# HG changeset patch
# User Durham Goode <durham@fb.com>
# Date 1389736159 28800
# Node ID 8dc254198a8f7ec51e459d3804fe0de91a17979a
# Parent  7d589d923b8a1ae7b920a88cf66a79d3faa330b5
changectx: increase perf of walk function

When running 'hg cat -r . <file>' it was doing an expensive ctx.walk(m) which
applied the regex to every file in the manifest.

This changes changectx.walk to iterate over just the files in the regex, if no
other patterns are specified. This cuts hg cat time by 50% in our repo and
probably benefits a few other commands as well.

diff -r 7d589d923b8a -r 8dc254198a8f mercurial/context.py
--- a/mercurial/context.py	Wed Jan 15 16:46:20 2014 -0800
+++ b/mercurial/context.py	Tue Jan 14 13:49:19 2014 -0800
@@ -410,6 +410,15 @@
         # for dirstate.walk, files=['.'] means "walk the whole tree".
         # follow that here, too
         fset.discard('.')
+
+        # avoid the entire walk if we're only looking for specific files
+        if fset and not match.anypats():
+            if util.all([fn in self for fn in fset]):
+                for fn in sorted(fset):
+                    if match(fn):
+                        yield fn
+                raise StopIteration
+
         for fn in self:
             if fn in fset:
                 # specified pattern is the exact name