; We don't have atoms built in, mutable vectors
; are our base building block. In order to make the
; following BF implementation nice, let's add atoms!
; They will be implmented as length 1 vectors with nice syntax for deref
(fun make-atom (x) [x])
(fun set-atom! (x y) (set-idx! x 0 y))
(fun get-atom (x) (idx x 0))
(add_grammar_rule 'form ["@" 'form] (lambda (_ x) `(get-atom ~x)))

; Now begin by defining our BF syntax & semantics
; Define our tokens as BF atoms
(add_grammar_rule 'bfs_atom ["<"]   (lambda (_) '(set-atom! cursor (- @cursor 1))))
(add_grammar_rule 'bfs_atom [">"]   (lambda (_) '(set-atom! cursor (+ @cursor 1))))
(add_grammar_rule 'bfs_atom ["\\+"] (lambda (_) '(set-idx! tape @cursor (+ (idx tape @cursor) 1))))
(add_grammar_rule 'bfs_atom ["-"]   (lambda (_) '(set-idx! tape @cursor (- (idx tape @cursor) 1))))
(add_grammar_rule 'bfs_atom [","]   (lambda (_) '(let (value (idx input @inptr))
                                                      (do (set-atom! inptr (+ 1 @inptr))
                                                          (set-idx! tape @cursor value)))))
(add_grammar_rule 'bfs_atom ["."]   (lambda (_) '(set-atom! output (concat [(idx tape @cursor)] @output))))

; Define strings of BF atoms
(add_grammar_rule 'bfs ['bfs_atom *] (lambda (x) x))

; Add loop as an atom
; (note that closure cannot yet close over itself by value, so we pass it in)
(add_grammar_rule 'bfs_atom ["\\[" 'bfs "]"] (lambda (_ x _)
                `(let (f (lambda (f)
                                (if (= 0 (idx tape @cursor))
                                    nil
                                    (do ,x (f f)))))
                       (f f))))

; For now, stick BFS rule inside an unambigious BFS block
; Also add setup code
(add_grammar_rule 'form ["bf" 'optional_WS "{" 'optional_WS 'bfs 'optional_WS "}"]
                        (lambda (_ _ _ _ x _ _)
                           `(lambda (input)
                             (let (
                                    tape   (vector 0 0 0 0 0)
                                    cursor (make-atom 0)
                                    inptr  (make-atom 0)
                                    output (make-atom (vector))
                                    )
                                (do (println "beginning bfs") ,x (idx output 0))))))

; Let's try it out! This BF program prints the input 3 times
(println (bf { ,>+++[<.>-] } [1337]))
; we can also have it compile into our main program
(fun main () (do (println "BF: " (bf { ,>+++[<.>-] } [1337])) 0))