diff mercurial/dagop.py @ 33080:a53bfc2845f2

revset: add depth limit to descendants() (issue5374)

This is naive implementation using two-pass scanning. Tracking descendants
isn't an easy problem if both start and stop depths are specified. It's
impractical to remember all possible depths of each node while scanning from
roots to descendants because the number of depths explodes. Instead, we could
cache (min, max) depths as a good approximation and track ancestors back when
needed, but that's likely to have off-by-one bug.

Since this implementation appears not significantly slower, and is quite
straightforward, I think it's good enough for practical use cases. The time
and space complexity is O(n) ish.

  revisions:
  0) 1-pass scanning with (min, max)-depth cache (worst-case quadratic)
  1) 2-pass scanning (this version)

  repository:
  mozilla-central

  # descendants(0) (for reference)
  *) 0.430353

  # descendants(0, depth=1000)
  0) 0.264889
  1) 0.398289

  # descendants(limit(tip:0, 1, offset=10000), depth=1000)
  0) 0.025478
  1) 0.029099

  # descendants(0, depth=2000, startdepth=1000)
  0) painfully slow (due to quadratic backtracking of ancestors)
  1) 1.531138

--- a/mercurial/dagop.py	Sat Jun 24 23:35:03 2017 +0900
+++ b/mercurial/dagop.py	Sat Jun 24 23:05:57 2017 +0900
@@ -125,10 +125,38 @@
                     yield i
                     break
 
-def revdescendants(repo, revs, followfirst):
+def _builddescendantsmap(repo, startrev, followfirst):
+    """Build map of 'rev -> child revs', offset from startrev"""
+    cl = repo.changelog
+    nullrev = node.nullrev
+    descmap = [[] for _rev in xrange(startrev, len(cl))]
+    for currev in cl.revs(startrev + 1):
+        p1rev, p2rev = cl.parentrevs(currev)
+        if p1rev >= startrev:
+            descmap[p1rev - startrev].append(currev)
+        if not followfirst and p2rev != nullrev and p2rev >= startrev:
+            descmap[p2rev - startrev].append(currev)
+    return descmap
+
+def _genrevdescendantsofdepth(repo, revs, followfirst, startdepth, stopdepth):
+    startrev = revs.min()
+    descmap = _builddescendantsmap(repo, startrev, followfirst)
+    def pfunc(rev):
+        return descmap[rev - startrev]
+    return _walkrevtree(pfunc, revs, startdepth, stopdepth, reverse=False)
+
+def revdescendants(repo, revs, followfirst, startdepth=None, stopdepth=None):
     """Like revlog.descendants() but supports additional options, includes
-    the given revs themselves, and returns a smartset"""
-    gen = _genrevdescendants(repo, revs, followfirst)
+    the given revs themselves, and returns a smartset
+
+    Scan ends at the stopdepth (exlusive) if specified. Revisions found
+    earlier than the startdepth are omitted.
+    """
+    if startdepth is None and stopdepth is None:
+        gen = _genrevdescendants(repo, revs, followfirst)
+    else:
+        gen = _genrevdescendantsofdepth(repo, revs, followfirst,
+                                        startdepth, stopdepth)
     return generatorset(gen, iterasc=True)
 
 def _reachablerootspure(repo, minroot, roots, heads, includepath):