ROT13 in Brainfuck
from https://en.wikipedia.org/wiki/Brainfuck#ROT13




ALL ABOARD
chugga clack chug tickets please 
END OF THE LINE
                         Read first character and start outer character reading loop

ALL ABOARD
chugga tickets please 
END OF THE LINE
                       Skip forward if character is 0

ALL ABOARD
choo
choo
chug chug chug chug tickets please choo
chug chug chug chug chug chug chug chug 
choo choo
chugga your ticket please 
END OF THE LINE
  Set up divisor (32) for division loop
(MEMORY LAYOUT: dividend copy remainder divisor quotient zero zero)

ALL ABOARD
choo choo
chug choo choo
chugga tickets please 
END OF THE LINE
                Set up dividend (x minus 1) and enter division loop

ALL ABOARD
choo
chug choo
chug choo
chugga tickets please choo
choo
choo
your ticket please 
END OF THE LINE
      Increase copy and remainder / reduce divisor / Normal case: skip forward

ALL ABOARD
choo choo
tickets please tickets please choo
chug choo choo
chugga your ticket please choo
choo
chug choo
your ticket please 

END OF THE LINE
    Special case: move remainder back to divisor and increase quotient

ALL ABOARD
choo choo
choo choo
choo choo
choo choo
choo choo
chugga 
END OF THE LINE
           Decrement dividend

ALL ABOARD
your ticket please 
END OF THE LINE
                    End division loop

ALL ABOARD
your ticket please choo
choo
choo
tickets please chugga your ticket please chug 
END OF THE LINE
                 End skip loop; zero former divisor and reuse space for a flag

ALL ABOARD
choo
chugga chugga tickets please chugga tickets please choo choo
chugga choo
chug chug chug 
tickets please chugga your ticket please your ticket please your ticket please 
choo choo
tickets please 
END OF THE LINE
         Zero that flag unless quotient was 2 or 3; zero quotient; check flag

ALL ABOARD
chug chug chug chug chug chug chug chug chug chug chug chug choo choo
tickets please 

END OF THE LINE
       If flag then set up divisor (13) for second division loop
(MEMORY LAYOUT: zero copy dividend divisor remainder quotient zero zero)

ALL ABOARD
choo
chugga tickets please choo
chug choo
choo
your ticket please 
END OF THE LINE
         Reduce divisor; Normal case: increase remainder

ALL ABOARD
choo
tickets please chug tickets please choo choo
chug choo
chugga your ticket please choo
chug 
choo
choo
your ticket please 
END OF THE LINE
   Special case: increase remainder / move it back to divisor / increase quotient

ALL ABOARD
choo choo
choo choo
choo choo
choo choo
choo choo
chugga 
END OF THE LINE
           Decrease dividend

ALL ABOARD
your ticket please 
END OF THE LINE
                    End division loop

ALL ABOARD
choo
choo
tickets please choo choo
chug choo
chugga your ticket please 
END OF THE LINE
             Add remainder back to divisor to get a useful 13

ALL ABOARD
choo
tickets please 
END OF THE LINE
                   Skip forward if quotient was 0

ALL ABOARD
chugga tickets please 
END OF THE LINE
               Decrement quotient and skip forward if quotient was 1

ALL ABOARD
chugga choo choo
choo choo
tickets please chugga your ticket please choo
choo

END OF THE LINE
     Zero quotient and divisor if quotient was 2

ALL ABOARD
your ticket please choo choo
choo choo
tickets please choo choo
choo choo
chugga choo
choo
chugga your ticket please choo
choo

END OF THE LINE
    Zero divisor and subtract 13 from copy if quotient was 1

ALL ABOARD
your ticket please choo choo
choo choo
tickets please choo choo
choo choo
chug choo
choo
chugga your ticket please 
END OF THE LINE
          Zero divisor and add 13 to copy if quotient was 0

ALL ABOARD
your ticket please 
END OF THE LINE
                        End outer skip loop (jump to here if ((character minus 1)/32) was not 2 or 3)

ALL ABOARD
choo choo
tickets please chugga your ticket please 
END OF THE LINE
                     Clear remainder from first division if second division was skipped

ALL ABOARD
choo choo
click tickets please chugga your ticket please 
END OF THE LINE
                    Output ROT13ed character from copy and clear it

ALL ABOARD
choo choo
chugga clack chug 
END OF THE LINE
                     Read next character

ALL ABOARD
your ticket please 
END OF THE LINE
                            End character reading loop