v ~$10 then (bban * ~$10) + v else (bban * ~$100) + v)
+          in
+          `In_bban (co, ch, bban'))
+  |> function
+  | Some (`In_bban (co, ch, bban)) ->
+      Some M97.((bban * ~$1000000) + (co * ~$100) + ch)
+  | _ -> None
+
+let check_iban s =
+  String.length s <= 34 && Option.exists (m97_of_iban s) ~f:M97.(equal ~$1)
+
+let make s : t option = if check_iban s then Some s else None
+let to_string = Fn.id
+
+let of_string s =
+  match make s with
+  | Some iban -> iban
+  | None -> Printf.failwithf "Iban.of_string: %S" s ()
+
+let equal = String.equal
-- 
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