Move toMillis(Duration) to Utils so that it can be reused. (#1114)

* Add toMillis() method to TimeUtils.

* Reuse toMillis() method in impl and exporters.

* Add a note about overflow and precision loss.

* Move toMillis() to Duration.

2 files changed, 26 insertions, 0 deletions

diff --git a/api/src/main/java/io/opencensus/common/Duration.java b/api/src/main/java/io/opencensus/common/Duration.java
index 1ee97191..57ca87fe 100644
--- a/api/src/main/java/io/opencensus/common/Duration.java
+++ b/api/src/main/java/io/opencensus/common/Duration.java
@@ -23,6 +23,7 @@ import static io.opencensus.common.TimeUtils.NANOS_PER_MILLI;
 
 import com.google.auto.value.AutoValue;
 import io.opencensus.internal.Utils;
+import java.util.concurrent.TimeUnit;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -79,6 +80,20 @@ public abstract class Duration implements Comparable<Duration> {
   }
 
   /**
+   * Converts a {@link Duration} to milliseconds.
+   *
+   * <p>Note that there could be overflow or loss of precision by making this conversion. See {@link
+   * TimeUnit#convert(long, TimeUnit)} for details.
+   *
+   * @param duration a {@code Duration}.
+   * @return the milliseconds representation of this {@code Duration}.
+   */
+  public static long toMillis(Duration duration) {
+    return TimeUnit.SECONDS.toMillis(duration.getSeconds())
+        + TimeUnit.NANOSECONDS.toMillis(duration.getNanos());
+  }
+
+  /**
    * Returns the number of seconds in the {@code Duration}.
    *
    * @return the number of seconds in the {@code Duration}.
diff --git a/api/src/test/java/io/opencensus/common/DurationTest.java b/api/src/test/java/io/opencensus/common/DurationTest.java
index 40682fc4..d46f2f7a 100644
--- a/api/src/test/java/io/opencensus/common/DurationTest.java
+++ b/api/src/test/java/io/opencensus/common/DurationTest.java
@@ -88,4 +88,15 @@ public class DurationTest {
     assertThat(Duration.create(-25, -42)).isNotEqualTo(Duration.create(-24, -42));
     assertThat(Duration.create(-24, -43)).isNotEqualTo(Duration.create(-24, -42));
   }
+
+  @Test
+  public void toMillis() {
+    assertThat(Duration.toMillis(Duration.create(10, 0))).isEqualTo(10000L);
+    assertThat(Duration.toMillis(Duration.create(10, 1000))).isEqualTo(10000L);
+    assertThat(Duration.toMillis(Duration.create(0, (int) 1e6))).isEqualTo(1L);
+    assertThat(Duration.toMillis(Duration.create(0, 0))).isEqualTo(0L);
+    assertThat(Duration.toMillis(Duration.create(-10, 0))).isEqualTo(-10000L);
+    assertThat(Duration.toMillis(Duration.create(-10, -1000))).isEqualTo(-10000L);
+    assertThat(Duration.toMillis(Duration.create(0, -(int) 1e6))).isEqualTo(-1L);
+  }
 }