rot13.ras

// function rot13(mem_in, mem_out) -> bytes written
.rot13:
  pop $4
  pop $5

  memlen $0 $4
  memlen $1 $5
  if $1 < $0
    load $0 $1

  load $1 0
  for $1 < $0 {
    peek $2 $4 $1 0

    if 'a' <= $2 {
      if $2 <= 'z' {
        load $3 'a'
        jump @__rot13__apply_rotate
      }
    }

    if 'A' <= $2 {
      if $2 <= 'Z' {
        load $3 'A'
        jump @__rot13__apply_rotate
      }
    }

    jump @__rot13__continue

    @__rot13__apply_rotate:
    sub $2 $2 $3
    add $2 $2 13
    imod $2 $2 26
    add $2 $2 $3

    @__rot13__continue:

    poke $5 $2 $1 0

    add $1 $1 1
  }
  load rp $0
  return