;;; json-sexp-mode.el --- Edit JSON in s-expression form.

;; Copyright (C) 2015  Free Software Foundation, Inc.

;; Author: Taylan Ulrich Bayırlı/Kammer <taylanbayirli@gmail.com>
;; Keywords: data, files

;; 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:

;; Activating `json-sexp-mode' will turn the JSON value in the buffer into an
;; s-expression, and arrange for it to be transparently converted back into a
;; (pretty-printed) JSON value whenever the buffer is saved.  Adding
;; `json-sexp-mode' to `auto-mode-alist' can provide a fully transparent
;; experience for editing JSON files as s-expressions.
;;
;; The symbol `false' is used to represent JSON's false value, because nil
;; stands for an empty object.  (However, t is used for JSON's true value.)
;;
;; Note that `json-sexp-mode' inherits from `emacs-lisp-mode' since it's for
;; editing JSON data as Elisp s-expressions.
;;
;;
;; The following commands can be used to manually convert a region or whole
;; buffer from a JSON value to an s-expression and from an s-expression to a
;; JSON value:
;;
;; `json-sexp-convert-region-to-sexp'
;; `json-sexp-convert-region-to-json'
;; `json-sexp-convert-buffer-to-sexp'
;; `json-sexp-convert-buffer-to-json'

;;; Code:
(require 'json)

(defun json-sexp-convert-region-to-sexp (start end)
  "Convert region from JSON to sexps."
  (interactive "r")
  (unless (= start end)
    (let ((data (let ((json-object-type 'alist)
                      (json-false 'false)
                      (json-null 'null))
                  (json-read-from-string (buffer-substring start end))))
          (point (point)))
      (delete-region start end)
      (goto-char start)
      (insert (pp-to-string data))
      ;; Only an approximation of our position in the JSON.
      (goto-char point))))

(defun json-sexp-convert-region-to-json (start end)
  "Convert region from sexps to JSON."
  (interactive "r")
  (unless (= start end)
    (let ((data (car (read-from-string (buffer-substring start end))))
          (point (point)))
      (delete-region start end)
      (goto-char start)
      (insert (let ((json-encoding-pretty-print t)
                    (json-false 'false)
                    (json-null 'null)
                    (json-nil 'object))
                (json-encode data)))
      ;; Only an approximation of our position in the s-expression.
      (goto-char point))))

(defun json-sexp-convert-buffer-to-sexp ()
  "Convert buffer from JSON to sexps."
  (interactive)
  (json-sexp-convert-region-to-sexp (point-min) (point-max)))

(defun json-sexp-convert-buffer-to-json ()
  "Convert buffer from sexps to JSON."
  (interactive)
  (json-sexp-convert-region-to-json (point-min) (point-max)))

(defvar json-sexp--saved-point nil)

(define-derived-mode json-sexp-mode emacs-lisp-mode "JSON-sexp"
  "Major mode for editing JSON in s-expression form.
The buffer-contents, which must be JSON, are transformed to
s-expressions when this mode is started, and transformed back
temporarily to JSON whenever the buffer is saved."
  (let ((was-modified (buffer-modified-p)))
    (json-sexp-convert-buffer-to-sexp)
    (set-buffer-modified-p was-modified))
  (add-hook 'before-save-hook 'json-sexp--before-save nil t)
  (add-hook 'after-save-hook 'json-sexp--after-save nil t))

(defun json-sexp--before-save ()
  (setq json-sexp--saved-point (point))
  (json-sexp-convert-buffer-to-json))

(defun json-sexp--after-save ()
  (json-sexp-convert-buffer-to-sexp)
  (set-buffer-modified-p nil)
  (goto-char json-sexp--saved-point))

(provide 'json-sexp-mode)
;;; json-sexp-mode.el ends here