Added a confidence sort order for comments

@@ -19,7 +19,7 @@
 # All portions of the code written by CondeNet are Copyright (c) 2006-2008
 # CondeNet, Inc. All Rights Reserved.
 ################################################################################
-from math import log
+from math import log, sqrt
 from datetime import datetime, timedelta
 from r2.config.databases import tz
 
@@ -46,3 +46,37 @@ def controversy(ups, downs):
     """The controversy sort."""
     return float(ups + downs) / max(abs(score(ups, downs)), 1)
 
+
+def _confidence(ups, downs):
+    """The confidence sort.
+       http://www.evanmiller.org/how-not-to-sort-by-average-rating.html"""
+    n = ups + downs
+
+    if n == 0:
+        return 0
+
+    z = 1.281551565545 # 80% confidence
+    p = float(ups) / n
+
+    left = p + 1/(2*n)*z*z
+    right = z*sqrt(p*(1-p)/n + z*z/(4*n*n))
+    under = 1+1/n*z*z
+
+    return (left - right) / under
+
+# precompute low values
+up_range = 400
+down_range = 100
+_confidences = []
+for _ups in xrange(up_range):
+    for _downs in xrange(down_range):
+        _confidences.append(_confidence(_ups, _downs))
+
+def confidence(ups, downs):
+    if ups + downs == 0:
+        return 0
+    elif ups < up_range and downs < down_range:  # check if pair is precomputed
+        return _confidences[downs + ups * down_range]
+    else:
+        return _confidence(ups, downs)
+

@@ -440,6 +440,10 @@ def _score(self):
     def _controversy(self):
         return sorts.controversy(self._ups, self._downs)
 
+    @property
+    def _confidence(self):
+        return sorts.confidence(self._ups, self._downs)
+
     @classmethod
     def _build(cls, id, bases):
         return cls(bases.ups, bases.downs, bases.date,

@@ -61,6 +61,7 @@ def __getattr__(self, attr):
                      more         = _('More'),
                      relevance    = _('Relevance'),
                      controversial  = _('Controversial'),
+                     confidence   = _('Best'),
                      saved        = _('Saved'),
                      recommended  = _('Recommended'),
                      rising       = _('Rising'),
@@ -435,10 +436,13 @@ def operator(self, sort):
             return operators.desc('_score')
         elif sort == 'controversial':
             return operators.desc('_controversy')
+        elif sort == 'confidence':
+            return operators.desc('_confidence')
 
 class CommentSortMenu(SortMenu):
     """Sort menu for comments pages"""
-    options   = ('hot', 'new', 'controversial', 'top', 'old')
+    default   = 'confidence'
+    options   = ('hot', 'new', 'controversial', 'top', 'old', 'confidence')
 
 class SearchSortMenu(SortMenu):
     """Sort menu for search pages."""