Improve point restoration of `cider--format-buffer`

Fix #2126

Also make `cider--format-region` the base format function and reduce duplication
in others, like `cider--format-buffer`.

2 files changed, 68 insertions, 42 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5606bdb7..7e853e02 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,7 +13,7 @@
 * [cider-nrepl#438](https://github.com/clojure-emacs/cider-nrepl/pull/438): Improve startup time by deferring loading CIDER's middleware until the first usage.
 * [#2078](https://github.com/clojure-emacs/cider/pull/2078): Improve startup time by bundling together sync requests during startup.
 * `cider-rotate-default-connection` will warn if you use it with only a single active connection.
-* `cider-format-buffer` preserves the point position.
+* `cider-format-buffer` tries to preserve the point position.
 
 ### Bugs Fixed
 
diff --git a/cider-interaction.el b/cider-interaction.el
index 3f24fe5a..58384e91 100644
--- a/cider-interaction.el
+++ b/cider-interaction.el
@@ -1763,36 +1763,8 @@ The heavy lifting is done by `cider-load-buffer'."
 (defalias 'cider-eval-buffer 'cider-load-buffer
   "A convenience alias as some people are confused by the load-* names.")
 
-(defun cider--format-buffer (formatter)
-  "Format the contents of the current buffer.
-
-Uses FORMATTER, a function of one argument, to convert the string contents
-of the buffer into a formatted string."
-  (let* ((original (substring-no-properties (buffer-string)))
-         (formatted (funcall formatter original)))
-    (unless (equal original formatted)
-      (let ((current-line (line-number-at-pos))
-            (current-column (current-column)))
-        (erase-buffer)
-        (insert formatted)
-        ;; we have to preserve our point location in the buffer,
-        ;; but save-excursion doesn't work, because of erase-buffer
-        (goto-char (point-min))
-        (forward-line (1- current-line))
-        (forward-char current-column)))))
-
-(defun cider-format-buffer ()
-  "Format the Clojure code in the current buffer."
-  (interactive)
-  (cider-ensure-connected)
-  (cider--format-buffer #'cider-sync-request:format-code))
-
-(defun cider-format-edn-buffer ()
-  "Format the EDN data in the current buffer."
-  (interactive)
-  (cider-ensure-connected)
-  (cider--format-buffer (lambda (edn)
-                          (cider-sync-request:format-edn edn (cider--pretty-print-width)))))
+
+;; Format
 
 (defun cider--format-reindent (formatted start)
   "Reindent FORMATTED to align with buffer position START."
@@ -1800,18 +1772,41 @@ of the buffer into a formatted string."
          (indent-line (concat "\n" (make-string start-column ? ))))
     (replace-regexp-in-string "\n" indent-line formatted)))
 
+
+;;; Format region
+
 (defun cider--format-region (start end formatter)
   "Format the contents of the given region.
 
 START and END represent the region's boundaries.
+
 FORMATTER is a function of one argument which is used to convert
-the string contents of the region into a formatted string."
+the string contents of the region into a formatted string.
+
+Uses the following heuristic to try to maintain point position:
+
+- Take a snippet of text starting at current position, up to 64 chars.
+- Search for the snippet, with lax whitespace, in the formatted text.
+  - If snippet is less than 64 chars (point was near end of buffer), search
+    from end instead of beginning.
+- Place point at match beginning, or `point-min' if no match."
   (let* ((original (buffer-substring-no-properties start end))
          (formatted (funcall formatter original))
          (indented (cider--format-reindent formatted start)))
     (unless (equal original indented)
-      (delete-region start end)
-      (insert indented))))
+      (let* ((pos (point))
+             (pos-max (1+ (buffer-size)))
+             (l 64)
+             (endp (> (+ pos l) pos-max))
+             (snippet (thread-last (buffer-substring-no-properties
+                                    pos (min (+ pos l) pos-max))
+                        (replace-regexp-in-string "[[:space:]\t\n\r]+" "[[:space:]\t\n\r]*"))))
+        (delete-region start end)
+        (insert indented)
+        (goto-char (if endp (point-max) (point-min)))
+        (funcall (if endp #'re-search-backward #'re-search-forward) snippet nil t)
+        (goto-char (or (match-beginning 0) start))
+        (when (looking-at-p "\n") (forward-char))))))
 
 (defun cider-format-region (start end)
   "Format the Clojure code in the current region.
@@ -1820,6 +1815,43 @@ START and END represent the region's boundaries."
   (cider-ensure-connected)
   (cider--format-region start end #'cider-sync-request:format-code))
 
+
+;;; Format defun
+
+(defun cider-format-defun ()
+  "Format the code in the current defun."
+  (interactive)
+  (cider-ensure-connected)
+  (save-excursion
+    (mark-defun)
+    (cider-format-region (region-beginning) (region-end))))
+
+
+;;; Format buffer
+
+(defun cider--format-buffer (formatter)
+  "Format the contents of the current buffer.
+
+Uses FORMATTER, a function of one argument, to convert the string contents
+of the buffer into a formatted string."
+  (cider--format-region 1 (1+ (buffer-size)) formatter))
+
+(defun cider-format-buffer ()
+  "Format the Clojure code in the current buffer."
+  (interactive)
+  (cider-ensure-connected)
+  (cider--format-buffer #'cider-sync-request:format-code))
+
+
+;;; Format EDN
+
+(defun cider-format-edn-buffer ()
+  "Format the EDN data in the current buffer."
+  (interactive)
+  (cider-ensure-connected)
+  (cider--format-buffer (lambda (edn)
+                          (cider-sync-request:format-edn edn (cider--pretty-print-width)))))
+
 (defun cider-format-edn-region (start end)
   "Format the EDN data in the current region.
 START and END represent the region's boundaries."
@@ -1831,15 +1863,9 @@ START and END represent the region's boundaries."
                           (lambda (edn)
                             (cider-sync-request:format-edn edn right-margin)))))
 
-(defun cider-format-defun ()
-  "Format the code in the current defun."
-  (interactive)
-  (cider-ensure-connected)
-  (save-excursion
-    (mark-defun)
-    (cider-format-region (region-beginning) (region-end))))
+
+;;; Interrupt evaluation
 
-;;; interrupt evaluation
 (defun cider-interrupt-handler (buffer)
   "Create an interrupt response handler for BUFFER."
   (nrepl-make-response-handler buffer nil nil nil nil))