1 files changed, 40 insertions, 0 deletions

diff --git a/app/Data/Iban.hs b/app/Data/Iban.hs
new file mode 100644
index 0000000..a42e192
--- /dev/null
+++ b/app/Data/Iban.hs
@@ -0,0 +1,40 @@
+module Data.Iban (Iban, mkIban) where
+
+import Control.Applicative ((<|>))
+import Data.Attoparsec.Text as AP
+import Data.Char
+  ( digitToInt,
+    ord,
+    toUpper,
+  )
+import Data.Text qualified as T
+
+newtype Iban = Iban T.Text deriving (Show, Eq)
+
+mkIban :: T.Text -> Either String Iban
+mkIban t = validateIban t >> return (Iban t)
+
+validateIban :: T.Text -> Either String ()
+validateIban t = AP.parseOnly ibanP t
+  where
+    ibanP = do
+      countryCode <- AP.count 2 ibanLetter
+      checkDigits <- AP.count 2 ibanDigit
+      chars <- AP.many1 ibanChar
+      endOfInput
+      if length chars < 30
+        then
+          if valid countryCode checkDigits chars
+            then return ()
+            else fail $ "IBAN checksum does not match (" ++ T.unpack t ++ ")"
+        else fail "IBAN has more than 34 characters"
+      where
+        ibanChar = ibanDigit <|> ibanLetter
+        ibanDigit = toInteger . digitToInt <$> AP.digit
+        ibanLetter = letterToInt <$> AP.letter
+        letterToInt c = toInteger (ord (toUpper c) - ord 'A' + 10)
+        charsToInteger = foldl' (\acc d -> if d >= 10 then acc * 100 + d else acc * 10 + d) 0
+        ibanToInteger countryCode checkDigits chars =
+          charsToInteger chars * 1000000 + charsToInteger countryCode * 100 + charsToInteger checkDigits
+        valid countryCode checkDigits chars =
+          ibanToInteger countryCode checkDigits chars `mod` 97 == 1