# HG changeset patch
# User Durham Goode <durham@fb.com>
# Date 2014-03-13 20:47:21
# Node ID 6a1a4c212d50b761c9abffabbd988d68aa95f607
# Parent  c152e538b85b099ce20b51104b8b7dd3666aad7c

revset: improve head revset performance

Previously the head() revset would iterate over every item in the subset and
check if it was a head.  Since the subset is often the entire repo, this was
slow on large repos. Now we iterate over each item in the head list and check if
it's in the subset, which results in much less work.

hg log -r 'head()' on a large repo:
Before: 0.95s
After: 0.28s

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -940,7 +940,7 @@ def head(repo, subset, x):
     hs = set()
     for b, ls in repo.branchmap().iteritems():
         hs.update(repo[h].rev() for h in ls)
-    return subset.filter(lambda r: r in hs)
+    return baseset(hs).filter(subset.__contains__)
 
 def heads(repo, subset, x):
     """``heads(set)``