EncodedBinaryToShortString.fsx

open System
let digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";;
let ex = "101010101011101010101011101010101011101010101011";;
let convert (chars:string) (ul) = 
    let baseLength = (int64 (chars.Length))
    let rec convert' (acc,rest) = 
        if rest = 0L then acc |> List.rev
        else convert' (chars.[int (rest % baseLength)]::acc,(rest/baseLength))
    convert' ([],ul)

let decimalToBinary (dec:Int64) = Convert.ToString(dec, 2)
let encodedToDecimal (digits:string) (c:char) = (int64 (digits.IndexOf(c)))
let sumCodedValue (digits:string) decimals =
    let multiplier = (float digits.Length)
    decimals 
    |> Seq.mapi (fun i v -> (int64 ((float v)*(multiplier**(float i))))) 
    |> Seq.sum

let stringToDec i = Convert.ToInt64(i, 2)
let listToString chars = chars |> Seq.map string |> String.concat ""
let stringToList (str:string) = str.ToCharArray() |> Array.toList

let encode = stringToDec >> convert digits >> listToString
let decode = Seq.map (encodedToDecimal digits) >> sumCodedValue digits >> decimalToBinary

let wat = ex |> encode
let actual = wat |> decode
actual = ex

With64ForChars.fsx

open System
open System.Text.RegularExpressions

let matches pattern str = Regex.Matches(str, pattern)
let chunk = matches ".{1,6}" >> Seq.cast >> Seq.map (fun (m:Match) -> m.Groups.[0].Captures.[0].ToString())
let encodeChunk bits = string digits.[Convert.ToInt32(bits, 2)]
let encode2 = chunk >> Seq.map encodeChunk >> String.concat ""

let indexOfChar (c:char) = digits.IndexOf(c)
let toBinary (i:int) = Convert.ToString(i, 2).PadLeft(6, '0')
let charArr (str:string) = str.ToCharArray()
let decode2 = charArr >> Seq.map indexOfChar >> Seq.map toBinary >> Seq.concat >> String.Concat

let a = "101110"
a |> encode2 |> decode2 = a