patch: rewrite reversehunks (issue5337)

The old reversehunks code accesses "crecord.uihunk._hunk", which is the raw
recordhunk without crecord selection information, therefore "revert -i"
cannot revert individual lines, aka. issue5337.

The patch rewrites related logic to return the right reverse hunk for
revert. Namely,

 1. "fromline" and "toline" are correctly swapped [1]
 2. crecord.uihunk generates a correct reverse hunk [2]

Besides, reversehunks(hunks) will no longer modify its input "hunks", which
is more expected.

[1]: To explain why "fromline" and "toline" need to be swapped, take the
     following example:

  $ cat > a <<EOF
  > 1
  > 2
  > 3
  > 4
  > EOF

  $ cat > b <<EOF
  > 2
  > 3
  > 5
  > EOF

  $ diff a b
  1d0   <---- "1" is "fromline" and "0" is "toline"
  < 1         and they are swapped if diff from the reversed direction
  4c3             |
  < 4             |
  ---             |
  > 5             |
                  |
  $ diff b a      |
  0a1   <---------+
  > 1
  3c4   <---- also "4c3" gets swapped to "3c4"
  < 5
  ---
  > 4

[2]: This is a bit tricky.

For example, given a file which is empty in working parent but has 3 lines
in working copy, and the user selection:

    select hunk to discard
    [x] +1
    [ ] +2
    [x] +3

The user intent is to drop "1" and "3" in working copy but keep "2", so the
reverse patch would be something like:

        -1
         2 (2 is a "context line")
        -3

We cannot just take all selected lines and swap "-" and "+", which will be:

        -1
        -3

That patch won't apply because of "2". So the correct way is to insert "2"
as a "context line" by inserting it first then deleting it:

        -2
        +2

Therefore, the correct revert patch is:

        -1
        -2
        +2
        -3

It could be reordered to look more like a common diff hunk:

        -1
        -2
        -3
        +2

Note: It's possible to return multiple hunks so there won't be lines like
"-2", "+2". But the current implementation is much simpler.

For deletions, like the working parent has "1\n2\n3\n" and it was changed to
empty in working copy:

    select hunk to discard
    [x] -1
    [ ] -2
    [x] -3

The user intent is to drop the deletion of 1 and 3 (in other words, keep
those lines), but still delete "2".

The reverse patch is meant to be applied to working copy which is empty.
So the patch would be:

        +1
        +3

That is to say, there is no need to special handle the unselected "2" like
the above insertion case.

mercurial/crecord.py

@@ -427,6 +427,54 b' class uihunk(patchnode):'
         self.pretty(x)
         return x.getvalue()
 
+    def reversehunk(self):
+        """return a recordhunk which is the reverse of the hunk
+
+        Assuming the displayed patch is diff(A, B) result. The returned hunk is
+        intended to be applied to B, instead of A.
+
+        For example, when A is "0\n1\n2\n6\n" and B is "0\n3\n4\n5\n6\n", and
+        the user made the following selection:
+
+                 0
+            [x] -1           [x]: selected
+            [ ] -2           [ ]: not selected
+            [x] +3
+            [ ] +4
+            [x] +5
+                 6
+
+        This function returns a hunk like:
+
+                 0
+                -3
+                -4
+                -5
+                +1
+                +4
+                 6
+
+        Note "4" was first deleted then added. That's because "4" exists in B
+        side and "-4" must exist between "-3" and "-5" to make the patch
+        applicable to B.
+        """
+        dels = []
+        adds = []
+        for line in self.changedlines:
+            text = line.linetext
+            if line.applied:
+                if text[0] == '+':
+                    dels.append(text[1:])
+                elif text[0] == '-':
+                    adds.append(text[1:])
+            elif text[0] == '+':
+                dels.append(text[1:])
+                adds.append(text[1:])
+        hunk = ['-%s' % l for l in dels] + ['+%s' % l for l in adds]
+        h = self._hunk
+        return patchmod.recordhunk(h.header, h.toline, h.fromline, h.proc,
+                                   h.before, hunk, h.after)
+
     def __getattr__(self, name):
         return getattr(self._hunk, name)
 

mercurial/patch.py

@@ -959,6 +959,18 b' class recordhunk(object):'
         rem = len([h for h in hunk if h[0] == '-'])
         return add, rem
 
+    def reversehunk(self):
+        """return another recordhunk which is the reverse of the hunk
+
+        If this hunk is diff(A, B), the returned hunk is diff(B, A). To do
+        that, swap fromline/toline and +/- signs while keep other things
+        unchanged.
+        """
+        m = {'+': '-', '-': '+'}
+        hunk = ['%s%s' % (m[l[0]], l[1:]) for l in self.hunk]
+        return recordhunk(self.header, self.toline, self.fromline, self.proc,
+                          self.before, hunk, self.after)
+
     def write(self, fp):
         delta = len(self.before) + len(self.after)
         if self.after and self.after[-1] == '\\ No newline at end of file\n':
@@ -1493,7 +1505,7 b' def reversehunks(hunks):'
      c
      1
      2
-    @@ -1,6 +2,6 @@
+    @@ -2,6 +1,6 @@
      c
      1
      2
@@ -1501,26 +1513,17 b' def reversehunks(hunks):'
     +4
      5
      d
-    @@ -5,3 +6,2 @@
+    @@ -6,3 +5,2 @@
      5
      d
     -lastline
 
     '''
 
-    from . import crecord as crecordmod
     newhunks = []
     for c in hunks:
-        if isinstance(c, crecordmod.uihunk):
-            # curses hunks encapsulate the record hunk in _hunk
-            c = c._hunk
-        if isinstance(c, recordhunk):
-            for j, line in enumerate(c.hunk):
-                if line.startswith("-"):
-                    c.hunk[j] = "+" + c.hunk[j][1:]
-                elif line.startswith("+"):
-                    c.hunk[j] = "-" + c.hunk[j][1:]
-            c.added, c.removed = c.removed, c.added
+        if util.safehasattr(c, 'reversehunk'):
+            c = c.reversehunk()
         newhunks.append(c)
     return newhunks