Re: feature request: view part of file

[Andrey Kotlarski <m00naticus@gmail.com>]

Sam Steingold <sds@gnu.org> writes:

> I put vlf.el by Mathias into elpa.

Hello,

thanks for putting this in gnu elpa, it's quite useful.

> I rewrote it almost completely (bzr still has the original version at
> revno 233).
>
> it works for me.
> I wish I knew how to hook it into files.el, but I don't...
>
> vlf.el has 2 issues:
>
> 1. insert-file-contents hangs emacs (see `vlf-next-batch'):
>
>     ;; replacing `erase-buffer' with replace arg to `insert-file-contents'
>     ;; hangs emacs
>     (unless append (erase-buffer))
>     (insert-file-contents buffer-file-name nil vlf-end-pos end)

I guess you've been using rather large file.  My attempts showed that
non nil REPLACE argument of insert-file-contents doesn't honour BEG END
and always inserts the whole file.

Anyway, I've made a few additions that I think are worth sharing.

* Option to start viewing from the end of file
* Ability to jump/insert given number of batches at once
* Ability to view newly added content if the file has grown meanwhile

Here is a changelog entry if necessary and a diff:

2012-08-04  Andrey Kotlarski  <m00naticus@gmail.com>

	* vlf.el (vlf-mode-map): Add decrease batch key binding.
	(vlf-change-batch-size): Fix doc string.
	(vlf-format-buffer-name): Fix indentation.
	(vlf-next-batch): Optionally jump/insert several batches based on
	prefix argument. If requested batch is past the end of file, get
	current file length and view as much as possible.
	(vlf-prev-batch): Optionally jump/insert several batches based on
	prefix argument.
	(vlf): Start viewing from end of file if prefix argument is
	given. Add autoload cookie.


diff -u vlf.el\~ vlf.el
--- vlf.el~	2012-07-23 17:34:54.000000000 +0300
+++ vlf.el	2012-07-27 23:44:43.696003339 +0300
@@ -2,7 +2,7 @@
 
 ;; Copyright (C) 2006, 2012  Free Software Foundation, Inc.
 
-;; Version: 0.2
+;; Version: 0.3
 ;; Keywords: large files, utilities
 ;; Authors: 2006 Mathias Dahl <mathias.dahl@gmail.com>
 ;;          2012 Sam Steingold <sds@gnu.org>
@@ -57,6 +57,10 @@
     (define-key map [M-next] 'vlf-next-batch)
     (define-key map [M-prior] 'vlf-prev-batch)
     (define-key map (kbd "C-+") 'vlf-change-batch-size)
+    (define-key map (kbd "C--")
+      (lambda () "Decrease vlf batch size by factor of 2."
+	(interactive)
+	(vlf-change-batch-size t)))
     map)
   "Keymap for `vlf-mode'.")
 
@@ -71,22 +75,22 @@
 (defun vlf-change-batch-size (decrease)
   "Change the buffer-local value of `vlf-batch-size'.
 Normally, the value is doubled;
-with the prefix argument it is halved."
+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)))
+	(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)[%d,%d](%d)"
-          (file-name-nondirectory buffer-file-name)
-          (file-size-human-readable vlf-file-size)
-          vlf-start-pos vlf-end-pos vlf-batch-size))
+	  (file-name-nondirectory buffer-file-name)
+	  (file-size-human-readable vlf-file-size)
+	  vlf-start-pos vlf-end-pos vlf-batch-size))
 
 (defun vlf-update-buffer-name ()
   "Update the current buffer name."
@@ -94,56 +98,80 @@
 
 (defun vlf-next-batch (append)
   "Display the next batch of file data.
-Append to the existing buffer when the prefix argument is supplied."
-  (interactive "P")
-  (when (= vlf-end-pos vlf-file-size)
-    (error "Already at EOF"))
-  (let ((inhibit-read-only t)
-        (end (min vlf-file-size (+ vlf-end-pos vlf-batch-size))))
-    (goto-char (point-max))
-    ;; replacing `erase-buffer' with replace arg to `insert-file-contents'
-    ;; hangs emacs
-    (unless append (erase-buffer))
-    (insert-file-contents buffer-file-name nil vlf-end-pos end)
-    (unless append
-      (setq vlf-start-pos vlf-end-pos))
-    (setq vlf-end-pos end)
-    (set-buffer-modified-p nil)
-    (vlf-update-buffer-name)))
+When prefix argument is supplied and positive
+ jump over APPEND number of batches.
+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)))
+      (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))
+    (setq vlf-end-pos end))
+  (set-buffer-modified-p nil)
+  (vlf-update-buffer-name))
 
 (defun vlf-prev-batch (prepend)
   "Display the previous batch of file data.
-Prepend to the existing buffer when the prefix argument is supplied."
-  (interactive "P")
-  (when (= vlf-start-pos 0)
-    (error "Already at BOF"))
+When prefix argument is supplied and positive
+ jump over PREPEND number of batches.
+When prefix argument is negative
+ append previous PREPEND number of batches to the existing buffer."
+  (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))))
-    (goto-char (point-min))
-    (unless prepend (erase-buffer))
-    (insert-file-contents buffer-file-name nil start vlf-start-pos)
-    (unless prepend
-      (setq vlf-end-pos vlf-start-pos))
-    (setq vlf-start-pos start)
-    (set-buffer-modified-p nil)
-    (vlf-update-buffer-name)))
+	(start (max 0 (- vlf-start-pos (* vlf-batch-size
+					  (abs prepend)))))
+	(do-prepend (< prepend 0)))
+    (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))
+    (setq vlf-start-pos start))
+  (set-buffer-modified-p nil)
+  (vlf-update-buffer-name))
 
-(defun vlf (file)
+;;;###autoload
+(defun vlf (from-end file)
   "View a Large File in Emacs.
+With FROM-END prefix, view from the back.
 FILE is the file to open.
 Batches of the file data from FILE will be displayed in a
  read-only buffer.
 You can customize the number of bytes to
  display by customizing `vlf-batch-size'."
-  (interactive "fFile to open: ")
+  (interactive "P\nfFile to open: ")
   (with-current-buffer (generate-new-buffer "*vlf*")
     (setq buffer-file-name file
-          vlf-start-pos 0
-          vlf-end-pos vlf-batch-size
-          vlf-file-size (nth 7 (file-attributes file)))
+	  vlf-file-size (nth 7 (file-attributes file)))
+    (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-update-buffer-name)
     (insert-file-contents buffer-file-name nil
-                          vlf-start-pos vlf-end-pos nil)
+			  vlf-start-pos vlf-end-pos)
     (vlf-mode)
     (display-buffer (current-buffer))))