view COPYING @ 21922:50e20154cb68

filemerge: add internal:tagmerge merge tool

Add a new internal:tagmerge merge tool which implements an automatic merge
algorithm for mercurial's tag files

The tagmerge algorithm is able to resolve most merge conflicts that
currently would trigger a .hgtags merge conflict. The only case that
it does not (and cannot) handle is that in which two tags point to
different revisions on each merge parent _and_ their corresponding tag
histories have the same rank (i.e. the same length). In all other
cases the merge algorithm will choose the revision belonging to the
parent with the highest ranked tag history. The merged tag history is
the combination of both tag histories (special care is taken to try to
combine common tag histories where possible).

The algorithm also handles cases in which tags have been manually
removed from the .hgtags file and other similar corner cases.

In addition to actually merging the tags from two parents, taking into
account the base, the algorithm also tries to minimize the difference
between the merged tag file and the first parent's tag file (i.e. it
tries to make the merged tag order as as similar as possible to the
first parent's tag file order).

The algorithm works as follows:
1. read the tags from p1, p2 and the base
    - when reading the p1 tags, also get the line numbers associated to each
      tag node (these will be used to sort the merged tags in a way that
      minimizes the diff to p1). Ignore the file numbers when reading p2 and
      the base
2. recover the "lost tags" (i.e. those that are found in the base but not on p1
   or p2) and add them back to p1 and/or p2
    - at this point the only tags that are on p1 but not on p2 are those new
      tags that were introduced in p1. Same thing for the tags that are on p2
      but not on p2
3. take all tags that are only on p1 or only on p2 (but not on the base)
    - Note that these are the tags that were introduced between base and p1 and
      between base and p2, possibly on separate clones
4. for each tag found both on p1 and p2 perform the following merge algorithm:
    - the tags conflict if their tag "histories" have the same "rank" (i.e.
      length) _AND_ the last (current) tag is _NOT_ the same
    - for non conflicting tags:
        - choose which are the high and the low ranking nodes
            - the high ranking list of nodes is the one that is longer.
              In case of draw favor p1
            - the merged node list is made of 3 parts:
                - first the nodes that are common to the beginning of both the
                  low and the high ranking nodes
                - second the non common low ranking nodes
                - finally the non common high ranking nodes (with the last one
                  being the merged tag node)
            - note that this is equivalent to putting the whole low ranking node
              list first, followed by the non common high ranking nodes
    - note that during the merge we keep the "node line numbers", which will
      be used when writing the merged tags to the tag file
5. write the merged tags taking into account to their positions in the first
   parent (i.e. try to keep the relative ordering of the nodes that come
   from p1). This minimizes the diff between the merged and the p1 tag files
   This is done by using the following algorithm
    - group the nodes for a given tag that must be written next to each other
        - A: nodes that come from consecutive lines on p1
        - B: nodes that come from p2 (i.e. whose associated line number is None)
             and are next to one of the a nodes in A
        - each group is associated with a line number coming from p1
    - generate a "tag block" for each of the groups
        - a tag block is a set of consecutive "node tag" lines belonging to the
          same tag and which will be written next to each other on the merged
          tags file
    - sort the "tag blocks" according to their associated number line
        - put blocks whose nodes come all from p2 first
    - write the tag blocks in the sorted order

Notes:
- A few tests have been added to test-tag.t. These tests are very specific to
the new internal:tagmerge tool, so perhaps they should be moved to their own
test file.
- The merge algorithm was discussed in a thread on the mercurial mailing list.
In http://markmail.org/message/anqaxldup4tmgyrx a slightly different algorithm
was suggested. In it the p1 and p2 tags would have been interleaved instead of
put one before the other. It would be possible to implement that but my tests
suggest that the merge result would be more confusing and harder to understand.

                    GNU GENERAL PUBLIC LICENSE
                       Version 2, June 1991

 Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
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