interpolate result if next point is also known

src/geotag/timeline/Track.kt

@@ -1,5 +1,6 @@
 package geotag.timeline
 
+import java.time.Duration.between
 import java.time.Instant
 
 data class Track(
@@ -8,6 +9,22 @@ data class Track(
   val points: List<TrackPoint>) {
 
   fun pointAt(time: Instant): TrackPoint? {
-    return points.findLast { it.time <= time }
+    val i = points.listIterator(points.size)
+    while (i.hasPrevious()) {
+      val point = i.previous()
+      if (time >= point.time) {
+        i.next()
+        if (i.hasNext()) return interpolate(point, i.next(), time)
+        else return point
+      }
+    }
+    return null
   }
+
+  private fun interpolate(p1: TrackPoint, p2: TrackPoint, time: Instant): TrackPoint {
+    val c = between(p1.time, time).toMillis().toFloat() / between(p1.time, p2.time).toMillis()
+    return TrackPoint(interpolate(p1.lat, p2.lat, c), interpolate(p1.lon, p2.lon, c), time)
+  }
+
+  private fun interpolate(v1: LatLon, v2: LatLon, c: Float) = v1.value + (v2.value - v1.value) * c
 }