X-Git-Url: https://code.delx.au/gnu-emacs-elpa/blobdiff_plain/43ed6c23230dcda20c48f6458d5af0082f4365a5..f0c48b253ea7903357d73eba6cc6ba3051ff8962:/packages/vlf/vlf.el

diff --git a/packages/vlf/vlf.el b/packages/vlf/vlf.el
index 20731f8a7..e6c34e641 100644
--- a/packages/vlf/vlf.el
+++ b/packages/vlf/vlf.el
@@ -2,11 +2,13 @@
 
 ;; Copyright (C) 2006, 2012, 2013  Free Software Foundation, Inc.
 
-;; Version: 0.4
+;; Version: 1.1
 ;; Keywords: large files, utilities
+;; Maintainer: Andrey Kotlarski <m00naticus@gmail.com>
 ;; Authors: 2006 Mathias Dahl <mathias.dahl@gmail.com>
 ;;          2012 Sam Steingold <sds@gnu.org>
 ;;          2013 Andrey Kotlarski <m00naticus@gmail.com>
+;; URL: https://github.com/m00natic/vlfi
 
 ;; This file is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by
@@ -26,11 +28,9 @@
 ;;; Commentary:
 
 ;; This package provides the M-x vlf command, which visits part of a
-;; large file in a read-only buffer without visiting the entire file.
-;; The buffer uses VLF mode, which defines the commands M-<next>
-;; (vlf-next-batch) and M-<prior> (vlf-prev-batch) to visit other
-;; parts of the file.  The option `vlf-batch-size' specifies the size
-;; of each batch, in bytes.
+;; large file without loading the entire file.
+;; The buffer uses VLF mode, which defines several commands for
+;; moving around, searching and editing selected part of file.
 
 ;; This package was inspired by a snippet posted by Kevin Rodgers,
 ;; showing how to use `insert-file-contents' to extract part of a
@@ -47,64 +47,240 @@
   "Defines how large each batch of file data is (in bytes)."
   :type 'integer
   :group 'vlf)
+(put 'vlf-batch-size 'permanent-local t)
 
-;; Keep track of file position.
-(defvar vlf-start-pos)
-(defvar vlf-end-pos)
-(defvar vlf-file-size)
+;;; Keep track of file position.
+(defvar vlf-start-pos 0
+  "Absolute position of the visible chunk start.")
+(put 'vlf-start-pos 'permanent-local t)
+
+(defvar vlf-end-pos 0 "Absolute position of the visible chunk end.")
+(put 'vlf-end-pos 'permanent-local t)
+
+(defvar vlf-file-size 0 "Total size of presented file.")
+(put 'vlf-file-size 'permanent-local t)
 
 (defvar vlf-mode-map
-  (let ((map (make-sparse-keymap)))
-    (define-key map [M-next] 'vlf-next-batch)
-    (define-key map [M-prior] 'vlf-prev-batch)
-    (define-key map (kbd "M-+") 'vlf-change-batch-size)
-    (define-key map (kbd "M--")
+  (let ((map-prefix (make-sparse-keymap))
+        (map (make-sparse-keymap)))
+    (define-key map [next] 'vlf-next-batch)
+    (define-key map [prior] 'vlf-prev-batch)
+    (define-key map "n" 'vlf-next-batch-from-point)
+    (define-key map "+" 'vlf-change-batch-size)
+    (define-key map "-"
       (lambda () "Decrease vlf batch size by factor of 2."
         (interactive)
         (vlf-change-batch-size t)))
     (define-key map "s" 'vlf-re-search-forward)
     (define-key map "r" 'vlf-re-search-backward)
-    (define-key map ">" (lambda () "Jump to end of file content."
-                          (interactive)
-                          (vlf-insert-file buffer-file-name t)))
-    (define-key map "<" (lambda () "Jump to beginning of file content."
-                          (interactive)
-                          (vlf-insert-file buffer-file-name)))
-    map)
+    (define-key map "o" 'vlf-occur)
+    (define-key map "[" 'vlf-beginning-of-file)
+    (define-key map "]" 'vlf-end-of-file)
+    (define-key map "j" 'vlf-jump-to-chunk)
+    (define-key map "l" 'vlf-goto-line)
+    (define-key map "g" 'vlf-refresh)
+    (define-key map-prefix "\C-c\C-v" map)
+    map-prefix)
   "Keymap for `vlf-mode'.")
 
-(define-derived-mode vlf-mode special-mode "VLF"
+(define-minor-mode vlf-mode
   "Mode to browse large files in."
-  (setq buffer-read-only t)
-  (set-buffer-modified-p nil)
-  (make-local-variable 'vlf-batch-size)
-  (make-local-variable 'vlf-start-pos)
-  (make-local-variable 'vlf-file-size))
+  :lighter " VLF"
+  :group 'vlf
+  :keymap vlf-mode-map
+  (if vlf-mode
+      (progn
+        (set (make-local-variable 'require-final-newline) nil)
+        (add-hook 'write-file-functions 'vlf-write nil t)
+        (set (make-local-variable 'revert-buffer-function)
+             'vlf-revert)
+        (make-local-variable 'vlf-batch-size)
+        (set (make-local-variable 'vlf-start-pos) -1)
+        (make-local-variable 'vlf-end-pos)
+        (set (make-local-variable 'vlf-file-size)
+             (vlf-get-file-size buffer-file-name))
+        (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)
+    (when (or (not large-file-warning-threshold)
+              (< vlf-file-size large-file-warning-threshold)
+              (y-or-n-p (format "Load whole file (%s)? "
+                                (file-size-human-readable
+                                 vlf-file-size))))
+      (kill-local-variable 'require-final-newline)
+      (remove-hook 'write-file-functions 'vlf-write t)
+      (let ((pos (+ vlf-start-pos (position-bytes (point)))))
+        (vlf-with-undo-disabled
+         (insert-file-contents buffer-file-name t nil nil t))
+        (goto-char (byte-to-position pos)))
+      (rename-buffer (file-name-nondirectory buffer-file-name) t))))
+
+;;;###autoload
+(defun vlf (file)
+  "View Large FILE in batches.
+You can customize number of bytes displayed by customizing
+`vlf-batch-size'."
+  (interactive "fFile to open: ")
+  (with-current-buffer (generate-new-buffer "*vlf*")
+    (set-visited-file-name file)
+    (set-buffer-modified-p nil)
+    (vlf-mode 1)
+    (switch-to-buffer (current-buffer))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; integration with other packages
+
+;;;###autoload
+(defun dired-vlf ()
+  "In Dired, visit the file on this line in VLF mode."
+  (interactive)
+  (vlf (dired-get-file-for-visit)))
+
+;;;###autoload
+(eval-after-load "dired"
+  '(define-key dired-mode-map "V" 'dired-vlf))
+
+;;;###autoload
+(defadvice abort-if-file-too-large (around vlf-if-file-too-large
+                                           (size op-type
+                                                 &optional filename)
+                                           compile activate)
+  "If file SIZE larger than `large-file-warning-threshold', \
+allow user to view file with `vlf', open it normally, or abort.
+OP-TYPE specifies the file operation being performed over FILENAME."
+  (and large-file-warning-threshold size
+       (> size large-file-warning-threshold)
+       (let ((char nil))
+         (while (not (memq (setq char
+                                 (read-event
+                                  (propertize
+                                   (format
+                                    "File %s is large (%s): \
+%s normally (o), %s with vlf (v) or abort (a)"
+                                    (if filename
+                                        (file-name-nondirectory filename)
+                                      "")
+                                    (file-size-human-readable size)
+                                    op-type op-type)
+                                   'face 'minibuffer-prompt)))
+                           '(?o ?O ?v ?V ?a ?A))))
+         (cond ((memq char '(?o ?O)))
+               ((memq char '(?v ?V))
+                (vlf filename)
+                (error ""))
+               ((memq char '(?a ?A))
+                (error "Aborted"))))))
+
+
+;; scroll auto batching
+(defadvice scroll-up (around vlf-scroll-up
+                             activate compile)
+  "Slide to next batch if at end of buffer in `vlf-mode'."
+  (if (and vlf-mode (pos-visible-in-window-p (point-max)))
+      (progn (vlf-next-batch 1)
+             (goto-char (point-min)))
+    ad-do-it))
+
+(defadvice scroll-down (around vlf-scroll-down
+                               activate compile)
+  "Slide to previous batch if at beginning of buffer in `vlf-mode'."
+  (if (and vlf-mode (pos-visible-in-window-p (point-min)))
+      (progn (vlf-prev-batch 1)
+             (goto-char (point-max)))
+    ad-do-it))
+
+;; non-recent Emacs
+;;;###autoload
+(unless (fboundp 'file-size-human-readable)
+  (defun file-size-human-readable (file-size)
+    "Print FILE-SIZE in MB."
+    (format "%.1fMB" (/ file-size 1048576.0))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; utilities
 
 (defun vlf-change-batch-size (decrease)
   "Change the buffer-local value of `vlf-batch-size'.
 Normally, the value is doubled;
 with the prefix argument DECREASE it is halved."
   (interactive "P")
-  (or (assq 'vlf-batch-size (buffer-local-variables))
-      (error "%s is not local in this buffer" 'vlf-batch-size))
-  (setq vlf-batch-size
-        (if decrease
-            (/ vlf-batch-size 2)
-          (* vlf-batch-size 2)))
-  (vlf-update-buffer-name))
-
-(defun vlf-format-buffer-name ()
-  "Return format for vlf buffer name."
-  (format "%s(%s)[%.2f%%%%](%d)"
-          (file-name-nondirectory buffer-file-name)
-          (file-size-human-readable vlf-file-size)
-          (/ (* 100 vlf-end-pos) (float vlf-file-size))
-          vlf-batch-size))
+  (setq vlf-batch-size (if decrease
+                           (/ vlf-batch-size 2)
+                         (* vlf-batch-size 2)))
+  (vlf-move-to-batch vlf-start-pos))
 
 (defun vlf-update-buffer-name ()
   "Update the current buffer name."
-  (rename-buffer (vlf-format-buffer-name) t))
+  (rename-buffer (format "%s(%d/%d)[%s]"
+                         (file-name-nondirectory buffer-file-name)
+                         (/ vlf-end-pos vlf-batch-size)
+                         (/ vlf-file-size vlf-batch-size)
+                         (file-size-human-readable vlf-batch-size))
+                 t))
+
+(defun vlf-get-file-size (file)
+  "Get size in bytes of FILE."
+  (nth 7 (file-attributes file)))
+
+(defun vlf-verify-size ()
+  "Update file size information if necessary and visited file time."
+  (unless (verify-visited-file-modtime (current-buffer))
+    (setq vlf-file-size (vlf-get-file-size buffer-file-name))
+    (set-visited-file-modtime)))
+
+(defun vlf-insert-file (&optional from-end)
+  "Insert first chunk of current file contents in current buffer.
+With FROM-END prefix, start from the back."
+  (let ((start 0)
+        (end vlf-batch-size))
+    (if from-end
+        (setq start (- vlf-file-size vlf-batch-size)
+              end vlf-file-size)
+      (setq end (min vlf-batch-size vlf-file-size)))
+    (vlf-move-to-chunk start end)))
+
+(defun vlf-beginning-of-file ()
+  "Jump to beginning of file content."
+  (interactive)
+  (vlf-insert-file))
+
+(defun vlf-end-of-file ()
+  "Jump to end of file content."
+  (interactive)
+  (vlf-insert-file t))
+
+(defun vlf-revert (&optional _ignore-auto noconfirm)
+  "Revert current chunk.  Ignore _IGNORE-AUTO.
+Ask for confirmation if NOCONFIRM is nil."
+  (if (or noconfirm
+          (yes-or-no-p (format "Revert buffer from file %s? "
+                               buffer-file-name)))
+      (vlf-move-to-chunk-2 vlf-start-pos vlf-end-pos)))
+
+(defun vlf-jump-to-chunk (n)
+  "Go to to chunk N."
+  (interactive "nGoto to chunk: ")
+  (vlf-move-to-batch (* (1- n) vlf-batch-size)))
+
+(defmacro vlf-with-undo-disabled (&rest body)
+  "Execute BODY with temporarily disabled undo."
+  `(let ((undo-enabled (not (eq buffer-undo-list t))))
+     (if undo-enabled
+         (buffer-disable-undo))
+     (unwind-protect (progn ,@body)
+       (if undo-enabled
+           (buffer-enable-undo)))))
+
+(defun vlf-no-modifications ()
+  "Ensure there are no buffer modifications."
+  (if (buffer-modified-p)
+      (error "Save or discard your changes first")
+    t))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; batch movement
 
 (defun vlf-next-batch (append)
   "Display the next batch of file data.
@@ -113,30 +289,14 @@ When prefix argument is supplied and positive
 When prefix argument is negative
  append next APPEND number of batches to the existing buffer."
   (interactive "p")
-  (let ((end (+ vlf-end-pos (* vlf-batch-size
-                                (abs append)))))
-    (when (< vlf-file-size end)		; re-check file size
-      (setq vlf-file-size (nth 7 (file-attributes buffer-file-name)))
-      (cond ((= vlf-end-pos vlf-file-size)
-             (error "Already at EOF"))
-            ((< vlf-file-size end)
-             (setq end vlf-file-size))))
-    (let ((inhibit-read-only t)
-          (do-append (< append 0))
-          (pos (point)))
-      (if do-append
-          (goto-char (point-max))
-        (setq vlf-start-pos (- end vlf-batch-size))
-        (erase-buffer))
-      (insert-file-contents buffer-file-name nil
-                            (if do-append
-                                vlf-end-pos
-                              vlf-start-pos)
-                            end)
-      (goto-char pos))
-    (setq vlf-end-pos end))
-  (set-buffer-modified-p nil)
-  (vlf-update-buffer-name))
+  (vlf-verify-size)
+  (let* ((end (min (+ vlf-end-pos (* vlf-batch-size
+                                     (abs append)))
+                   vlf-file-size))
+         (start (if (< append 0)
+                    vlf-start-pos
+                  (- end vlf-batch-size))))
+    (vlf-move-to-chunk start end)))
 
 (defun vlf-prev-batch (prepend)
   "Display the previous batch of file data.
@@ -147,228 +307,687 @@ When prefix argument is negative
   (interactive "p")
   (if (zerop vlf-start-pos)
       (error "Already at BOF"))
-  (let ((inhibit-read-only t)
-        (start (max 0 (- vlf-start-pos (* vlf-batch-size
+  (let* ((start (max 0 (- vlf-start-pos (* vlf-batch-size
                                            (abs prepend)))))
-        (do-prepend (< prepend 0))
-        (pos (- (point-max) (point))))
-    (if do-prepend
-        (goto-char (point-min))
-      (setq vlf-end-pos (+ start vlf-batch-size))
-      (erase-buffer))
-    (insert-file-contents buffer-file-name nil start
-                          (if do-prepend
-                              vlf-start-pos
-                            vlf-end-pos))
-    (goto-char (- (point-max) pos))
-    (setq vlf-start-pos start))
-  (set-buffer-modified-p nil)
-  (vlf-update-buffer-name))
+         (end (if (< prepend 0)
+                  vlf-end-pos
+                (+ start vlf-batch-size))))
+    (vlf-move-to-chunk start end)))
+
+(defun vlf-move-to-batch (start &optional minimal)
+  "Move to batch determined by START.
+Adjust according to file start/end and show `vlf-batch-size' bytes.
+When given MINIMAL flag, skip non important operations."
+  (vlf-verify-size)
+  (let* ((start (max 0 start))
+         (end (min (+ start vlf-batch-size) vlf-file-size)))
+    (if (= vlf-file-size end)          ; re-adjust start
+        (setq start (max 0 (- end vlf-batch-size))))
+    (vlf-move-to-chunk start end minimal)))
+
+(defun vlf-next-batch-from-point ()
+  "Display batch of file data starting from current point."
+  (interactive)
+  (vlf-move-to-batch (+ vlf-start-pos (position-bytes (point)) -1))
+  (goto-char (point-min)))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; primitive chunk operations
 
-(defun vlf-move-to-chunk (start end)
-  "Move to chunk determined by START END."
-  (if (< vlf-file-size end)          ; re-check file size
-      (setq vlf-file-size (nth 7
-                                (file-attributes buffer-file-name))))
+(defun vlf-move-to-chunk (start end &optional minimal)
+  "Move to chunk determined by START END.
+When given MINIMAL flag, skip non important operations.
+If same as current chunk is requested, do nothing."
+  (unless (and (= start vlf-start-pos)
+               (= end vlf-end-pos))
+    (vlf-verify-size)
+    (if (vlf-move-to-chunk-1 start end)
+        (or minimal (vlf-update-buffer-name)))))
+
+(defun vlf-move-to-chunk-1 (start end)
+  "Move to chunk determined by START END keeping as much edits if any.
+Return t if move hasn't been canceled."
+  (let ((modified (buffer-modified-p))
+        (start (max 0 start))
+        (end (min end vlf-file-size))
+        (edit-end (+ (position-bytes (point-max)) vlf-start-pos)))
+    (cond
+     ((and (= start vlf-start-pos) (= end edit-end))
+      (unless modified
+        (vlf-move-to-chunk-2 start end)
+        t))
+     ((or (<= edit-end start) (<= end vlf-start-pos))
+      (when (or (not modified)
+                (y-or-n-p "Chunk modified, are you sure? ")) ;full chunk renewal
+        (set-buffer-modified-p nil)
+        (vlf-move-to-chunk-2 start end)
+        t))
+     ((or (and (<= start vlf-start-pos) (<= edit-end end))
+          (not modified)
+          (y-or-n-p "Chunk modified, are you sure? "))
+      (let ((pos (+ (position-bytes (point)) vlf-start-pos))
+            (shift-start 0)
+            (shift-end 0)
+            (inhibit-read-only t))
+        (cond ((< end edit-end)
+               (let* ((del-pos (1+ (byte-to-position
+                                    (- end vlf-start-pos))))
+                      (del-len (length (encode-coding-region
+                                        del-pos (point-max)
+                                        buffer-file-coding-system
+                                        t))))
+                 (setq end (- vlf-end-pos del-len))
+                 (vlf-with-undo-disabled
+                  (delete-region del-pos (point-max)))))
+              ((< edit-end end)
+               (let ((edit-end-pos (point-max)))
+                 (goto-char edit-end-pos)
+                 (vlf-with-undo-disabled
+                  (insert-file-contents buffer-file-name nil
+                                        vlf-end-pos end)
+                  (setq shift-end (cdr (vlf-adjust-chunk
+                                        vlf-end-pos end nil t
+                                        edit-end-pos)))))))
+        (cond ((< vlf-start-pos start)
+               (let* ((del-pos (1+ (byte-to-position
+                                    (- start vlf-start-pos))))
+                      (del-len (length (encode-coding-region
+                                        (point-min) del-pos
+                                        buffer-file-coding-system
+                                        t))))
+                 (setq start (+ vlf-start-pos del-len))
+                 (vlf-with-undo-disabled
+                  (delete-region (point-min) del-pos))))
+              ((< start vlf-start-pos)
+               (let ((edit-end-pos (point-max)))
+                 (goto-char edit-end-pos)
+                 (vlf-with-undo-disabled
+                  (insert-file-contents buffer-file-name nil
+                                        start vlf-start-pos)
+                  (setq shift-start (car
+                                     (vlf-adjust-chunk start
+                                                       vlf-start-pos
+                                                       t nil
+                                                       edit-end-pos)))
+                  (goto-char (point-min))
+                  (insert (delete-and-extract-region edit-end-pos
+                                                     (point-max)))))))
+        (setq vlf-start-pos (- start shift-start)
+              vlf-end-pos (+ end shift-end))
+        (goto-char (or (byte-to-position (- pos vlf-start-pos))
+                       (point-max))))
+      (set-buffer-modified-p modified)
+      t))))
+
+(defun vlf-move-to-chunk-2 (start end)
+  "Unconditionally move to chunk determined by START END."
   (setq vlf-start-pos (max 0 start)
         vlf-end-pos (min end vlf-file-size))
-  (let ((inhibit-read-only t))
-    (erase-buffer)
-    (insert-file-contents buffer-file-name nil
-                          vlf-start-pos vlf-end-pos))
+  (let ((inhibit-read-only t)
+        (pos (position-bytes (point))))
+    (vlf-with-undo-disabled
+     (erase-buffer)
+     (insert-file-contents buffer-file-name nil
+                           vlf-start-pos vlf-end-pos)
+     (let ((shifts (vlf-adjust-chunk vlf-start-pos vlf-end-pos t
+                                     t)))
+       (setq vlf-start-pos (- vlf-start-pos (car shifts))
+             vlf-end-pos (+ vlf-end-pos (cdr shifts)))
+       (goto-char (or (byte-to-position (+ pos (car shifts)))
+                      (point-max))))))
   (set-buffer-modified-p nil)
-  (vlf-update-buffer-name))
+  (set-visited-file-modtime))
 
-(defun vlf-insert-file (file &optional from-end)
-  "Insert first chunk of FILE contents in current buffer.
-With FROM-END prefix, start from the back."
-  (if from-end
-      (setq vlf-start-pos (max 0 (- vlf-file-size vlf-batch-size))
-            vlf-end-pos vlf-file-size)
-    (setq vlf-start-pos 0
-          vlf-end-pos (min vlf-batch-size vlf-file-size)))
-  (vlf-move-to-chunk vlf-start-pos vlf-end-pos))
+(defun vlf-adjust-chunk (start end &optional adjust-start adjust-end
+                               position)
+  "Adjust chunk at absolute START to END till content can be\
+properly decoded.  ADJUST-START determines if trying to prepend bytes\
+ to the beginning, ADJUST-END - append to the end.
+Use buffer POSITION as start if given.
+Return number of bytes moved back for proper decoding and number of
+bytes added to the end."
+  (let ((shift-start 0)
+        (shift-end 0))
+    (if adjust-start
+        (let ((position (or position (point-min)))
+              (chunk-size (- end start)))
+          (while (and (not (zerop start))
+                      (< shift-start 4)
+                      (< 4 (abs (- chunk-size
+                                   (length (encode-coding-region
+                                            position (point-max)
+                                            buffer-file-coding-system
+                                            t))))))
+            (setq shift-start (1+ shift-start)
+                  start (1- start)
+                  chunk-size (1+ chunk-size))
+            (delete-region position (point-max))
+            (goto-char position)
+            (insert-file-contents buffer-file-name nil start end))))
+    (if adjust-end
+        (cond ((vlf-partial-decode-shown-p) ;remove raw bytes from end
+               (goto-char (point-max))
+               (while (eq (char-charset (preceding-char)) 'eight-bit)
+                 (setq shift-end (1- shift-end))
+                 (delete-char -1)))
+              ((< end vlf-file-size) ;add bytes until new character is displayed
+               (let ((position (or position (point-min)))
+                     (expected-size (buffer-size)))
+                 (while (and (progn
+                               (setq shift-end (1+ shift-end)
+                                     end (1+ end))
+                               (delete-region position (point-max))
+                               (goto-char position)
+                               (insert-file-contents buffer-file-name
+                                                     nil start end)
+                               (< end vlf-file-size))
+                             (= expected-size (buffer-size))))))))
+    (cons shift-start shift-end)))
 
-;;;###autoload
-(defun vlf (file &optional from-end)
-  "View Large FILE.  With FROM-END prefix, view from the back.
-Batches of the file data from FILE will be displayed in a read-only
-buffer.  You can customize number of bytes displayed by customizing
-`vlf-batch-size'."
-  (interactive "fFile to open: \nP")
-  (with-current-buffer (generate-new-buffer "*vlf*")
-    (buffer-disable-undo)
-    (setq buffer-file-name file
-          vlf-file-size (nth 7 (file-attributes file)))
-    (vlf-insert-file file from-end)
-    (vlf-mode)
-    (switch-to-buffer (current-buffer))))
+(defun vlf-partial-decode-shown-p ()
+  "Determine if partial decode codes are displayed.
+This seems to be the case with GNU/Emacs before 24.4."
+  (cond ((< emacs-major-version 24) t)
+        ((< 24 emacs-major-version) nil)
+        (t ;; TODO: use (< emacs-minor-version 4) after 24.4 release
+         (string-lessp emacs-version "24.3.5"))))
 
-;;;###autoload
-(defun dired-vlf (from-end)
-  "In Dired, visit the file on this line in VLF mode.
-With FROM-END prefix, view from the back."
-  (interactive "P")
-  (vlf (dired-get-file-for-visit) from-end))
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; search
 
-;;;###autoload
-(eval-after-load "dired"
-  '(define-key dired-mode-map "V" 'dired-vlf))
+(defun vlf-re-search (regexp count backward batch-step)
+  "Search for REGEXP COUNT number of times forward or BACKWARD.
+BATCH-STEP is amount of overlap between successive chunks."
+  (if (<= count 0)
+      (error "Count must be positive"))
+  (let* ((case-fold-search t)
+         (match-chunk-start vlf-start-pos)
+         (match-chunk-end vlf-end-pos)
+         (match-start-pos (+ vlf-start-pos (position-bytes (point))))
+         (match-end-pos match-start-pos)
+         (to-find count)
+         (reporter (make-progress-reporter
+                    (concat "Searching for " regexp "...")
+                    (if backward
+                        (- vlf-file-size vlf-end-pos)
+                      vlf-start-pos)
+                    vlf-file-size)))
+    (vlf-with-undo-disabled
+     (unwind-protect
+         (catch 'end-of-file
+           (if backward
+               (while (not (zerop to-find))
+                 (cond ((re-search-backward regexp nil t)
+                        (setq to-find (1- to-find)
+                              match-chunk-start vlf-start-pos
+                              match-chunk-end vlf-end-pos
+                              match-start-pos (+ vlf-start-pos
+                                                 (position-bytes
+                                                  (match-beginning 0)))
+                              match-end-pos (+ vlf-start-pos
+                                               (position-bytes
+                                                (match-end 0)))))
+                       ((zerop vlf-start-pos)
+                        (throw 'end-of-file nil))
+                       (t (let ((batch-move (- vlf-start-pos
+                                               (- vlf-batch-size
+                                                  batch-step))))
+                            (vlf-move-to-batch
+                             (if (< match-start-pos batch-move)
+                                 (- match-start-pos vlf-batch-size)
+                               batch-move) t))
+                          (goto-char (if (< match-start-pos
+                                            vlf-end-pos)
+                                         (or (byte-to-position
+                                              (- match-start-pos
+                                                 vlf-start-pos))
+                                             (point-max))
+                                       (point-max)))
+                          (progress-reporter-update
+                           reporter (- vlf-file-size
+                                       vlf-start-pos)))))
+             (while (not (zerop to-find))
+               (cond ((re-search-forward regexp nil t)
+                      (setq to-find (1- to-find)
+                            match-chunk-start vlf-start-pos
+                            match-chunk-end vlf-end-pos
+                            match-start-pos (+ vlf-start-pos
+                                               (position-bytes
+                                                (match-beginning 0)))
+                            match-end-pos (+ vlf-start-pos
+                                             (position-bytes
+                                              (match-end 0)))))
+                     ((= vlf-end-pos vlf-file-size)
+                      (throw 'end-of-file nil))
+                     (t (let ((batch-move (- vlf-end-pos batch-step)))
+                          (vlf-move-to-batch
+                           (if (< batch-move match-end-pos)
+                               match-end-pos
+                             batch-move) t))
+                        (goto-char (if (< vlf-start-pos match-end-pos)
+                                       (or (byte-to-position
+                                            (- match-end-pos
+                                               vlf-start-pos))
+                                           (point-min))
+                                     (point-min)))
+                        (progress-reporter-update reporter
+                                                  vlf-end-pos)))))
+           (progress-reporter-done reporter))
+       (set-buffer-modified-p nil)
+       (if backward
+           (vlf-goto-match match-chunk-start match-chunk-end
+                           match-end-pos match-start-pos
+                           count to-find)
+         (vlf-goto-match match-chunk-start match-chunk-end
+                         match-start-pos match-end-pos
+                         count to-find))))))
 
-;;;###autoload
-(defun vlf-if-file-too-large (size op-type &optional filename)
-  "If file SIZE larger than `large-file-warning-threshold', \
-allow user to view file with `vlf', open it normally or abort.
-OP-TYPE specifies the file operation being performed over FILENAME."
-  (and large-file-warning-threshold size
-       (> size large-file-warning-threshold)
-       (let ((char nil))
-         (while (not (memq (setq char
-                                 (read-event
-                                  (propertize
-                                   (format "File %s is large (%s): \
-%s normally (o), %s with vlf (v) or abort (a)"
-                                           (file-name-nondirectory filename)
-                                           (file-size-human-readable size)
-                                           op-type op-type)
-                                   'face 'minibuffer-prompt)))
-                           '(?o ?O ?v ?V ?a ?A))))
-         (cond ((memq char '(?o ?O)))
-               ((memq char '(?v ?V))
-                (vlf nil filename)
-                (error ""))
-               ((memq char '(?a ?A))
-                (error "Aborted"))))))
-
-;;; hijack `abort-if-file-too-large'
-;;;###autoload
-(fset 'abort-if-file-too-large 'vlf-if-file-too-large)
+(defun vlf-goto-match (match-chunk-start match-chunk-end
+                                         match-pos-start
+                                         match-pos-end
+                                         count to-find)
+  "Move to MATCH-CHUNK-START MATCH-CHUNK-END surrounding \
+MATCH-POS-START and MATCH-POS-END.
+According to COUNT and left TO-FIND, show if search has been
+successful.  Return nil if nothing found."
+  (if (= count to-find)
+      (progn (vlf-move-to-chunk match-chunk-start match-chunk-end)
+             (goto-char (or (byte-to-position (- match-pos-start
+                                                 vlf-start-pos))
+                            (point-max)))
+             (message "Not found")
+             nil)
+    (let ((success (zerop to-find)))
+      (if success
+          (vlf-update-buffer-name)
+        (vlf-move-to-chunk match-chunk-start match-chunk-end))
+      (let* ((match-end (or (byte-to-position (- match-pos-end
+                                                 vlf-start-pos))
+                            (point-max)))
+             (overlay (make-overlay (byte-to-position
+                                     (- match-pos-start
+                                        vlf-start-pos))
+                                    match-end)))
+        (overlay-put overlay 'face 'match)
+        (unless success
+          (goto-char match-end)
+          (message "Moved to the %d match which is last"
+                   (- count to-find)))
+        (unwind-protect (sit-for 3)
+          (delete-overlay overlay))
+        t))))
 
 (defun vlf-re-search-forward (regexp count)
-  "Search forward for REGEXP prefix COUNT number of times."
-  (interactive (list (read-regexp "Search whole file"
-                                  (if regexp-history
-                                      (car regexp-history))
-                                  'regexp-history)
-                     (or current-prefix-arg 1)))
-  (let ((match-chunk-start vlf-start-pos)
-        (match-chunk-end vlf-end-pos)
-        (match-start-pos (point))
-        (match-end-pos (point))
-        (to-find count)
-        (search-reporter (make-progress-reporter
-                          (concat "Searching for " regexp)
-                          vlf-start-pos vlf-file-size))
-        (initial-chunk t))
-    (unwind-protect
-        (catch 'end-of-file
-          (while (not (zerop to-find))
-            (cond ((re-search-forward regexp nil t)
-                   (setq to-find (if (= match-start-pos
-                                        (match-beginning 0))
-                                     to-find
-                                   (1- to-find))
-                         match-start-pos (match-beginning 0)
-                         match-end-pos (match-end 0)
-                         match-chunk-start vlf-start-pos
-                         match-chunk-end vlf-end-pos)
-                   (if (and (< vlf-batch-size match-start-pos)
-                            (> (- vlf-end-pos vlf-start-pos)
-                               vlf-batch-size))
-                       (setq match-chunk-start
-                             (+ match-chunk-start vlf-batch-size)
-                             match-start-pos (- match-start-pos
-                                                vlf-batch-size)
-                             match-end-pos (- match-end-pos
-                                              vlf-batch-size))))
-                  ((= vlf-end-pos vlf-file-size)
-                   (throw 'end-of-file nil))
-                  (t (if initial-chunk
-                         (progn (setq initial-chunk nil)
-                                (vlf-next-batch -1))
-                       (vlf-move-to-chunk (+ vlf-start-pos
-                                              vlf-batch-size)
-                                           (+ vlf-end-pos
-                                              vlf-batch-size)))
-                     (goto-char (if (< vlf-start-pos match-chunk-end)
-                                    match-start-pos
-                                  (point-min)))
-                     (goto-char match-start-pos)
-                     (progress-reporter-update search-reporter
-                                               vlf-end-pos))))
-          (progress-reporter-done search-reporter))
-      (vlf-end-search match-chunk-start match-chunk-end
-                       match-end-pos count to-find))))
+  "Search forward for REGEXP prefix COUNT number of times.
+Search is performed chunk by chunk in `vlf-batch-size' memory."
+  (interactive (if (vlf-no-modifications)
+                   (list (read-regexp "Search whole file"
+                                      (if regexp-history
+                                          (car regexp-history)))
+                         (or current-prefix-arg 1))))
+  (vlf-re-search regexp count nil (/ vlf-batch-size 8)))
 
 (defun vlf-re-search-backward (regexp count)
-  "Search backward for REGEXP prefix COUNT number of times."
-  (interactive (list (read-regexp "Search whole file"
+  "Search backward for REGEXP prefix COUNT number of times.
+Search is performed chunk by chunk in `vlf-batch-size' memory."
+  (interactive (if (vlf-no-modifications)
+                   (list (read-regexp "Search whole file backward"
+                                      (if regexp-history
+                                          (car regexp-history)))
+                         (or current-prefix-arg 1))))
+  (vlf-re-search regexp count t (/ vlf-batch-size 8)))
+
+(defun vlf-goto-line (n)
+  "Go to line N.  If N is negative, count from the end of file."
+  (interactive (if (vlf-no-modifications)
+                   (list (read-number "Go to line: "))))
+  (let ((start-pos vlf-start-pos)
+        (end-pos vlf-end-pos)
+        (pos (point))
+        (success nil))
+    (unwind-protect
+        (if (< 0 n)
+            (progn (vlf-beginning-of-file)
+                   (goto-char (point-min))
+                   (setq success (vlf-re-search "[\n\C-m]" (1- n)
+                                                nil 0)))
+          (vlf-end-of-file)
+          (goto-char (point-max))
+          (setq success (vlf-re-search "[\n\C-m]" (- n) t 0)))
+      (if success
+          (message "Onto line %s" n)
+        (vlf-move-to-chunk start-pos end-pos)
+        (goto-char pos)))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; occur
+
+(defvar vlf-occur-mode-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map "n" 'vlf-occur-next-match)
+    (define-key map "p" 'vlf-occur-prev-match)
+    (define-key map "\C-m" 'vlf-occur-visit)
+    (define-key map "\M-\r" 'vlf-occur-visit-new-buffer)
+    (define-key map [mouse-1] 'vlf-occur-visit)
+    (define-key map "o" 'vlf-occur-show)
+    map)
+  "Keymap for command `vlf-occur-mode'.")
+
+(define-derived-mode vlf-occur-mode special-mode "VLF[occur]"
+  "Major mode for showing occur matches of VLF opened files.")
+
+(defun vlf-occur-next-match ()
+  "Move cursor to next match."
+  (interactive)
+  (if (eq (get-char-property (point) 'face) 'match)
+      (goto-char (next-single-property-change (point) 'face)))
+  (goto-char (or (text-property-any (point) (point-max) 'face 'match)
+                 (text-property-any (point-min) (point)
+                                    'face 'match))))
+
+(defun vlf-occur-prev-match ()
+  "Move cursor to previous match."
+  (interactive)
+  (if (eq (get-char-property (point) 'face) 'match)
+      (goto-char (previous-single-property-change (point) 'face)))
+  (while (not (eq (get-char-property (point) 'face) 'match))
+    (goto-char (or (previous-single-property-change (point) 'face)
+                   (point-max)))))
+
+(defun vlf-occur-show (&optional event)
+  "Visit current `vlf-occur' link in a vlf buffer but stay in the \
+occur buffer.  If original VLF buffer has been killed,
+open new VLF session each time.
+EVENT may hold details of the invocation."
+  (interactive (list last-nonmenu-event))
+  (let ((occur-buffer (if event
+                          (window-buffer (posn-window
+                                          (event-end event)))
+                        (current-buffer))))
+    (vlf-occur-visit event)
+    (pop-to-buffer occur-buffer)))
+
+(defun vlf-occur-visit-new-buffer ()
+  "Visit `vlf-occur' link in new vlf buffer."
+  (interactive)
+  (let ((current-prefix-arg t))
+    (vlf-occur-visit)))
+
+(defun vlf-occur-visit (&optional event)
+  "Visit current `vlf-occur' link in a vlf buffer.
+With prefix argument or if original VLF buffer has been killed,
+open new VLF session.
+EVENT may hold details of the invocation."
+  (interactive (list last-nonmenu-event))
+  (when event
+    (set-buffer (window-buffer (posn-window (event-end event))))
+    (goto-char (posn-point (event-end event))))
+  (let* ((pos (point))
+         (pos-relative (- pos (line-beginning-position) 1))
+         (file (get-char-property pos 'file)))
+    (if file
+        (let ((chunk-start (get-char-property pos 'chunk-start))
+              (chunk-end (get-char-property pos 'chunk-end))
+              (vlf-buffer (get-char-property pos 'buffer))
+              (occur-buffer (current-buffer))
+              (match-pos (+ (get-char-property pos 'line-pos)
+                            pos-relative)))
+          (cond (current-prefix-arg
+                 (setq vlf-buffer (vlf file))
+                 (switch-to-buffer occur-buffer))
+                ((not (buffer-live-p vlf-buffer))
+                 (or (catch 'found
+                       (dolist (buf (buffer-list))
+                         (set-buffer buf)
+                         (and vlf-mode (equal file buffer-file-name)
+                              (setq vlf-buffer buf)
+                              (throw 'found t))))
+                     (setq vlf-buffer (vlf file)))
+                 (switch-to-buffer occur-buffer)))
+          (pop-to-buffer vlf-buffer)
+          (vlf-move-to-chunk chunk-start chunk-end)
+          (goto-char match-pos)))))
+
+(defun vlf-occur (regexp)
+  "Make whole file occur style index for REGEXP.
+Prematurely ending indexing will still show what's found so far."
+  (interactive (list (read-regexp "List lines matching regexp"
                                   (if regexp-history
-                                      (car regexp-history))
-                                  'regexp-history)
-                     (or current-prefix-arg 1)))
-  (let ((match-chunk-start vlf-start-pos)
-        (match-chunk-end vlf-end-pos)
-        (match-start-pos (point))
-        (match-end-pos (point))
-        (to-find count)
-        (search-reporter (make-progress-reporter
-                          (concat "Searching for " regexp)
-                          (- vlf-file-size vlf-end-pos)
-                          vlf-file-size))
-        (initial-chunk t))
+                                      (car regexp-history)))))
+  (if (buffer-modified-p) ;use temporary buffer not to interfere with modifications
+      (let ((vlf-buffer (current-buffer))
+            (file buffer-file-name)
+            (batch-size vlf-batch-size))
+        (with-temp-buffer
+          (setq buffer-file-name file)
+          (set-buffer-modified-p nil)
+          (set (make-local-variable 'vlf-batch-size) batch-size)
+          (vlf-mode 1)
+          (goto-char (point-min))
+          (vlf-with-undo-disabled
+           (vlf-build-occur regexp vlf-buffer))))
+    (let ((start-pos vlf-start-pos)
+          (end-pos vlf-end-pos)
+          (pos (point)))
+      (vlf-beginning-of-file)
+      (goto-char (point-min))
+      (vlf-with-undo-disabled
+       (unwind-protect (vlf-build-occur regexp (current-buffer))
+         (vlf-move-to-chunk start-pos end-pos)
+         (goto-char pos))))))
+
+(defun vlf-build-occur (regexp vlf-buffer)
+  "Build occur style index for REGEXP over VLF-BUFFER."
+  (let ((case-fold-search t)
+        (line 1)
+        (last-match-line 0)
+        (last-line-pos (point-min))
+        (file buffer-file-name)
+        (total-matches 0)
+        (match-end-pos (+ vlf-start-pos (position-bytes (point))))
+        (occur-buffer (generate-new-buffer
+                       (concat "*VLF-occur " (file-name-nondirectory
+                                              buffer-file-name)
+                               "*")))
+        (line-regexp (concat "\\(?5:[\n\C-m]\\)\\|\\(?10:"
+                             regexp "\\)"))
+        (batch-step (/ vlf-batch-size 8))
+        (end-of-file nil)
+        (reporter (make-progress-reporter
+                   (concat "Building index for " regexp "...")
+                   vlf-start-pos vlf-file-size)))
     (unwind-protect
-        (catch 'start-of-file
-          (while (not (zerop to-find))
-            (cond ((re-search-backward regexp nil t)
-                   (setq to-find (if (= match-end-pos
-                                        (match-end 0))
-                                     to-find
-                                   (1- to-find))
-                         match-start-pos (match-beginning 0)
-                         match-end-pos (match-end 0)
-                         match-chunk-start vlf-start-pos
-                         match-chunk-end vlf-end-pos)
-                   (if (and (< match-end-pos vlf-batch-size)
-                            (> (- vlf-end-pos vlf-start-pos)
-                               vlf-batch-size))
-                       (setq match-chunk-end
-                             (- match-chunk-end
-                                vlf-batch-size))))
-                  ((zerop vlf-start-pos)
-                   (throw 'start-of-file nil))
-                  (t (if initial-chunk
-                         (progn (setq initial-chunk nil)
-                                (vlf-prev-batch -1))
-                       (vlf-move-to-chunk (- vlf-start-pos
-                                              vlf-batch-size)
-                                           (- vlf-end-pos
-                                              vlf-batch-size)))
-                     (goto-char (if (< match-chunk-start vlf-end-pos)
-                                    match-end-pos
-                                  (point-max)))
-                     (setq last-chunk-match nil)
-                     (progress-reporter-update search-reporter
-                                               (- vlf-file-size
-                                                  vlf-start-pos)))))
-          (progress-reporter-done search-reporter))
-      (vlf-end-search match-chunk-start match-chunk-end
-                       match-start-pos count to-find))))
-
-(defun vlf-end-search (match-chunk-start match-chunk-end
-                                          match-pos count to-find)
-  "Move to chunk determined by MATCH-CHUNK-START and MATCH-CHUNK-END.
-Go to MATCH-POS and according to COUNT and left TO-FIND show if search
-has been successful.  Return nil if nothing found."
-  (vlf-move-to-chunk match-chunk-start match-chunk-end)
-  (goto-char match-pos)
-  (cond ((zerop to-find) t)
-        ((< to-find count)
-         (message "Moved to the %d match which is last found"
-                  (- count to-find))
-         t)
-        (t (message "Not found")
-           nil)))
+        (progn
+          (while (not end-of-file)
+            (if (re-search-forward line-regexp nil t)
+                (progn
+                  (setq match-end-pos (+ vlf-start-pos
+                                         (position-bytes
+                                          (match-end 0))))
+                  (if (match-string 5)
+                      (setq line (1+ line) ; line detected
+                            last-line-pos (point))
+                    (let* ((chunk-start vlf-start-pos)
+                           (chunk-end vlf-end-pos)
+                           (line-pos (line-beginning-position))
+                           (line-text (buffer-substring
+                                       line-pos (line-end-position))))
+                      (with-current-buffer occur-buffer
+                        (unless (= line last-match-line) ;new match line
+                          (insert "\n:") ; insert line number
+                          (let* ((overlay-pos (1- (point)))
+                                 (overlay (make-overlay
+                                           overlay-pos
+                                           (1+ overlay-pos))))
+                            (overlay-put overlay 'before-string
+                                         (propertize
+                                          (number-to-string line)
+                                          'face 'shadow)))
+                          (insert (propertize line-text ; insert line
+                                              'file file
+                                              'buffer vlf-buffer
+                                              'chunk-start chunk-start
+                                              'chunk-end chunk-end
+                                              'mouse-face '(highlight)
+                                              'line-pos line-pos
+                                              'help-echo
+                                              (format "Move to line %d"
+                                                      line))))
+                        (setq last-match-line line
+                              total-matches (1+ total-matches))
+                        (let ((line-start (1+
+                                           (line-beginning-position)))
+                              (match-pos (match-beginning 10)))
+                          (add-text-properties ; mark match
+                           (+ line-start match-pos (- last-line-pos))
+                           (+ line-start (match-end 10)
+                              (- last-line-pos))
+                           (list 'face 'match
+                                 'help-echo
+                                 (format "Move to match %d"
+                                         total-matches))))))))
+              (setq end-of-file (= vlf-end-pos vlf-file-size))
+              (unless end-of-file
+                (let ((batch-move (- vlf-end-pos batch-step)))
+                  (vlf-move-to-batch (if (< batch-move match-end-pos)
+                                         match-end-pos
+                                       batch-move) t))
+                (goto-char (if (< vlf-start-pos match-end-pos)
+                               (or (byte-to-position (- match-end-pos
+                                                        vlf-start-pos))
+                                   (point-min))
+                             (point-min)))
+                (setq last-match-line 0
+                      last-line-pos (line-beginning-position))
+                (progress-reporter-update reporter vlf-end-pos))))
+          (progress-reporter-done reporter))
+      (set-buffer-modified-p nil)
+      (if (zerop total-matches)
+          (progn (with-current-buffer occur-buffer
+                   (set-buffer-modified-p nil))
+                 (kill-buffer occur-buffer)
+                 (message "No matches for \"%s\"" regexp))
+        (with-current-buffer occur-buffer
+          (goto-char (point-min))
+          (insert (propertize
+                   (format "%d matches in %d lines for \"%s\" \
+in file: %s" total-matches line regexp file)
+                   'face 'underline))
+          (set-buffer-modified-p nil)
+          (forward-char 2)
+          (vlf-occur-mode))
+        (display-buffer occur-buffer)))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; editing
+
+(defun vlf-refresh ()
+  "Discard edit and refresh chunk from file."
+  (interactive)
+  (set-buffer-modified-p nil)
+  (vlf-move-to-chunk-2 vlf-start-pos vlf-end-pos))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; saving
+
+(defun vlf-write ()
+  "Write current chunk to file.  Always return true to disable save.
+If changing size of chunk, shift remaining file content."
+  (interactive)
+  (when (and (buffer-modified-p)
+             (or (verify-visited-file-modtime (current-buffer))
+                 (y-or-n-p "File has changed since visited or saved.  \
+Save anyway? ")))
+    (let ((pos (point))
+          (size-change (- vlf-end-pos vlf-start-pos
+                          (length (encode-coding-region
+                                   (point-min) (point-max)
+                                   buffer-file-coding-system t)))))
+      (cond ((zerop size-change)
+             (write-region nil nil buffer-file-name vlf-start-pos t))
+            ((< 0 size-change)
+             (vlf-file-shift-back size-change))
+            (t (vlf-file-shift-forward (- size-change))))
+      (vlf-move-to-chunk-2 vlf-start-pos vlf-end-pos)
+      (goto-char pos)))
+  t)
+
+(defun vlf-file-shift-back (size-change)
+  "Shift file contents SIZE-CHANGE bytes back."
+  (write-region nil nil buffer-file-name vlf-start-pos t)
+  (let ((read-start-pos vlf-end-pos)
+        (coding-system-for-write 'no-conversion)
+        (reporter (make-progress-reporter "Adjusting file content..."
+                                          vlf-end-pos
+                                          vlf-file-size)))
+    (vlf-with-undo-disabled
+     (while (vlf-shift-batch read-start-pos (- read-start-pos
+                                               size-change))
+       (setq read-start-pos (+ read-start-pos vlf-batch-size))
+       (progress-reporter-update reporter read-start-pos))
+     ;; pad end with space
+     (erase-buffer)
+     (vlf-verify-size)
+     (insert-char 32 size-change))
+    (write-region nil nil buffer-file-name (- vlf-file-size
+                                              size-change) t)
+    (progress-reporter-done reporter)))
+
+(defun vlf-shift-batch (read-pos write-pos)
+  "Read `vlf-batch-size' bytes from READ-POS and write them \
+back at WRITE-POS.  Return nil if EOF is reached, t otherwise."
+  (erase-buffer)
+  (vlf-verify-size)
+  (let ((read-end (+ read-pos vlf-batch-size)))
+    (insert-file-contents-literally buffer-file-name nil
+                                    read-pos
+                                    (min vlf-file-size read-end))
+    (write-region nil nil buffer-file-name write-pos 0)
+    (< read-end vlf-file-size)))
+
+(defun vlf-file-shift-forward (size-change)
+  "Shift file contents SIZE-CHANGE bytes forward.
+Done by saving content up front and then writing previous batch."
+  (let ((read-size (max (/ vlf-batch-size 2) size-change))
+        (read-pos vlf-end-pos)
+        (write-pos vlf-start-pos)
+        (reporter (make-progress-reporter "Adjusting file content..."
+                                          vlf-start-pos
+                                          vlf-file-size)))
+    (vlf-with-undo-disabled
+     (when (vlf-shift-batches read-size read-pos write-pos t)
+       (setq write-pos (+ read-pos size-change)
+             read-pos (+ read-pos read-size))
+       (progress-reporter-update reporter write-pos)
+       (let ((coding-system-for-write 'no-conversion))
+         (while (vlf-shift-batches read-size read-pos write-pos nil)
+           (setq write-pos (+ read-pos size-change)
+                 read-pos (+ read-pos read-size))
+           (progress-reporter-update reporter write-pos)))))
+    (progress-reporter-done reporter)))
+
+(defun vlf-shift-batches (read-size read-pos write-pos hide-read)
+  "Append READ-SIZE bytes of file starting at READ-POS.
+Then write initial buffer content to file at WRITE-POS.
+If HIDE-READ is non nil, temporarily hide literal read content.
+Return nil if EOF is reached, t otherwise."
+  (vlf-verify-size)
+  (let ((read-more (< read-pos vlf-file-size))
+        (start-write-pos (point-min))
+        (end-write-pos (point-max)))
+    (when read-more
+      (goto-char end-write-pos)
+      (insert-file-contents-literally buffer-file-name nil read-pos
+                                      (min vlf-file-size
+                                           (+ read-pos read-size))))
+    ;; write
+    (if hide-read ; hide literal region if user has to choose encoding
+        (narrow-to-region start-write-pos end-write-pos))
+    (write-region start-write-pos end-write-pos
+                  buffer-file-name write-pos 0)
+    (delete-region start-write-pos end-write-pos)
+    (if hide-read (widen))
+    read-more))
 
 (provide 'vlf)