--- a/mercurial/dagop.py	Fri Mar 09 21:59:07 2018 -0500
+++ b/mercurial/dagop.py	Wed Feb 28 15:09:05 2018 -0500
@@ -9,6 +9,9 @@
 
 import heapq
 
+from .thirdparty import (
+    attr,
+)
 from . import (
     error,
     mdiff,
@@ -358,6 +361,74 @@
         if inrange:
             yield c, linerange1
 
+@attr.s(slots=True, frozen=True)
+class annotateline(object):
+    fctx = attr.ib()
+    lineno = attr.ib(default=False)
+    # Whether this annotation was the result of a skip-annotate.
+    skip = attr.ib(default=False)
+
+def _annotatepair(parents, childfctx, child, skipchild, diffopts):
+    r'''
+    Given parent and child fctxes and annotate data for parents, for all lines
+    in either parent that match the child, annotate the child with the parent's
+    data.
+
+    Additionally, if `skipchild` is True, replace all other lines with parent
+    annotate data as well such that child is never blamed for any lines.
+
+    See test-annotate.py for unit tests.
+    '''
+    pblocks = [(parent, mdiff.allblocks(parent[1], child[1], opts=diffopts))
+               for parent in parents]
+
+    if skipchild:
+        # Need to iterate over the blocks twice -- make it a list
+        pblocks = [(p, list(blocks)) for (p, blocks) in pblocks]
+    # Mercurial currently prefers p2 over p1 for annotate.
+    # TODO: change this?
+    for parent, blocks in pblocks:
+        for (a1, a2, b1, b2), t in blocks:
+            # Changed blocks ('!') or blocks made only of blank lines ('~')
+            # belong to the child.
+            if t == '=':
+                child[0][b1:b2] = parent[0][a1:a2]
+
+    if skipchild:
+        # Now try and match up anything that couldn't be matched,
+        # Reversing pblocks maintains bias towards p2, matching above
+        # behavior.
+        pblocks.reverse()
+
+        # The heuristics are:
+        # * Work on blocks of changed lines (effectively diff hunks with -U0).
+        # This could potentially be smarter but works well enough.
+        # * For a non-matching section, do a best-effort fit. Match lines in
+        #   diff hunks 1:1, dropping lines as necessary.
+        # * Repeat the last line as a last resort.
+
+        # First, replace as much as possible without repeating the last line.
+        remaining = [(parent, []) for parent, _blocks in pblocks]
+        for idx, (parent, blocks) in enumerate(pblocks):
+            for (a1, a2, b1, b2), _t in blocks:
+                if a2 - a1 >= b2 - b1:
+                    for bk in xrange(b1, b2):
+                        if child[0][bk].fctx == childfctx:
+                            ak = min(a1 + (bk - b1), a2 - 1)
+                            child[0][bk] = attr.evolve(parent[0][ak], skip=True)
+                else:
+                    remaining[idx][1].append((a1, a2, b1, b2))
+
+        # Then, look at anything left, which might involve repeating the last
+        # line.
+        for parent, blocks in remaining:
+            for a1, a2, b1, b2 in blocks:
+                for bk in xrange(b1, b2):
+                    if child[0][bk].fctx == childfctx:
+                        ak = min(a1 + (bk - b1), a2 - 1)
+                        child[0][bk] = attr.evolve(parent[0][ak], skip=True)
+    return child
+
 def toposort(revs, parentsfunc, firstbranch=()):
     """Yield revisions from heads to roots one (topo) branch at a time.