From ae718618d9256b0db3c1b16e66ee12145ea0af4f Mon Sep 17 00:00:00 2001
From: swrup <swrup@protonmail.com>
Date: Sun, 1 Mar 2026 00:35:59 +0100
Subject: [PATCH]

---
 src/amount.ml | 131 ++++++++++++++++++++++++++++++--------------------
 1 file changed, 79 insertions(+), 52 deletions(-)

diff --git a/src/amount.ml b/src/amount.ml
index 9b8b34ff..473fb4ec 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -14,67 +14,96 @@ type t = {
   fraction: Int32.t;
 }
 
+let maximum_currency_length = 11
 let value_upper_bound = Int64.of_float @@ Float.pow 2. 52.
+let fraction_max_number_of_digits = 8
 
-(* TODO
-   the constraint is on the number of digits,
-   so this wrong if leading 0s
-   this depends on currency..? *)
-let fraction_upper_bound = Int32.of_int 100_000_000
+(* TODO choose to either include or exclude upper limit for all bounds
+   need a -1 here *)
+let fraction_upper_bound =
+  Int32.of_float @@ Float.pow 10. (Int.to_float fraction_max_number_of_digits)
+
+let check_currency s =
+  if not @@ String.for_all is_letter s then
+    Error "currency contains forbidden character"
+  else
+    let len = String.length s in
+    if len = 0 then Error "currency is empty string"
+    else if len > maximum_currency_length then
+      Fmt.error "currency is more than %d characters long"
+        maximum_currency_length
+    else Ok ()
+
+let check_value v =
+  if Int64.unsigned_compare v value_upper_bound > 0 then
+    Error "value is greater than 2^52"
+  else Ok v
+
+let check_fraction v =
+  if Int32.unsigned_compare v fraction_upper_bound > 0 then
+    Fmt.error "fraction has more than %d digits" fraction_max_number_of_digits
+  else Ok v
 
 let make ~sign ~currency ~value ~fraction =
-  if value < Int64.zero then Error "value is negative"
-  else if fraction < Int32.zero then Error "fraction is negative"
-  else if value > value_upper_bound then Error "value is greater than 2^52-1"
-  else if fraction >= fraction_upper_bound then
-    Error "fraction has more than 8 decimal digits"
-  else Ok { sign; currency; value; fraction }
+  Ok { sign; currency; value; fraction }
 
-module Parse = struct
-  open Angstrom
+open Angstrom
 
-  let sign =
-    char '+' *> return (Some Sign_plus)
-    <|> char '-' *> return (Some Sign_minus)
-    <|> return None
+let is_letter = function 'a' .. 'z' | 'A' .. 'Z' -> true | _ -> false
+let is_digit = function '0' .. '9' -> true | _ -> false
+let letters = take_while1 is_letter
+let digits = take_while1 is_digit
 
-  let currency =
-    take_while1 (function 'a' .. 'z' | 'A' .. 'Z' -> true | _ -> false)
-    >>= fun s ->
-    match String.length s < 12 with
-    | false -> fail "currency is more than 11 characters"
-    | true -> return s
+let sign =
+  choice
+    [
+      char '+' *> return (Some Sign_plus);
+      char '-' *> return (Some Sign_minus);
+      return None;
+    ]
 
-  let int64 =
-    take_while1 (function '0' .. '9' -> true | _ -> false)
-    >>| Int64.of_string_opt
-    >>= function
-    | None -> fail "value is not a valid int64"
-    | Some n when n >= value_upper_bound -> fail "value is greater than 2^52-1"
-    | Some n -> return n
+let currency_of_string s =
+  if not @@ String.for_all is_letter s then
+    Error "currency contains forbidden character"
+  else if String.length s > maximum_currency_length then
+    Fmt.error "currency is more than %d characters long" maximum_currency_length
+  else Ok (String.uppercase_ascii s)
 
-  let int32 =
-    take_while1 (function '0' .. '9' -> true | _ -> false)
-    >>| Int32.of_string_opt
-    >>= function
-    | None -> fail "fraction is not a valid int32"
-    | Some n when n >= fraction_upper_bound ->
-        fail "fraction is greater than 10^8-1"
-    | Some n -> return n
+let value_of_string s =
+  if not @@ String.for_all is_digit s then
+    Error "value contains forbidden character"
+  else
+    match Int64.of_string_opt s with
+    | None -> Error "value is not a valid integer"
+    | Some v ->
+        if v > value_upper_bound then Error "value is greater than 2^52"
+        else if v < Int64.zero then Error "value is negative"
+        else Ok v
 
-  let amount =
-    lift4
-      (fun sign currency value fraction ->
-        make ~sign ~currency ~value ~fraction)
-      sign currency
-      (char ':' *> int64)
-      (char '.' *> int32 <|> return 0_l)
-    <* end_of_input
+let fraction_of_string s =
+  if not @@ String.for_all is_digit s then
+    Error "fraction contains forbidden character"
+  else if String.length s > fraction_max_number_of_digits then
+    Fmt.error "fraction has more than %d digits" fraction_max_number_of_digits
+  else
+    match Int32.of_string_opt s with
+    | None -> Error "fraction is not a valid integer"
+    | Some v -> if v < Int32.zero then Error "fraction is negative" else Ok v
 
-  let f s = parse_string ~consume:Consume.All amount s |> Result.join
-end
+let amount =
+  lift4
+    (fun sign currency value fraction ->
+      let open Syntax in
+      let* currency = currency_of_string currency in
+      let* value = value_of_string value in
+      let* fraction = fraction_of_string fraction in
+      Ok { sign; currency; value; fraction })
+    sign letters
+    (char ':' *> digits)
+    (option "" (char '.' *> digits))
+  <* end_of_input
 
-let of_string = Parse.f
+let of_string s = parse_string ~consume:Consume.All amount s |> Result.join
 
 let pp =
   let open Fmt in
@@ -92,9 +121,7 @@ let to_string = Fmt.str "%a" pp
 (* - *)
 
 let jsont = Jsont.of_of_string ~kind:"Amount" of_string ~enc:to_string
-
-(* byte length of currency string *)
-let currency_len = 12
+let currency_length = 12
 
 let pad_currency_string s =
   let len = String.length s in