open System
let decodeBase36 (input: string) = 
    if String.IsNullOrEmpty(input) then
        raise (new ArgumentException("decodeBase36 input string was null or empty"))
    
    let zeroChar = uint64 '0' // int 48
    let lowPunc  = uint64 ':' // int 58
    let highPunc = uint64 '@' // int 64
    let maxChar  = uint64 'Z' // int 90

    let mutable result = 0UL
    let mutable expBase = 1UL

    for i = input.Length - 1 downto 0 do
        let cv = 
            let c = uint64 input.[i] 
            if (c >= lowPunc && c <= highPunc) || c < zeroChar || c > maxChar then
                raise (new ArgumentException(sprintf "decodeBase36 found an unexpected character in string: %c" input.[i]))
            let v = c - zeroChar
            if v > 10UL then v - 7UL else v   
        
        result <- result + (cv * expBase)
        expBase <- expBase * 36UL
   
    result