Extract upperbound calculation

The upperbound is constant for a given quantile.
By calculating this once we save O(iterator size * quantiles size) calculations of this constant.

Signed-off-by: Jens <jenskat@gmail.com>

Author: DieBauer

benchmarks/src/main/java/io/prometheus/client/CKMSQuantileBenchmark.java

@@ -97,6 +97,10 @@ public void setup() {
             for (Double l : shuffle) {
                 ckmsQuantiles.insert(l);
             }
+            // make sure we inserted all 'hanging' samples (count % 128)
+            ckmsQuantiles.get(0);
+            // compress everything so we have a similar samples size regardless of n.
+            ckmsQuantiles.compress();
             System.out.println("Sample size is: " + ckmsQuantiles.samples.size());
         }
 

simpleclient/src/main/java/io/prometheus/client/CKMSQuantiles.java

@@ -152,11 +152,12 @@ public double get(double q) {
 
         int r = 0; // sum of g's left of the current sample
         int desiredRank = (int) Math.ceil(q * n);
+        int upperBound = desiredRank + f(desiredRank) / 2;
 
         ListIterator<Sample> iterator = samples.listIterator();
         while (iterator.hasNext()) {
             Sample sample = iterator.next();
-            if (r + sample.g + sample.delta > desiredRank + f(desiredRank) / 2) {
+            if (r + sample.g + sample.delta > upperBound) {
                 iterator.previous(); // roll back the item.next() above
                 if (iterator.hasPrevious()) {
                     Sample result = iterator.previous();