;;; go-mode.el --- Major mode for the Go programming language

;;; Commentary:

;; For installation instructions, see go-mode-load.el

;;; To do:

;; * Indentation is *almost* identical to gofmt
;; ** We think struct literal keys are labels and outdent them
;; ** We disagree on the indentation of function literals in arguments
;; ** There are bugs with the close brace of struct literals
;; * Highlight identifiers according to their syntactic context: type,
;;   variable, function call, or tag
;; * Command for adding an import
;; ** Check if it's already there
;; ** Factor/unfactor the import line
;; ** Alphabetize
;; * Remove unused imports
;; ** This is hard, since I have to be aware of shadowing to do it
;;    right
;; * Format region using gofmt

;;; Code:

(eval-when-compile (require 'cl))

(defvar go-mode-syntax-table
  (let ((st (make-syntax-table)))
    ;; Symbols
    (modify-syntax-entry ?_  "_" st)

    ;; Operators (punctuation)
    (modify-syntax-entry ?+  "." st)
    (modify-syntax-entry ?-  "." st)
    (modify-syntax-entry ?*  "." st)
    (modify-syntax-entry ?/  "." st)
    (modify-syntax-entry ?%  "." st)
    (modify-syntax-entry ?&  "." st)
    (modify-syntax-entry ?|  "." st)
    (modify-syntax-entry ?^  "." st)
    (modify-syntax-entry ?!  "." st)
    (modify-syntax-entry ?=  "." st)
    (modify-syntax-entry ?<  "." st)
    (modify-syntax-entry ?>  "." st)

    ;; Strings
    (modify-syntax-entry ?\" "\"" st)
    (modify-syntax-entry ?\' "\"" st)
    (modify-syntax-entry ?`  "\"" st)
    (modify-syntax-entry ?\\ "\\" st)

    ;; Comments
    (modify-syntax-entry ?/  ". 124b" st)
    (modify-syntax-entry ?*  ". 23"   st)
    (modify-syntax-entry ?\n "> b"    st)
    (modify-syntax-entry ?\^m "> b"   st)

    st)
  "Syntax table for Go mode.")

(defvar go-mode-font-lock-keywords
  (let ((keywords '("import" "package"
                    "var" "const" "type" "func"
                    "struct" "interface"
                    "chan" "map"
                    "if" "else" "for" "switch" "select"
                    "range" "case" "default"
                    "return" "continue" "break" "fallthrough" "goto"
                    "go" "defer"))
        (builtins '("cap" "close" "closed" "len" "make" "new"
                    "panic" "panicln" "print" "println"))
        (constants '("nil" "true" "false" "iota"))
        (type-name "\\s *\\(?:[*(]\\s *\\)*\\(?:\\w+\\s *\\.\\s *\\)?\\(\\w+\\)")
        )
    `((,(regexp-opt keywords 'words) . font-lock-keyword-face)
      (,(regexp-opt builtins 'words) . font-lock-builtin-face)
      (,(regexp-opt constants 'words) . font-lock-constant-face)
      ;; Function names in declarations
      ("\\<func\\>\\s *\\(\\w+\\)" 1 font-lock-function-name-face)
      ;; Function names in methods are handled by function call pattern
      ;; Function names in calls
      ;; XXX Doesn't match if function name is surrounded by parens
      ("\\(\\w+\\)\\s *(" 1 font-lock-function-name-face)
      ;; Type names
      ("\\<type\\>\\s *\\(\\w+\\)" 1 font-lock-type-face)
      (,(concat "\\<type\\>\\s *\\w+\\s *" type-name) 1 font-lock-type-face)
      ;; Arrays/slices/map value type
      ;; XXX Wrong.  Marks 0 in expression "foo[0] * x"
;;      (,(concat "]" type-name) 1 font-lock-type-face)
      ;; Map key type
      (,(concat "\\<map\\s *\\[" type-name) 1 font-lock-type-face)
      ;; Channel value type
      (,(concat "\\<chan\\s *\\(?:<-\\)?" type-name) 1 font-lock-type-face)
      ;; new/make type
      (,(concat "\\<\\(?:new\\|make\\)\\>\\(?:\\s \\|)\\)*(" type-name) 1 font-lock-type-face)
      ;; Type conversion
      (,(concat "\\.\\s *(" type-name) 1 font-lock-type-face)
      ;; Method receiver type
      (,(concat "\\<func\\>\\s *(\\w+\\s +" type-name) 1 font-lock-type-face)
      ;; Labels
      ;; XXX Not quite right.  Also marks compound literal fields.
      ("^\\s *\\(\\w+\\)\\s *:\\(\\S.\\|$\\)" 1 font-lock-constant-face)
      ("\\<\\(goto\\|break\\|continue\\)\\>\\s *\\(\\w+\\)" 2 font-lock-constant-face)))
  "Basic font lock keywords for Go mode.  Highlights keywords,
built-ins, functions, and some types.")

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Key map
;;

(defvar go-mode-map
  (let ((m (make-sparse-keymap)))
    (define-key m "}" #'go-mode-insert-and-indent)
    (define-key m ")" #'go-mode-insert-and-indent)
    (define-key m ":" #'go-mode-delayed-electric)
    ;; In case we get : indentation wrong, correct ourselves
    (define-key m "=" #'go-mode-insert-and-indent)
    m)
  "Keymap used by Go mode to implement electric keys.")

(defun go-mode-insert-and-indent (key)
  "Invoke the global binding of KEY, then reindent the line."

  (interactive (list (this-command-keys)))
  (call-interactively (lookup-key (current-global-map) key))
  (indent-according-to-mode))

(defvar go-mode-delayed-point nil
  "The point following the previous insertion if the insertion
was a delayed electric key.  Used to communicate between
`go-mode-delayed-electric' and `go-mode-delayed-electric-hook'.")
(make-variable-buffer-local 'go-mode-delayed-point)

(defun go-mode-delayed-electric (p)
  "Perform electric insertion, but delayed by one event.

This inserts P into the buffer, as usual, then waits for another key.
If that second key causes a buffer modification starting at the
point after the insertion of P, reindents the line containing P."

  (interactive "p")
  (self-insert-command p)
  (setq go-mode-delayed-point (point)))

(defun go-mode-delayed-electric-hook (b e l)
  "An after-change-function that implements `go-mode-delayed-electric'."

  (when (and go-mode-delayed-point
             (= go-mode-delayed-point b))
    (save-excursion
      (save-match-data
        (goto-char go-mode-delayed-point)
        (indent-according-to-mode))))
  (setq go-mode-delayed-point nil))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Parser
;;

(defvar go-mode-mark-cs-end 1
  "The point at which the comment/string cache ends.  The buffer
will be marked from the beginning up to this point (that is, up
to and including character (1- go-mode-mark-cs-end)).")
(make-variable-buffer-local 'go-mode-mark-cs-end)

(defvar go-mode-mark-cs-state nil
  "The `parse-partial-sexp' state of the comment/string parser as
of the point `go-mode-mark-cs-end'.")
(make-variable-buffer-local 'go-mode-mark-cs-state)

(defvar go-mode-mark-nesting-end 1
  "The point at which the nesting cache ends.  The buffer will be
marked from the beginning up to this point.")
(make-variable-buffer-local 'go-mode-mark-nesting-end)

(defun go-mode-mark-clear-cache (b e l)
  "An after-change-function that clears the comment/string and
nesting caches from the modified point on."

  (when (< b go-mode-mark-cs-end)
    (remove-text-properties b (min go-mode-mark-cs-end (buffer-size)) '(go-mode-cs nil))
    (setq go-mode-mark-cs-end b
          go-mode-mark-cs-state nil))

  (when (< b go-mode-mark-nesting-end)
    (remove-text-properties b (min go-mode-mark-nesting-end (buffer-size)) '(go-mode-nesting nil))
    (setq go-mode-mark-nesting-end b)))

(defmacro go-mode-parser (&rest body)
  "Evaluate BODY in an environment set up for parsers that use
text properties to mark text.  This inhibits changes to the undo
list or the buffer's modification status and inhibits calls to
the modification hooks.  It also saves the excursion and
restriction and widens the buffer, since most parsers are
context-sensitive."

  (let ((modified-var (make-symbol "modified")))
    `(let ((buffer-undo-list t)
           (,modified-var (buffer-modified-p))
           (inhibit-modification-hooks t)
           (inhibit-read-only t))
       (save-excursion
         (save-restriction
           (widen)
           (unwind-protect
               (progn ,@body)
             (set-buffer-modified-p ,modified-var)))))))

(defsubst go-mode-cs (&optional pos)
  "Return the comment/string state at point POS.  If point is
inside a comment or string (including the delimiters), this
returns a pair (START . END) indicating the extents of the
comment or string."

  (unless pos
    (setq pos (point)))
  (if (= pos 1)
      nil
    (when (> pos go-mode-mark-cs-end)
      (go-mode-mark-cs pos))
    (get-text-property (- pos 1) 'go-mode-cs)))

(defun go-mode-mark-cs (end)
  "Mark comments and strings up to point END.  Don't call this
directly; use `go-mode-cs'."

  (setq end (min end (point-max)))
  (go-mode-parser
   (let* ((pos go-mode-mark-cs-end)
          (state (or go-mode-mark-cs-state (syntax-ppss pos))))
     ;; Mark comments and strings
     (when (nth 8 state)
       ;; Get to the beginning of the comment/string
       (setq pos (nth 8 state)
             state nil))
     (while (> end pos)
       ;; Find beginning of comment/string
       (while (and (> end pos)
                   (progn
                     (setq state (parse-partial-sexp pos end nil nil state 'syntax-table)
                           pos (point))
                     (not (nth 8 state)))))
       ;; Find end of comment/string
       (let ((start (nth 8 state)))
         (when start
           (setq state (parse-partial-sexp pos (point-max) nil nil state 'syntax-table)
                 pos (point))
           ;; Mark comment
           (put-text-property start (- pos 1) 'go-mode-cs (cons start pos))
           (when nil
             (put-text-property start (- pos 1) 'face
                                `((:background "midnight blue")))))))
     ;; Update state
     (setq go-mode-mark-cs-end   pos
           go-mode-mark-cs-state state))))

(defsubst go-mode-nesting (&optional pos)
  "Return the nesting at point POS.  The nesting is a list
of (START . END) pairs for all braces, parens, and brackets
surrounding POS, starting at the inner-most nesting.  START is
the location of the open character.  END is the location of the
close character or nil if the nesting scanner has not yet
encountered the close character."

  (unless pos
    (setq pos (point)))
  (if (= pos 1)
      '()
    (when (> pos go-mode-mark-nesting-end)
      (go-mode-mark-nesting pos))
    (get-text-property (- pos 1) 'go-mode-nesting)))

(defun go-mode-mark-nesting (pos)
  "Mark nesting up to point END.  Don't call this directly; use
`go-mode-nesting'."

  (go-mode-cs pos)
  (go-mode-parser
   ;; Mark depth
   (goto-char go-mode-mark-nesting-end)
   (let ((nesting (go-mode-nesting))
         (last (point)))
     (while (< last pos)
       ;; Find the next depth-changing character
       (skip-chars-forward "^(){}[]" pos)
       ;; Mark everything up to this character with the current
       ;; nesting
       (put-text-property last (point) 'go-mode-nesting nesting)
       (when nil
         (let ((depth (length nesting)))
           (put-text-property last (point) 'face
                              `((:background
                                 ,(format "gray%d" (* depth 10)))))))
       (setq last (point))
       ;; Update nesting
       (unless (eobp)
         (let ((ch (unless (go-mode-cs) (char-after))))
           (forward-char 1)
           (case ch
             ((?\( ?\{ ?\[)
              (setq nesting (cons (cons (- (point) 1) nil)
                                  nesting)))
             ((?\) ?\} ?\])
              (when nesting
                (setcdr (car nesting) (- (point) 1))
                (setq nesting (cdr nesting))))))))
     ;; Update state
     (setq go-mode-mark-nesting-end last))))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Indentation
;;

(defun go-mode-indentation ()
  "Compute the ideal indentation level of the current line.

To the first order, this is the brace depth of the current line,
plus parens that follow certain keywords.  case, default, and
labels are outdented one level, and continuation lines are
indented one level."

  (save-excursion
    (back-to-indentation)
    (let ((cs (go-mode-cs)))
      ;; Treat comments and strings differently only if the beginning
      ;; of the line is contained within them
      (when (and cs (= (point) (car cs)))
        (setq cs nil))
      ;; What type of context am I in?
      (cond
       ((and cs (save-excursion
                  (goto-char (car cs))
                  (looking-at "\\s\"")))
        ;; Inside a multi-line string.  Don't mess with indentation.
        nil)
       (cs
        ;; Inside a multi-line comment
        (goto-char (car cs))
        (forward-char 1)
        (current-column))
       ((not (go-mode-nesting))
        ;; Top-level
        (if (or (eolp)
                (looking-at "\\<\\(import\\|package\\|const\\|var\\|type\\|func\\)\\>")
                (looking-at "//\\|/\\*"))
            0
          ;; Continuation line
          ;; XXX If you start typing a new continuation line, nothing
          ;; will cause it to be indented.
          tab-width))
       (t
        ;; Neither top-level nor in a multi-line string or comment
        (let ((indent 0)
              (inside-indenting-paren nil))
          ;; Count every enclosing brace, plus parens that follow
          ;; import, const, var, or type and indent according to
          ;; depth.  This simple rule does quite well, but also has a
          ;; very large extent.  It would be better if we could mimic
          ;; some nearby indentation.
          (save-excursion
            (skip-chars-forward "})")
            (let ((first t))
              (dolist (nest (go-mode-nesting))
                (case (char-after (car nest))
                  ((?\{)
                   (incf indent tab-width))
                  ((?\()
                   (goto-char (car nest))
                   (forward-comment (- (buffer-size)))
                   ;; Really just want the token before
                   (when (looking-back "\\<import\\|const\\|var\\|type"
                                       (max (- (point) 7) (point-min)))
                     (incf indent tab-width)
                     (when first
                       (setq inside-indenting-paren t)))))
                (setq first nil))))

          ;; case, default, and labels are outdented 1 level
          (when (looking-at "\\<case\\>\\|\\<default\\>\\|\\w+\\s *:\\(\\S.\\|$\\)")
            (decf indent tab-width))

          ;; Continuation lines are indented 1 level
          (forward-comment (- (buffer-size)))
          (when (case (char-before)
                  ((?\{ ?\} ?\; ?:)
                   ;; Not a continuation line
                   nil)
                  ((?\()
                   ;; Usually a continuation line, unless this paren
                   ;; counted towards our indentation already
                   (not inside-indenting-paren))
                  ((?,)
                   ;; Could be inside a literal.  We're a little
                   ;; conservative here and consider any comma within
                   ;; curly braces (as opposed to parens) to be a
                   ;; literal separator.  This will fail to recognize
                   ;; line-breaks in parallel assignments as
                   ;; continuation lines.
                   (let ((depth (go-mode-nesting)))
                     (and depth
                          (not (eq (char-after (caar depth)) ?\{)))))
                  (t
                   ;; Anything else is always a continuation line
                   t))
            (incf indent tab-width))
          (max indent 0)))))))

(defun go-mode-indent-line ()
  "Indent the current line according to `go-mode-indentation'."
  (interactive)

  (let ((col (go-mode-indentation)))
    (when col
      (let ((offset (- (current-column) (current-indentation))))
        (indent-line-to col)
        (when (> offset 0)
          (forward-char offset))))))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Go mode
;;

;;;###autoload
(define-derived-mode go-mode nil "Go"
  "Major mode for editing Go source text.

This provides basic syntax highlighting for keywords, built-ins,
functions, and some types.  It also provides indentation that is
\(almost) identical to gofmt."

  ;; Font lock
  (set (make-local-variable 'font-lock-defaults)
       '(go-mode-font-lock-keywords nil nil nil nil))

  ;; Remove stale text properties
  (save-restriction
    (widen)
    (remove-list-of-text-properties 1 (+ (buffer-size) 1)
                                    '(go-mode-cs go-mode-nesting)))

  ;; Reset the syntax mark caches
  (setq go-mode-mark-cs-end      1
        go-mode-mark-cs-state    nil
        go-mode-mark-nesting-end 1)
  (add-hook 'after-change-functions #'go-mode-mark-clear-cache nil t)

  ;; Indentation
  (set (make-local-variable 'indent-line-function)
       #'go-mode-indent-line)
  (add-hook 'after-change-functions #'go-mode-delayed-electric-hook nil t)

  ;; Comments
  (setq comment-start "// "
        comment-end   "")

  ;; Go style
  (setq indent-tabs-mode t))

;;;###autoload
(add-to-list 'auto-mode-alist (cons "\\.go$" #'go-mode))

(defun go-mode-reload ()
  "Reload go-mode.el and put the current buffer into Go mode.
Useful for development work."

  (interactive)
  (unload-feature 'go-mode)
  (require 'go-mode)
  (go-mode))

(provide 'go-mode)