handrolled_hash.fsx

let hex = 
  seq {
    yield! Seq.zip { 0uy..9uy } { '0'..'9' }
    yield! Seq.zip { 10uy..15uy } { 'a'..'f' }
  }
  |> Map.ofSeq

let hash (input : string) =
  let bytes = input |> Encoding.UTF8.GetBytes |> md5.ComputeHash
  let chars = Array.zeroCreate<char> (bytes.Length * 2)
  bytes |> Seq.iteri (fun i b -> 
    chars.[i*2] <- hex.[b / 16uy]
    chars.[i*2+1] <- hex.[b % 16uy])

  new String(chars)