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;;; Brainfuck for GNU Guile
;; Copyright (C) 2009, 2013 Free Software Foundation, Inc.
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 3 of the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;; Code:
(define-module (language brainfuck compile-scheme)
#:export (compile-scheme))
;; Compilation of Brainfuck to Scheme is pretty straight-forward. For all of
;; brainfuck's instructions, there are basic representations in Scheme we
;; only have to generate.
;;
;; Brainfuck's pointer and data-tape are stored in the variables pointer and
;; tape, where tape is a vector of integer values initially set to zero. Pointer
;; starts out at position 0.
;; Our tape is thus of finite length, with an address range of 0..n for
;; some defined upper bound n depending on the length of our tape.
;; Define the length to use for the tape.
(define tape-size 30000)
;; This compiles a whole brainfuck program. This constructs a Scheme code like:
;; (let ((pointer 0)
;; (tape (make-vector tape-size 0)))
;; (begin
;; <body>
;; (write-char #\newline)))
;;
;; So first the pointer and tape variables are set up correctly, then the
;; program's body is executed in this context, and finally we output an
;; additional newline character in case the program does not output one.
;;
;; TODO: Find out and explain the details about env, the three return values and
;; how to use the options. Implement options to set the tape-size, maybe.
(define (compile-scheme exp env opts)
(values
`(let ((pointer 0)
(tape (make-vector ,tape-size 0)))
,@(compile-body (cdr exp))
(write-char #\newline))
env
env))
;; Compile a list of instructions to get a list of Scheme codes. As we always
;; strip off the car of the instructions-list and cons the result onto the
;; result-list, it will get out in reversed order first; so we have to (reverse)
;; it on return.
(define (compile-body instructions)
(let iterate ((cur instructions)
(result '()))
(if (null? cur)
(reverse result)
(let ((compiled (compile-instruction (car cur))))
(iterate (cdr cur) (cons compiled result))))))
;; Compile a single instruction to Scheme, using the direct representations
;; all of Brainfuck's instructions have.
(define (compile-instruction ins)
(case (car ins)
;; Pointer moval >< is done simply by something like:
;; (set! pointer (+ pointer +-1))
((<bf-move>)
(let ((dir (cadr ins)))
`(set! pointer (+ pointer ,dir))))
;; Cell increment +- is done as:
;; (vector-set! tape pointer (+ (vector-ref tape pointer) +-1))
((<bf-increment>)
(let ((inc (cadr ins)))
`(vector-set! tape pointer (+ (vector-ref tape pointer) ,inc))))
;; Output . is done by converting the cell's integer value to a character
;; first and then printing out this character:
;; (write-char (integer->char (vector-ref tape pointer)))
((<bf-print>)
'(write-char (integer->char (vector-ref tape pointer))))
;; Input , is done similarly, read in a character, get its ASCII code and
;; store it into the current cell:
;; (vector-set! tape pointer (char->integer (read-char)))
((<bf-read>)
'(vector-set! tape pointer (char->integer (read-char))))
;; For loops [...] we use a named let construction to execute the body until
;; the current cell gets zero. The body is compiled via a recursive call
;; back to (compile-body).
;; (let iterate ()
;; (if (not (= (vector-ref! tape pointer) 0))
;; (begin
;; <body>
;; (iterate))))
((<bf-loop>)
`(let iterate ()
(if (not (= (vector-ref tape pointer) 0))
(begin
,@(compile-body (cdr ins))
(iterate)))))
(else (error "unknown brainfuck instruction " (car ins)))))
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