-,+[                          ; Read first character and start outer character reading loop
    -[                        ; Skip forward if character is 0
        >>++++[>++++++++<-]   ; Set up divisor (32) for division loop
                              ;   (MEMORY LAYOUT: dividend copy remainder divisor quotient zero zero)
        <+<-[                 ; Set up dividend (x minus 1) and enter division loop
            >+>+>-[>>>]       ; Increase copy and remainder / reduce divisor / Normal case: skip forward
            <[[>+<-]>>+>]     ; Special case: move remainder back to divisor and increase quotient
            <<<<<-            ; Decrement dividend
        ]                     ; End division loop
    ]>>>[-]+                  ; End skip loop; zero former divisor and reuse space for a flag
    >--[-[<->+++[-]]]<[       ;    Zero that flag unless quotient was 2 or 3; zero quotient; check flag
        ++++++++++++<[        ; If flag then set up divisor (13) for second division loop
                              ;   (MEMORY LAYOUT: zero copy dividend divisor remainder quotient zero zero)
            >-[>+>>]          ; Reduce divisor; Normal case: increase remainder
            >[+[<+>-]>+>>]    ; Special case: increase remainder / move it back to divisor / increase quotient
            <<<<<-            ; Decrease dividend
        ]                     ; End division loop
        >>[<+>-]              ; Add remainder back to divisor to get a useful 13
        >[                    ; Skip forward if quotient was 0
            -[                ; Decrement quotient and skip forward if quotient was 1
                -<<[-]>>      ; Zero quotient and divisor if quotient was 2
            ]<<[<<->>-]>>     ; Zero divisor and subtract 13 from copy if quotient was 1
        ]<<[<<+>>-]           ; Zero divisor and add 13 to copy if quotient was 0
    ]                         ; End outer skip loop (jump to here if ((character minus 1)/32) was not 2 or 3)
    <[-]                      ; Clear remainder from first division if second division was skipped
    <.[-]                     ; Output ROT13ed character from copy and clear it
    <-,+                      ; Read next character
]                             ; End character reading loop