Editing Incubator escapee wiki:History merging (section)

==Repair process (for admins)==
===Using the MergeHistory special page===
{{shortcut|WP:MERGEHISTORY}}
Administrators can use a special page, [[Special:MergeHistory]], to perform history merges. It differs from the manual methods, as follows:
# It automatically detects the latest version of the source page which is older than the oldest version of the target page, and won't allow the user to move later revisions. This feature is good if the source page eventually became something else, but can be bad if the target page had started out as a redirect to the source. When a redirect is blocking a full MergeHistory merge, the redirect and any older edits will need to be either deleted or merged to another redirect. Deletion and restoration of pages with lengthy edit histories is very time- and resource-intensive, and administrators are not allowed to delete pages with more than 5000 edits in their history. An easier option in these cases may be to history-merge the redirect and any earlier history to another redirect that was created later. See {{section link||Clearing away merge-blocking redirects}}.
# The user can, however, tell it to only move earlier revisions than that – it is possible to select the latest revision it should move.
# It doesn't mix deleted and non-deleted versions of the target page.
# It retains any protection the target page may have.
# It doesn't create a new revision of the old page.
# If the user moves all non-deleted revisions of the source, a hard redirect is automatically created. This can't be overridden.
# The logs for this action aren't in the move log - they're in [[Special:Log/merge|a separate log]].

====Clearing away merge-blocking redirects====
* To clear a blocking redirect by deleting it:
*# Check for deleted history at the target, and take note of all deleted edits there
*#*WARNING: Beware the [[#Revisions with same timestamp|co-mingled revisions issue]]. WORKAROUND:
*#*# Move the page to draft: namespace before deleting it, with the rationale: "history-merging process".
*#*# Restore the oldest (redirect) revision(s) – these would have been deleted by a regular move when it "moved over the redirect"
*#*# Move the page with the oldest (redirect) revision(s) back to mainspace, then delete it (adding to the already existing deleted revisions)
*#*# Restore the remaining history and move it back to mainspace
*# Delete the target page with the rationale "setting up for a history merge"
*# Restore all but the previously deleted edits and the oldest (redirect) revision(s) – these would have been deleted by a regular move when it "moved over the redirect"
*#* Deletion and restoration operations often time out with errors on pages with long edit histories. Simply try the delete or restore again; it virtually always succeeds on the second try
*#* Now MergeHistory can do the merge; this technique avoids making a new edit that needs to be reverted, which happens when the source is moved to the target
* To clear a blocking redirect by history-merging it:
*# Find other redirects to the target page using [[Special:WhatLinksHere]], while hiding links and transclusions: [https://en.wikipedia.org/w/index.php?title=Special%3AWhatLinksHere&target=&hidelinks=1&hidetrans=1 What redirects here] Example: [https://en.wikipedia.org/w/index.php?title=Special%3AWhatLinksHere&target=Yasser_Arafat_International_Airport&hidelinks=1&hidetrans=1 Pages that redirect to "Yasser Arafat International Airport"]
*# Find an appropriate redirect, whose oldest revision (creation date) is newer than the most recent revision of the redirect history that needs to be cleared
*# Merge the blocking redirect history to that redirect. Example: [[Special:PageHistory/Gaza Airport]]. The April and July 2005‎ revisions were merged here from [[Yasser Arafat International Airport]] ([https://en.wikipedia.org/wiki/Special:Log?type=&user=Wbm1058&page=Yasser+Arafat+International+Airport&wpdate=&tagfilter= log])
*#* Now MergeHistory can do the merge; this technique avoids making a new edit that needs to be reverted, which happens when the source is moved to the target
* Instead of finding redirects, you can make a temporary page (for example, in draftspace), merge the blocking redirect history there, and then delete the temporary page when you're done.

===Manual process===
'''''Warning''': this procedure may only be '''undone''' by spending quite silly amounts of time. To undo a merge, see [[#How to undo a history merge|below]]. Do not do this if you're not sure what you're doing.''

====An easy case====
[[File:Page history merge Simple.png|border|500px|right|Steps for a simple case]]

The following procedure merges the page histories in the case of a hypothetical example:

Suppose [[Alabama/History]] (old title) was the only article on that subject, and that the article developed in the course of a number of edits, until a decision that [[History of Alabama]] (new title) was a better style of name for the article. Suppose further that for whatever reason, the contents of the old article were
* cut from the old article,
* replaced in it with a redirect to the new title, and
* pasted into a newly created article bearing the new title.
(That is, the move tool was not available or not used to simultaneously transfer the Wiki text and the history of edits to the new title.)
And suppose this replacement (new-title) article develops further and reflects the new history of these further edits. Our goal is to graft the (old) edit history from [[Alabama/History]] (article with old title) onto the new history in [[History of Alabama]] (article with new title) where those partial histories can complement each other. The process is as follows:

# Move [[Alabama/History]] to [[History of Alabama]], using the move tool. The admin approves deletion of [[History of Alabama]] to allow the move. (Now the old versions are the whole of the new title's history.)
# [[Wikipedia:Viewing and restoring deleted pages by sysops|Undelete]] the [[History of Alabama]] article, by
## Viewing the Page history,
## Linking via "View or restore ... deleted edits?", and
## Clicking on "Restore". (Now the new title's history has both the old and new versions, including an extra copy of the most recent version of [[Alabama/History]], created by the move tool.)
# At this stage, [[History of Alabama]] will only show the text "#redirect [[History of Alabama]]" (assuming a redirect was the most recent version of [[Alabama/History]], the [[History of Alabama]] page will now be showing whatever the most recent version of [[Alabama/History]] was). The last step is to revert to the last version of [[History of Alabama]] from before the move, by
## Clicking "Page history" on [[History of Alabama]].
## Make a [[WP:Bypass your cache|hard reload]] (Shift+Control+R in Mozilla or Opera, Ctrl+F5 in Internet Explorer, and Ctrl+R in Firefox) to see an up-to-date history reflecting the undeletion.<sup>[[#Lost history bug|1]]</sup>
## [[Help:Reverting|Reverting]] to the last pre-move version.

====Merging page histories of pages with many revisions====
Suppose that the page [[History of Alabama]] had too many revisions to be deleted or deleting it may cause other disruption. The following procedure can be used to merge page histories in this situation:
# Move [[History of Alabama]] to [[Alabama/History]] with a move summary like "history merge, will be back at correct title soon". Answer yes when asked to delete the Alabama/History page.
# Undelete the revisions of [[Alabama/History]] containing the page history.
# Move [[Alabama/History]] back to [[History of Alabama]].
# If needed, undelete the remaining revisions at [[Alabama/History]].

====A more complex case====
Sometimes, after a cut-and-paste move is performed, the article at the old title is then edited for some other purpose (e.g., turning it into a [[Wikipedia:Disambiguation|disambiguation]] page). That causes the article now at NewTitle to have part of its history there, and part at OldTitle, but the history at OldTitle also contains the history of NewMeaning. Use of the [[Wikipedia:Selective deletion|selective deletion]] function allows these to be repaired as well.

[[File:Page history merge Complex.png|border|500px|right|Steps for a complex case]]

''To select more than one revision for undeletion, click on the tick box of the first revision to be undeleted, then shift-click on the last revision to be undeleted. Every intermediate revision will then be selected.''

An example of this was [[Military of Japan]]; the original was moved to [[Japan Self-Defense Forces]] with a cut-and-paste move, and the article [[Military of Japan]] was then turned into a disambiguation page. This was repaired with the following procedure:

# [[Military of Japan]] is deleted.
# Selective undelete is used to undelete only those versions of [[Military of Japan]] which belonged to "Japan Self-Defense Forces".
# The versions of "Japan Self-Defense Forces" at [[Military of Japan]] are moved to [[Japan Self-Defense Forces]], using the normal page-move function. For this to happen, [[Japan Self-Defense Forces]] must be deleted, although this can be done as part of the move.
# Undeletion of [[Japan Self-Defense Forces]] restores the rest of the versions of that article to its history.<sup>[[#Lost history bug|1]]</sup>
# However, the most recent version in the history of [[Japan Self-Defense Forces]] is now the most recent version of the old history from [[Military of Japan]] (it's a copy of that version, created by the page-move function). So, go into the history of [[Japan Self-Defense Forces]], select the next-most-recent version, click on it, and when it appears, click on "Edit this page", ignore the "WARNING: You are editing an out-of-date revision" message, type something suitable (e.g., "restoring most recent version after merging histories") in the [[Wikipedia:Edit summary|edit summary]], and hit "Save page". That article is now restored to its condition prior to this procedure, and now also has its complete history.
# Step 3 above (the move) will have left a history containing just a redirect at [[Military of Japan]]. Delete the redirect.
# Undeletion of all the ''other'' versions of [[Military of Japan]] restores the more recent history of that article; no additional steps are needed, as the most recent version should now be the current version.<sup>[[#Lost history bug|1]]</sup>

====A troublesome case====
{{shortcut|WP:PV}}
{{redirect|WP:PV|pageviews|Wikipedia:Pageview statistics}}
However, the examples just described only work well if the two pieces of the history of one 'article' are disjoint; i.e. one ends before the other begins. These procedures are inadequate if this condition does not apply, e.g., if the copy of the article at the old title has been edited ''after'' the pasting of its contents into the new title. For example, it is not uncommon for:

# an article at (old) page ''A'' to be cut and pasted into (new) page ''B'', and
# page ''A'' later to be reverted to an article on the same topic, with a sequence of edits there as well.

In this case, the time periods of the two series of edits will overlap.

If someone then page-history merges pages ''A'' and ''B'' using the method described above, the result will sequence the versions of ''A'' and ''B'' strictly by time, with the result that various versions of ''A'' will be interleaved between versions in the page history of page ''B'' (and/or vice-versa). Inspecting this merged history without means of distinguishing between the two overlapping progressions (since nothing in this history indicates which version belongs to which sequence) invites severe confusion.

An appropriate procedure for such a case is to forego the history merge, and instead handle the situation much like a normal merge; put a note pointing to the other version of the page on the article's talk page. If it is inappropriate to leave the second copy in the main article space, you can archive the duplicate page to Talk: space (i.e. by moving it to some suitable title, such as Talk:RandomArticle/OldVersion).

The MediaWiki software does not allow page history to be publicly archived at a page title that does not host a live page or redirect. Therefore, if two pages with parallel histories are merged but it is undesirable to keep a redirect from the deprecated page title to the destination page title, the old page history needs to move. This is sometimes done by moving the page history to a subpage of the talk page of the destination page. An example can be found at [[Talk:Compilation of Final Fantasy VII#Old page history]]. Use the {{T|Parallel version}} template for tagging parallel versions found on talk pages.

Also, if page A is to be history-merged into page B, before the process, make sure that there are no deleted edits in page B, as then deleting B will shuffle the deleted and non-deleted edits attached to the page together. The deleted history should first be rescued from under B by some process such as this: Move B to some other name, say B_zxcvbnm (without making a redirect). Undelete B. Move B to some other name, say B/old_version . If necessary, re-delete B/old_version . Move B_zxcvbnm back to B (without making a redirect).

Likewise, if a page must be deleted and then partly undeleted for a history-split, first check in case it is sitting over a deleted parallel history.

====History splitting====
{{shortcut|WP:HISTSPLIT}}
Over time, articles may change from one underlying topic to a completely separate topic. Normally this should be accomplished through [[WP:move|moves]] and [[WP:disambiguation|disambiguation]] pages. However, if a user is unfamiliar with those processes they may simply [[WP:AHIJACK|change the topic of an article]] (overwriting the old) and continue editing. If this is not caught immediately it is very easy for the new topic to build up a substantial edit history of its own. Admins can use the following steps to fix this problem and maintain separate histories for the separate topics:
# Delete the article (original article name)
# Restore previous revisions up to (but not including) the point where the topic was changed.
# Move [without redirect] the restored versions (old topic) to a new name (see also [[WP:disambiguation|disambiguation]])
#* If there is already an article under the new name and you wish to histmerge into it:
#: a) select the "delete the existing article" option, while moving;
#: b) restore deleted revisions of new name.
# Restore new revisions of new topic (still at original article name)
# Revert to latest good versions as needed
# Establish a disambiguation page for the different topics

==== How to handle the left-over redirect ====
In most cases, you will be moving all non-redirect versions of one page into the history of another and leaving a redirect.  Please keep the following situations in mind when deciding what to do with the redirect:
* Is the resulting redirect eligible for speedy deletion (see [[WP:SPEEDY#General]] and [[WP:SPEEDY#Redirects]])?  As with regular page moves, consider waiting a few days before deleting the leftover redirect even if it is eligible for speedy deletion.
* Are all incoming links to the leftover redirect fixed?  If not, don't delete the redirect until they are.
* Is it likely the most recent editors of the moved revisions are watchlisting the page?  Consider notifying them of the change.
* Is the leftover redirect in User: or User_talk: space?  If you do delete it, notify the affected user unless there is a [[WP:IAR|good reason]] not to.  Consider leaving the redirect unless doing so would cause problems, such as in the case of: 
** A redirect from the "main" user page or "main" user talk page to somewhere other than another page in that user's userspace.
** A redirect to another user's pages or non-user space in a way that may cause confusion or is otherwise inappropriate.

===History-merging a transcluded page===
If page X is transcluded in page Y, and page X is marked to be the recipient in a history-merge, then page X and page Y will both appear in [[:Category:Candidates for history merging]], and both pages will display the request to perform a history-merge. An admin should not try to perform a history-merge on page Y, but only on page X. This is most likely to happen if page X is a template, but it may happen to any page that is transcluded. To avoid this, <code><nowiki>{{history merge}}</nowiki></code> should be placed in <code>&lt;noinclude&gt;</code> tags on page X.