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# Brainfuck2Scheme
A simple compiler from Brainfuck to R5RS. The compiler will output a Scheme
list that is a lambda with two arguments:
* `data`: The data vector.
* `dptr`: The initial data pointer.
To turn the list into an executable Scheme function, just give it to
`eval`.
## How It Works
Brainfuck is a very simple Harvard architecture computer. The data is
stored as a vector `data` and the data pointer is an integer `dptr`.
The big idea is that all the code is compiled to a big lambda form, but
there are some wrinkles due to jumps.
Data access brainfuck instructions are translated like
* `+` -> `(vector-set! data dptr (+ 1 (vector-ref data dptr)))`
* `-` -> `(vector-set! data dptr (+ -1 (vector-ref data dptr)))`
* `>` -> `(set! dptr (+ dptr 1))`
* `<` -> `(set! dptr (- dptr 1))`
* `.` -> `(display (vector-ref data dptr))`
* `,` -> `(vector-set! data dptr (read-char))`
Branches are trickier. Basically, all code that will be executed in a block
is in a lambda. Given `[code...]`, the `code...` will be compiled to a
lambda in a `letrec`, with a conditional at the end that will tail-call the
lambda if the current data pointer is not zero.
The transformation then goes like
`[ CODE ... ] REST ...]` ->
(letrec ((between (lambda ()
(TRANSLATE CODE ...)
(if (not (zero? vector-ref data dptr)
(between))))))
(if (not (zero? (vector-ref data dptr)))
(between)))
(TRANSLATE REST ...)
where `(TRANSLATE CODE ...)` translates `CODE` to Scheme instructions
like above.

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;;; Brainfuck->Scheme compiler.
;;;
;;; Copyright 2024 Peter McGoron
;;;
;;; Licensed under the Apache License, Version 2.0 (the "License");
;;; you may not use this file except in compliance with the License.
;;; You may obtain a copy of the License at
;;;
;;; http://www.apache.org/licenses/LICENSE-2.0
;;;
;;; Unless required by applicable law or agreed to in writing, software
;;; distributed under the License is distributed on an "AS IS" BASIS,
;;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;;; See the License for the specific language governing permissions and
;;; limitations under the License.
;;; Turn a reversed list of Scheme commands into a thunk.
(define (assemble-function ins)
`(lambda ()
,@(reverse ins)))
;;; Compile all brainfuck instructions in LST, coming after the Scheme
;;; commands in INS.
(define (compile lst ins)
(if (null? lst)
(assemble-function ins)
(case (car lst)
((#\>) (compile (cdr lst)
(cons '(set! dptr (+ dptr 1)) ins)))
((#\<) (compile (cdr lst)
(cons '(set! dptr (- dptr 1)) ins)))
((#\+) (compile (cdr lst)
(cons
'(vector-set! data dptr
(+ (vector-ref data dptr) 1))
ins)))
((#\-) (compile (cdr lst)
(cons
'(vector-set! data dptr
(- (vector-ref data dptr) 1))
ins)))
((#\.) (compile (cdr lst)
(cons '(display (integer->char (vector-ref data dptr))) ins)))
((#\,) (compile (cdr lst)
(cons '(vector-set! data dptr
(char->integer (read-char))) ins)))
((#\[) (let ((rest (compile (cdr lst) '())))
(if (not (pair? rest))
(error "unmatched [")
(let ((between (car rest))
(after-uncompiled (cdr rest)))
(compile after-uncompiled
(cons `(letrec ((between ,between))
(if (not (zero? (vector-ref data dptr)))
(between)))
ins))))))
((#\]) (cons (assemble-function
(cons '(if (not (zero? (vector-ref data dptr)))
(between))
ins))
(cdr lst)))
(else (compile (cdr lst) ins)))))
(define (brainfuck->scheme str)
`(lambda (data dptr)
(,(compile (string->list str) '()))))
(define (brainfuck->scheme-from-file filename)
(brainfuck->scheme
(call-with-port (open-input-file filename)
(lambda (port)
(let loop ((str ""))
(if (eof-object? (peek-char port))
str
(loop (string-append str (read-line port)))))))))
(define (execute scheme)
((eval scheme) (make-vector 4096) 0))