Retrieve only digit from TokenName and use onchain

Question

How can I make the following function to retrieve only digit portion from the TokenName and use it on chain?

For example I want to retrieve the number 123 from "AAA123" (TokenName)

{-# INLINABLE getIdFromTokenName #-}
getIdFromTokenName :: TokenName -> Integer
getIdFromTokenName tn = ...

and I have the check function for state machine, as follows:

{-# INLINABLE check #-}
check :: Game -> GameDatum -> GameRedeemer -> ScriptContext -> Bool
check g _ (Buy tn) _ = ix > 0 && gMaxItems g >= ix
                            where
                                ix = getIdFromTokenName tn

Answer 1

given the request on way would be the following:

getIdFromTokenName :: TokenName -> Integer
getIdFromTokenName tn = getIdFromStr 0 $ (BS8.unpack . unTokenName) tn -- BS8.unpack :: ByteString -> String
  where
    getIdFromStr :: Integer -> String -> Integer
    getIdFromStr accumulatedNum strName =
      case strName of
        (char: restStr) ->
          case maybeIntegerFormChar char of
            Nothing  -> getIdFromStr accumulatedNum restStr
            Just int -> getIdFromStr (10 * accumulatedNum + int) restStr
        _               -> accumulatedNum

    maybeIntegerFormChar :: Char -> Maybe Integer
    maybeIntegerFormChar ch =
      case ch of
        '0' -> Just 0
        '1' -> Just 1
        '2' -> Just 2
        '3' -> Just 3
        '4' -> Just 4
        '5' -> Just 5
        '6' -> Just 6
        '7' -> Just 7
        '8' -> Just 8
        '9' -> Just 9
        _   -> Nothing

this should satisfy the codition of getting out 123 from "AAA123" but note that "AAA123BB45" will result in 12345

Answer 2

Token name is a Bytestring therefore it has to handle like that. I am learning and my attempt at extracting the integer value is like this:-

import Data.Char ( ord )
import Data.ByteString.Char8 (ByteString, unpack,pack)

getIdFromTokenName :: ByteString -> Integer
getIdFromTokenName tn = numbers (reverse (unpack tn)) 0 1

numbers :: String->Integer->Integer -> Integer 
numbers [] x y = x
numbers (l:ls) x y = if ord l >= 48 && ord l <= 57
    then numbers ls ((toInteger (ord l) - 48)* y + x) (y*10)
    else numbers ls x y

I used pack unpacked from the module imported above in ghci it was working like this

*Main> getIdFromTokenName (pack "NUM123") 
123

This is not a safe function to extract numbers and is also not the best method I feel. It's not checked in the Plutus setting also.