Avoid computing negative values for offsets, making zero the lowest possible value

Fixes #25

build.gradle

@@ -7,7 +7,7 @@ apply plugin: 'application'
 
 group = "com.github.lookout"
 description = "A utility for monitoring the delay of Kafka consumers"
-version = '0.1.7'
+version = '0.1.8'
 mainClassName = 'com.github.lookout.verspaetung.Main'
 defaultTasks 'clean', 'check'
 sourceCompatibility = '1.7'

src/main/groovy/com/github/lookout/verspaetung/metrics/ConsumerGauge.groovy

@@ -39,7 +39,15 @@ class ConsumerGauge implements Gauge<Integer>, Tagged {
             (!this.topics.containsKey(topicPartition))) {
             return 0
         }
-        return ((Integer)this.topics[topicPartition]) - this.consumers[consumer]
+
+        /*
+         * Returning the maximum value of the computation and zero, there are
+         * some cases where we can be "behind" on the Kafka latest offset
+         * polling and this could result in an erroneous negative value. See:
+         * <https://github.com/lookout/verspaetung/issues/25> for more details
+         */
+        return Math.max(0,
+                        ((Integer)this.topics[topicPartition]) - this.consumers[consumer])
     }
 
     @Override

src/test/groovy/com/github/lookout/verspaetung/metrics/ConsumerGaugeSpec.groovy

@@ -35,7 +35,17 @@ class ConsumerGaugeSpec extends Specification {
 
     def "getValue() should return zero for a consumer not in the map"() {
         given:
-        ConsumerGauge gauge = new ConsumerGauge(consumer, [:], [:])
+        ConsumerGauge gauge = new ConsumerGauge(this.consumer, [:], [:])
+
+        expect:
+        gauge.value == 0
+    }
+
+    def "getValue() should return zero instead of a negative number"() {
+        given:
+        ConsumerGauge gauge = new ConsumerGauge(this.consumer,
+                                                    [(this.consumer) : 10],
+                                                    [(this.tp) : 5])
 
         expect:
         gauge.value == 0