dirhistory.txt

操作系统文件管理

							1999-06-08
							Emil Brink

			Directory History

1. INTRODUCTION
This little document talks about the "directory history" mechanisms
in gentoo. These mechanisms are responsible for keeping track of
which rows were selected in a certain directory the last time you
visited it in a given pane, the vertical position last used, and
such.
	This document was written during development of gentoo
0.11.8, since that version (hopefully) includes a reworked directory
history system.


2. PROBLEMS WITH PRE-0.11.8's HISTORY HANDLING
The history code included in releases prior to 0.11.8 has several
problems. Perhaps the most severe is that it always keeps the list
of paths you have visited sorted in reverse chronological order,
leaving the most recent path on top. This is fine for simple inter-
action, but makes it difficult to step backwards through the
history (since each step causes it to re-sort).
	As of version 0.11.7, symbolic links are fairly well supp-
orted by gentoo, and it becomes necessary to have some way of
following links "backwards". The old directory history system
doesn't do this very well, so a new one is needed.


3. THE NEW HISTORY
The new directory history is modeled after the URL history in
programs such as Netscape Navigator. It will not be exactly like
a browser's history, however. There are certain differences between
file system directories and web pages (yes, really, no matter what
Micros~1 says!).
	The main idea of the dirhistory system is to keep the data
that describes a directory around until the next time you enter
that directory. A secondary goal is to arrange the data in a way
that preserves the order you visited the directories in, thus making
it easy to provide back- and forward movement through the history.

3.1 The Current Directory
There is always a history data entry for the current directory.
It is not on any of the lists (mentioned below); it's stored in a
special single-entry "slot" designed for the purpose. This makes
it easy to always find the current directory's history entry.

3.2 The Backward, Forward and Cache Lists
In addition to the current directory slot, there are three lists
that also hold history entries: the backward list; the forward
list; and the cache.
	The backward list holds entries that you have already
visited once during a session. The last entry on the backward
list is normally the directory that was current previous to the
one that is currently current (!), and so on. If it sounds comp-
licated, that's entierly my fault. :)
	The forward list holds entries for directories that
you entered after the current directory. The forward list is
only used by the DirBackward and DirForward commands; it is
left blank by normal (absolute) navigation.
	The cache holds entries for directories that have been
visited, but are no longer part of the sequence available by
moving backward and forward. See the command descriptions below
for more details.

3.3 History Commands
The following commands operate on the history state:

DirEnter
	Changes the current directory by simply replacing it with
	a new path.
		Appends the current dir to the back list first, and
	then clears the forward list. Detailed steps:
	1. Find current dir entry, and update the history state in
	   it.
	2. Find entry for target directory. If found on the back
	   list, move all entries _after_ it to the cache list.
	3. Enter target dir, apply found entry (if any).
	4. Append initial current dir's entry (from step #1) to
	   the back list.
	4. Clear forward list.
	5. Done!

DirRescan
	Refreshes the display of a directory, by reading the dir's
	contents back from the disk. Does not alter the length or
	contents of any history lists, but makes use of the information
	in them nevertheless. Detailed steps of operation:
	1. Find the current directory's history entry. Should be
	   really simple, since it should be in the "current" slot.
	2. Update history information.
	3. Reread the directory from disk.
	4. Apply previously stored history info.
	5. Done!
DirBack
	Changes to the previous directory, i.e. the one that is
	last in the back list. Painfully detailed description:
	1. Find current dir's slot.
	2. Update history information.
	3. Prepend current to forward history list.
	4. Change to directory at end of backward list.
	5. Done!
DirForward
	Goes one step forward in the directory history, to the dir
	specified by the first item in the forward list. Steps:
	1. Find current.
	2. Update it.
	3. Append current to backward history listt.
	4. Change to directory at beginning of forward list.
	5. Done!

3.4 Initial Example
	Illustration, showing a command, the back list, current
dir, and forward list _after_ the command has executed:

Command			Back		Current		Forward
(start, initial state)	()		""		()
DirEnter "/"		()		"/"		()
DirEnter "/home"	("/")		"/home"		()
DirEnter "/home/emil"	("/" "/home")	"/home/emil"	()
DirBack			("/")		"/home"		("/home/emil")
DirBack			()		"/"		("/home" "/home/emil")
DirForward		("/")		"/home"		("/home/emil")
DirForward		("/" "/home")	"/home/emil"	()

3.5 No Duplicates
The new history system (just like the old) will not allow duplicate
entries in the history. If there is an entry for a path in one of
the history lists, that entry will always be used whenever that path
becomes current again. This is unlike e.g. Netscape's behaviour, and
motivated by two main factors: 1) it's necessary in order to preserve
selection info, and 2) it's simpler and I think I understand it. :)

3.6 Second Example
The example continues, from the last state shown above:
Command		Back List			Current Dir	Forward List
(initial)	("/" "/home")			"/home/emil"	()
DirEnter "/usr"	("/" "/home" "/home/emil")	"/usr"		()
DirBack		("/" "/home")			"/home/emil"	("/usr")
DirBack		("/")				"/home"		("/home/emil" "/usr")
DirEnter "/usr"	("/" "/home")			"/usr"		()

Note that for the last line here, the "/usr" record on the forward
list would NOT be completely ignored: both lists are searched be-
fore changing the directory, so that any old selection in "/usr"
could be found and applied.
	When the record has been found and used, though, it will
be deleted together with the rest of the forward list.