From 4134de068fe97e308dff2c684adce9fd2af468ca Mon Sep 17 00:00:00 2001
From: Andrey Kotlarski <m00naticus@gmail.com>
Date: Fri, 13 Dec 2013 17:22:10 +0200
Subject: [PATCH] Add intelligent recenter chunk around point functionality.

---
 README.org | 13 +++++++++++++
 vlf.el     | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/README.org b/README.org
index 045ab0c..9acd286 100644
--- a/README.org
+++ b/README.org
@@ -79,6 +79,9 @@ refresh with *C-c C-v g*.
 
 ** Move around
 
+Scrolling automatically triggers moving to previous or next chunk at
+the beginning or end respectively of the current one.
+
 *C-c C-v n* and *C-c C-v p* move batch by batch.  With positive
 prefix argument they move prefix number of batches.  With negative -
 append prefix number of batches.
@@ -93,6 +96,16 @@ how many chunks there are (using the current batch size), look at the
 parenthesized part of the buffer name, batch size is also indicated at
 the end.
 
+** Follow point
+
+Intelligent continuous recenter around point in current buffer can be
+enabled with:
+
+*M-x vlf-start-following*
+
+You are asked for number of idle seconds interval for automatic
+update.  To cancel following invoke *M-x vlf-stop-following*.
+
 ** Search whole file
 
 *C-c C-v s* and *C-c C-v r* search forward and backward respectively
diff --git a/vlf.el b/vlf.el
index 73b133f..d38aa35 100644
--- a/vlf.el
+++ b/vlf.el
@@ -73,6 +73,11 @@ Possible values are: nil to never use it;
 (defvar vlf-file-size 0 "Total size of presented file.")
 (put 'vlf-file-size 'permanent-local t)
 
+(defvar vlf-follow-timer nil
+  "Contains timer and it's repeat interval if vlf buffer is set to\
+continuously recenter.")
+(put 'vlf-follow-timer 'permanent-local t)
+
 (defvar vlf-mode-map
   (let ((map (make-sparse-keymap)))
     (define-key map "n" 'vlf-next-batch)
@@ -125,11 +130,13 @@ Possible values are: nil to never use it;
              (vlf-get-file-size buffer-file-truename))
         (set (make-local-variable 'vlf-start-pos) 0)
         (set (make-local-variable 'vlf-end-pos) 0)
+        (set (make-local-variable 'vlf-follow-timer) nil)
         (let* ((pos (position-bytes (point)))
                (start (* (/ pos vlf-batch-size) vlf-batch-size)))
           (goto-char (byte-to-position (- pos start)))
           (vlf-move-to-batch start)))
     (kill-local-variable 'revert-buffer-function)
+    (vlf-stop-following)
     (when (or (not large-file-warning-threshold)
               (< vlf-file-size large-file-warning-threshold)
               (y-or-n-p (format "Load whole file (%s)? "
@@ -563,6 +570,50 @@ This seems to be the case with GNU/Emacs before 24.4."
         (t ;; TODO: use (< emacs-minor-version 4) after 24.4 release
          (string-lessp emacs-version "24.3.5"))))
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; follow point
+
+(defun vlf-recenter (vlf-buffer)
+  "Recenter chunk around current point in VLF-BUFFER."
+  (and vlf-follow-timer
+       (eq (current-buffer) vlf-buffer)
+       (or (pos-visible-in-window-p (point-min))
+           (pos-visible-in-window-p (point-max)))
+       (let ((current-pos (+ vlf-start-pos (position-bytes (point))))
+             (half-batch (/ vlf-batch-size 2)))
+         (if (buffer-modified-p)
+             (progn
+               (let ((edit-end (+ (position-bytes (point-max))
+                                  vlf-start-pos)))
+                 (vlf-move-to-chunk (min vlf-start-pos
+                                         (- current-pos half-batch))
+                                    (max edit-end
+                                         (+ current-pos half-batch))))
+               (goto-char (byte-to-position (- current-pos
+                                               vlf-start-pos))))
+           (vlf-move-to-batch (- current-pos half-batch))
+           (and (< half-batch current-pos)
+                (< half-batch (- vlf-file-size current-pos))
+                (goto-char (byte-to-position half-batch)))))))
+
+(defun vlf-stop-following ()
+  "Stop continuous recenter."
+  (interactive)
+  (when vlf-follow-timer
+    (cancel-timer (car vlf-follow-timer))
+    (setq vlf-follow-timer nil)))
+
+(defun vlf-start-following (interval)
+  "Continuously recenter chunk around point every INTERVAL seconds."
+  (interactive "nNumber of seconds: ")
+  (when vlf-mode
+    (vlf-stop-following)
+    (setq vlf-follow-timer (cons (run-with-idle-timer interval interval
+                                                      'vlf-recenter
+                                                      (current-buffer))
+                                 interval))
+    (add-hook 'kill-buffer-hook 'vlf-stop-following nil t)))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; search