Base64 encoder

Base64.tq

type State =
  | S0
  | S1 Number
  | S2 Number

let charOfNumber n =
  if n<26 then Char.ofNumber(n+65)
  else if n<52 then Char.ofNumber(n-26+97)
  else if n<62 then Char.ofNumber(n-52+48)
  else if n=62 then '+' else '/'

let encode bytes =
  let append = String.Unsafe.appendChar in
  let emit(s, bits) = charOfNumber bits @ append s in
  let div(p, q) = floor(p/q) in
  Array.fold
    [ (s, S0), byte -> emit(s, div(byte, 4)), mod(byte, 4) @ S1
    | (s, S1 b), byte -> emit(s, b*16+div(byte,16)), mod(byte, 16) @ S2
    | (s, S2 b), byte -> emit(emit(s, b*4+mod(byte/64, 4)), mod(byte, 64)), S0 ]
    ("", S0) bytes
  @ [ s, S0 -> s
    | s, S1 b -> append (append (emit(s, b*16)) '=') '='
    | s, S2 b -> append (emit(s, b*4)) '=' ]