;;; Sex semantic engine

(import
  scheme
  (chicken base)
  (chicken keyword)
  (chicken string)
  (chicken module)
  fmt
  sex-macros
  sex-modules
  types
  matchable                         ; pattern matching
  srfi-1                            ; list routines
  srfi-69                           ; hash tables
  utils
  )

(export/rename (process semen-process))
(export closure-env-declaration)

;;; for lambda extraction, docstring processing, macro expansion,
;;; injection of module headers, i.e. all things that rearrange code
;;; structurally, add or remove forms
;;;
;;; The algorithm: feed toplevel forms to appropriate handlers, then
;;; append their return to the resulting list. Each handler can return
;;; multiple forms, e.g. lambdas collected from a function may result
;;; in auxiliary structures and functions.
(define (process raw-sex-forms)
  (process-rec raw-sex-forms (list)))

(define (process-rec forms acc)
  (cond
   ((null? forms) (reverse acc))
   ((macro? (car forms))
    (process-rec
     (macroexpand (car forms) (cdr forms))
     acc))
   (else
    ;; A closure type's struct is emitted before the toplevel form that
    ;; first mentioned it, which is why the new forms are lifted off and
    ;; the structs slide underneath them
    (let* ((processed (match-sex-form (car forms) acc))
           (new (take-until processed acc)))
      (process-rec (cdr forms)
                   (append new (flush-closure-structs!) acc))))))

(define (take-until forms tail)
  (if (eq? forms tail)
      (list)
      (cons (car forms) (take-until (cdr forms) tail))))

(define (macroexpand macro-form rest-forms)
  ;; We want to replace macro with its expansion. The problem is,
  ;; top-level macro can return either a single form, or a list of
  ;; forms, when it for example generates some aux
  ;; structures/functions/typedefs.
  ;;
  ;; Single form we just cons to the top of rest-forms, but multiple
  ;; forms have to be appended to the rest-forms.
  (let ((res (apply-macro macro-form))
        (src (form-source macro-form)))
    ;; An expansion is fresh structure with no location of its own. Give
    ;; it the call site's, the way cpp attributes a macro body to where
    ;; the macro was used
    (if (list? (car res))
        (append (map (lambda (f) (stamp-form-source! f src)) res)
                rest-forms)
        (cons (stamp-form-source! res src) rest-forms))))

(define (match-sex-form sex-form acc)
  (match sex-form
    ((or ('fn . _)
         ('pub 'fn . _)
         ('extern 'fn . _)) (process-fn sex-form acc))
    ((or ('struct . _)
         ('pub 'struct . _)) (process-struct sex-form acc))
    ((or ('union . _)
         ('pub 'union . _)) (process-struct sex-form acc))
    ((or ('enum . _)
         ('pub 'enum . _)) (process-struct sex-form acc))
    ((or ('var . _)
         ('pub 'var . _)
         ('extern 'var . _)) (process-global-var sex-form acc))
    (('include . includes) (process-includes sex-form includes acc))
    ((or ('define name . _)
         ('pub 'define name . _))
     (add-define name sex-form)
     (cons sex-form acc))
    (('comment . _) (cons sex-form acc))

    (('import . modules)
     (process-imports (get-modules-public-forms modules) acc))

    ((or ('defmacro . rest)
         ('pub 'defmacro . rest)) (defmacro rest) acc)

    ((or ('typedef new-type target)
         ('pub 'typedef new-type target))
     (add-typedef new-type sex-form)
     (process-typedef sex-form new-type target acc))

    (else (sex-error sex-form "unknown top level form" sex-form))))

(define (process-includes sex-form includes acc)
  ;; consume (include ...) form and add to acc
  ;; (include <inc>) for each include
  (let process ((includes includes)
                (acc acc))
    (if (null? includes)
        acc
        (process (cdr includes)
                 (cons (copy-form-source! sex-form `(include ,(car includes)))
                       acc)))))

(define (process-imports module-public-forms acc)
  ;; consume (import ...) form and process imports so
  ;; data types end up in types db
  (fold match-sex-form acc module-public-forms))

(define (macro-expand form)
  "Walk the form recursively and expand all macros, until none is left."
  (walk-form
   form
   (lambda (subform env)
     (if (macro? subform)
         (cons walk-embed-result (macroexpand subform (list)))
         subform))
   #f))

;;; walk-form and friends: form walker with various abilities.
;;; By default, replaces walked form with walk-fn result But may
;;; perform additional operations depending of what the walk function
;;; has requested.

;;; For inspiration, see SBCL's walk.lisp and their template
;;; system.

(define walk-embed-result (gensym)
  ;; For cases when result is a list which must be embedded in the
  ;; form, e.g. when it returned from a macro
  )

(define (walk-form form walk-fn env)
  (if (atom? form) form
      (let ((new-form (walk-fn form env)))
        (cond ((not (eq? form new-form))
               (walk-form new-form walk-fn env))
              (else
               (let ((new-car (walk-form (car new-form) walk-fn env))
                     (new-cdr (walk-form (cdr new-form) walk-fn env)))
                 (cond ((and (pair? new-car)
                             (eq? (car new-car) walk-embed-result))
                        (append (cdr new-car) new-cdr))
                       (else
                        (recons new-form new-car new-cdr)))))))))

;;; Typdef

(define (process-typedef form new-type target acc)
  (cons (copy-form-source! form `(typedef ,target ,new-type)) acc))

;;; Fn processing
;;;
;;; A string as the first body form is a docstring. In the generated
;;; C code it will be placed as a C commentary just before the function
;;; definition (actually that works for all blocky things: enum, struct, union as well).

(define (fn-header-length fn-form)
  (if (memq (first fn-form) '(pub extern)) 5 4))

(define (fn-core form)
  ;; The (fn name args rettype . body) list, without pub/extern
  (if (memq (first form) '(pub extern))
      (cdr form)
      form))

(define (take-leading-docstring forms)
  ;; If FORMS starts with a string, possibly after comment forms, return
  ;; that string and FORMS without it. Otherwise #f and FORMS unchanged
  (let loop ((fs forms) (prefix (list)))
    (match fs
      (() (values #f forms))
      (((and cmt ('comment . _)) . rest)
       (loop rest (cons cmt prefix)))
      (((? string? doc) . rest)
       (values doc (append (reverse prefix) rest)))
      (_ (values #f forms)))))

(define (extract-fn-docstring fn-form)
  (let ((lift
         (lambda (proto body)
           (let-values (((doc rest) (take-leading-docstring body)))
             (if doc
                 (values doc (copy-form-source! fn-form (append proto rest)))
                 (values #f fn-form))))))
    (match fn-form
      (('pub 'fn name args ret . body)
       (lift `(pub fn ,name ,args ,ret) body))
      (('extern 'fn name args ret . body)
       (lift `(extern fn ,name ,args ,ret) body))
      (('fn name args ret . body)
       (lift `(fn ,name ,args ,ret) body))
      (_ (values #f fn-form)))))

(define (extract-aggregate-docstring form)
  ;; A string immediately after the name is the docstring; comments
  ;; between name and fields are not skipped, they already confuse the
  ;; writer
  (match form
    (('pub (and kind (or 'struct 'union 'enum))
           (? symbol? name) (? string? doc) . rest)
     (values doc (copy-form-source! form `(pub ,kind ,name ,@rest))))
    (((and kind (or 'struct 'union 'enum))
      (? symbol? name) (? string? doc) . rest)
     (values doc (copy-form-source! form `(,kind ,name ,@rest))))
    (_ (values #f form))))

(define (with-docstring doc form acc)
  ;; acc is newest-first; FORM is consed last so the final reverse
  ;; emits the comment immediately before the declaration
  (cons form
        (if doc
            (cons (list 'comment doc) acc)
            acc)))

(define (strip-fn-header-comments fn-form)
  ;; ([pub|extern] fn name arglist rettype). Comments in the body are
  ;; left in place as ordinary statements and preserved into the
  ;; generated C.
  (strip-header-comments fn-form (fn-header-length fn-form)))

(define (process-fn sex-fn-raw acc)
  (let-values (((doc sex-fn)
                (extract-fn-docstring (strip-fn-header-comments sex-fn-raw))))
    (let* ((expanded (macro-expand sex-fn))
           (env (make-hash-table))
           (lifted
            (walk-form
             expanded
             fn-walker
             (begin
               (set! (hash-table-ref env :fn-name) (sex-fn-name expanded))
               (set! (hash-table-ref env :lambda-counter) 0)
               (set! (hash-table-ref env :lambda-aux-code) (list))
               (set! (hash-table-ref env :var-types)
                 (declared-types (sex-fn-arglist expanded)))
               env)))
           (processed (rewrite-closure-calls-in-body lifted env)))
      (with-docstring doc processed
                      (append (hash-table-ref env :lambda-aux-code) acc)))))

(define (declared-types arglist)
  (let ((types (make-hash-table)))
    (for-each (lambda (param)
                (when (and (pair? param) (pair? (cdr param)))
                  (hash-table-set! types (first param) (second param))))
              arglist)
    types))

(define (aux-name! env make)
  (let ((counter (hash-table-ref env :lambda-counter)))
    (set! (hash-table-ref env :lambda-counter) (+ counter 1))
    (make (hash-table-ref env :fn-name) counter)))

(define (add-aux-code! env forms)
  (set! (hash-table-ref env :lambda-aux-code)
    (append forms (hash-table-ref env :lambda-aux-code))))

(define (fn-walker form env)
  (let ((head (car form)))
    (cond
     ((eq? 'lambda head)
      (let ((name (aux-name! env make-lambda-name)))
        (add-aux-code! env (lift-lambda name form))
        name))

     ;; a type, not an expression -- becomes the struct for its signature
     ((closure-type? form)
      (copy-form-source! form `(struct ,(register-closure-type! form form))))

     ((eq? 'closure head)
      (let ((base (aux-name! env make-closure-name)))
        (let-values (((construct forms) (lift-closure base form env)))
          (add-aux-code! env (fold match-sex-form (list) forms))
          (copy-form-source! form `(,construct ,@(fourth form))))))

     ;; every binding site is written, so tracking the declarations is
     ;; enough to know a receiver's type without inference
     ((and (eq? 'var head) (>= (length form) 3))
      (hash-table-set! (hash-table-ref env :var-types) (second form) (third form))
      form)

     (else form))))

(define (rewrite-closure-calls-in-body fn-form env)
  (let ((header (fn-header-length fn-form)))
    (append (take fn-form header)
            (map (lambda (form) (rewrite-closure-calls form env))
                 (drop fn-form header)))))

(define (rewrite-closure-calls form env)
  (if (not (list? form))
      form
      (let ((type (and (pair? form) (receiver-closure-type (car form) env)))
            (rewrite (lambda (sub) (rewrite-closure-calls sub env))))
        (copy-form-source!
         form
         (if type
             `(,(register-closure-call! type form)
               ,(rewrite (car form))
               ,@(map rewrite (cdr form)))
             (map rewrite form))))))

;;; A closure type has two spellings: `(closure ...)' as written, and
;;; `(struct ƛ...)' once resolved -- which is what a struct
;;; field holds, since the type database is populated after resolution.
;;; Both name the same thing, so a receiver is recognised either way.
(define (as-closure-type type)
  (cond
   ((closure-type? type) type)
   ((and (list? type) (= 2 (length type)) (eq? 'struct (car type)))
    (hash-table-ref/default +closure-structs+ (second type) #f))
   (else #f)))

(define (receiver-closure-type expr env)
  (as-closure-type (expression-type expr env)))

;;; The type of an lvalue path, from known declarations -- a name, and
;;; what can be reached from one by subscripting, dereferencing and
;;; member access.
(define (expression-type expr env)
  (cond
   ((symbol? expr)
    (hash-table-ref/default (hash-table-ref env :var-types) expr #f))
   ((not (and (list? expr) (>= (length expr) 2))) #f)
   (else
    (case (car expr)
      ((¤) (let ((base (expression-type (second expr) env)))
             (and (list? base) (>= (length base) 2) (eq? '¤ (car base))
                  (second base))))
      ((*) (and (= 2 (length expr))
                (let ((base (expression-type (second expr) env)))
                  (and (list? base) (= 2 (length base)) (eq? '* (car base))
                       (second base)))))
      ((dot-access) (member-path-type (expression-type (second expr) env)
                                      (cddr expr)))
      ((->) (let ((base (expression-type (second expr) env)))
              (and (list? base) (= 2 (length base)) (eq? '* (car base))
                   (member-path-type (second base) (cddr expr)))))
      (else #f)))))

(define (member-path-type type fields)
  (if (null? fields)
      type
      (member-path-type (field-type type (car fields)) (cdr fields))))

(define (field-type type field)
  (let ((name (cond ((symbol? type) type)
                    ((and (list? type)
                          (= 2 (length type))
                          (memq (car type) '(struct union)))
                     (second type))
                    (else #f))))
    (and name
         (let* ((fields (get-fields name))
                (entry (and fields (assq field fields))))
           (and entry (second entry))))))

(define (make-lambda-name enclosing-fn-name counter)
  (string->symbol
   (fmt #f "λ" counter "_" enclosing-fn-name)))

(define (lift-lambda name form)
  (match form
    (('lambda arglist ret-type . body)
     (process-fn (copy-form-source! form `(fn ,name ,arglist ,ret-type ,@body))
                 (list)))
    (else (sex-error form "malformed lambda" form))))

;;; Closures
;;;
;;; A closure is a function pointer and an inline environment, so the
;;; value owns its captures and nothing is allocated. The type
;;; `(closure ((int)) int)' becomes one struct per signature, shared by
;;; every closure with that signature. The captures live in `env' as a record
;;; only the lifted body knows the shape of, which is why `env' is
;;; max_align_t rather than char -- it has to be aligned for whatever
;;; ends up in it.
;;;
;;; The expression becomes three hoisted definitions -- the capture
;;; struct, the lifted body, and a constructor -- and is replaced by a
;;; call to the constructor, so the captures are evaluated as ordinary
;;; arguments at the point the closure is written.

;;; How much of a closure is environment, in bytes. Counted in bytes
;;; so a closure that fits where it was written will also fit
;;; elsewhere. Captures are only by value, and never allocated on
;;; heap. Anything that is more than 16 bytes should be stored as a
;;; pointer, and memory management is entirely up to caller

(define +closure-env-bytes+ 16)

;;; +closure-env-bytes+ for maximum capacity, max-align-t for
;;; effectiveness, hence union
(define +closure-env-type+ 'ƛenv)

(define (closure-env-declaration)
  `(union ,+closure-env-type+ ((align max-align-t)
                               (bytes (¤ char ,+closure-env-bytes+)))))

(define +closure-structs+ (make-hash-table))
(define +closure-forwards+ (make-hash-table))
(define *pending-closure-structs* (list))

(define (closure-type? form)
  (and (pair? form)
       (eq? 'closure (car form))
       (= 3 (length form))))

;;; A type spelling becomes an identifier deterministically, so two
;;; translation units have the same signatures for the same closure
;;; types: (* const char) -> p_const_char, (closure ((int)) int) ->
;;; closure_int_int.
(define (mangle-type type)
  (cond
   ((symbol? type) (mangle-word (symbol->string type)))
   ((number? type) (number->string type))
   ((null? type) "void")
   ((pair? type) (string-intersperse (map mangle-type (mangle-head type)) "_"))
   (else (sex-error type "cannot mangle type" type))))

(define (mangle-head type)
  (case (car type)
    ((*) (cons 'p (cdr type)))
    ((¤) (cons 'a (cdr type)))
    (else type)))

(define (mangle-word word)
  (list->string
   (map (lambda (c)
          (if (or (char-alphabetic? c) (char-numeric? c)) c #\_))
        (string->list word))))

(define (aggregates-in type)
  (cond
   ((not (list? type)) (list))
   ((and (= 2 (length type))
         (memq (car type) '(struct union))
         (symbol? (second type)))
    (list type))
   (else (append-map aggregates-in type))))

;;; Extract aggregate types from the closure's signature to forward
;;; declare them before the closure, so they can be referenced in the
;;; closure. Particularly useful for complex cases like fixed point
;;; combinator, etc.
(define (forward-declare-aggregates! type src-form)
  (for-each
   (lambda (aggregate)
     (let ((name (second aggregate)))
       (unless (or (hash-table-exists? +closure-forwards+ name)
                   (get-tag-info name))
         (hash-table-set! +closure-forwards+ name #t)
         (set! *pending-closure-structs*
           (cons (copy-form-source! src-form aggregate)
                 *pending-closure-structs*)))))
   (delete-duplicates (aggregates-in type))))

(define (closure-struct-name type)
  ;; the glyph says `closure' already, so the tag is just the signature
  (string->symbol (string-append "ƛ"
                                 (mangle-type (second type))
                                 "_"
                                 (mangle-type (third type)))))

;;; The code pointer takes the environment first; everything else is
;;; the closure's own signature.
(define (closure-code-type type)
  `(fn (((* void)) ,@(second type)) ,(third type)))

;;; Emitted once per signature, before the toplevel form that first
;;; needed it.
(define (register-closure-type! type src-form)
  (let ((name (closure-struct-name type)))
    (unless (hash-table-exists? +closure-structs+ name)
      (forward-declare-aggregates! type src-form)
      (hash-table-set! +closure-structs+ name type)
      (let ((form (copy-form-source!
                   src-form
                   `(struct ,name ((code ,(closure-code-type type))
                                   (env (union ,+closure-env-type+)))))))
        (register-aggregate! form)
        (set! *pending-closure-structs*
          (cons form *pending-closure-structs*))))
    name))

;;; Every closure type in FORM becomes the struct for its signature,
;;; registering it on the way. The walker does this for function bodies
;;; and headers; globals come through here instead.
(define (resolve-closure-types form)
  (cond
   ((closure-type? form)
    (copy-form-source! form `(struct ,(register-closure-type! form form))))
   ((list? form)
    (copy-form-source! form (map resolve-closure-types form)))
   (else form)))

(define +closure-calls+ (make-hash-table))

;;; The helper a closure call is routed through: `(f 1)' becomes
;;; `ƛint_int_call(f, 1)', which unpacks the receiver into
;;; `ƛc.code(&ƛc.env, ƛa0)' inside. The receiver arrives as an argument,
;;; so it is evaluated once: unpacked at the call site instead,
;;; `([table (++ i)] 10)' would read
;;; `table[++i].code(&table[++i].env, 10)' and bump `i' twice.
(define (register-closure-call! type src-form)
  (let* ((closure (closure-struct-name type))
         (helper  (suffixed closure "_call"))
         (returns (third type))
         (params  (map (lambda (arg index)
                         (list (string->symbol (fmt #f "ƛa" index))
                               (unwrap-type arg)))
                       (second type)
                       (iota (length (second type))))))
    (unless (hash-table-ref/default +closure-calls+ helper #f)
      (hash-table-set! +closure-calls+ helper #t)
      (let ((call `((dot-access ƛc code)
                    (& (dot-access ƛc env))
                    ,@(map first params))))
        (set! *pending-closure-structs*
          (cons (copy-form-source!
                 src-form
                 `(fn ,helper ((ƛc (struct ,closure)) ,@params) ,returns
                      ,(if (eq? 'void returns) call `(return ,call))))
                *pending-closure-structs*))))
    helper))

;;; An argument type is written wrapped: `(int)' in `((int) (float))'
(define (unwrap-type type)
  (if (and (list? type) (= 1 (length type)))
      (car type)
      type))

(define (flush-closure-structs!)
  (let ((pending *pending-closure-structs*))
    (set! *pending-closure-structs* (list))
    pending))

;;; Lowering

(define (make-closure-name enclosing-fn-name counter)
  (string->symbol (fmt #f "ƛ" counter "_" enclosing-fn-name)))

(define (suffixed name suffix)
  (string->symbol (string-append (symbol->string name) suffix)))

;;; A capture is a plain name, whose type comes from the declarations
;;; the walker has passed. `(name expr)' captures want the type of an
;;; expression, which is inference, and wait for it.
(define (capture-binding capture form env)
  (unless (symbol? capture)
    (sex-error form "a closure capture must be a plain name for now" capture))
  (let ((type (hash-table-ref/default (hash-table-ref env :var-types) capture #f)))
    (unless type
      (sex-error form "closure captures an undeclared name" capture))
    (list capture type)))

(define (lift-closure base form env)
  (match form
    (('closure arglist ret-type captures . body)
     (let* ((caps      (map (lambda (c) (capture-binding c form env)) captures))
            (record    (suffixed base "_captures"))
            (code      (suffixed base "_code"))
            (construct (suffixed base "_make"))
            (type      `(closure ,(map (lambda (p) (list (second p))) arglist)
                                 ,ret-type))
            (closure   (register-closure-type! type form)))
       (values
        construct
        (map (lambda (f) (copy-form-source! form f))
             ;; C has no empty struct, and a closure over nothing needs
             ;; no record to point at
             (append
              (if (null? caps)
                  (list)
                  (list `(struct ,record ,caps)))

              (list
               `(fn ,code ((ƛe (* void)) ,@arglist) ,ret-type
                    ,@(if (null? caps)
                          (list)
                          `((var ƛcaptures (* (struct ,record)) ƛe)
                            ,@(map (lambda (cap)
                                     `(var ,(first cap) ,(second cap)
                                           (-> ƛcaptures ,(first cap))))
                                   caps)))
                    ,@body)

               `(fn ,construct ,caps (struct ,closure)
                    (var ƛc (struct ,closure))
                    ,@(if (null? caps)
                          (list)
                          `((static-assert
                             (<= (sizeof (struct ,record))
                                 (sizeof (dot-access ƛc env)))
                             "closure captures do not fit the inline environment")))
                    (= (dot-access ƛc code) ,code)
                    ,@(if (null? caps)
                          (list)
                          `((var ƛcaptures (* (struct ,record))
                                 (cast (& (dot-access ƛc env)) (* (struct ,record))))
                            ,@(map (lambda (cap)
                                     `(= (-> ƛcaptures ,(first cap)) ,(first cap)))
                                   caps)))
                    (return ƛc))))))))
    (else (sex-error form "malformed closure" form))))

;;; Structs

;;; Record the named structs, unions and enums in the type database
(define (process-struct sex-struct acc)
  (let-values (((doc form) (extract-aggregate-docstring sex-struct)))
    ;; Resolve before registering: a field of closure type has to reach
    ;; the type database as the struct it becomes, or member access
    ;; through it finds nothing
    (let ((form (resolve-closure-types form)))
      (register-aggregate! form)
      (with-docstring doc form acc))))

(define (register-aggregate! form)
  (let* ((f (if (eq? (car form) 'pub) (cdr form) form))
         (name (and (pair? (cdr f)) (symbol? (cadr f)) (cadr f))))
    ;; An anonymous aggregate has a field list where the name would be,
    ;; and nothing can refer to it by name anyway
    (when name
      (case (car f)
        ((struct) (add-struct name form))
        ((union) (add-union name form))
        ((enum) (add-enum name form))))))

(define (process-global-var sex-var acc)
  ;; A global is not walked for lambdas, but its type still has to stop
  ;; saying `closure' before the writer sees it
  (cons (resolve-closure-types sex-var) acc))

;;; Utils
(define (non-empty-list? form)
  (and (list? form)
       (not (null? form))))

(define (sex-fn? form)
  "The `form` must be toplevel.
Returns #f if the form is not a function, returns the form otherwise"
  (match form
    ((or ('fn . _)
         ('pub 'fn . _)
         ('extern 'fn . _)) form)
    (else #f)))

(define (sex-fn-public? fn-form)
  (eq? (first fn-form) 'pub))

(define (sex-fn-name fn-form)
  (assert (sex-fn? fn-form)
          (fmt #f "Form " fn-form " is not a function"))
  (second (fn-core fn-form)))

(define (sex-fn-arglist fn-form)
  (assert (sex-fn? fn-form)
          (fmt #f "Form " fn-form " is not a function"))
  (third (fn-core fn-form)))

(define (sex-fn-return-type fn-form)
  (assert (sex-fn? fn-form)
          (fmt #f "Form " fn-form " is not a function"))
  (fourth (fn-core fn-form)))

(define (sex-fn-prototype fn-form)
  "Returns all except body"
  (assert (sex-fn? fn-form)
          (fmt #f "Form " fn-form " is not a function"))
  (take fn-form (fn-header-length fn-form)))

(define (sex-fn-body fn-form)
  (assert (sex-fn? fn-form)
          (fmt #f "Form " fn-form " is not a function"))
  (drop fn-form (fn-header-length fn-form)))