6
Essential characteristics
7
-------------------------
9
In the general case (no criss-cross), it is a three-way merge. When
10
there is a criss-cross at the tree level, but not for the particular
11
file, it is still a three-way merge. When there's a file-level
12
criss-cross, it's superior to a three-way merge.
17
First, we compare the files we are trying to merge, and find the lines
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that differ. Next, we try to determine why they differ; this is
19
essential to the merge operation, because it affects how we resolve the
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differences. In this merger, there are three possible outcomes:
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1. The line was added in this version: "new-this"
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2. The line was deleted in the other version: "killed-other"
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3. The line was preserved as part of merge resolution in this version,
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but deleted in the other version: "conflicted-this"
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Option 3 is new, but I believe it is essential. When each side has made
28
a conflicting merge resolution, we should let the user decide how to
29
combine the two resolutions, i.e. we should emit a conflict. We cannot
30
silently drop the line, or silently keep the line, which can happen if
31
we choose options 1 or 2. If we choose options 1 or 2, there's also a
32
possibility that a conflict will be produced, but no guarantee. We need
33
a guarantee, which is why we need a new possible outcome.
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To decide whether a line is "new-this", "killed-other" or
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"conflicted-this", we compare this version against the versions from
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each "least common ancestor" (LCA), in graph terminology. For each LCA
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version, if the line is not present in the LCA version, we add it to the
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"new" set. If the line is present in the LCA version, we add it to the
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When we are done going through each LCA version, each unique line will
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be in at least one of the sets. If it is only in the "new" set, it's
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handled as "new-this". If it is only in the "killed" set, it's handled
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as "killed-other". If it is in both sets, it's handled as
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The logic here is a bit tricky: first, we know that the line is present
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in some, but not all, LCAs. We can assume that all LCAs were produced
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by merges of the same sets of revisions. That means that in those LCAs,
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there were different merge resolutions. Since THIS and OTHER disagree
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about whether the line is present, those differences have propogated
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into THIS and OTHER. Therefore, we should declare that the lines are in
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conflict, and let the user handle the issue.
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LCA merge and Three-way merge
57
-----------------------------
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Now, in the common case, there's a single LCA, and LCA merge behaves as
60
a three-way merge. Since there's only one LCA, we cannot get the
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"conflicted-this" outcome, only "new-this" or "killed-other. Let's look
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at the typical description of three-way merges:
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+-----+------+-------+------------+
65
|THIS | BASE | OTHER | OUT |
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+-----+------+-------+------------+
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+-----+------+-------+------------+
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+-----+------+-------+------------+
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+-----+------+-------+------------+
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+-----+------+-------+------------+
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|A | B | C |\*conflict\*|
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+-----+------+-------+------------+
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Now, let's assume that BASE is a common ancestor, as is typically the
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case. In fact, for best-case merges, BASE is the sole LCA.
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We always pick the version that represents a change from BASE, if there
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is one. For the AAAA line, there is no change, so the output is
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rightfully BASE/THIS/OTHER. For ABAA, the THIS and OTHER are changes
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from BASE, and they are the same change so they both win. (This case is
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sometimes called convergence.) For ABBA, THIS is a change from BASE, so
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THIS wins. For AABB, OTHER is a change from BASE, so OTHER wins. For
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ABC*, THIS and OTHER are both changes to BASE, but they are different
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changes, so they can't both win cleanly. Instead, we have a conflict.
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Now in three-way merging, we typically talk about regions of text. In
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weave/knit/newness/lca merge, we also have regions. Each contiguous
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group of "unchanged" lines is a region, and the areas between them are
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Let's assign a to THIS and b to OTHER. "unchanged" regions represent
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the AAAA or ABAA cases; it doesn't matter which, because the outcome is
97
the same regardless. Regions which consist of only "new-a" or
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"killed-a" represent the ABBA case. Regions which consist of only
99
"new-b" or "killed-b" represent the AABB case. Regions which have
100
(new-a or killed-a) AND (new-b or killed-b) are the ABC* case-- both
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sides have made changes, and they are different changes, so a conflict
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This is what I mean when I say that it is a three-way merge in the
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common case; if there is only one LCA, then it is merely an alternative
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implementation of three-way. (One that happens to automatically do
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``--reprocess``, ftw).
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1. It was time. Although knit / annotate merge and newness merge have
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tried to emulate the behavior of the original weave merge algorithm,
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``--merge-type=weave`` hasn't been based on weaves for a long time.
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2. Behavior differences. This algorithm should behave like a three-way
116
merge in the common case, while its predecessors did not. It also has
117
explicit support for handling conflicting merge resolutions, so it
118
should behave better in criss-cross merge scenarios.
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Unlike the current "weave" merge implementation, lca merge does not
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perform any whole-history operations. LCA selection should scale with
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the number of uncommon revisions. Text comparison time should scale
126
mO(n\ :sup:`2`\ ), where m is the number of LCAs, and n is the number of lines
127
in the file. The current weave merge compares each uncommon ancestor,
128
potentially several times, so it is >= kO(n\ :sup:`2`\ ), where k is the
129
number of uncommon ancestors. So "lca" should beat "weave" both in history
130
analysis time and in text comparison time.
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1. Inaccurate LCA selection. Our current LCA algorithm uses
136
``Graph.heads()``, which is known to be flawed. It may occasionally give
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bad results. This risk is mitigated by the fact that the per-file graphs
138
tend to be simpler than the revision graph. And since we're already using
139
this LCA algorithm, this is not an additional risk. I hope that John Meinel
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will soon have a fixed version of ``Graph.heads`` for us.
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2. False matches. Weaves have a concept of line identity, but knits and
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later formats do not. So a line may appear to be common to two files, when
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in fact it was introduced separately into each for entirely different
144
reasons. This risk is the same for three-way merging. It is mitigated by
145
using Patience sequence matching, which a longest-common-subsequence match.
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I think this could be a great merge algorithm, and a candidate to make
151
our default, but this work would not have been possible without the work
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of others, especially:
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- Martin Pool's weave merge and knit/annotate merge algorithms.
155
- Bram Cohen's discussions of merge algorithms
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- Andrew Tridgell's dissection of BitKeeper merge
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- Nathaniel Smith's analysis of why criss-cross histories necessarily
158
produce poor three-way merges.
added
removed
doc/developers/lca-merge.txt
