# HG changeset patch
# User Lucas Moscovicz <lmoscovicz@fb.com>
# Date 2014-02-19 20:56:41
# Node ID 4849f574aa24b6619e0613d3b6e9f04f54447bc4
# Parent  87e52e6425625ea4f7645cfe2fc491a21f9a6b51

revset: changed last implementation to use lazy classes

Instead of using getitem just reverse the revision list and get the first
'lim' elements. With classes like spanset which are easily reversible this
will work faster.

Performance Benchmarking:

$ time hg log -qr "last(all())"
...

real  0m0.569s
user  0m0.447s
sys 0m0.122s

$ time ./hg log -qr "last(all())"
...

real  0m0.215s
user  0m0.150s
sys 0m0.063s

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -972,8 +972,18 @@ def last(repo, subset, x):
         # i18n: "last" is a keyword
         raise error.ParseError(_("last expects a number"))
     ss = subset.set()
-    os = getset(repo, spanset(repo), l[0])[-lim:]
-    return baseset([r for r in os if r in ss])
+    os = getset(repo, spanset(repo), l[0])
+    os.reverse()
+    bs = baseset([])
+    it = iter(os)
+    for x in xrange(lim):
+        try:
+            y = it.next()
+            if y in ss:
+                bs.append(y)
+        except (StopIteration):
+            break
+    return bs
 
 def maxrev(repo, subset, x):
     """``max(set)``