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

appengine/findit/crash/scorers/min_distance.py

+# 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.
+
+"""MinDistance scorer applies to MatchResult objects.
+
+It represents a heuristic rule:
+  1. Highest score if the result changed the crashed lines.
+  2. 0 score if changed lines are too far away from crashed lines.
+"""
+
+import logging
+
+from crash.scorers.scorer import Scorer
+
+_MAX_DISTANCE = 50
+
+
+class MinDistance(Scorer):
+
+  def __init__(self, max_distance=_MAX_DISTANCE):
+    self.max_distance = max_distance
+
+  def GetMetric(self, result):
+    if not hasattr(result, 'min_distance'):
+      logging.warning('Scorer %s only applies to MatchResult', self.name)
+      return None
+
+    return result.min_distance
+
+  def Score(self, min_distance):
+    if min_distance > self.max_distance:
+      return 0
+
+    if min_distance == 0:
+      return 1
+
+    # TODO(katesonia): This number is randomly picked from a reasonable range,
+    # best value to use still needs be experimented out.
+    return 0.8
+
+  def Reason(self, min_distance, score):
+    if score == 0:
+      return ''
+
+    return 'Minimum distance to crashed line is %d' % min_distance