# HG changeset patch
# User David Soria Parra <davidsp@fb.com>
# Date 2017-11-23 22:13:14
# Node ID 8287df8b7be545fdafa22b771012ac65f6264d12
# Parent  ec25c8275cfaac673ccb9e1ac22d0c554b9fe672

hbisect: use a defaultdict to avoid large allocations for a large changelogs

We can avoid a SPACE(len(changelog)) allocation by using a defaultdict.

Test Plan:
    python run-tests.py test-bisect*

Differential Revision: https://phab.mercurial-scm.org/D1499

diff --git a/mercurial/hbisect.py b/mercurial/hbisect.py
--- a/mercurial/hbisect.py
+++ b/mercurial/hbisect.py
@@ -38,7 +38,7 @@ def bisect(repo, state):
 
     def buildancestors(bad, good):
         badrev = min([changelog.rev(n) for n in bad])
-        ancestors = [None] * (len(changelog) + 1)
+        ancestors = collections.defaultdict(lambda: None)
         for rev in repo.revs("descendants(%ln) - ancestors(%ln)", good, good):
             ancestors[rev] = []
         if ancestors[badrev] is None: