Cleaner and more optimized implementation (+4 squashed commits)

Squashed commits:
[ce4b687] Cleaner implementation
[fd64536] Cleaner implementation
[a951435] Performance optimization
[fae6416] Possibly cleaner code

Author: mikulatomas

concepts/algorithms/covering_edges.py

@@ -5,29 +5,23 @@
 John Wiley & Sons, 2004.
 """
 
+from collections import Counter
+
 from .fcbo import fast_generate_from
 
 
 def lattice(context):
-    edges = covering_edges(fast_generate_from(context), context)
+    concepts = tuple(fast_generate_from(context))
+    edges = covering_edges(concepts, context)
 
-    mapping = {}
+    mapping = dict([(extent, (extent, intent, [], [])) for extent, intent in concepts])
 
     for concept, lower_neighbor in edges:
-        extent, intent = concept
-        lower_extent, lower_intent = lower_neighbor
-
-        if extent in mapping:
-            _, _, _, lower = mapping[extent]
-            lower.append(lower_extent)
-        else:
-            mapping[extent] = (extent, intent, [], [lower_extent])
+        extent, _ = concept
+        lower_extent, _ = lower_neighbor
 
-        if lower_extent in mapping:
-            _, _, upper, _ = mapping[lower_extent]
-            upper.append(extent)
-        else:
-            mapping[lower_extent] = (lower_extent, lower_intent, [extent], [])
+        mapping[extent][3].append(lower_extent)
+        mapping[lower_extent][2].append(extent)
 
     return mapping.values()
 
@@ -97,7 +91,7 @@ def covering_edges(concept_list, context):
     concept_index = dict(concept_list)
 
     for extent, intent in concept_index.items():
-        candidate_counter = dict.fromkeys(concept_index, 0)
+        candidate_counter = Counter()
 
         property_candidates = Properties.fromint(Properties.supremum & ~intent)
 
@@ -107,4 +101,4 @@ def covering_edges(concept_list, context):
             candidate_counter[extent_candidate] += 1
 
             if (intent_candidate.count() - intent.count()) == candidate_counter[extent_candidate]:
-                yield (extent, intent), (extent_candidate, intent_candidate)
+                yield (extent, intent), (extent_candidate, intent_candidate)