Add issue section to the README.

of problem characters.

README.org

@@ -1,12 +1,22 @@
-* View Large File Improved
+* View Large Files Improved
 
 An Emacs mode that allows viewing files in chunks.  This is a fork
-that builds on the GNU ELPA vlf.el.  It adds the following
+that builds on the bare bones GNU ELPA vlf.el.  It adds the following
 improvements:
 
-- by chunk search
-- options to jump to end or beginning of file
+- regular expression search on whole file (in constant memory
+  determined by current batch size)
+- chunk editing (if size has changed, saving is done in constant
+  memory determined by current batch size)
+- options to jump to beginning, end or arbitrary file chunk
 - ability to jump/insert given number of batches at once
-- ability to view newly added content if the file has grown meanwhile
+- newly added content is acknowledged if file has changed size
+  meanwhile
 - vlfi is added as an option when opening large files
 
+* Known issues
+
+- When chunk starts with only part of multi-byte character, encoding
+  for this chunk is clobbered visibly, don't edit/save in such case.
+  Workaround is to move back to the first non-clobbered chunk and
+  enlarge batch size until this chunk comes into range.

vlfi.el

@@ -1,9 +1,8 @@
-;;; vlfi.el --- View Large Files Improved
-;;; -*- lexical-bind: t -*-
+;;; vlfi.el --- View Large Files Improved  -*- lexical-binding: t -*-
 
 ;; Copyright (C) 2006, 2012, 2013  Free Software Foundation, Inc.
 
-;; Version: 0.4
+;; Version: 0.6
 ;; Keywords: large files, utilities
 ;; Authors: 2006 Mathias Dahl <mathias.dahl@gmail.com>
 ;;          2012 Sam Steingold <sds@gnu.org>
@@ -28,10 +27,8 @@
 
 ;; This package provides the M-x vlfi command, which visits part of a
 ;; large file in a read-only buffer without visiting the entire file.
-;; The buffer uses VLFI mode, which defines the commands M-<next>
-;; (vlfi-next-batch) and M-<prior> (vlfi-prev-batch) to visit other
-;; parts of the file.  The option `vlfi-batch-size' specifies the size
-;; of each batch, in bytes.
+;; The buffer uses VLFI mode, which defines several commands for
+;; moving around, searching and editing selected chunk of file.
 
 ;; This package is an improved fork of the vlf.el package.
 
@@ -47,28 +44,28 @@
   :type 'integer
   :group 'vlfi)
 
-;; Keep track of file position.
-(defvar vlfi-start-pos)
-(defvar vlfi-end-pos)
-(defvar vlfi-file-size)
+;;; Keep track of file position.
+(defvar vlfi-start-pos 0
+  "Absolute position of the visible chunk start.")
+(defvar vlfi-end-pos vlfi-batch-size
+  "Absolute position of the visible chunk end.")
+(defvar vlfi-file-size 0 "Total size of presented file.")
 
 (defvar vlfi-mode-map
   (let ((map (make-sparse-keymap)))
     (define-key map [M-next] 'vlfi-next-batch)
     (define-key map [M-prior] 'vlfi-prev-batch)
-    (define-key map (kbd "M-+") 'vlfi-change-batch-size)
-    (define-key map (kbd "M--")
+    (define-key map "+" 'vlfi-change-batch-size)
+    (define-key map "-"
       (lambda () "Decrease vlfi batch size by factor of 2."
         (interactive)
         (vlfi-change-batch-size t)))
     (define-key map "s" 'vlfi-re-search-forward)
     (define-key map "r" 'vlfi-re-search-backward)
-    (define-key map ">" (lambda () "Jump to end of file content."
-                          (interactive)
-                          (vlfi-insert-file buffer-file-name t)))
-    (define-key map "<" (lambda () "Jump to beginning of file content."
-                          (interactive)
-                          (vlfi-insert-file buffer-file-name)))
+    (define-key map "[" 'vlfi-beginning-of-file)
+    (define-key map "]" 'vlfi-end-of-file)
+    (define-key map "e" 'vlfi-edit-mode)
+    (define-key map "j" 'vlfi-jump-to-chunk)
     map)
   "Keymap for `vlfi-mode'.")
 
@@ -76,9 +73,86 @@
   "Mode to browse large files in."
   (setq buffer-read-only t)
   (set-buffer-modified-p nil)
+  (buffer-disable-undo)
+  (add-hook 'write-contents-functions 'vlfi-write)
+  (setq revert-buffer-function 'vlfi-revert)
   (make-local-variable 'vlfi-batch-size)
+  (put 'vlfi-batch-size 'permanent-local t)
   (make-local-variable 'vlfi-start-pos)
-  (make-local-variable 'vlfi-file-size))
+  (put 'vlfi-start-pos 'permanent-local t)
+  (make-local-variable 'vlfi-end-pos)
+  (put 'vlfi-end-pos 'permanent-local t)
+  (make-local-variable 'vlfi-file-size)
+  (put 'vlfi-file-size 'permanent-local t))
+
+;;;###autoload
+(defun vlfi (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
+`vlfi-batch-size'."
+  (interactive "fFile to open: \nP")
+  (with-current-buffer (generate-new-buffer "*vlfi*")
+    (setq buffer-file-name file
+          vlfi-file-size (nth 7 (file-attributes file)))
+    (vlfi-insert-file from-end)
+    (vlfi-mode)
+    (switch-to-buffer (current-buffer))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; integration with other packages
+
+;;;###autoload
+(defun dired-vlfi (from-end)
+  "In Dired, visit the file on this line in VLFI mode.
+With FROM-END prefix, view from the back."
+  (interactive "P")
+  (vlfi (dired-get-file-for-visit) from-end))
+
+;;;###autoload
+(eval-after-load "dired"
+  '(define-key dired-mode-map "V" 'dired-vlfi))
+
+;;;###autoload
+(defun vlfi-if-file-too-large (size op-type &optional filename)
+  "If file SIZE larger than `large-file-warning-threshold', \
+allow user to view file with `vlfi', 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 vlfi (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))
+                (vlfi filename nil)
+                (error ""))
+               ((memq char '(?a ?A))
+                (error "Aborted"))))))
+
+;; hijack `abort-if-file-too-large'
+;;;###autoload
+(fset 'abort-if-file-too-large 'vlfi-if-file-too-large)
+
+;; non recent Emacs
+(unless (fboundp 'file-size-human-readable)
+  (defun file-size-human-readable (file-size)
+    "Print FILE-SIZE in MB."
+    (format "%.1fMB" (/ file-size 1024.0))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; utilities
 
 (defun vlfi-change-batch-size (decrease)
   "Change the buffer-local value of `vlfi-batch-size'.
@@ -87,24 +161,57 @@ with the prefix argument DECREASE it is halved."
   (interactive "P")
   (or (assq 'vlfi-batch-size (buffer-local-variables))
       (error "%s is not local in this buffer" 'vlfi-batch-size))
-  (setq vlfi-batch-size
-        (if decrease
-            (/ vlfi-batch-size 2)
-          (* vlfi-batch-size 2)))
-  (vlfi-update-buffer-name))
+  (setq vlfi-batch-size (if decrease
+                            (/ vlfi-batch-size 2)
+                          (* vlfi-batch-size 2)))
+  (vlfi-move-to-batch vlfi-start-pos))
 
 (defun vlfi-format-buffer-name ()
   "Return format for vlfi buffer name."
-  (format "%s(%s)[%.2f%%%%](%d)"
+  (format "%s(%s)[%d/%d](%d)"
           (file-name-nondirectory buffer-file-name)
           (file-size-human-readable vlfi-file-size)
-          (/ (* 100 vlfi-end-pos) (float vlfi-file-size))
+          (/ vlfi-end-pos vlfi-batch-size)
+          (/ vlfi-file-size vlfi-batch-size)
           vlfi-batch-size))
 
 (defun vlfi-update-buffer-name ()
   "Update the current buffer name."
   (rename-buffer (vlfi-format-buffer-name) t))
 
+(defun vlfi-insert-file (&optional from-end)
+  "Insert first chunk of current file contents in current buffer.
+With FROM-END prefix, start from the back."
+  (if from-end
+      (setq vlfi-start-pos (max 0 (- vlfi-file-size vlfi-batch-size))
+            vlfi-end-pos vlfi-file-size)
+    (setq vlfi-start-pos 0
+          vlfi-end-pos (min vlfi-batch-size vlfi-file-size)))
+  (vlfi-move-to-chunk vlfi-start-pos vlfi-end-pos))
+
+(defun vlfi-beginning-of-file ()
+  "Jump to beginning of file content."
+  (interactive)
+  (vlfi-insert-file))
+
+(defun vlfi-end-of-file ()
+  "Jump to end of file content."
+  (interactive)
+  (vlfi-insert-file t))
+
+(defun vlfi-revert (&rest args)
+  "Revert current chunk.  Ignore ARGS."
+  (ignore args)
+  (vlfi-move-to-chunk vlfi-start-pos vlfi-end-pos))
+
+(defun vlfi-jump-to-chunk (n)
+  "Go to to chunk N."
+  (interactive "nGoto to chunk: ")
+  (vlfi-move-to-batch (* (1- n) vlfi-batch-size)))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; batch movement
+
 (defun vlfi-next-batch (append)
   "Display the next batch of file data.
 When prefix argument is supplied and positive
@@ -114,7 +221,7 @@ When prefix argument is negative
   (interactive "p")
   (let ((end (+ vlfi-end-pos (* vlfi-batch-size
                                 (abs append)))))
-    (when (< vlfi-file-size end)		; re-check file size
+    (when (< vlfi-file-size end)        ; re-check file size
       (setq vlfi-file-size (nth 7 (file-attributes buffer-file-name)))
       (cond ((= vlfi-end-pos vlfi-file-size)
              (error "Already at EOF"))
@@ -127,13 +234,13 @@ When prefix argument is negative
           (goto-char (point-max))
         (setq vlfi-start-pos (- end vlfi-batch-size))
         (erase-buffer))
-      (insert-file-contents buffer-file-name nil
-                            (if do-append
-                                vlfi-end-pos
-                              vlfi-start-pos)
+      (insert-file-contents buffer-file-name nil (if do-append
+                                                     vlfi-end-pos
+                                                   vlfi-start-pos)
                             end)
       (goto-char pos))
     (setq vlfi-end-pos end))
+  (set-visited-file-modtime)
   (set-buffer-modified-p nil)
   (vlfi-update-buffer-name))
 
@@ -161,213 +268,295 @@ When prefix argument is negative
                             vlfi-end-pos))
     (goto-char (- (point-max) pos))
     (setq vlfi-start-pos start))
+  (set-visited-file-modtime)
+  (set-buffer-modified-p nil)
+  (vlfi-update-buffer-name))
+
+(defun vlfi-move-to-batch (start)
+  "Move to batch determined by START.
+Adjust according to file start/end and show `vlfi-batch-size' bytes."
+  (setq vlfi-start-pos (max 0 start)
+        vlfi-end-pos (+ vlfi-start-pos vlfi-batch-size))
+  (if (< vlfi-file-size vlfi-end-pos)   ; re-check file size
+      (setq vlfi-file-size
+            (nth 7 (file-attributes buffer-file-name))
+            vlfi-end-pos (min vlfi-end-pos vlfi-file-size)
+            vlfi-start-pos (max 0 (- vlfi-end-pos vlfi-batch-size))))
+  (let ((inhibit-read-only t)
+        (pos (point)))
+    (erase-buffer)
+    (insert-file-contents buffer-file-name nil
+                          vlfi-start-pos vlfi-end-pos)
+    (goto-char pos))
+  (set-visited-file-modtime)
   (set-buffer-modified-p nil)
   (vlfi-update-buffer-name))
 
 (defun vlfi-move-to-chunk (start end)
   "Move to chunk determined by START END."
-  (if (< vlfi-file-size end)          ; re-check file size
+  (if (< vlfi-file-size end)            ; re-check file size
       (setq vlfi-file-size (nth 7
                                 (file-attributes buffer-file-name))))
   (setq vlfi-start-pos (max 0 start)
         vlfi-end-pos (min end vlfi-file-size))
-  (let ((inhibit-read-only t))
+  (let ((inhibit-read-only t)
+        (pos (point)))
     (erase-buffer)
     (insert-file-contents buffer-file-name nil
-                          vlfi-start-pos vlfi-end-pos))
+                          vlfi-start-pos vlfi-end-pos)
+    (goto-char pos))
+  (set-visited-file-modtime)
   (set-buffer-modified-p nil)
   (vlfi-update-buffer-name))
 
-(defun vlfi-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 vlfi-start-pos (max 0 (- vlfi-file-size vlfi-batch-size))
-            vlfi-end-pos vlfi-file-size)
-    (setq vlfi-start-pos 0
-          vlfi-end-pos (min vlfi-batch-size vlfi-file-size)))
-  (vlfi-move-to-chunk vlfi-start-pos vlfi-end-pos))
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; search
 
-;;;###autoload
-(defun vlfi (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
-`vlfi-batch-size'."
-  (interactive "fFile to open: \nP")
-  (with-current-buffer (generate-new-buffer "*vlfi*")
-    (buffer-disable-undo)
-    (setq buffer-file-name file
-          vlfi-file-size (nth 7 (file-attributes file)))
-    (vlfi-insert-file file from-end)
-    (vlfi-mode)
-    (switch-to-buffer (current-buffer))))
-
-;;;###autoload
-(defun dired-vlfi (from-end)
-  "In Dired, visit the file on this line in VLFI mode.
-With FROM-END prefix, view from the back."
-  (interactive "P")
-  (vlfi (dired-get-file-for-visit) from-end))
-
-;;;###autoload
-(eval-after-load "dired"
-  '(define-key dired-mode-map "V" 'dired-vlfi))
-
-;;;###autoload
-(defun vlfi-if-file-too-large (size op-type &optional filename)
-  "If file SIZE larger than `large-file-warning-threshold', \
-allow user to view file with `vlfi', 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 vlfi (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))
-                (vlfi nil filename)
-                (error ""))
-               ((memq char '(?a ?A))
-                (error "Aborted"))))))
-
-;;; hijack `abort-if-file-too-large'
-;;;###autoload
-(fset 'abort-if-file-too-large 'vlfi-if-file-too-large)
-
-(defun vlfi-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 vlfi-start-pos)
-        (match-chunk-end vlfi-end-pos)
-        (match-start-pos (point))
-        (match-end-pos (point))
-        (to-find count)
-        (search-reporter (make-progress-reporter
-                          (concat "Searching for " regexp)
-                          vlfi-start-pos vlfi-file-size))
-        (initial-chunk t))
+(defun vlfi-re-search (regexp count backward)
+  "Search for REGEXP COUNT number of times forward or BACKWARD."
+  (let* ((match-start-pos (+ vlfi-start-pos (point)))
+         (match-end-pos match-start-pos)
+         (to-find count)
+         (search-reporter (make-progress-reporter
+                           (concat "Searching for " regexp)
+                           (if backward
+                               (- vlfi-file-size vlfi-end-pos)
+                             vlfi-start-pos)
+                           vlfi-file-size))
+         (batch-step (/ vlfi-batch-size 8))) ; amount of chunk overlap
     (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 vlfi-start-pos
-                         match-chunk-end vlfi-end-pos)
-                   (if (and (< vlfi-batch-size match-start-pos)
-                            (> (- vlfi-end-pos vlfi-start-pos)
-                               vlfi-batch-size))
-                       (setq match-chunk-start
-                             (+ match-chunk-start vlfi-batch-size)
-                             match-start-pos (- match-start-pos
-                                                vlfi-batch-size)
-                             match-end-pos (- match-end-pos
-                                              vlfi-batch-size))))
-                  ((= vlfi-end-pos vlfi-file-size)
-                   (throw 'end-of-file nil))
-                  (t (if initial-chunk
-                         (progn (setq initial-chunk nil)
-                                (vlfi-next-batch -1))
-                       (vlfi-move-to-chunk (+ vlfi-start-pos
-                                              vlfi-batch-size)
-                                           (+ vlfi-end-pos
-                                              vlfi-batch-size)))
-                     (goto-char (if (< vlfi-start-pos match-chunk-end)
-                                    match-start-pos
-                                  (point-min)))
-                     (goto-char match-start-pos)
-                     (progress-reporter-update search-reporter
-                                               vlfi-end-pos))))
+          (if backward
+              (while (not (zerop to-find))
+                (cond ((re-search-backward regexp nil t)
+                       (setq to-find (1- to-find)
+                             match-start-pos (+ vlfi-start-pos
+                                                (match-beginning 0))
+                             match-end-pos (+ vlfi-start-pos
+                                              (match-end 0))))
+                      ((zerop vlfi-start-pos)
+                       (throw 'end-of-file nil))
+                      (t (let ((batch-move (- vlfi-start-pos
+                                              (- vlfi-batch-size
+                                                 batch-step))))
+                           (vlfi-move-to-batch
+                            (if (< match-start-pos batch-move)
+                                (- match-start-pos vlfi-batch-size)
+                              batch-move)))
+                         (goto-char (if (< match-start-pos
+                                           vlfi-end-pos)
+                                        (- match-start-pos
+                                           vlfi-start-pos)
+                                      (point-max)))
+                         (progress-reporter-update search-reporter
+                                                   vlfi-start-pos))))
+            (while (not (zerop to-find))
+              (cond ((re-search-forward regexp nil t)
+                     (setq to-find (1- to-find)
+                           match-start-pos (+ vlfi-start-pos
+                                              (match-beginning 0))
+                           match-end-pos (+ vlfi-start-pos
+                                            (match-end 0))))
+                    ((= vlfi-end-pos vlfi-file-size)
+                     (throw 'end-of-file nil))
+                    (t (let ((batch-move (- vlfi-end-pos batch-step)))
+                         (vlfi-move-to-batch
+                          (if (< batch-move match-end-pos)
+                              match-end-pos
+                            batch-move)))
+                       (goto-char (if (< vlfi-start-pos match-end-pos)
+                                      (- match-end-pos vlfi-start-pos)
+                                    (point-min)))
+                       (progress-reporter-update search-reporter
+                                                 vlfi-end-pos)))))
           (progress-reporter-done search-reporter))
-      (vlfi-end-search match-chunk-start match-chunk-end
-                       match-end-pos count to-find))))
+      (if backward
+          (vlfi-goto-match match-end-pos match-start-pos
+                           count to-find)
+        (vlfi-goto-match match-start-pos match-end-pos
+                         count to-find)))))
 
-(defun vlfi-re-search-backward (regexp count)
-  "Search backward for REGEXP prefix COUNT number of times."
+(defun vlfi-goto-match (match-pos-start match-pos-end count to-find)
+  "Move to chunk 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."
+  (let ((success (zerop to-find)))
+    (or success
+        (vlfi-move-to-batch (- match-pos-start
+                               (/ vlfi-batch-size 2))))
+    (let* ((match-end (- match-pos-end vlfi-start-pos))
+           (overlay (make-overlay (- match-pos-start vlfi-start-pos)
+                                  match-end)))
+      (overlay-put overlay 'face 'region)
+      (or success (goto-char match-end))
+      (prog1 (cond (success t)
+                   ((< to-find count)
+                    (message "Moved to the %d match which is last"
+                             (- count to-find))
+                    t)
+                   (t (message "Not found")
+                      nil))
+        (sit-for 0.1)
+        (delete-overlay overlay)))))
+
+(defun vlfi-re-search-forward (regexp count)
+  "Search forward for REGEXP prefix COUNT number of times.
+Search is performed chunk by chunk in `vlfi-batch-size' memory."
   (interactive (list (read-regexp "Search whole file"
                                   (if regexp-history
                                       (car regexp-history))
                                   'regexp-history)
                      (or current-prefix-arg 1)))
-  (let ((match-chunk-start vlfi-start-pos)
-        (match-chunk-end vlfi-end-pos)
-        (match-start-pos (point))
-        (match-end-pos (point))
-        (to-find count)
-        (search-reporter (make-progress-reporter
-                          (concat "Searching for " regexp)
-                          (- vlfi-file-size vlfi-end-pos)
-                          vlfi-file-size))
-        (initial-chunk t))
-    (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 vlfi-start-pos
-                         match-chunk-end vlfi-end-pos)
-                   (if (and (< match-end-pos vlfi-batch-size)
-                            (> (- vlfi-end-pos vlfi-start-pos)
-                               vlfi-batch-size))
-                       (setq match-chunk-end
-                             (- match-chunk-end
-                                vlfi-batch-size))))
-                  ((zerop vlfi-start-pos)
-                   (throw 'start-of-file nil))
-                  (t (if initial-chunk
-                         (progn (setq initial-chunk nil)
-                                (vlfi-prev-batch -1))
-                       (vlfi-move-to-chunk (- vlfi-start-pos
-                                              vlfi-batch-size)
-                                           (- vlfi-end-pos
-                                              vlfi-batch-size)))
-                     (goto-char (if (< match-chunk-start vlfi-end-pos)
-                                    match-end-pos
-                                  (point-max)))
-                     (setq last-chunk-match nil)
-                     (progress-reporter-update search-reporter
-                                               (- vlfi-file-size
-                                                  vlfi-start-pos)))))
-          (progress-reporter-done search-reporter))
-      (vlfi-end-search match-chunk-start match-chunk-end
-                       match-start-pos count to-find))))
+  (vlfi-re-search regexp count nil))
 
-(defun vlfi-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."
-  (vlfi-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)))
+(defun vlfi-re-search-backward (regexp count)
+  "Search backward for REGEXP prefix COUNT number of times.
+Search is performed chunk by chunk in `vlfi-batch-size' memory."
+  (interactive (list (read-regexp "Search whole file backward"
+                                  (if regexp-history
+                                      (car regexp-history))
+                                  'regexp-history)
+                     (or current-prefix-arg 1)))
+  (vlfi-re-search regexp count t))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; editing
+
+(defvar vlfi-edit-mode-map
+  (let ((map (make-sparse-keymap)))
+    (set-keymap-parent map text-mode-map)
+    (define-key map "\C-c\C-c" 'vlfi-write)
+    (define-key map "\C-c\C-q" 'vlfi-discard-edit)
+    map)
+  "Keymap for command `vlfi-edit-mode'.")
+
+(define-derived-mode vlfi-edit-mode vlfi-mode "VLFI[edit]"
+  "Major mode for editing large file chunks."
+  (setq buffer-read-only nil)
+  (buffer-enable-undo)
+  (message (substitute-command-keys
+            "Editing: Type \\[vlfi-write] to write chunk \
+or \\[vlfi-discard-edit] to discard changes.")))
+
+(defun vlfi-discard-edit ()
+  "Discard edit and refresh chunk from file."
+  (interactive)
+  (vlfi-move-to-chunk vlfi-start-pos vlfi-end-pos)
+  (vlfi-mode)
+  (message "Switched to VLFI mode."))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; saving
+
+(defun vlfi-write ()
+  "Write current chunk to file.  Always return true to disable save.
+If changing size of chunk shift remaining file content."
+  (interactive)
+  (when (and (derived-mode-p 'vlfi-mode)
+             (buffer-modified-p)
+             (or (verify-visited-file-modtime)
+                 (y-or-n-p "File has changed since visited or saved.  \
+Save anyway? ")))
+    (let ((size-change (- vlfi-end-pos vlfi-start-pos
+                          (length (encode-coding-region
+                                   (point-min) (point-max)
+                                   buffer-file-coding-system t))))
+          (pos (point)))
+      (cond ((zerop size-change)
+             (write-region nil nil buffer-file-name vlfi-start-pos t))
+            ((< 0 size-change)
+             (vlfi-file-shift-back size-change))
+            (t (vlfi-file-shift-forward (- size-change))))
+      (goto-char pos))
+    (vlfi-move-to-chunk vlfi-start-pos vlfi-end-pos)
+    (vlfi-mode)
+    t))
+
+(defun vlfi-file-shift-back (size-change)
+  "Shift file contents SIZE-CHANGE bytes back."
+  (let ((coding-system buffer-file-coding-system))
+    (write-region nil nil buffer-file-name vlfi-start-pos t)
+    (setq buffer-file-coding-system nil)
+    (let ((read-start-pos vlfi-end-pos)
+          (reporter (make-progress-reporter "Adjusting file content"
+                                            vlfi-end-pos
+                                            vlfi-file-size)))
+      (while (vlfi-shift-batch read-start-pos (- read-start-pos
+                                                 size-change))
+        (setq read-start-pos (+ read-start-pos vlfi-batch-size))
+        (progress-reporter-update reporter read-start-pos))
+      ;; pad end with space
+      (erase-buffer)
+      (insert-char 32 size-change)
+      (write-region nil nil buffer-file-name (- vlfi-file-size
+                                                size-change) t)
+      (progress-reporter-done reporter))
+    (setq buffer-file-coding-system coding-system)))
+
+(defun vlfi-shift-batch (read-pos write-pos)
+  "Read `vlfi-batch-size' bytes from READ-POS and write them \
+back at WRITE-POS.  Return nil if EOF is reached, t otherwise."
+  (erase-buffer)
+  (setq vlfi-file-size (nth 7 (file-attributes buffer-file-name)))
+  (let ((read-end (+ read-pos vlfi-batch-size)))
+    (insert-file-contents-literally buffer-file-name nil
+                                    read-pos
+                                    (min vlfi-file-size read-end))
+    (write-region nil nil buffer-file-name write-pos t)
+    (< read-end vlfi-file-size)))
+
+(defun vlfi-file-shift-forward (size-change)
+  "Shift file contents SIZE-CHANGE bytes forward.
+Done by saving content up front and then writing previous batch."
+  (let ((vlfi-buffer (current-buffer))
+        (temp-buffer (generate-new-buffer (concat " "
+                                                  (buffer-name))))
+        (coding-system buffer-file-coding-system))
+    (let ((file buffer-file-name))
+      (set-buffer temp-buffer)
+      (setq buffer-file-name file))
+    (set-buffer vlfi-buffer)
+    (let ((read-buffer temp-buffer)
+          (write-buffer vlfi-buffer)
+          (size (+ vlfi-batch-size size-change))
+          (read-pos vlfi-end-pos)
+          (write-pos vlfi-start-pos)
+          swap-buffer
+          (reporter (make-progress-reporter "Adjusting file content"
+                                            vlfi-start-pos
+                                            vlfi-file-size)))
+      (while (vlfi-shift-batches size read-buffer read-pos
+                                 write-buffer write-pos)
+        (setq swap-buffer read-buffer
+              read-buffer write-buffer
+              write-buffer swap-buffer
+              write-pos (+ read-pos size-change)
+              read-pos (+ read-pos size))
+        (progress-reporter-update reporter write-pos))
+      (progress-reporter-done reporter))
+    (kill-buffer temp-buffer)
+    (set-buffer vlfi-buffer)
+    (setq buffer-file-coding-system coding-system)))
+
+(defun vlfi-shift-batches (size read-buffer read-pos
+                                write-buffer write-pos)
+  "Read SIZE bytes in READ-BUFFER starting from READ-POS.
+Then write contents of WRITE-BUFFER to buffer file at WRITE-POS.
+Return nil if EOF is reached, t otherwise."
+  (let* ((file-size (nth 7 (file-attributes buffer-file-name)))
+         (read-more (< read-pos file-size)))
+    (when read-more
+      ;; read
+      (set-buffer read-buffer)
+      (erase-buffer)
+      (setq buffer-file-coding-system nil)
+      (insert-file-contents-literally buffer-file-name nil read-pos
+                                      (min file-size (+ read-pos
+                                                        size))))
+    ;; write
+    (set-buffer write-buffer)
+    (write-region nil nil buffer-file-name write-pos t)
+    read-more))
 
 (provide 'vlfi)