#lang scheme/base

#|

Note: the patterns described in the doc.txt file are
slightly different than the patterns processed here.
The difference is in the form of the side-condition
expressions. Here they are procedures that accept
binding structures, instead of expressions. The
reduction (And other) macros do this transformation
before the pattern compiler is invoked.

|#
(require scheme/list
         scheme/match
         scheme/contract
         "underscore-allowed.rkt")

(define-struct compiled-pattern (cp))

(define caching-enabled? (make-parameter #t))

;; lang = (listof nt)
;; nt = (make-nt sym (listof rhs))
;; rhs = (make-rhs single-pattern)
;; single-pattern = sexp
(define-struct nt (name rhs) #:inspector (make-inspector))
(define-struct rhs (pattern) #:inspector  (make-inspector))

;; var = (make-var sym sexp)
;; patterns are sexps with `var's embedded
;; in them. It means to match the
;; embedded sexp and return that binding

;; bindings = (make-bindings (listof rib))
;; rib = (make-bind sym sexp)
;; if a rib has a pair, the first element of the pair should be treated as a prefix on the identifer
;; NOTE: the bindings may contain mismatch-ribs temporarily, but they are all removed
;;       by merge-multiples/remove, a helper function called from match-pattern
(define-values (make-bindings bindings-table bindings?)
  (let () 
    (define-struct bindings (table) #:inspector (make-inspector)) ;; for testing, add inspector
    (define mt-bindings (make-bindings null))
    (values (lambda (table) (if (null? table) mt-bindings (make-bindings table)))
            bindings-table
            bindings?)))

(define-struct bind (name exp) #:inspector (make-inspector)) ;; for testing, add inspector
(define-struct mismatch-bind (name exp) #:inspector (make-inspector)) ;; for testing, add inspector

;; repeat = (make-repeat compiled-pattern (listof rib) (union #f symbol) boolean)
(define-struct repeat (pat empty-bindings suffix mismatch?) #:inspector (make-inspector)) ;; inspector for tests below

;; compiled-pattern : exp hole-info -> (union #f (listof mtch))
;; mtch = (make-mtch bindings sexp[context] (union none sexp[hole]))
;; hole-info = boolean
;;               #f means we're not in a `in-hole' context
;;               #t means we're looking for a hole
(define-values (mtch-bindings mtch-context mtch-hole make-mtch mtch?)
  (let ()
    (define-struct mtch (bindings context hole) #:inspector (make-inspector))
    (values mtch-bindings
            mtch-context
            mtch-hole
            (lambda (a b c)
              (unless (bindings? a)
                (error 'make-mtch "expected bindings for first agument, got ~e" a))
              (make-mtch a b c))
            mtch?)))

(define none
  (let ()
    (define-struct none ())
    (make-none)))
(define (none? x) (eq? x none))

;; compiled-lang : (make-compiled-lang (listof nt) 
;;                                     hash[sym -o> compiled-pattern]
;;                                     hash[sym -o> compiled-pattern]
;;                                     hash[sym -o> compiled-pattern]
;;                                     hash[sym -o> boolean])
;;                                     hash[sexp[pattern] -o> (cons compiled-pattern boolean)]
;;                                     hash[sexp[pattern] -o> (cons compiled-pattern boolean)]
;;                                     pict-builder
;;                                     (listof symbol)
;;                                     (listof (listof symbol))) -- keeps track of `primary' non-terminals

(define-struct compiled-lang (lang cclang ht list-ht across-ht across-list-ht
                                   has-hole-ht cache bind-names-cache pict-builder
                                   literals nt-map))

;; lookup-binding : bindings (union sym (cons sym sym)) [(-> any)] -> any
(define (lookup-binding bindings
                        sym
                        [fail (lambda () 
                                (error 'lookup-binding "didn't find ~e in ~e" sym bindings))])
  (let loop ([ribs (bindings-table bindings)])
    (cond
      [(null? ribs) (fail)]
      [else
       (let ([rib (car ribs)])
         (if (and (bind? rib) (equal? (bind-name rib) sym))
             (bind-exp rib)
             (loop (cdr ribs))))])))

;; compile-language : language-pict-info[see pict.rkt] (listof nt) (listof (listof sym)) -> compiled-lang
(define (compile-language pict-info lang nt-map)
  (let* ([clang-ht (make-hasheq)]
         [clang-list-ht (make-hasheq)]
         [across-ht (make-hasheq)]
         [across-list-ht (make-hasheq)]
         [has-hole-ht (build-has-hole-ht lang)]
         [cache (make-hash)]
         [bind-names-cache (make-hash)]
         [literals (extract-literals lang)]
         [clang (make-compiled-lang lang #f clang-ht clang-list-ht 
                                    across-ht across-list-ht
                                    has-hole-ht 
                                    cache bind-names-cache
                                    pict-info
                                    literals
                                    nt-map)]
         [non-list-nt-table (build-non-list-nt-label lang)]
         [list-nt-table (build-list-nt-label lang)]
         [do-compilation
          (lambda (ht list-ht lang prefix-cross?)
            (for-each
             (lambda (nt)
               (for-each
                (lambda (rhs)
                  (let-values ([(compiled-pattern has-hole?) 
                                (compile-pattern/cross? clang (rhs-pattern rhs) prefix-cross? #f)])
                    (let ([add-to-ht
                           (lambda (ht)
                             (hash-set!
                              ht
                              (nt-name nt)
                              (cons compiled-pattern (hash-ref ht (nt-name nt)))))]
                          [may-be-non-list? (may-be-non-list-pattern? (rhs-pattern rhs) non-list-nt-table)]
                          [may-be-list? (may-be-list-pattern? (rhs-pattern rhs) list-nt-table)])
                      (when may-be-non-list? (add-to-ht ht))
                      (when may-be-list? (add-to-ht list-ht))
                      (unless (or may-be-non-list? may-be-list?)
                        (error 'compile-language 
                               "internal error: unable to determine whether pattern matches lists, non-lists, or both: ~s"
                               (rhs-pattern rhs))))))
                (nt-rhs nt)))
             lang))]
         [init-ht
          (lambda (ht)
            (for-each (lambda (nt) (hash-set! ht (nt-name nt) null))
                      lang))])
    
    (init-ht clang-ht)
    (init-ht clang-list-ht)
    
    (hash-for-each
     clang-ht
     (lambda (nt rhs)
       (when (has-underscore? nt)
         (error 'compile-language "cannot use underscore in nonterminal name, ~s" nt))))
    
    (let ([compatible-context-language
           (build-compatible-context-language clang-ht lang)])
      (for-each (lambda (nt)
                  (hash-set! across-ht (nt-name nt) null)
                  (hash-set! across-list-ht (nt-name nt) null))
                compatible-context-language)
      (do-compilation clang-ht clang-list-ht lang #t)
      (do-compilation across-ht across-list-ht compatible-context-language #f)
      (struct-copy compiled-lang clang [cclang compatible-context-language]))))

;; extract-literals : (listof nt) -> (listof symbol)
(define (extract-literals nts)
  (let ([literals-ht (make-hasheq)]
        [nt-names (map nt-name nts)])
    (for-each (λ (nt) 
                (for-each (λ (rhs) (extract-literals/pat nt-names (rhs-pattern rhs) literals-ht))
                          (nt-rhs nt)))
              nts)
    (hash-map literals-ht (λ (x y) x))))

;; extract-literals/pat : (listof sym) pattern ht -> void
;; inserts the literals mentioned in pat into ht
(define (extract-literals/pat nts pat ht)
  (let loop ([pat pat])
    (match pat
      [`any (void)]
      [`number (void)]
      [`string (void)]
      [`natural (void)]
      [`integer (void)]
      [`real (void)]
      [`variable (void)]
      [`(variable-except ,s ...) (void)]
      [`(variable-prefix ,s) (void)]
      [`variable-not-otherwise-mentioned (void)]
      [`hole (void)]
      [(? symbol? s) 
       (unless (regexp-match #rx"_" (symbol->string s))
         (unless (regexp-match #rx"^\\.\\.\\." (symbol->string s))
           (unless (memq s nts)
             (hash-set! ht s #t))))]
      [`(name ,name ,pat) (loop pat)]
      [`(in-hole ,p1 ,p2) 
       (loop p1)
       (loop p2)]
      [`(hide-hole ,p) (loop p)]
      [`(side-condition ,p ,g ,e)
       (loop p)]
      [`(cross ,s) (void)]
      [_
       (let l-loop ([l-pat pat])
         (when (pair? l-pat) 
           (loop (car l-pat))
           (l-loop (cdr l-pat))))])))

; build-has-hole-ht : (listof nt) -> hash[symbol -o> boolean]
; produces a map of nonterminal -> whether that nonterminal could produce a hole
(define (build-has-hole-ht lang)
  (build-nt-property 
   lang
   (lambda (pattern recur)
     (match pattern
       [`any #f]
       [`number #f]
       [`string #f]
       [`variable #f] 
       [`natural #f]
       [`integer #f]
       [`real #f]
       [`(variable-except ,vars ...) #f]
       [`(variable-prefix ,var) #f]
       [`variable-not-otherwise-mentioned #f]
       [`hole #t]
       [(? string?) #f]
       [(? symbol?)
        ;; cannot be a non-terminal, otherwise this function isn't called
        #f]
       [`(name ,name ,pat)
        (recur pat)]
       [`(in-hole ,context ,contractum)
        (recur contractum)]
       [`(hide-hole ,arg) #f]
       [`(side-condition ,pat ,condition ,expr)
        (recur pat)]
       [(? list?)
        (ormap recur pattern)]
       [else #f]))
   #t
   (lambda (lst) (ormap values lst))))

;; build-nt-property : lang (pattern[not-non-terminal] (pattern -> boolean) -> boolean) boolean
;;                  -> hash[symbol[nt] -> boolean]
(define (build-nt-property lang test-rhs conservative-answer combine-rhss)
  (let ([ht (make-hasheq)]
        [rhs-ht (make-hasheq)])
    (for-each
     (lambda (nt)
       (hash-set! rhs-ht (nt-name nt) (nt-rhs nt))
       (hash-set! ht (nt-name nt) 'unknown))
     lang)
    (let ()
      (define (check-nt nt-sym)
        (let ([current (hash-ref ht nt-sym)])
          (case current
            [(unknown)
             (hash-set! ht nt-sym 'computing)
             (let ([answer (combine-rhss 
                            (map (lambda (x) (check-rhs (rhs-pattern x)))
                                 (hash-ref rhs-ht nt-sym)))])
               (hash-set! ht nt-sym answer)
               answer)]
            [(computing) conservative-answer]
            [else current])))
      (define (check-rhs rhs)
        (cond
          [(hash-maps? ht rhs)
           (check-nt rhs)]
          [else (test-rhs rhs check-rhs)]))
      (for-each (lambda (nt) (check-nt (nt-name nt)))
                lang)
      ht)))

;; build-compatible-context-language : lang -> lang
(define (build-compatible-context-language clang-ht lang)
  (remove-empty-compatible-contexts
   (apply
    append
    (map 
     (lambda (nt1)
       (map
        (lambda (nt2)
          (let ([compat-nt (build-compatible-contexts/nt clang-ht (nt-name nt1) nt2)])
            (if (eq? (nt-name nt1) (nt-name nt2))
                (make-nt (nt-name compat-nt)
                         (cons
                          (make-rhs 'hole)
                          (nt-rhs compat-nt)))
                compat-nt)))
        lang))
     lang))))

;; remove-empty-compatible-contexts : lang -> lang
;; Removes the empty compatible context non-terminals and the 
;; rhss that reference them.
(define (remove-empty-compatible-contexts lang)
  (define (has-cross? pattern crosses)
    (match pattern
      [`(cross ,(? symbol? nt)) (memq nt crosses)]
      [(list-rest p '... rest) (has-cross? rest crosses)]
      [(cons first rest) (or (has-cross? first crosses)
                             (has-cross? rest crosses))]
      [_ #f]))
  (define (delete-empty nts)
    (for/fold ([deleted null] [kept null]) ([nt nts])
      (if (null? (nt-rhs nt))
          (values (cons nt deleted) kept)
          (values deleted (cons nt kept)))))
  (define (delete-references deleted-names remaining-nts)
    (map (λ (nt) 
           (make-nt (nt-name nt)
                    (filter (λ (rhs) (not (has-cross? (rhs-pattern rhs) deleted-names))) 
                            (nt-rhs nt))))
         remaining-nts))
  
  (let loop ([nts lang])
    (let-values ([(deleted kept) (delete-empty nts)])
      (if (null? deleted)
          kept
          (loop (delete-references (map nt-name deleted) kept))))))

;; build-compatible-contexts : clang-ht prefix nt -> nt
;; constructs the compatible closure evaluation context from nt.
(define (build-compatible-contexts/nt clang-ht prefix nt)
  (make-nt
   (symbol-append prefix '- (nt-name nt))
   (apply append
          (map
           (lambda (rhs)
             (let-values ([(maker count) (build-compatible-context-maker clang-ht
                                                                         (rhs-pattern rhs)
                                                                         prefix)])
               (let loop ([i count])
                 (cond
                   [(zero? i) null]
                   [else (let ([nts (build-across-nts (nt-name nt) count (- i 1))])
                           (cons (make-rhs (maker (box nts)))
                                 (loop (- i 1))))]))))
           (nt-rhs nt)))))

(define (symbol-append . args)
  (string->symbol (apply string-append (map symbol->string args))))

;; build-across-nts : symbol number number -> (listof pattern)
(define (build-across-nts nt count i)
  (let loop ([j count])
    (cond
      [(zero? j) null]
      [else
       (cons (= i (- j 1)) 
             (loop (- j 1)))])))

;; build-compatible-context-maker : symbol pattern -> (values ((box (listof pattern)) -> pattern) number)
;; when the result function is applied, it takes each element
;; of the of the boxed list and plugs them into the places where
;; the nt corresponding from this rhs appeared in the original pattern. 
;; The number result is the number of times that the nt appeared in the pattern.
(define (build-compatible-context-maker clang-ht pattern prefix)
  (let ([count 0])
    (define maker
     (let loop ([pattern pattern])
       (define (untouched-pattern _) 
         (values pattern #f))
       (match pattern
         [`any untouched-pattern]
         [`number untouched-pattern]
         [`string untouched-pattern]
         [`natural untouched-pattern]
         [`integer untouched-pattern]
         [`real untouched-pattern]
         [`variable untouched-pattern] 
         [`(variable-except ,vars ...) untouched-pattern]
         [`(variable-prefix ,var) untouched-pattern]
         [`variable-not-otherwise-mentioned untouched-pattern]
         [`hole untouched-pattern]
         [(? string?) untouched-pattern]
         [(? symbol?) 
          (cond
            [(hash-ref clang-ht pattern #f)
             (set! count (+ count 1))
             (lambda (l)
               (let ([fst (car (unbox l))])
                 (set-box! l (cdr (unbox l)))
                 (if fst
                     (values `(cross ,(symbol-append prefix '- pattern)) #t)
                     (values pattern #f))))]
            [else untouched-pattern])]
         [`(name ,name ,pat)
          (let ([patf (loop pat)])
            (lambda (l)
              (let-values ([(p h?) (patf l)])
                (values `(name ,name ,p) h?))))]
         [`(in-hole ,context ,contractum)
          (let ([match-context (loop context)]
                [match-contractum (loop contractum)])
            (lambda (l)
              (let-values ([(ctxt _) (match-context l)]
                           [(ctct h?) (match-contractum l)])
                (values `(in-hole ,ctxt ,ctct) h?))))]
         [`(hide-hole ,p)
          (let ([m (loop p)])
            (lambda (l)
              (let-values ([(p h?) (m l)])
                (if h?
                    (values p #t)
                    (values `(hide-hole ,p) #f)))))]
         [`(side-condition ,pat ,condition ,expr)
          (let ([patf (loop pat)])
            (lambda (l)
              (let-values ([(p h?) (patf l)])
                (values `(side-condition ,p ,condition ,expr) h?))))]
         [(? list?)
          (define pre-cross
            (let l-loop ([ps pattern])
              (match ps
                ['() '()]
                [(list-rest p '... ps*)
                 (cons (list (loop p) p) (l-loop ps*))]
                [(cons p ps*)
                 (cons (list (loop p) #f) (l-loop ps*))])))
          (λ (l)
            (define any-cross? #f)
            (define post-cross
              (map (match-lambda 
                     [(list f r?)
                      (let-values ([(p h?) (f l)])
                        (set! any-cross? (or any-cross? h?))
                        (list p h? r?))])
                   pre-cross))
            (define (hide p)
              (if any-cross? `(hide-hole ,p) p))
            (values
             (foldr (λ (post tail)
                      (match post
                        [(list p* #t (and (not #f) p))
                         `(,(hide p) ... ,p* ,(hide p) ... . ,tail)]
                        [(list p #f (not #f))
                         `(,(hide p) ... . ,tail)]
                        [(list p* #t #f)
                         `(,p* . ,tail)]
                        [(list p #f #f)
                         `(,(hide p) . ,tail)]))
                    '()
                    post-cross)
             any-cross?))]
         [else untouched-pattern])))
    (values (λ (l) (let-values ([(p _) (maker l)]) p))
            count)))

;; build-list-nt-label : lang -> hash[symbol -o> boolean]
(define (build-list-nt-label lang)
  (build-nt-property 
   lang
   (lambda (pattern recur)
     (may-be-list-pattern?/internal pattern
                                    (lambda (sym) #f)
                                    recur))
   #t
   (lambda (lst) (ormap values lst))))

(define (may-be-list-pattern? pattern list-nt-table)
  (let loop ([pattern pattern])
    (may-be-list-pattern?/internal
     pattern
     (lambda (sym) 
       (hash-ref list-nt-table (symbol->nt sym) #t))
     loop)))

(define (may-be-list-pattern?/internal pattern handle-symbol recur)
  (match pattern
    [`any #t]
    [`number #f]
    [`string #f]
    [`variable #f] 
    [`natural #f]
    [`integer #f]
    [`real #f]
    [`(variable-except ,vars ...) #f]
    [`variable-not-otherwise-mentioned #f]
    [`(variable-prefix ,var) #f]
    [`hole  #t]
    [(? string?) #f]
    [(? symbol?)
     (handle-symbol pattern)]
    [`(name ,name ,pat)
     (recur pat)]
    [`(in-hole ,context ,contractum)
     (recur context)]
    [`(hide-hole ,p)
     (recur p)]
    [`(side-condition ,pat ,condition ,expr)
     (recur pat)]
    [(? list?)
     #t]
    [else 
     ;; is this right?!
     (or (null? pattern) (pair? pattern))]))


;; build-non-list-nt-label : lang -> hash[symbol -o> boolean]
(define (build-non-list-nt-label lang)
  (build-nt-property 
   lang
   (lambda (pattern recur)
     (may-be-non-list-pattern?/internal pattern
                                        (lambda (sym) #t)
                                        recur))
   #t
   (lambda (lst) (ormap values lst))))

(define (may-be-non-list-pattern? pattern non-list-nt-table)
  (let loop ([pattern pattern])
    (may-be-non-list-pattern?/internal
     pattern
     (lambda (sym)
       (hash-ref non-list-nt-table (symbol->nt sym) #t))
     loop)))

(define (may-be-non-list-pattern?/internal pattern handle-sym recur)
  (match pattern
    [`any #t]
    [`number #t]
    [`string #t]
    [`variable #t]
    [`natural #t]
    [`integer #t]
    [`real #t]
    [`(variable-except ,vars ...) #t]
    [`variable-not-otherwise-mentioned #t]
    [`(variable-prefix ,prefix) #t]
    [`hole #t]
    [(? string?) #t]
    [(? symbol?) (handle-sym pattern)]
    [`(name ,name ,pat)
     (recur pat)]
    [`(in-hole ,context ,contractum)
     (recur context)]
    [`(hide-hole ,p)
     (recur p)]
    [`(side-condition ,pat ,condition ,expr)
     (recur pat)]
    [(? list?)
     #f]
    [else 
     ;; is this right?!
     (not (or (null? pattern) (pair? pattern)))]))

;; match-pattern : compiled-pattern exp -> (union #f (listof bindings))
(define (match-pattern compiled-pattern exp)
  (let ([results ((compiled-pattern-cp compiled-pattern) exp #f)])
    (and results
         (let ([filtered (filter-multiples results)])
           (and (not (null? filtered))
                filtered)))))

;; filter-multiples : (listof mtch) -> (listof mtch)
(define (filter-multiples matches)
  (let loop ([matches matches]
             [acc null])
    (cond
      [(null? matches) acc]
      [else
       (let ([merged (merge-multiples/remove (car matches))])
         (if merged
             (loop (cdr matches) (cons merged acc))
             (loop (cdr matches) acc)))])))

;; merge-multiples/remove : bindings -> (union #f bindings)
;; returns #f if all duplicate bindings don't bind the same thing
;; returns a new bindings 
(define (merge-multiples/remove match)
  (let/ec fail
    (let (
          ;; match-ht : sym -o> sexp
          [match-ht (make-hash)]
          
          ;; mismatch-ht : sym -o> hash[sexp -o> #t]
          [mismatch-ht (make-hash)]
          
          [ribs (bindings-table (mtch-bindings match))])
      (for-each
       (lambda (rib)
         (cond
           [(bind? rib)
            (let ([name (bind-name rib)]
                  [exp (bind-exp rib)])
              (let ([previous-exp (hash-ref match-ht name uniq)])
                (cond
                  [(eq? previous-exp uniq)
                   (hash-set! match-ht name exp)]
                  [else
                   (unless (equal? exp previous-exp)
                     (fail #f))])))]
           [(mismatch-bind? rib)
            (let* ([name (mismatch-bind-name rib)]
                   [exp (mismatch-bind-exp rib)]
                   [priors (hash-ref mismatch-ht name uniq)])
              (when (eq? priors uniq)
                (let ([table (make-hash)])
                  (hash-set! mismatch-ht name table)
                  (set! priors table)))
              (when (hash-ref priors exp #f)
                (fail #f))
              (hash-set! priors exp #t))]))
       ribs)
      (make-mtch
       (make-bindings (hash-map match-ht make-bind))
       (mtch-context match)
       (mtch-hole match)))))

;; compile-pattern : compiled-lang pattern boolean -> compiled-pattern
(define (compile-pattern clang pattern bind-names?)
  (let-values ([(pattern has-hole?) (compile-pattern/cross? clang pattern #t bind-names?)])
    (make-compiled-pattern pattern)))

;; name-to-key/binding : hash[symbol -o> key-wrap]
(define name-to-key/binding (make-hasheq))
(define-struct key-wrap (sym) #:inspector (make-inspector))

;; compile-pattern/cross? : compiled-lang pattern boolean boolean -> (values compiled-pattern boolean)
(define (compile-pattern/cross? clang pattern prefix-cross? bind-names?)
  (define clang-ht (compiled-lang-ht clang))
  (define clang-list-ht (compiled-lang-list-ht clang))
  (define has-hole-ht (compiled-lang-has-hole-ht clang))
  (define across-ht (compiled-lang-across-ht clang))
  (define across-list-ht (compiled-lang-across-list-ht clang))
  
  (define (compile-pattern/default-cache pattern)
    (compile-pattern/cache pattern 
                           (if bind-names?
                               (compiled-lang-bind-names-cache clang)
                               (compiled-lang-cache clang))))
  
  (define (compile-pattern/cache pattern compiled-pattern-cache)
    (let ([compiled-cache (hash-ref compiled-pattern-cache pattern uniq)])
      (cond 
        [(eq? compiled-cache uniq)
         (let-values ([(compiled-pattern has-hole?)
                       (true-compile-pattern pattern)])
           (let ([val (list (memoize compiled-pattern has-hole?) has-hole?)])
             (hash-set! compiled-pattern-cache pattern val)
             (apply values val)))]
        [else
         (apply values compiled-cache)])))
  
  (define (true-compile-pattern pattern)
    (match pattern
      [(? (lambda (x) (eq? x '....)))
       (error 'compile-language "the pattern .... can only be used in extend-language")]
      [`(variable-except ,vars ...)
       (values
        (lambda (exp hole-info)
          (and (symbol? exp)
               (not (memq exp vars))
               (list (make-mtch (make-bindings null)
                                (build-flat-context exp)
                                none))))
        #f)]
      [`(variable-prefix ,var)
       (values
        (let* ([prefix-str (symbol->string var)]
               [prefix-len (string-length prefix-str)])
          (lambda (exp hole-info)
            (and (symbol? exp)
                 (let ([str (symbol->string exp)])
                   (and ((string-length str) . >= . prefix-len)
                        (string=? (substring str 0 prefix-len) prefix-str)
                        (list (make-mtch (make-bindings null)
                                         (build-flat-context exp)
                                         none)))))))
        #f)]
      [`hole
       (values match-hole #t)]
      [(? string?)
       (values
        (lambda (exp hole-info)
          (and (string? exp)
               (string=? exp pattern)
               (list (make-mtch (make-bindings null)
                                (build-flat-context exp)
                                none))))
        #f)]
      [(? symbol?)
       (cond
         [(has-underscore? pattern)
          (let*-values ([(binder before-underscore)
                         (let ([before (split-underscore pattern)])
                           (values pattern before))]
                        [(match-raw-name has-hole?)
                         (compile-id-pattern before-underscore)])
            (values
             (match-named-pat binder match-raw-name)
             has-hole?))]
         [else 
          (let-values ([(match-raw-name has-hole?) (compile-id-pattern pattern)])
            (values (if (non-underscore-binder? pattern)
                        (match-named-pat pattern match-raw-name)
                        match-raw-name)
                    has-hole?))])]
      [`(cross ,(? symbol? pre-id))
       (let ([id (if prefix-cross?
                     (symbol-append pre-id '- pre-id)
                     pre-id)])
         (cond
           [(hash-maps? across-ht id)
            (values
             (lambda (exp hole-info)
               (match-nt (hash-ref across-list-ht id)
                         (hash-ref across-ht id)
                         id exp hole-info))
             #t)]
           [else
            (error 'compile-pattern "unknown cross reference ~a" id)]))]
      
      [`(name ,name ,pat)
       (let-values ([(match-pat has-hole?) (compile-pattern/default-cache pat)])
         (values (match-named-pat name match-pat)
                 has-hole?))]
      [`(in-hole ,context ,contractum) 
       (let-values ([(match-context ctxt-has-hole?) (compile-pattern/default-cache context)]
                    [(match-contractum contractum-has-hole?) (compile-pattern/default-cache contractum)])
         (values
          (match-in-hole context contractum exp match-context match-contractum)
          (or ctxt-has-hole? contractum-has-hole?)))]
      [`(hide-hole ,p)
       (let-values ([(match-pat has-hole?) (compile-pattern/default-cache p)])
         (values
          (lambda (exp hole-info)
            (let ([matches (match-pat exp #f)])
              (and matches
                   (map (λ (match) (make-mtch (mtch-bindings match) (hole->not-hole (mtch-context match)) none))
                        matches))))
          #f))]
      
      [`(side-condition ,pat ,condition ,expr)
       (let-values ([(match-pat has-hole?) (compile-pattern/default-cache pat)])
         (values
          (lambda (exp hole-info)
            (let ([matches (match-pat exp hole-info)])
              (and matches
                   (let ([filtered (filter (λ (m) (condition (mtch-bindings m))) 
                                           (filter-multiples matches))])
                     (if (null? filtered)
                         #f
                         filtered)))))
          has-hole?))]
      [(? list?)
       (let-values ([(rewritten has-hole?) (rewrite-ellipses non-underscore-binder? pattern compile-pattern/default-cache)])
         (let ([count (and (not (ormap repeat? rewritten))
                           (length rewritten))])
           (values
            (lambda (exp hole-info)
              (cond
                [(list? exp)
                 ;; shortcircuit: if the list isn't the right length, give up immediately.
                 (if (and count
                          (not (= (length exp) count)))
                     #f
                     (match-list rewritten exp hole-info))]
                [else #f]))
            has-hole?)))]
      
      ;; an already comiled pattern
      [(? compiled-pattern?)
       ;; return #t here as a failsafe; no way to check better.
       (values (compiled-pattern-cp pattern)
               #t)]
      
      [else 
       (values
        (lambda (exp hole-info)
          (and (eqv? pattern exp)
               (list (make-mtch (make-bindings null)
                                (build-flat-context exp)
                                none))))
        #f)]))
  
  (define (non-underscore-binder? pattern)
    (and bind-names?
         (or (hash-maps? clang-ht pattern)
             (memq pattern underscore-allowed))))
  
  ;; compile-id-pattern : symbol[with-out-underscore] -> (values <compiled-pattern-proc> boolean)
  (define (compile-id-pattern pat)
    (match pat
      [`any (simple-match (λ (x) #t))]
      [`number (simple-match number?)]
      [`string (simple-match string?)]
      [`variable (simple-match symbol?)]
      [`variable-not-otherwise-mentioned
       (let ([literals (compiled-lang-literals clang)])
         (simple-match
          (λ (exp)
            (and (symbol? exp) 
                 (not (memq exp literals))))))]
      [`natural (simple-match (λ (x) (and (integer? x) (exact? x) (not (negative? x)))))]
      [`integer (simple-match (λ (x) (and (integer? x) (exact? x))))]
      [`real (simple-match real?)]
      [(? is-non-terminal?)
       (values
        (lambda (exp hole-info)
          (match-nt (hash-ref clang-list-ht pat)
                    (hash-ref clang-ht pat)
                    pat exp hole-info))
        (hash-ref has-hole-ht pat))]
      [else
       (values
        (lambda (exp hole-info) 
          (and (eq? exp pat)
               (list (make-mtch (make-bindings null)
                                (build-flat-context exp)
                                none))))
        #f)]))
  
  (define (is-non-terminal? sym) (hash-maps? clang-ht sym))
  
  ;; simple-match : (any -> bool) -> (values <compiled-pattern> boolean)
  ;; does a match based on a built-in Scheme predicate
  (define (simple-match pred)
    (values (lambda (exp hole-info) 
              (and (pred exp) 
                   (list (make-mtch
                          (make-bindings null)
                          (build-flat-context exp)
                          none))))
            #f))
  
  (compile-pattern/default-cache pattern))

;; match-named-pat : symbol <compiled-pattern> -> <compiled-pattern>
(define (match-named-pat name match-pat)
  (let ([mismatch-bind? (regexp-match #rx"_!_" (symbol->string name))])
    (lambda (exp hole-info)
      (let ([matches (match-pat exp hole-info)])
        (and matches 
             (map (lambda (match)
                    (make-mtch
                     (make-bindings (cons (if mismatch-bind?
                                              (make-mismatch-bind name (mtch-context match))
                                              (make-bind name (mtch-context match)))
                                          (bindings-table (mtch-bindings match))))
                     (mtch-context match)
                     (mtch-hole match)))
                  matches))))))

;; split-underscore : symbol -> symbol
;; returns the text before the underscore in a symbol (as a symbol)
;; raise an error if there is more than one underscore in the input
(define (split-underscore sym)
  (let ([str (symbol->string sym)])
    (cond
      [(regexp-match #rx"^([^_]*)_[^_]*$" str)
       =>
       (λ (m) (string->symbol (cadr m)))]
      [(regexp-match #rx"^([^_]*)_!_[^_]*$" str)
       =>
       (λ (m) (string->symbol (cadr m)))]
      [else
       (error 'compile-pattern "found a symbol with multiple underscores: ~s" sym)])))

;; has-underscore? : symbol -> boolean
(define (has-underscore? sym)
  (memq #\_ (string->list (symbol->string sym))))

;; symbol->nt : symbol -> symbol
;; strips the trailing underscore from a symbol, if one is there.
(define (symbol->nt sym)
  (cond
    [(has-underscore? sym)
     (split-underscore sym)]
    [else sym]))

(define (memoize f needs-all-args?)
  (if needs-all-args?
      (memoize2 f)
      (memoize1 f)))

; memoize1 : (x y -> w) -> x y -> w
; memoizes a function of two arguments under the assumption
; that the function is constant w.r.t the second
(define (memoize1 f) (memoize/key f (lambda (x y) x) nohole))
(define (memoize2 f) (memoize/key f cons w/hole))

(define cache-size 350)
(define (set-cache-size! cs) (set! cache-size cs))

;; original version, but without closure allocation in hash lookup
(define (memoize/key f key-fn statsbox)
  (let ([ht (make-hash)]
        [entries 0])
    (lambda (x y)
      (cond
        [(not (caching-enabled?)) (f x y)]
        [else
         (let* ([key (key-fn x y)])
           ;(record-cache-test! statsbox)
           (unless (< entries cache-size)
             (set! entries 0)
             (set! ht (make-hash)))
           (let ([ans (hash-ref ht key uniq)])
             (cond
               [(eq? ans uniq)
                ;(record-cache-miss! statsbox)
                (set! entries (+ entries 1))
                (let ([res (f x y)])
                  (hash-set! ht key res)
                  res)]
               [else
                ans])))]))))

;; hash version, but with an extra hash that tells when to evict cache entries
#;
(define (memoize/key f key-fn statsbox)
  (let* ([cache-size 50]
         [ht (make-hash)]
         [uniq (gensym)]
         [when-to-evict-table (make-hasheq)]
         [pointer 0])
    (lambda (x y)
      (record-cache-test! statsbox)
      (let* ([key (key-fn x y)]
             [value-in-cache (hash-ref ht key uniq)])
        (cond
          [(eq? value-in-cache uniq)
           (record-cache-miss! statsbox)
           (let ([res (f x y)])
             (let ([to-remove (hash-ref when-to-evict-table pointer uniq)])
               (unless (eq? uniq to-remove)
                 (hash-remove! ht to-remove)))
             (hash-set! when-to-evict-table pointer key)
             (hash-set! ht key res)
             (set! pointer (modulo (+ pointer 1) cache-size))
             res)]
          [else
           value-in-cache])))))

;; lru cache
;; for some reason, this seems to hit *less* than the "just dump stuff out" strategy!
#;
(define (memoize/key f key-fn statsbox)
  (let* ([cache-size 50]
         [cache '()])
    (lambda (x y)
      (record-cache-test! statsbox)
      (let ([key (key-fn x y)])
        (cond
          [(null? cache)
           ;; empty cache
           (let ([ans (f x y)])
             (record-cache-miss! statsbox)
             (set! cache (cons (cons key ans) '()))
             ans)]
          [(null? (cdr cache))
           ;; one element cache
           (if (equal? (car (car cache)) key)
               (cdr (car cache))
               (let ([ans (f x y)])
                 (record-cache-miss! statsbox)
                 (set! cache (cons (cons key ans) cache))
                 ans))]
          [else
           ;; two of more element cache
           (cond
             [(equal? (car (car cache)) key)
              ;; check first element
              (cdr (car cache))]
             [(equal? (car (cadr cache)) key)
              ;; check second element
              (cdr (cadr cache))]
             [else
              ;; iterate from the 3rd element onwards
              (let loop ([previous2 cache]
                         [previous1 (cdr cache)]
                         [current (cddr cache)]
                         [i 0])
                (cond
                  [(null? current)
                   ;; found the end of the cache -- need to drop the last element if the cache is too full,
                   ;; and put the current value at the front of the cache.
                   (let ([ans (f x y)])
                     (record-cache-miss! statsbox)
                     (set! cache (cons (cons key ans) cache))
                     (unless (< i cache-size)
                       ;; drop the last element from the cache
                       (set-cdr! previous2 '()))
                     ans)]
                  [else
                   (let ([entry (car current)])
                     (cond
                       [(equal? (car entry) key)
                        ;; found a hit 
                        
                        ; remove this element from the list where it is.
                        (set-cdr! previous1 (cdr current))
                        
                        ; move it to the front of the cache
                        (set! cache (cons current cache))
                        
                        ; return the found element
                        (cdr entry)]
                       [else
                        ;; didn't hit yet, continue searching
                        (loop previous1 current (cdr current) (+ i 1))]))]))])])))))

;; hash version, but with a vector that tells when to evict cache entries
#;
(define (memoize/key f key-fn statsbox)
  (let* ([cache-size 50]
         [ht (make-hash)]
         [uniq (gensym)]
         [vector (make-vector cache-size uniq)] ;; vector is only used to evict things from the hash
         [pointer 0])
    (lambda (x y)
      (let* ([key (key-fn x y)]
             [value-in-cache (hash-ref ht key uniq)])
        (cond
          [(eq? value-in-cache uniq)
           (let ([res (f x y)])
             (let ([to-remove (vector-ref vector pointer)])
               (unless (eq? uniq to-remove)
                 (hash-remove! ht to-remove)))
             (vector-set! vector pointer key)
             (hash-set! ht key res)
             (set! pointer (modulo (+ pointer 1) cache-size))
             res)]
          [else
           value-in-cache])))))

;; vector-based version, with a cleverer replacement strategy
#;
(define (memoize/key f key-fn statsbox)
  (let* ([cache-size 20]
         ;; cache : (vector-of (union #f (cons key val)))
         ;; the #f correspond to empty spots in the cache
         [cache (make-vector cache-size #f)] 
         [pointer 0])
    (lambda (x y)
      (let ([key (key-fn x y)])
        (let loop ([i 0])
          (cond
            [(= i cache-size)
             (unless (vector-ref cache pointer)
               (vector-set! cache pointer (cons #f #f)))
             (let ([pair (vector-ref cache pointer)]
                   [ans (f x y)])
               (set-car! pair key)
               (set-cdr! pair ans)
               (set! pointer (modulo (+ 1 pointer) cache-size))
               ans)]
            [else
             (let ([entry (vector-ref cache i)])
               (if entry
                   (let ([e-key (car entry)]
                         [e-val (cdr entry)])
                     (if (equal? e-key key)
                         e-val
                         (loop (+ i 1))))
                   
                   ;; if we hit a #f, just skip ahead and store this in the cache
                   (loop cache-size)))]))))))

;; original version
#;
(define (memoize/key f key-fn statsbox)
  (let ([ht (make-hash)]
        [entries 0])
    (lambda (x y)
      (record-cache-test! statsbox)
      (let* ([key (key-fn x y)]
             [compute/cache
              (lambda ()
                (set! entries (+ entries 1))
                (record-cache-miss! statsbox)
                (let ([res (f x y)])
                  (hash-set! ht key res)
                  res))])
        (unless (< entries 200) ; 10000 was original size
          (set! entries 0)
          (set! ht (make-hash)))
        (hash-ref ht key compute/cache)))))

(define (record-cache-miss! statsbox)
  (set-cache-stats-hits! statsbox (sub1 (cache-stats-hits statsbox)))
  (set-cache-stats-misses! statsbox (add1 (cache-stats-misses statsbox))))

(define (record-cache-test! statsbox)
  (set-cache-stats-hits! statsbox (add1 (cache-stats-hits statsbox))))

(define-struct cache-stats (name misses hits) #:mutable)
(define (new-cache-stats name) (make-cache-stats name 0 0))

(define w/hole (new-cache-stats "hole"))
(define nohole (new-cache-stats "no-hole"))

(define (print-stats)
  (let ((stats (list w/hole nohole)))
    (for-each 
     (lambda (s) 
       (when (> (+ (cache-stats-hits s) (cache-stats-misses s)) 0)
         (printf "~a has ~a hits, ~a misses (~a% miss rate)\n" 
                 (cache-stats-name s)
                 (cache-stats-hits s)
                 (cache-stats-misses s)
                 (floor
                  (* 100 (/ (cache-stats-misses s)
                            (+ (cache-stats-hits s) (cache-stats-misses s))))))))
     stats)
    (let ((overall-hits (apply + (map cache-stats-hits stats)))
          (overall-miss (apply + (map cache-stats-misses stats))))
      (printf "---\nOverall hits: ~a\n" overall-hits)
      (printf "Overall misses: ~a\n" overall-miss)
      (when (> (+ overall-hits overall-miss) 0)
        (printf "Overall miss rate: ~a%\n" 
                (floor (* 100 (/ overall-miss (+ overall-hits overall-miss)))))))))

;; match-hole : compiled-pattern
(define (match-hole exp hole-info)
  (if hole-info
      (list (make-mtch (make-bindings '())
                       the-hole
                       exp))
      (and (hole? exp)
           (list (make-mtch (make-bindings '())
                            the-hole
                            none)))))

;; match-in-hole : sexp sexp sexp compiled-pattern compiled-pattern -> compiled-pattern
(define (match-in-hole context contractum exp match-context match-contractum)
  (lambda (exp old-hole-info)
    (let ([mtches (match-context exp #t)])
      (and mtches
           (let loop ([mtches mtches]
                      [acc null])
             (cond
               [(null? mtches) acc]
               [else 
                (let* ([mtch (car mtches)]
                       [bindings (mtch-bindings mtch)]
                       [hole-exp (mtch-hole mtch)]
                       [contractum-mtches (match-contractum hole-exp old-hole-info)])
                  (when (eq? none hole-exp)
                    (error 'matcher.rkt "found zero holes when matching a decomposition"))
                  (if contractum-mtches
                      (let i-loop ([contractum-mtches contractum-mtches]
                                   [acc acc])
                        (cond
                          [(null? contractum-mtches) (loop (cdr mtches) acc)]
                          [else (let* ([contractum-mtch (car contractum-mtches)]
                                       [contractum-bindings (mtch-bindings contractum-mtch)])
                                  (i-loop
                                   (cdr contractum-mtches)
                                   (cons
                                    (make-mtch (make-bindings
                                                (append (bindings-table contractum-bindings)
                                                        (bindings-table bindings)))
                                               (build-nested-context 
                                                (mtch-context mtch)
                                                (mtch-context contractum-mtch))
                                               (mtch-hole contractum-mtch))
                                    acc)))]))
                      (loop (cdr mtches) acc)))]))))))

;; match-list : (listof (union repeat compiled-pattern)) sexp hole-info -> (union #f (listof bindings))
(define (match-list patterns exp hole-info)
  (let (;; raw-match : (listof (listof (listof mtch)))
        [raw-match (match-list/raw patterns exp hole-info)])
    
    (and (not (null? raw-match))
         (let loop ([raw-match raw-match])
	   (cond
	    [(null? raw-match) '()]
	    [else (append (combine-matches (car raw-match))
			  (loop (cdr raw-match)))])))))

;; match-list/raw : (listof (union repeat compiled-pattern)) 
;;                  sexp
;;                  hole-info
;;               -> (listof (listof (listof mtch)))
;; the result is the raw accumulation of the matches for each subpattern, as follows:
;;  (listof (listof (listof mtch)))
;;  \       \       \-------------/  a match for one position in the list (failures don't show up)
;;   \       \-------------------/   one element for each position in the pattern list
;;    \-------------------------/    one element for different expansions of the ellipses
;; the failures to match are just removed from the outer list before this function finishes
;; via the `fail' argument to `loop'.
(define (match-list/raw patterns exp hole-info)
  (let/ec k
    (let loop ([patterns patterns]
               [exp exp]
               ;; fail : -> alpha
               ;; causes one possible expansion of ellipses to fail
               ;; initially there is only one possible expansion, so
               ;; everything fails.
               [fail (lambda () (k null))])
      (cond
        [(pair? patterns)
         (let ([fst-pat (car patterns)])
           (cond
             [(repeat? fst-pat)
              (if (or (null? exp) (pair? exp))
                  (let ([r-pat (repeat-pat fst-pat)]
                        [r-mt (make-mtch (make-bindings (repeat-empty-bindings fst-pat))
                                         (build-flat-context '())
                                         none)])
                    (apply 
                     append
                     (cons (let/ec k
                             (let ([mt-fail (lambda () (k null))])
                               (map (lambda (pat-ele) 
                                      (cons (add-ellipses-index (list r-mt) (repeat-suffix fst-pat) (repeat-mismatch? fst-pat) 0)
                                            pat-ele))
                                    (loop (cdr patterns) exp mt-fail))))
                           (let r-loop ([exp exp]
                                        ;; past-matches is in reverse order
                                        ;; it gets reversed before put into final list
                                        [past-matches (list r-mt)]
                                        [index 1])
                             (cond
                               [(pair? exp)
                                (let* ([fst (car exp)]
                                       [m (r-pat fst hole-info)])
                                  (if m
                                      (let* ([combined-matches (collapse-single-multiples m past-matches)]
                                             [reversed 
                                              (add-ellipses-index 
                                               (reverse-multiples combined-matches)
                                               (repeat-suffix fst-pat)
                                               (repeat-mismatch? fst-pat)
                                               index)])
                                        (cons 
                                         (let/ec fail-k
                                           (map (lambda (x) (cons reversed x))
                                                (loop (cdr patterns) 
                                                      (cdr exp)
                                                      (lambda () (fail-k null)))))
                                         (r-loop (cdr exp)
                                                 combined-matches
                                                 (+ index 1))))
                                      (list null)))]
                               ;; what about dotted pairs?
                               [else (list null)])))))
                  (fail))]
             [else
              (cond
                [(pair? exp)
                 (let* ([fst-exp (car exp)]
                        [match (fst-pat fst-exp hole-info)])
                   (if match
                       (let ([exp-match (map (λ (mtch) (make-mtch (mtch-bindings mtch)
                                                                  (build-list-context (mtch-context mtch))
                                                                  (mtch-hole mtch)))
                                             match)])
                         (map (lambda (x) (cons exp-match x))
                              (loop (cdr patterns) (cdr exp) fail)))
                       (fail)))]
                [else
                 (fail)])]))]
        [else
         (if (null? exp)
             (list null)
             (fail))]))))

;; add-ellipses-index : (listof mtch) sym boolean number -> (listof mtch)
(define (add-ellipses-index mtchs key mismatch-bind? i)
  (if key
      (let ([rib (if mismatch-bind? 
                     (make-mismatch-bind key i)
                     (make-bind key i))])
        (map (λ (mtch) (make-mtch (make-bindings (cons rib (bindings-table (mtch-bindings mtch))))
                                  (mtch-context mtch)
                                  (mtch-hole mtch)))
             mtchs))
      mtchs))

;; collapse-single-multiples : (listof mtch) (listof mtch[to-lists]) -> (listof mtch[to-lists])
(define (collapse-single-multiples bindingss multiple-bindingss)
  (apply append 
         (map
          (lambda (multiple-match)
            (let ([multiple-bindings (mtch-bindings multiple-match)])
              (map
               (lambda (single-match)
                 (let ([single-bindings (mtch-bindings single-match)])
                   (make-mtch (make-bindings 
                               (map (match-lambda* 
                                      [`(,(struct bind (name sing-exp)) ,(struct bind (name mult-exp)))
                                       (make-bind name (cons sing-exp mult-exp))]
                                      [`(,(struct mismatch-bind (name sing-exp)) ,(struct mismatch-bind (name mult-exp)))
                                       (make-mismatch-bind name (cons sing-exp mult-exp))]
                                      [else 
                                       (error 'collapse-single-multiples
                                              "internal error: expected matches' bindings in same order; got ~e ~e"
                                              single-bindings
                                              multiple-bindings)])
                                    (bindings-table single-bindings)
                                    (bindings-table multiple-bindings)))
                              (build-cons-context
                               (mtch-context single-match)
                               (mtch-context multiple-match))
                              (pick-hole (mtch-hole single-match)
                                         (mtch-hole multiple-match)))))
               bindingss)))
          multiple-bindingss)))

;; pick-hole : (union none sexp) (union none sexp) -> (union none sexp)
(define (pick-hole s1 s2)
  (cond
    [(eq? none s1) s2]
    [(eq? none s2) s1]
    [(error 'matcher.rkt "found two holes")]))

;; reverse-multiples : (listof mtch[to-lists]) -> (listof mtch[to-lists])
;; reverses the rhs of each rib in the bindings and reverses the context.
(define (reverse-multiples matches)
  (map (lambda (match)
         (let ([bindings (mtch-bindings match)])
           (make-mtch
            (make-bindings
             (map (lambda (rib)
                    (cond
                      [(bind? rib)
                       (make-bind (bind-name rib)
                                  (reverse (bind-exp rib)))]
                      [(mismatch-bind? rib)
                       (make-mismatch-bind (mismatch-bind-name rib)
                                           (reverse (mismatch-bind-exp rib)))]))
                  (bindings-table bindings)))
            (reverse-context (mtch-context match))
            (mtch-hole match))))
       matches))

;; match-nt : (listof compiled-rhs) (listof compiled-rhs) sym exp hole-info
;;        -> (union #f (listof bindings))
(define (match-nt list-rhs non-list-rhs nt term hole-info)
  (let loop ([rhss (if (or (null? term) (pair? term))
                       list-rhs
                       non-list-rhs)]
             [ht #f])
    (cond
      [(null? rhss) 
       (if ht
           (hash-map ht (λ (k v) k))
           #f)]
      [else
       (let ([mth (remove-bindings/filter ((car rhss) term hole-info))])
         (cond
           [mth
            (let ([ht (or ht (make-hash))])
              (for-each (λ (x) (hash-set! ht x #t)) mth)
              (loop (cdr rhss) ht))]
           [else 
            (loop (cdr rhss) ht)]))])))

;; remove-bindings/filter : (union #f (listof mtch)) -> (union #f (listof mtch))
(define (remove-bindings/filter matches)
  (and matches
       (let ([filtered (filter-multiples matches)])
         (and (not (null? filtered))
              (map (λ (match)
                     (make-mtch (make-bindings '())
                                (mtch-context match)
                                (mtch-hole match)))
                   matches)))))

;; rewrite-ellipses : (symbol -> boolean)
;;                    (listof pattern) 
;;                    (pattern -> (values compiled-pattern boolean))
;;                 -> (values (listof (union repeat compiled-pattern)) boolean)
;; moves the ellipses out of the list and produces repeat structures
(define (rewrite-ellipses non-underscore-binder? pattern compile)
  (let loop ([exp-eles pattern]
             [fst dummy])
    (cond
      [(null? exp-eles)
       (if (eq? fst dummy)
           (values empty #f)
           (let-values ([(compiled has-hole?) (compile fst)])
             (values (list compiled) has-hole?)))]
      [else
       (let ([exp-ele (car exp-eles)])
         (cond
           [(or (eq? '... exp-ele)
                (prefixed-with? "..._" exp-ele))
            (when (eq? fst dummy)
              (error 'match-pattern "bad ellipses placement: ~s" pattern))
            (let-values ([(compiled has-hole?) (compile fst)]
                         [(rest rest-has-hole?) (loop (cdr exp-eles) dummy)])
              (let ([underscore-key (if (eq? exp-ele '...) #f exp-ele)]
                    [mismatch? (and (regexp-match #rx"_!_" (symbol->string exp-ele)) #t)])
                (values
                 (cons (make-repeat compiled (extract-empty-bindings non-underscore-binder? fst) underscore-key mismatch?) 
                       rest)
                 (or has-hole? rest-has-hole?))))]
           [(eq? fst dummy)
            (loop (cdr exp-eles) exp-ele)]
           [else
            (let-values ([(compiled has-hole?) (compile fst)]
                         [(rest rest-has-hole?) (loop (cdr exp-eles) exp-ele)])
              (values
               (cons compiled rest)
               (or has-hole? rest-has-hole?)))]))])))

(define (prefixed-with? prefix exp)
  (and (symbol? exp)
       (let* ([str (symbol->string exp)]
              [len (string-length str)])
         (and (len . >= . (string-length prefix))
              (string=? (substring str 0 (string-length prefix))
                        prefix)))))

(define dummy (box 0))

;; extract-empty-bindings : (symbol -> boolean) pattern -> (listof rib)
(define (extract-empty-bindings non-underscore-binder? pattern)
  (let loop ([pattern pattern]
             [ribs null])
    (match pattern
      [`(variable-except ,vars ...) ribs]
      [`(variable-prefix ,vars) ribs]
      [`variable-not-otherwise-mentioned ribs]
      
      [`hole ribs]
      [(? symbol?) 
       (cond
         [(regexp-match #rx"_!_" (symbol->string pattern))
          (cons (make-mismatch-bind pattern '()) ribs)]
         [(or (has-underscore? pattern)
              (non-underscore-binder? pattern))
          (cons (make-bind pattern '()) ribs)]
         [else ribs])]
      [`(name ,name ,pat) 
       (cons (if (regexp-match #rx"_!_" (symbol->string name))
                 (make-mismatch-bind name '())
                 (make-bind name '()))
             (loop pat ribs))]
      [`(in-hole ,context ,contractum) (loop contractum (loop context ribs))]
      [`(hide-hole ,p) (loop p ribs)]
      [`(side-condition ,pat ,test ,expr) (loop pat ribs)]
      [(? list?)
       (let-values ([(rewritten has-hole?) (rewrite-ellipses non-underscore-binder? pattern (lambda (x) (values x #f)))])
         (let i-loop ([r-exps rewritten]
                      [ribs ribs])
           (cond
             [(null? r-exps) ribs]
             [else (let ([r-exp (car r-exps)])
                     (cond
                       [(repeat? r-exp)
                        (append (if (repeat-suffix r-exp)
                                    (list ((if (repeat-mismatch? r-exp)
                                               make-mismatch-bind
                                               make-bind)
                                           (repeat-suffix r-exp)
                                           '()))
                                    null)
                                (repeat-empty-bindings r-exp)
                                (i-loop (cdr r-exps) ribs))]
                       [else
                        (loop (car r-exps) (i-loop (cdr r-exps) ribs))]))])))]
      [else ribs])))

;; combine-matches : (listof (listof mtch)) -> (listof mtch)
;; input is the list of bindings corresonding to a piecewise match
;; of a list. produces all of the combinations of complete matches
(define (combine-matches matchess)
  (let loop ([matchess matchess])
    (cond
      [(null? matchess) combine-matches-base-case]
      [else (combine-pair (car matchess) (loop (cdr matchess)))])))

;; this 'inlines' build-flat-context so that the definition can remain here, near where it is used.
(define combine-matches-base-case (list (make-mtch (make-bindings null)
						   '() #;(build-flat-context '()) 
						   none)))

;; combine-pair : (listof mtch) (listof mtch) -> (listof mtch)
(define (combine-pair fst snd)
  (let ([mtchs null])
    (for-each 
     (lambda (mtch1)
       (for-each
        (lambda (mtch2)
          (set! mtchs (cons (make-mtch 
                             (make-bindings (append (bindings-table (mtch-bindings mtch1))
                                                    (bindings-table (mtch-bindings mtch2))))
                             (build-append-context (mtch-context mtch1) (mtch-context mtch2))
                             (pick-hole (mtch-hole mtch1) 
                                        (mtch-hole mtch2)))
                            mtchs)))
        snd))
     fst)
    mtchs))

(define (hash-maps? ht key)
  (not (eq? (hash-ref ht key uniq) uniq)))


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; context adt
;;

#|
  ;; This version of the ADT isn't right yet -- 
  ;; need to figure out what to do about (name ...) patterns.

  (define-values (struct:context make-context context? context-ref context-set!)
    (make-struct-type 'context #f 1 0 #f '() #f 0))
  (define hole values)
  (define (build-flat-context exp) (make-context (lambda (x) exp)))
  (define (build-cons-context c1 c2) (make-context (lambda (x) (cons (c1 x) (c2 x)))))
  (define (build-append-context l1 l2) (make-context (lambda (x) (append (l1 x) (l2 x)))))
  (define (build-list-context l) (make-context (lambda (x) (list (l x)))))
  (define (build-nested-context c1 c2) (make-context (lambda (x) (c1 (c2 x)))))
  (define (plug exp hole-stuff) (exp hole-stuff))
  (define (reverse-context c) (make-context (lambda (x) (reverse (c x)))))

|#
(define (context? x) #t)
(define-values (the-hole the-not-hole hole?)
  (let ()
    (define-struct hole () #:inspector #f)
    (define the-hole (make-hole))
    (define the-not-hole (make-hole))
    (values the-hole the-not-hole hole?)))

(define hole->not-hole
  (match-lambda
    [(? hole?) the-not-hole]
    [(list-rest f r)
     (cons (hole->not-hole f) (hole->not-hole r))]
    [x x]))

(define (build-flat-context exp) exp)
(define (build-cons-context e1 e2) (cons e1 e2))
(define (build-append-context e1 e2) (append e1 e2))
(define (build-list-context x) (list x))
(define (reverse-context x) (reverse x))
(define (build-nested-context c1 c2) 
  (plug c1 c2))
(define (plug exp hole-stuff)
  (let ([done? #f])
    (let loop ([exp exp])
      (cond
        [(pair? exp) 
         (cons (loop (car exp))
               (loop (cdr exp)))]
        [(eq? the-not-hole exp)
         the-not-hole]
        [(eq? the-hole exp)
         (if done?
             exp
             (begin (set! done? #t)
                    hole-stuff))]
        [else exp]))))

;;
;; end context adt
;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; used in hash lookups to tell when something isn't in the table
(define uniq (gensym))

(provide/contract
 (match-pattern (compiled-pattern? any/c . -> . (or/c false/c (listof mtch?))))
 (compile-pattern (-> compiled-lang? any/c boolean?
                      compiled-pattern?))
 
 (set-cache-size! (-> (and/c integer? positive?) void?))
 (cache-size (and/c integer? positive?))
 
 (make-bindings ((listof bind?) . -> . bindings?))
 (bindings-table (bindings? . -> . (listof bind?)))
 (bindings? (any/c . -> . boolean?))
 
 (mtch? (any/c . -> . boolean?))
 (make-mtch (bindings? any/c any/c . -> . mtch?))
 (mtch-bindings (mtch? . -> . bindings?))
 (mtch-context (mtch? . -> . any/c))
 (mtch-hole (mtch? . -> . (or/c none? any/c)))
 
 (make-bind (symbol? any/c . -> . bind?))
 (bind? (any/c . -> . boolean?))
 (bind-name (bind? . -> . symbol?))
 (bind-exp (bind? . -> . any/c))
 (compile-language (-> any/c (listof nt?) (listof (listof symbol?)) compiled-lang?))
 (symbol->nt (symbol? . -> . symbol?))
 (has-underscore? (symbol? . -> . boolean?))
 (split-underscore (symbol? . -> . symbol?)))
(provide compiled-pattern? 
         print-stats)

;; for test suite
(provide build-cons-context
         build-flat-context
         context?
         extract-empty-bindings
         (rename-out [bindings-table bindings-table-unchecked])
         (struct-out mismatch-bind)
         (struct-out compiled-pattern))

(provide (struct-out nt)
         (struct-out rhs)
         (struct-out compiled-lang)
         
         lookup-binding
         
         compiled-pattern
         
         plug
         none? none
         
         make-repeat
         the-not-hole the-hole hole?
         rewrite-ellipses
         build-compatible-context-language
         caching-enabled?)
