;;; myrddin-mode.el --- Major mode for editing Myrddin source files -*- lexical-binding: t; -*-

;; Copyright (C) 2019 Jakob L. Kreuze

;; Author: Jakob L. Kreuze <zerodaysfordays@sdf.lonestar.org>
;; Version: 0.1
;; Package-Requires: ((emacs "24.3"))
;; Keywords: languages
;; URL: https://git.sr.ht/~jakob/myrddin-mode

;; This program is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or (at
;; your option) any later version.

;; This program is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; myrddin-mode provides support for editing Myrddin, including automatic
;; indentation and syntactical font-locking.

;;; Code:

(require 'rx)

(defgroup myrddin nil
  "Support for Myrddin code."
  :link '(url-link "https://git.sr.ht/~jakob/myrddin-mode")
  :group 'languages)

(defcustom myrddin-indent-offset 8
  "Indent Myrddin code by this number of spaces."
  :type 'integer
  :group 'myrddin
  :safe #'integerp)


;;;
;;; Syntax tables.
;;;

(defvar myrddin-mode-syntax-table
  (let ((table (make-syntax-table)))
    ;; Operators.
    (dolist (i '(?+ ?- ?* ?/ ?% ?& ?| ?^ ?! ?< ?> ?~ ?@))
      (modify-syntax-entry i "." table))

    ;; Strings.
    (modify-syntax-entry ?\" "\"" table)
    (modify-syntax-entry ?\\ "\\" table)

    ;; Angle brackets.
    (modify-syntax-entry ?< "(>" table)
    (modify-syntax-entry ?> ")<" table)

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

    table))


;;;
;;; Font locking.
;;;

(defconst myrddin-mode-keywords
  '("$noret"
    "break"
    "const" "continue"
    "elif" "else" "extern"
    "for"
    "generic" "goto"
    "if" "impl" "in"
    "match"
    "pkg" "pkglocal"
    "sizeof" "struct"
    "trait" "type"
    "union"
    "use"
    "var"
    "while"))

(defconst myrddin-mode-constants
  '("true" "false" "void"))

(defconst myrddin-mode-block-keywords
  '("elif" "else" "for" "if" "match" "struct" "trait" "while"))

(defconst myrddin-mode-identifier-rx
  '(and (or alpha ?_)
        (one-or-more (or alnum ?_)))
  "`rx' expression matching a Myrddin identifier.")

(defconst myrddin-mode-type-name-rx
  '(and
    (eval myrddin-mode-identifier-rx)
    (optional (or (and ?[ (zero-or-more any) ?])
                  ?#)))
  "`rx' expression matching a Myrddin type specification.
Some examples include \"int\", \"foo#\", and \"foo[:]\".")

(defconst myrddin-mode-variable-declaration-regexp
  (rx
   (and (or "const" "var" "generic")
        (zero-or-more (or "extern" "pkglocal" "#noret"))
        (and (one-or-more whitespace)
             (group
              symbol-start
              (eval myrddin-mode-identifier-rx)
              symbol-end)))))

(defconst myrddin-mode-type-specification-regexp
  (rx
   (and (or ?: "->")
        (zero-or-more whitespace)
        (group (eval myrddin-mode-type-name-rx)))))

(defconst myrddin-mode-label-regexp
  (rx (and ?: (eval myrddin-mode-identifier-rx) symbol-end)))

(defvar myrddin-mode-font-lock-keywords
  `((,(regexp-opt myrddin-mode-keywords 'symbols) . font-lock-keyword-face)
    (,(regexp-opt myrddin-mode-constants 'symbols) . font-lock-constant-face)
    (,myrddin-mode-label-regexp . font-lock-constant-face)
    (,myrddin-mode-type-specification-regexp 1 font-lock-type-face)
    (,myrddin-mode-variable-declaration-regexp 1 font-lock-variable-name-face)))


;;;
;;; Indentation.
;;;

(defun myrddin-mode-indent-line ()
  "Indent current line for Myrddin mode.
Return the amount the indentation changed by."
  (interactive)
  (let* ((prev-level (save-excursion
                       (previous-line)
                       (back-to-indentation)
                       (/ (current-column) myrddin-indent-offset)))
         (prev-line (save-excursion
                      (previous-line)
                      (back-to-indentation)
                      (buffer-substring-no-properties
                       (line-beginning-position)
                       (line-end-position)))))
    (let ((indent
           (save-excursion
             (back-to-indentation)
             (cond
              ((looking-at
                (rx (or "}" ";;" "elif" "else")))
               (max 0 (* myrddin-indent-offset (1- prev-level))))
              ((string-match-p
                (rx (eval (cons 'or (cons "{" myrddin-mode-block-keywords))))
                prev-line)
               (* myrddin-indent-offset (1+ prev-level)))
              (t (* myrddin-indent-offset prev-level))))))

      (if (<= (current-column) (current-indentation))
          (indent-line-to indent)
        (save-excursion (indent-line-to indent)))
      indent)))


;;;
;;; Mode declaration.
;;;

;;;###autoload
(define-derived-mode myrddin-mode prog-mode "Myrdin"
  "Major mode for Myrddin code."
  :group 'myrddin
  :syntax-table myrddin-mode-syntax-table
  (setq-local font-lock-defaults '(myrddin-mode-font-lock-keywords
                                   nil nil nil nil))
  (setq-local indent-line-function 'myrddin-mode-indent-line)
  (setq comment-start "/*")
  (setq comment-end "*/"))

;;;###autoload
(add-to-list 'auto-mode-alist '("\\.myr\\'" . myrddin-mode))

(provide 'myrddin-mode)
;;; myrddin-mode.el ends here