# HG changeset patch
# User Martin von Zweigbergk <martinvonz@google.com>
# Date 2020-12-09 00:45:13
# Node ID bdc2bf68f19e456574cefe36bc5c9b165bb2fbfc
# Parent  aa4dbc14f7354557933072c99967d43f6d9c3c22

mergetools: add new conflict marker format with diffs in

I use 3-way conflict markers. Often when I resolve them, I manually
compare one the base with one side and apply the differences to the
other side. That can be hard when the conflict marker is large. This
patch introduces a new type of conflict marker, which I'm hoping will
make it easier to resolve conflicts.

The new format uses `<<<<<<<` and `>>>>>>>` to open and close the
markers, just like our existing 2-way and 3-way conflict
markers. Instead of having 2 or 3 snapshots (left+right or
left+base+right), it has a sequence of diffs. A diff looks like this:

```
------- base
+++++++ left
 a
-b
+c
 d
```

A diff that adds one side ("diff from nothing") has a `=======` header
instead and does not have have `+` prefixed on its lines. A regular
3-way merge can be viewed as adding one side plus a diff between the
base and the other side. It thus has two ways of being represented,
depending on which side is being diffed:

```
<<<<<<<
======= left
contents
on
left
------- base
+++++++ right
 contents
 on
-left
+right
>>>>>>>
```
or
```
<<<<<<<
------- base
+++++++ left
 contents
 on
-right
+left
======= right
contents
on
right
>>>>>>>
```

I've made it so the new merge tool tries to pick a version that has
the most common lines (no difference in the example above).


I've called the new tool "mergediff" to stick to the convention of
starting with "merge" if the tool tries a regular 3-way merge.

The idea came from my pet VCS (placeholder name `jj`), which has
support for octopus merges and other ways of ending up with merges of
more than 3 versions. I wanted to be able to represent such conflicts
in the working copy and therefore thought of this format (although I
have not yet implemented it in my VCS). I then attended a meeting with
Larry McVoy, who said BitKeeper has an option (`bk smerge -g`) for
showing a similar format, which reminded me to actually attempt this
in Mercurial.

Differential Revision: https://phab.mercurial-scm.org/D9551

diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -532,6 +532,33 @@ def _imerge3(repo, mynode, orig, fcd, fc
     return _imerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels)
 
 
+@internaltool(
+    b'mergediff',
+    fullmerge,
+    _(
+        b"warning: conflicts while merging %s! "
+        b"(edit, then use 'hg resolve --mark')\n"
+    ),
+    precheck=_mergecheck,
+)
+def _imerge_diff(
+    repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None
+):
+    """
+    Uses the internal non-interactive simple merge algorithm for merging
+    files. It will fail if there are any conflicts and leave markers in
+    the partially merged file. The marker will have two sections, one with the
+    content from one side of the merge, and one with a diff from the base
+    content to the content on the other side. (experimental)"""
+    if not labels:
+        labels = _defaultconflictlabels
+    if len(labels) < 3:
+        labels.append(b'base')
+    return _merge(
+        repo, mynode, orig, fcd, fco, fca, toolconf, files, labels, b'mergediff'
+    )
+
+
 def _imergeauto(
     repo,
     mynode,
diff --git a/mercurial/simplemerge.py b/mercurial/simplemerge.py
--- a/mercurial/simplemerge.py
+++ b/mercurial/simplemerge.py
@@ -455,6 +455,68 @@ def is_not_null(ctx):
     return ctx.node() != nodemod.nullid
 
 
+def _mergediff(m3, name_a, name_b, name_base):
+    lines = []
+    conflicts = False
+    for group in m3.merge_groups():
+        if group[0] == b'conflict':
+            base_lines, a_lines, b_lines = group[1:]
+            base_text = b''.join(base_lines)
+            b_blocks = list(
+                mdiff.allblocks(
+                    base_text,
+                    b''.join(b_lines),
+                    lines1=base_lines,
+                    lines2=b_lines,
+                )
+            )
+            a_blocks = list(
+                mdiff.allblocks(
+                    base_text,
+                    b''.join(a_lines),
+                    lines1=base_lines,
+                    lines2=b_lines,
+                )
+            )
+
+            def matching_lines(blocks):
+                return sum(
+                    block[1] - block[0]
+                    for block, kind in blocks
+                    if kind == b'='
+                )
+
+            def diff_lines(blocks, lines1, lines2):
+                for block, kind in blocks:
+                    if kind == b'=':
+                        for line in lines1[block[0] : block[1]]:
+                            yield b' ' + line
+                    else:
+                        for line in lines1[block[0] : block[1]]:
+                            yield b'-' + line
+                        for line in lines2[block[2] : block[3]]:
+                            yield b'+' + line
+
+            lines.append(b"<<<<<<<\n")
+            if matching_lines(a_blocks) < matching_lines(b_blocks):
+                lines.append(b"======= %s\n" % name_a)
+                lines.extend(a_lines)
+                lines.append(b"------- %s\n" % name_base)
+                lines.append(b"+++++++ %s\n" % name_b)
+                lines.extend(diff_lines(b_blocks, base_lines, b_lines))
+            else:
+                lines.append(b"------- %s\n" % name_base)
+                lines.append(b"+++++++ %s\n" % name_a)
+                lines.extend(diff_lines(a_blocks, base_lines, a_lines))
+                lines.append(b"======= %s\n" % name_b)
+                lines.extend(b_lines)
+            lines.append(b">>>>>>>\n")
+            conflicts = True
+        else:
+            lines.extend(group[1])
+    return lines, conflicts
+
+
 def simplemerge(ui, localctx, basectx, otherctx, **opts):
     """Performs the simplemerge algorithm.
 
@@ -499,9 +561,15 @@ def simplemerge(ui, localctx, basectx, o
         extrakwargs[b'name_base'] = name_base
         extrakwargs[b'minimize'] = False
 
-    lines = m3.merge_lines(
-        name_a=name_a, name_b=name_b, **pycompat.strkwargs(extrakwargs)
-    )
+    if mode == b'mergediff':
+        lines, conflicts = _mergediff(m3, name_a, name_b, name_base)
+    else:
+        lines = list(
+            m3.merge_lines(
+                name_a=name_a, name_b=name_b, **pycompat.strkwargs(extrakwargs)
+            )
+        )
+        conflicts = m3.conflicts
 
     # merge flags if necessary
     flags = localctx.flags()
@@ -519,5 +587,5 @@ def simplemerge(ui, localctx, basectx, o
     else:
         localctx.write(mergedtext, flags)
 
-    if m3.conflicts and not mode == b'union':
+    if conflicts and not mode == b'union':
         return 1
diff --git a/relnotes/next b/relnotes/next
--- a/relnotes/next
+++ b/relnotes/next
@@ -27,6 +27,13 @@
  * The memory footprint per changeset during pull/unbundle
    operations has been further reduced.
 
+ * There is a new internal merge tool called `internal:mergediff` (can
+   be set as the value for the `merge` config in the `[ui]`
+   section). It resolves merges the same was as `internal:merge` and
+   `internal:merge3`, but it shows conflicts differently. Instead of
+   showing 2 or 3 snapshots of the conflicting pieces of code, it
+   shows one snapshot and a diff. This may be useful when at least one
+   side of the conflict is similar to the base.
 
 == New Experimental Features ==
 
diff --git a/tests/test-conflict.t b/tests/test-conflict.t
--- a/tests/test-conflict.t
+++ b/tests/test-conflict.t
@@ -282,6 +282,80 @@ internal:merge3
   >>>>>>> merge rev
   Hop we are done.
 
+internal:mergediff
+
+  $ hg co -C 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat << EOF > a
+  > Small Mathematical Series.
+  > 1
+  > 2
+  > 3
+  > 4
+  > 4.5
+  > 5
+  > Hop we are done.
+  > EOF
+  $ hg co -m 2 -t internal:mergediff
+  merging a
+  warning: conflicts while merging a! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges
+  [1]
+  $ cat a
+  Small Mathematical Series.
+  1
+  2
+  3
+  <<<<<<<
+  ------- base
+  +++++++ working copy
+   4
+  +4.5
+   5
+  ======= destination
+  6
+  8
+  >>>>>>>
+  Hop we are done.
+Test the same thing as above but modify a bit more so we instead get the working
+copy in full and the diff from base to destination.
+  $ hg co -C 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat << EOF > a
+  > Small Mathematical Series.
+  > 1
+  > 2
+  > 3.5
+  > 4.5
+  > 5.5
+  > Hop we are done.
+  > EOF
+  $ hg co -m 2 -t internal:mergediff
+  merging a
+  warning: conflicts while merging a! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges
+  [1]
+  $ cat a
+  Small Mathematical Series.
+  1
+  2
+  <<<<<<<
+  ======= working copy
+  3.5
+  4.5
+  5.5
+  ------- base
+  +++++++ destination
+   3
+  -4
+  -5
+  +6
+  +8
+  >>>>>>>
+  Hop we are done.
+
 Add some unconflicting changes on each head, to make sure we really
 are merging, unlike :local and :other
 
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -2057,6 +2057,13 @@ Test dynamic list of merge tools only sh
         partially merged file. Marker will have three sections, one from each
         side of the merge and one for the base content.
   
+      ":mergediff"
+        Uses the internal non-interactive simple merge algorithm for merging
+        files. It will fail if there are any conflicts and leave markers in the
+        partially merged file. The marker will have two sections, one with the
+        content from one side of the merge, and one with a diff from the base
+        content to the content on the other side. (experimental)
+  
       ":other"
         Uses the other 'p2()' version of files as the merged version.