1 files changed, 120 insertions, 0 deletions
diff --git a/kamon-core/src/legacy-main/scala/kamon/metric/instrument/Gauge.scala b/kamon-core/src/legacy-main/scala/kamon/metric/instrument/Gauge.scala
new file mode 100644
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+++ b/kamon-core/src/legacy-main/scala/kamon/metric/instrument/Gauge.scala
@@ -0,0 +1,120 @@
+/*
+ * =========================================================================================
+ * Copyright © 2013-2015 the kamon project <http://kamon.io/>
+ *
+ * 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 kamon.metric.instrument
+
+import java.util.concurrent.atomic.{AtomicLong, AtomicLongFieldUpdater, AtomicReference}
+
+import akka.actor.Cancellable
+import kamon.metric.instrument.Gauge.CurrentValueCollector
+import kamon.metric.instrument.Histogram.DynamicRange
+
+import scala.concurrent.duration.FiniteDuration
+
+trait Gauge extends Instrument {
+  type SnapshotType = Histogram.Snapshot
+
+  def record(value: Long): Unit
+  def record(value: Long, count: Long): Unit
+  def refreshValue(): Unit
+}
+
+object Gauge {
+
+  def apply(dynamicRange: DynamicRange, refreshInterval: FiniteDuration, scheduler: RefreshScheduler, valueCollector: CurrentValueCollector): Gauge = {
+    val underlyingHistogram = Histogram(dynamicRange)
+    val gauge = new HistogramBackedGauge(underlyingHistogram, valueCollector)
+    val refreshValuesSchedule = scheduler.schedule(refreshInterval, () ⇒ {
+      gauge.refreshValue()
+    })
+
+    gauge.automaticValueCollectorSchedule.set(refreshValuesSchedule)
+    gauge
+  }
+
+  def create(dynamicRange: DynamicRange, refreshInterval: FiniteDuration, scheduler: RefreshScheduler, valueCollector: CurrentValueCollector): Gauge =
+    apply(dynamicRange, refreshInterval, scheduler, valueCollector)
+
+  trait CurrentValueCollector {
+    def currentValue: Long
+  }
+
+  implicit def functionZeroAsCurrentValueCollector(f: () ⇒ Long): CurrentValueCollector = new CurrentValueCollector {
+    def currentValue: Long = f.apply()
+  }
+
+  implicit def callByNameLongAsCurrentValueCollector(f: ⇒ Long): CurrentValueCollector = new CurrentValueCollector {
+    def currentValue: Long = f
+  }
+}
+
+/**
+ *  Helper for cases in which a gauge shouldn't store the current value of a observed value but the difference between
+ *  the current observed value and the previously observed value. Should only be used if the observed value is always
+ *  increasing or staying steady, but is never able to decrease.
+ *
+ *  Note: The first time a value is collected, this wrapper will always return zero, afterwards, the difference between
+ *        the current value and the last value will be returned.
+ */
+class DifferentialValueCollector(wrappedValueCollector: CurrentValueCollector) extends CurrentValueCollector {
+  @volatile private var _readAtLeastOnce = false
+  private val _lastObservedValue = new AtomicLong(0)
+
+  def currentValue: Long = {
+    if (_readAtLeastOnce) {
+      val wrappedCurrent = wrappedValueCollector.currentValue
+      val diff = wrappedCurrent - _lastObservedValue.getAndSet(wrappedCurrent)
+
+      if (diff >= 0) diff else 0L
+
+    } else {
+      _lastObservedValue.set(wrappedValueCollector.currentValue)
+      _readAtLeastOnce = true
+      0L
+    }
+
+  }
+}
+
+object DifferentialValueCollector {
+  def apply(wrappedValueCollector: CurrentValueCollector): CurrentValueCollector =
+    new DifferentialValueCollector(wrappedValueCollector)
+
+  def apply(wrappedValueCollector: ⇒ Long): CurrentValueCollector =
+    new DifferentialValueCollector(new CurrentValueCollector {
+      def currentValue: Long = wrappedValueCollector
+    })
+}
+
+class HistogramBackedGauge(underlyingHistogram: Histogram, currentValueCollector: Gauge.CurrentValueCollector) extends Gauge {
+  private[kamon] val automaticValueCollectorSchedule = new AtomicReference[Cancellable]()
+
+  def record(value: Long): Unit = underlyingHistogram.record(value)
+
+  def record(value: Long, count: Long): Unit = underlyingHistogram.record(value, count)
+
+  def collect(context: CollectionContext): Histogram.Snapshot = underlyingHistogram.collect(context)
+
+  def cleanup: Unit = {
+    if (automaticValueCollectorSchedule.get() != null)
+      automaticValueCollectorSchedule.get().cancel()
+  }
+
+  def refreshValue(): Unit =
+    underlyingHistogram.record(currentValueCollector.currentValue)
+
+}
+