Merge pull request #22 from Netflix/feature/fast_sqrt

Optimized function for calculating square roots

concurrency-limits-core/src/main/java/com/netflix/concurrency/limits/limit/functions/SquareRootFunction.java

@@ -0,0 +1,34 @@
+package com.netflix.concurrency.limits.limit.functions;
+
+import java.util.function.Function;
+import java.util.stream.IntStream;
+
+/**
+ * Specialized utility function used by limiters to calculate thredsholds using square root
+ * of the current limit.  Here we pre-compute the square root of numbers up to 1000 because
+ * the square root operation can be slow.
+ */
+public final class SquareRootFunction implements Function<Integer, Integer> {
+    static final int[] lookup = new int[1000];
+
+    static {
+        IntStream.range(0, 1000).forEach(i -> lookup[i] = (int)Math.sqrt(i));
+    }
+
+    private static final SquareRootFunction INSTANCE = new SquareRootFunction();
+
+    /**
+     * Create an instance of a function that returns : baseline + sqrt(limit)
+     * 
+     * @param baseline
+     * @return
+     */
+    public static Function<Integer, Integer> create(int baseline) {
+        return INSTANCE.andThen(t -> t + baseline);
+    }
+
+    @Override
+    public Integer apply(Integer t) {
+        return t < 1000 ? lookup[t] : (int)Math.sqrt(t);
+    }
+}

concurrency-limits-core/src/test/java/com/netflix/concurrency/limits/limit/functions/SquareRootFunctionTest.java

@@ -0,0 +1,26 @@
+package com.netflix.concurrency.limits.limit.functions;
+
+import java.util.function.Function;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SquareRootFunctionTest {
+    @Test
+    public void confirm0Index() {
+        Function<Integer, Integer> func = SquareRootFunction.create(4);
+        Assert.assertEquals(4, func.apply(0).intValue());
+    }
+
+    @Test
+    public void confirmMaxIndex() {
+        Function<Integer, Integer> func = SquareRootFunction.create(4);
+        Assert.assertEquals(35, func.apply(999).intValue());
+    }
+
+    @Test
+    public void confirmOutofLookupRange() {
+        Function<Integer, Integer> func = SquareRootFunction.create(4);
+        Assert.assertEquals(35, func.apply(1005).intValue());
+    }
+}