# HG changeset patch
# User Siddharth Agarwal <sid0@fb.com>
# Date 1354905463 28800
# Node ID 34a1a639d8358e43f4bcba7b0cff19f4e4e6958d
# Parent  1471f5e83686107c8ebbbec47d05d3fbda40959f
revset.children: ignore rev numbers that are too low

This replaces unnecessary parentrevs() calls with calculating min(parentset).
Even though the min operation is O(size of parentset), since parentrevs is
relatively expensive, this tradeoff almost always works in our favour. In a
repository with over 400,000 changesets, hg perfrevset "children(X)" takes:

       Set X       Before    After
       -1           0.51s    0.06s
    -1000:          0.55s    0.08s
   -10000:          0.56s    0.10s
  -100000:          0.60s    0.25s
  -100000:-99000    0.55s    0.19s
        0:100000    0.60s    0.61s
      all()         0.72s    0.74s

The relative performance is similar for Mercurial's own repository -- several
times faster in most cases, slightly slower for revisions close to 0 and
all().

diff -r 1471f5e83686 -r 34a1a639d835 mercurial/revset.py
--- a/mercurial/revset.py	Thu Dec 06 11:21:11 2012 -0800
+++ b/mercurial/revset.py	Fri Dec 07 10:37:43 2012 -0800
@@ -488,8 +488,13 @@
 
 def _children(repo, narrow, parentset):
     cs = set()
+    if not parentset:
+        return cs
     pr = repo.changelog.parentrevs
+    minrev = min(parentset)
     for r in narrow:
+        if r <= minrev:
+            continue
         for p in pr(r):
             if p in parentset:
                 cs.add(r)