fix backoff calucuration #245

Author: nkzawa

src/main/java/io/socket/backo/Backoff.java

@@ -1,5 +1,8 @@
 package io.socket.backo;
 
+import java.math.BigDecimal;
+import java.math.BigInteger;
+
 public class Backoff {
 
     private long ms = 100;
@@ -11,17 +14,16 @@ public class Backoff {
     public Backoff() {}
 
     public long duration() {
-        long ms = this.ms * (long) Math.pow(this.factor, this.attempts++);
+        BigInteger ms = BigInteger.valueOf(this.ms)
+                .multiply(BigInteger.valueOf(this.factor).pow(this.attempts++));
         if (jitter != 0.0) {
             double rand = Math.random();
-            int deviation = (int) Math.floor(rand * this.jitter * ms);
-            ms = (((int) Math.floor(rand * 10)) & 1) == 0 ? ms - deviation : ms + deviation;
-        }
-        if (ms < this.ms) {
-            // overflow happened
-            ms = Long.MAX_VALUE;
+            BigInteger deviation = BigDecimal.valueOf(rand)
+                    .multiply(BigDecimal.valueOf(jitter))
+                    .multiply(new BigDecimal(ms)).toBigInteger();
+            ms = (((int) Math.floor(rand * 10)) & 1) == 0 ? ms.subtract(deviation) : ms.add(deviation);
         }
-        return Math.min(ms, this.max);
+        return ms.min(BigInteger.valueOf(this.max)).longValue();
     }
 
     public void reset() {

src/test/java/io/socket/backo/BackoffTest.java

@@ -2,6 +2,9 @@
 
 import org.junit.Test;
 
+import java.math.BigDecimal;
+import java.math.BigInteger;
+
 import static org.junit.Assert.assertTrue;
 
 public class BackoffTest {
@@ -22,14 +25,23 @@ public void durationShouldIncreaseTheBackoff() {
 
     @Test
     public void durationOverflow() {
-        Backoff b = new Backoff();
-        b.setMin(100);
-        b.setMax(10000);
-        b.setJitter(1.0);
-
-        for (int i = 0; i < 100; i++) {
-            long duration = b.duration();
-            assertTrue(100 <= duration && duration <= 10000);
+        for (int i = 0; i < 10; i++) {
+            Backoff b = new Backoff();
+            b.setMin(100);
+            b.setMax(10000);
+            b.setJitter(0.5);
+
+            // repeats to make it overflow (a long can have 2 ** 63 - 1)
+            for (int j = 0; j < 100; j++) {
+                BigInteger ms = BigInteger.valueOf(100).multiply(BigInteger.valueOf(2).pow(j));
+                BigInteger deviation = new BigDecimal(ms).multiply(BigDecimal.valueOf(0.5)).toBigInteger();
+                BigInteger duration = BigInteger.valueOf(b.duration());
+
+                BigInteger min = ms.subtract(deviation).min(BigInteger.valueOf(10000));
+                BigInteger max = ms.add(deviation).min(BigInteger.valueOf(10001));
+                assertTrue(min + " <= " + duration + " < " + max,
+                        min.compareTo(duration) <= 0 && max.compareTo(duration) == 1);
+            }
         }
     }
 }