[Findit] Initial code of findit for crash. Add scorers to apply heuristic rules.

appengine/findit/crash/scorers/top_frame_index.py

Index: appengine/findit/crash/scorers/top_frame_index.py
diff --git a/appengine/findit/crash/scorers/top_frame_index.py b/appengine/findit/crash/scorers/top_frame_index.py
new file mode 100644
index 0000000000000000000000000000000000000000..0ec9d1511373435a6abf2162fc7c5cdde8631072
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+++ b/appengine/findit/crash/scorers/top_frame_index.py
@@ -0,0 +1,45 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""TopFrameIndex scorer applies to all Result objects.
+
+It represents a heuristic rule:
+  The less the top frame index (this result changed) is, the higher score.
+"""
+
+from crash.scorers.scorer import Scorer
+
+# TODO(katesonia): Move this to the config saved in datastore.
+_MAX_TOP_N_FRAMES = 7
+_INFINITY = 1000
+
+
+class TopFrameIndex(Scorer):
+
+  def __init__(self, max_top_n=_MAX_TOP_N_FRAMES):
+    self.max_top_n = max_top_n
+
+  def GetMetric(self, result):
+    if not result.file_to_stack_infos:
+      return None
+
+    top_frame_index = _INFINITY
+    for _, stack_infos in result.file_to_stack_infos.iteritems():
+      for frame, _ in stack_infos:
+        top_frame_index = min(top_frame_index, frame.index)
+
+    return top_frame_index
+
+  def Score(self, top_frame_index):
+    # TODO(katesonia): experiment the model and parameters later.
+    if top_frame_index < self.max_top_n:
+      return 1 - top_frame_index / float(self.max_top_n)
+
+    return 0
+
+  def Reason(self, top_frame_index, score):
+    if score == 0:
+      return ''
+
+    return 'Top frame changed is frame #%d' % top_frame_index