Rot13 (Forth)
From LiteratePrograms
Simple
: r13 ( c -- o )
dup 32 or \ tolower
dup [char] a [char] z 1+ within if
[char] m > if -13 else 13 then +
else drop then ;
Table based
This implementation of rot13 constructs a translation table which maps 8-bit characters to 8-bit characters. This moves the logic of the translation from run-time to compile time.
xlate is a defining word for a table which does a character lookup (+ c@) when invoked. xlate rot13 and the following lines create the specific table for the rot13 mapping.
,chars is a utility for laying down a run of consecutive characters in the dictionary.
rot13-string applies the rot13 character transform to a string, modifying it in-place.
<<rot13 characters and strings>>= : xlate create does> ( c -- c' ) + c@ ; : ,chars ( end start -- ) do i c, loop ; xlate rot13 char A 0 ,chars char Z 1+ char N ,chars char N char A ,chars char a char Z 1+ ,chars char z 1+ char n ,chars char n char a ,chars 256 char z 1+ ,chars : rot13-string ( addr len -- ) over + swap do i c@ rot13 i c! loop ;
.rot13" is a front-end to rot13-string which makes use on the Forth command line more convenient.
<<rot13 a parsed string>>= : .rot13" ( string" -- ) [char] " parse 2dup rot13-string type ; .rot13" abjurer NOWHERE" \ nowhere ABJURER
rot13-file will apply the rot13 transform to the contents of a file. The begin...while...repeat loop is the typical wrapper for doing something to each line of a file. pad is the built-in scratch buffer.
<<rot13 a file>>= : rot13-file ( file -- ) begin dup pad 256 rot read-line throw while pad over rot13-string pad swap type cr repeat 2drop ; \ Examples: \ s" foo" r/o open-file throw rot13-file \ stdin rot13-file
<<rot13.f>>= rot13 characters and strings rot13 a parsed string rot13 a file
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