account for all same values in rank

cache.go

@@ -273,6 +273,11 @@ func (c *RankCache) recalculate() {
 			if cnt <= c.thresholdValue {
 				delete(c.entries, id)
 			}
+			// prevent from deleting the whole cache if all the same value
+			if len(c.entries) <= int(c.maxEntries) {
+				break
+			}
+
 		}
 	}
 }

cache_test.go

@@ -0,0 +1,35 @@
+// Copyright 2017 Pilosa Corp.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pilosa_test
+
+import (
+	"testing"
+
+	"github.com/pilosa/pilosa"
+)
+
+// Ensure a bitmap query can be executed.
+func TestCache_Rank(t *testing.T) {
+	cacheSize := uint32(3)
+	cache := pilosa.NewRankCache(cacheSize)
+	for i := 1; i < int(2*cacheSize); i++ {
+		cache.Add(uint64(i), 3)
+	}
+	cache.Recalculate()
+	if cache.Len() != int(cacheSize) {
+		t.Fatalf("unexpected cache Size: %d!=%d expected\n", cache.Len(), cacheSize)
+	}
+
+}