apheleia.el

;;; apheleia.el --- Reformat buffer stably -*- lexical-binding: t -*-

;; Copyright (C) 2019 Radon Rosborough

;; Author: Radon Rosborough <radon.neon@gmail.com>
;; Created: 7 Jul 2019
;; Homepage: https://github.com/raxod502/apheleia
;; Keywords: tools
;; Package-Requires: ((emacs "25.2"))
;; SPDX-License-Identifier: MIT
;; Version: 1.1.1

;;; Commentary:

;; Apheleia is an Emacs Lisp package which allows you to reformat a
;; buffer without moving point. This solves the usual problem of
;; running a tool like Prettier or Black on `before-save-hook', namely
;; that it resets point to the beginning of the buffer. Apheleia
;; maintains the position of point relative to its surrounding text
;; even if the buffer is modified by the reformatting.

;; Please see https://github.com/raxod502/apheleia for more information.

;;; Code:

(require 'cl-lib)
(require 'map)
(require 'subr-x)

(defgroup apheleia nil
  "Reformat buffer without moving point."
  :group 'external
  :link '(url-link :tag "GitHub" "https://github.com/raxod502/apheleia")
  :link '(emacs-commentary-link :tag "Commentary" "apheleia"))

(cl-defun apheleia--edit-distance-table (s1 s2)
  "Align strings S1 and S2 for minimum edit distance.
Return the dynamic programming table as has table which maps cons
of integers (I1 . I2) to the edit distance between the first I1
characters of S1 and the first I2 characters of S2."
  (let ((table (make-hash-table :test #'equal)))
    (dotimes (i1 (1+ (length s1)))
      (puthash (cons i1 0) i1 table))
    (dotimes (i2 (1+ (length s2)))
      (puthash (cons 0 i2) i2 table))
    (dotimes (i1 (length s1))
      ;; Iterate from 1 to length+1.
      (cl-incf i1)
      (dotimes (i2 (length s2))
        (cl-incf i2)
        (let ((ins (1+ (gethash (cons i1 (1- i2)) table)))
              (del (1+ (gethash (cons (1- i1) i2) table)))
              (sub (gethash (cons (1- i1) (1- i2)) table)))
          (unless (= (aref s1 (1- i1)) (aref s2 (1- i2)))
            (cl-incf sub))
          (puthash (cons i1 i2) (min ins del sub) table))))
    table))

(defun apheleia--align-point (s1 s2 p1)
  "Given strings S1 and S2 and index P1 in S1, return matching index P2 in S2.
If S1 and S2 are the same, then P1 and P2 will also be the same.
Otherwise, the text of S2 surrounding P2 is \"similar\" to the
text of S1 surrounding P1."
  (let* ((table (apheleia--edit-distance-table s1 s2))
         (i1 (length s1))
         (i2 (length s2)))
    (while (> i1 p1)
      (let ((ins (1+ (gethash (cons i1 (1- i2)) table)))
            (del (1+ (gethash (cons (1- i1) i2) table)))
            (sub (gethash (cons (1- i1) (1- i2)) table)))
        (unless (= (aref s1 (1- i1)) (aref s2 (1- i2)))
          (cl-incf sub))
        (let ((cost (min ins del sub)))
          (cond
           ((= cost ins)
            (cl-decf i2))
           ((= cost del)
            (cl-decf i1))
           ((= cost sub)
            (cl-decf i1)
            (cl-decf i2))))
        ))
    i2))

(defun apheleia--map-rcs-patch (func)
  "Map over the RCS patch in the current buffer.
For each RCS patch command, FUNC is called with an alist that has
the following keys:

- `command': either `addition' or `deletion'
- `start': line number, an integer
- `lines': number of lines to be inserted or removed
- `text': the string to be inserted, only for `addition'

See <https://tools.ietf.org/doc/tcllib/html/rcs.html#section4>
for documentation on the RCS patch format."
  (save-excursion
    (goto-char (point-min))
    (while (not (= (point) (point-max)))
      (unless (looking-at "$\\|\\([ad]\\)\\([0-9]+\\) \\([0-9]+\\)")
        (error "Malformed RCS patch: %S" (point)))
      (forward-line)
      (when-let ((command (match-string 1)))
        (let ((start (string-to-number (match-string 2)))
              (lines (string-to-number (match-string 3))))
          (pcase command
            ("a"
             (let ((text-start (point)))
               (forward-line lines)
               (funcall
                func
                `((command . addition)
                  (start . ,start)
                  (lines . ,lines)
                  (text . ,(buffer-substring-no-properties
                            text-start (point)))))))
            ("d"
             (funcall
              func
              `((command . deletion)
                (start . ,start)
                (lines . ,lines))))))))))

(defcustom apheleia-max-alignment-size 400
  "Maximum size for diff regions that will have point aligned.
Apheleia uses a dynamic programming algorithm to determine where
point should be placed within a diff region, but this algorithm
has quadratic runtime so it will lock up Emacs if it is run on a
diff region that is too large. The value of this variable serves
as a limit on the input size to the algorithm; larger diff
regions will still be applied, but Apheleia won't try to move
point correctly."
  :type 'integer)

(defun apheleia--apply-rcs-patch (content-buffer patch-buffer)
  "Apply RCS patch.
CONTENT-BUFFER contains the text to be patched, and PATCH-BUFFER
contains the patch."
  (let ((commands nil)
        (point-list nil)
        (window-line-list nil))
    (with-current-buffer content-buffer
      (push (cons nil (point)) point-list)
      (dolist (w (get-buffer-window-list nil nil t))
        (push (cons w (window-point w)) point-list)
        (push (cons w (count-lines (window-start w) (point)))
              window-line-list)))
    (with-current-buffer patch-buffer
      (apheleia--map-rcs-patch
       (lambda (command)
         (with-current-buffer content-buffer
           ;; Could be optimized significantly by moving only as many
           ;; lines as needed, rather than returning to the beginning
           ;; of the buffer first.
           (save-excursion
             (goto-char (point-min))
             (forward-line (1- (alist-get 'start command)))
             ;; Account for the off-by-one error in the RCS patch spec
             ;; (namely, text is added *after* the line mentioned in
             ;; the patch).
             (when (eq (alist-get 'command command) 'addition)
               (forward-line))
             (push `(marker . ,(point-marker)) command)
             (push command commands)
             ;; If we delete a region just before inserting new text
             ;; at the same place, then it is a replacement. In this
             ;; case, check if the replaced region includes the window
             ;; point for any window currently displaying the content
             ;; buffer. If so, figure out where that window point
             ;; should be moved to, and record the information in an
             ;; additional command.
             ;;
             ;; See <https://www.gnu.org/software/emacs/manual/html_node/elisp/Window-Point.html>.
             ;;
             ;; Note that the commands get pushed in reverse order
             ;; because of how linked lists work.
             (let ((deletion (nth 1 commands))
                   (addition (nth 0 commands)))
               (when (and (eq (alist-get 'command deletion) 'deletion)
                          (eq (alist-get 'command addition) 'addition)
                          ;; Again with the weird off-by-one
                          ;; computations. For example, if you replace
                          ;; lines 68 through 71 inclusive, then the
                          ;; deletion is for line 68 and the addition
                          ;; is for line 70. Blame RCS.
                          (= (+ (alist-get 'start deletion)
                                (alist-get 'lines deletion)
                                -1)
                             (alist-get 'start addition)))
                 (let ((text-start (alist-get 'marker deletion)))
                   (forward-line (alist-get 'lines deletion))
                   (let ((text-end (point)))
                     (dolist (entry point-list)
                       ;; Check if the (window) point is within the
                       ;; replaced region.
                       (cl-destructuring-bind (w . p) entry
                         (when (and (< text-start p)
                                    (< p text-end))
                           (let* ((old-text (buffer-substring-no-properties
                                             text-start text-end))
                                  (new-text (alist-get 'text addition))
                                  (old-relative-point (- p text-start))
                                  (new-relative-point
                                   (if (> (max (length old-text)
                                               (length new-text))
                                          apheleia-max-alignment-size)
                                       old-relative-point
                                     (apheleia--align-point
                                      old-text new-text old-relative-point))))
                             (goto-char text-start)
                             (push `((marker . ,(point-marker))
                                     (command . set-point)
                                     (window . ,w)
                                     (relative-point . ,new-relative-point))
                                   commands))))))))))))))
    (with-current-buffer content-buffer
      (let ((move-to nil))
        (save-excursion
          (dolist (command (nreverse commands))
            (goto-char (alist-get 'marker command))
            (pcase (alist-get 'command command)
              (`addition
               (insert (alist-get 'text command)))
              (`deletion
               (let ((text-start (point)))
                 (forward-line (alist-get 'lines command))
                 (delete-region text-start (point))))
              (`set-point
               (let ((new-point
                      (+ (point) (alist-get 'relative-point command))))
                 (if-let ((w (alist-get 'window command)))
                     (set-window-point w new-point)
                   (setq move-to new-point)))))))
        (when move-to
          (goto-char move-to))))
    ;; Restore the scroll position of each window displaying the
    ;; buffer.
    (dolist (entry window-line-list)
      (cl-destructuring-bind (w . old-window-line) entry
        (let ((new-window-line
               (count-lines (window-start w) (point))))
          (with-selected-window w
            ;; Sometimes if the text is less than a buffer long, and
            ;; we do a deletion, it might not be possible to keep the
            ;; vertical position of point the same by scrolling.
            ;; That's okay. We just go as far as we can.
            (ignore-errors
              (scroll-down (- old-window-line new-window-line)))))))))

(defvar apheleia--current-process nil
  "Current process that Apheleia is running, or nil.
Keeping track of this helps avoid running more than one process
at once.")

(cl-defun apheleia--make-process (&key command stdin callback exit-status)
  "Wrapper for `make-process' that behaves a bit more nicely.
COMMAND is as in `make-process'. STDIN, if given, is a buffer
whose contents are fed to the process on stdin. CALLBACK is
invoked with one argument, the buffer containing the text from
stdout, when the process terminates (if it succeeds). EXIT-STATUS
is a function which is called with the exit status of the
command; it should return non-nil to indicate that the command
succeeded. If EXIT-STATUS is omitted, then the command succeeds
provided that its exit status is 0."
  (when (process-live-p apheleia--current-process)
    (interrupt-process apheleia--current-process)
    (accept-process-output apheleia--current-process 0.1 nil 'just-this-one)
    (when (process-live-p apheleia--current-process)
      (kill-process apheleia--current-process)))
  (let* ((name (file-name-nondirectory (car command)))
         (stdout (get-buffer-create
                  (format " *apheleia-%s-stdout*" name)))
         (stderr (get-buffer-create
                  (format " *apheleia-%s-stderr*" name))))
    (dolist (buf (list stdout stderr))
      (with-current-buffer buf
        (erase-buffer)))
    (condition-case-unless-debug e
        (progn
          (setq apheleia--current-process
                (make-process
                 :name (format "aphelieia-%s" name)
                 :buffer stdout
                 :stderr stderr
                 :command command
                 :noquery t
                 :sentinel
                 (lambda (proc _event)
                   (unless (process-live-p proc)
                     (with-current-buffer stderr
                       (when (= 0 (buffer-size))
                         (insert "[No output received on stderr]\n")))
                     (if (funcall
                          (or exit-status
                              (lambda (status)
                                (= 0 status)))
                          (process-exit-status proc))
                         (when callback
                           (funcall callback stdout))
                       (message
                        (concat
                         "Failed to run %s: exit status %s "
                         "(see hidden buffer%s)")
                        (car command)
                        (process-exit-status proc)
                        stderr))))))
          (set-process-sentinel (get-buffer-process stderr) #'ignore)
          (set-process-coding-system
           apheleia--current-process
           nil
           (buffer-local-value 'buffer-file-coding-system stdin))
          (when stdin
            (process-send-string
             apheleia--current-process
             (with-current-buffer stdin
               (buffer-string))))
          (process-send-eof apheleia--current-process))
      (error (message "Failed to run %s: %s" name (error-message-string e))))))

(defun apheleia--write-region-silently
    (start end filename &optional
           append visit lockname mustbenew write-region)
  "Like `write-region', but silent.
START, END, FILENAME, APPEND, VISIT, LOCKNAME, and MUSTBENEW are
as in `write-region'. WRITE-REGION is used instead of the actual
`write-region' function, if provided."
  (funcall (or write-region #'write-region)
           start end filename append 0 lockname mustbenew)
  (when (or (eq visit t) (stringp visit))
    (setq buffer-file-name (if (eq visit t)
                               filename
                             visit))
    (set-visited-file-modtime)
    (set-buffer-modified-p nil)))

(defun apheleia--write-file-silently (&optional filename)
  "Write contents of current buffer into file FILENAME, silently.
FILENAME defaults to value of variable `buffer-file-name'. Do not
mark the buffer as visiting FILENAME."
  (cl-letf* ((write-region (symbol-function #'write-region))
             ((symbol-function #'write-region)
              (lambda (start end filename &optional
                             append _visit lockname mustbenew)
                (apheleia--write-region-silently
                 start end filename append t lockname mustbenew write-region)))
             (message (symbol-function #'message))
             ((symbol-function #'message)
              (lambda (format &rest args)
                (unless (equal format "Saving file %s...")
                  (apply message format args)))))
    ;; Avoid infinite loop.
    (let ((after-save-hook
           (remq #'apheleia--format-after-save after-save-hook)))
      (write-file (or filename buffer-file-name)))))

(defun apheleia--create-rcs-patch (old-buffer new-buffer callback)
  "Generate RCS patch from text in OLD-BUFFER to text in NEW-BUFFER.
Once finished, invoke CALLBACK with a buffer containing the patch
as its sole argument."
  ;; Make sure at least one of the two buffers is saved to a file. The
  ;; other one we can feed on stdin.
  (let ((old-fname
         (with-current-buffer old-buffer
           (and (not (buffer-modified-p)) buffer-file-name)))
        (new-fname
         (with-current-buffer new-buffer
           (and (not (buffer-modified-p)) buffer-file-name))))
    (unless (or old-fname new-fname)
      (with-current-buffer new-buffer
        (setq new-fname (make-temp-file "apheleia"))
        (apheleia--write-region-silently (point-min) (point-max) new-fname)))
    (with-current-buffer (get-buffer-create " *apheleia-patch*")
      (erase-buffer)
      (apheleia--make-process
       :command `("diff" "--rcs" "--"
                  ,(or old-fname "-")
                  ,(or new-fname "-"))
       :stdin (if new-fname old-buffer new-buffer)
       :callback callback
       :exit-status (lambda (status)
                      ;; Exit status is 0 if no changes, 1 if some
                      ;; changes, and 2 if error.
                      (memq status '(0 1)))))))

(defun apheleia--run-formatter (command callback)
  "Run a code formatter on the current buffer.
The formatter is specified by COMMAND, a list of strings or
symbols (see `apheleia-format-buffer'). Invoke CALLBACK with one
argument, a buffer containing the output of the formatter.

If COMMAND uses the symbol `file' and the current buffer is
modified from what is written to disk, then don't do anything."
  (cl-block nil
    (let ((input-fname nil)
          (output-fname nil)
          (npx nil))
      (when (memq 'npx command)
        (setq npx t)
        (setq command (remq 'npx command)))
      (unless (stringp (car command))
        (error "Command cannot start with %S" (car command)))
      (when npx
        (when-let ((project-dir
                    (locate-dominating-file
                     default-directory "node_modules")))
          (let ((binary
                 (expand-file-name
                  (car command)
                  (expand-file-name
                   ".bin"
                   (expand-file-name
                    "node_modules"
                    project-dir)))))
            (when (file-executable-p binary)
              (setcar command binary)))))
      (when (or (memq 'file command) (memq 'filepath command))
        (setq command (mapcar (lambda (arg)
                                (if (memq arg '(file filepath))
                                    (prog1 buffer-file-name
                                      (when (buffer-modified-p)
                                        (cl-return)))
                                  arg))
                              command)))
      (when (memq 'input command)
        (let ((input-fname (make-temp-file
                            "apheleia" nil
                            (and buffer-file-name
                                 (file-name-extension
                                  buffer-file-name 'period)))))
          (apheleia--write-region-silently nil nil input-fname)
          (setq command (mapcar (lambda (arg)
                                  (if (eq arg 'input)
                                      input-fname
                                    arg))
                                command))))
      (when (memq 'output command)
        (setq output-fname (make-temp-file "apheleia"))
        (setq command (mapcar (lambda (arg)
                                (if (eq arg 'output)
                                    output-fname
                                  arg))
                              command)))
      (apheleia--make-process
       :command command
       :stdin (unless input-fname
                (current-buffer))
       :callback (lambda (stdout)
                   (when output-fname
                     (erase-buffer)
                     (insert-file-contents-literally output-fname))
                   (funcall callback stdout))))))

(defcustom apheleia-formatters
  '((black . ("black" "-"))
    (brittany . ("brittany" file))
    (prettier . (npx "prettier" "--stdin-filepath" filepath))
    (gofmt . ("gofmt"))
    (ocamlformat . ("ocamlformat" file))
    (terraform . ("terraform" "fmt" "-")))
  "Alist of code formatting commands.
The keys may be any symbols you want, and the values are
commands, lists of strings and symbols, in the format of
`apheleia-format-buffer' (which see)."
  :type '(alist
          :key-type symbol
          :value-type
          (repeat
           (choice
            (string :tag "Argument")
            (const :tag "Name of temporary file used for input" input)
            (const :tag "Name of temporary file used for output" output)))))

(defcustom apheleia-mode-alist
  '((css-mode . prettier)
    (go-mode . gofmt)
    (js-mode . prettier)
    (js3-mode . prettier)
    (json-mode . prettier)
    (html-mode . prettier)
    (python-mode . black)
    (sass-mode . prettier)
    (typescript-mode . prettier)
    (web-mode . prettier)
    (yaml-mode . prettier)
    (terraform-mode . terraform)
    (tuareg-mode . ocamlformat)
    (haskell-mode . brittany))
  "Alist mapping major mode names to formatters to use in those modes.
This determines what formatter to use in buffers without a
setting for `apheleia-formatter'. The keys are major mode
symbols (matched against `major-mode' with `derived-mode-p') or
strings (matched against value of variable `buffer-file-name'
with `string-match-p'), and the values are symbols with entries
in `apheleia-formatters' (or equivalently, they are allowed
values for `apheleia-formatter'). Earlier entries take precedence
over later ones.

Be careful when writing regexps to include \"\\'\" and to escape
\"\\.\" in order to properly match a file extension. For example,
to match \".jsx\" files you might use \"\\.jsx\\'\"."
  :type '(alist
          :key-type symbol
          :value-type symbol))

(defvar-local apheleia-formatter nil
  "Name of formatter to use in current buffer, a symbol or nil.
If non-nil, then `apheleia-formatters' should have a matching
entry. This overrides `apheleia-mode-alist'.")

(defun apheleia--get-formatter-command (&optional interactive)
  "Return the formatter command to use for the current buffer.
This is a value suitable for `apheleia--run-formatter', or nil if
no formatter is configured for the current buffer. Consult the
values of `apheleia-mode-alist' and `apheleia-formatter' to
determine which formatter is configured.

If INTERACTIVE is non-nil, then prompt the user for which
formatter to run if none is configured, instead of returning nil.
If INTERACTIVE is the special symbol `prompt', then prompt
even if a formatter is configured."
  (when-let ((formatter
              (or (and (not (eq interactive 'prompt))
                       (or apheleia-formatter
                           (cl-dolist (entry apheleia-mode-alist)
                             (when (or (and (symbolp (car entry))
                                            (derived-mode-p (car entry)))
                                       (and (stringp (car entry))
                                            buffer-file-name
                                            (string-match-p
                                             (car entry) buffer-file-name)))
                               (cl-return (cdr entry))))))
                  (and interactive
                       (intern
                        (completing-read
                         "Formatter: "
                         (or (map-keys apheleia-formatters)
                             (user-error
                              "No formatters in `apheleia-formatters'"))
                         nil 'require-match))))))
    (or (alist-get formatter apheleia-formatters)
        (user-error "No configuration for formatter `%S'"
                    formatter))))

(defun apheleia--buffer-hash ()
  "Compute hash of current buffer."
  (if (fboundp 'buffer-hash)
      (buffer-hash)
    (md5 (current-buffer))))

(defvar apheleia--buffer-hash nil
  "Return value of `buffer-hash' when formatter started running.")

(defun apheleia--disallowed-p ()
  "Return an error message if Apheleia cannot be run, else nil."
  (when (and buffer-file-name (file-remote-p buffer-file-name))
    "Apheleia does not support remote files"))

;;;###autoload
(defun apheleia-format-buffer (command &optional callback)
  "Run code formatter asynchronously on current buffer, preserving point.

Interactively, run the currently configured formatter (see
`apheleia-formatter' and `apheleia-mode-alist'), or prompt from
`apheleia-formatters' if there is none configured for the current
buffer. With a prefix argument, prompt always.

In Lisp code, COMMAND is similar to what you pass to
`make-process', except as follows. Normally, the contents of the
current buffer are passed to the command on stdin, and the output
is read from stdout. However, if you use the symbol `file' as one
of the elements of COMMAND, then the filename of the current
buffer is substituted for it. (Use `filepath' instead of `file'
if you need the filename of the current buffer, but you still
want its contents to be passed on stdin.) If you instead use the
symbol `input' as one of the elements of COMMAND, then the
contents of the current buffer are written to a temporary file
and its name is substituted for `input'. Also, if you use the
symbol `output' as one of the elements of COMMAND, then it is
substituted with the name of a temporary file. In that case, it
is expected that the command writes to that file, and the file is
then read into an Emacs buffer. Finally, if you use the symbol
`npx' as one of the elements of COMMAND, then the first string
element of COMMAND is resolved inside node_modules/.bin if such a
directory exists anywhere above the current `default-directory'.

In any case, after the formatter finishes running, the diff
utility is invoked to determine what changes it made. That diff
is then used to apply the formatter's changes to the current
buffer without moving point or changing the scroll position in
any window displaying the buffer. If the buffer has been modified
since the formatter started running, however, the operation is
aborted.

If the formatter actually finishes running and the buffer is
successfully updated (even if the formatter has not made any
changes), CALLBACK, if provided, is invoked with no arguments."
  (interactive (progn
                 (when-let ((err (apheleia--disallowed-p)))
                   (user-error err))
                 (list (apheleia--get-formatter-command
                        (if current-prefix-arg
                            'prompt
                          'interactive)))))
  ;; Fail silently if disallowed, since we don't want to throw an
  ;; error on `post-command-hook'.
  (unless (apheleia--disallowed-p)
    (setq-local apheleia--buffer-hash (apheleia--buffer-hash))
    (let ((cur-buffer (current-buffer)))
      (apheleia--run-formatter
       command
       (lambda (formatted-buffer)
         (with-current-buffer cur-buffer
           ;; Short-circuit.
           (when (equal apheleia--buffer-hash (apheleia--buffer-hash))
             (apheleia--create-rcs-patch
              (current-buffer) formatted-buffer
              (lambda (patch-buffer)
                (with-current-buffer cur-buffer
                  (when (equal apheleia--buffer-hash (apheleia--buffer-hash))
                    (apheleia--apply-rcs-patch
                     (current-buffer) patch-buffer)
                    (when callback
                      (funcall callback)))))))))))))

(defcustom apheleia-post-format-hook nil
  "Normal hook run after Apheleia formats a buffer."
  :type 'hook)

;; Handle recursive references.
(defvar apheleia-mode)

;; Autoload because the user may enable `apheleia-mode' without
;; loading Apheleia; thus this function may be invoked as an autoload.
;;;###autoload
(defun apheleia--format-after-save ()
  "Run code formatter for current buffer if any configured, then save."
  (when apheleia-mode
    (when-let ((command (apheleia--get-formatter-command)))
      (apheleia-format-buffer
       command
       (lambda ()
         (with-demoted-errors "Apheleia: %s"
           (apheleia--write-file-silently buffer-file-name)
           (run-hooks 'apheleia-post-format-hook)))))))

;; Use `progn' to force the entire minor mode definition to be copied
;; into the autoloads file, so that the minor mode can be enabled
;; without pulling in all of Apheleia during init.
;;;###autoload
(progn

  (define-minor-mode apheleia-mode
    "Minor mode for reformatting code on save without moving point.
It is customized by means of the variables `apheleia-mode-alist'
and `apheleia-formatters'."
    :lighter " Apheleia"
    (if apheleia-mode
        (add-hook 'after-save-hook #'apheleia--format-after-save nil 'local)
      (remove-hook 'after-save-hook #'apheleia--format-after-save 'local)))

  (define-globalized-minor-mode apheleia-global-mode
    apheleia-mode apheleia-mode)

  (put 'apheleia-mode 'safe-local-variable #'booleanp))

(provide 'apheleia)

;; Local Variables:
;; outline-regexp: ";;;;* "
;; End:

;;; apheleia.el ends here