Fix annotation moving to the left when X axis PV changes

When on plot the X axis PV changes the way that
there is no anymore the same x value, the annotation was choosing
the last index before.
When X axis PV is changing every time, it was causing the issue
that annotation was slowly going to the left and finally ending at the
beginning.
Finding the closest sample instead should fix the issue.

Author: Mateusz Nabywaniec

app/rtplot/src/main/java/org/csstudio/javafx/rtplot/data/PlotDataSearch.java

@@ -7,6 +7,9 @@
  ******************************************************************************/
 package org.csstudio.javafx.rtplot.data;
 
+import java.util.Date;
+import java.time.Instant;
+
 /** Search for samples in a haystack.
  *  @author Kay Kasemir
  */
@@ -125,4 +128,67 @@ final public int findSampleGreaterThan(final PlotDataProvider<XTYPE> data, final
         }
         return -1;
     }
+
+    /** Find the sample closest to the given value.
+     * @param data Data, must already be locked
+     * @param x The value to look for.
+     * @return Index of sample closest to x, or -1 if data is empty or type unsupported.
+     */
+    public int findClosestSample(final PlotDataProvider<XTYPE> data, final XTYPE x) {
+        int low = 0;
+        int high = data.size() - 1;
+
+        if (high < 0) return -1;
+
+        int bestIndex = -1;
+        double bestDistance = Double.MAX_VALUE;
+
+        double target;
+        try {
+            target = toDouble(x);
+        } catch (IllegalArgumentException e) {
+            return -1;
+        }
+
+        double distance;
+
+        while (low <= high) {
+            int mid = (low + high) / 2;
+            PlotDataItem<XTYPE> sample = data.get(mid);
+            XTYPE sampleX = sample.getPosition();
+            int cmp = sampleX.compareTo(x);
+
+            try{
+                distance = Math.abs(toDouble(sampleX) - target);
+            } catch (IllegalArgumentException e) {
+                return -1;
+            }
+
+            if (distance < bestDistance) {
+                bestDistance = distance;
+                bestIndex = mid;
+            }
+
+            if (cmp == 0) {
+                return mid;
+            } else if (cmp < 0) {
+                low = mid + 1;
+            } else {
+                high = mid - 1;
+            }
+        }
+
+        return bestIndex;
+    }
+
+    /** Convert supported XTYPE to double for distance comparison */
+    private double toDouble(XTYPE x) {
+        if (x instanceof Number)
+            return ((Number) x).doubleValue();
+        if (x instanceof Instant)
+            return ((Instant) x).toEpochMilli();
+        if (x instanceof Date)
+            return ((Date) x).getTime();
+        throw new IllegalArgumentException("Unsupported XTYPE: " + x.getClass());
+    }
 }

app/rtplot/src/main/java/org/csstudio/javafx/rtplot/internal/AnnotationImpl.java

@@ -179,7 +179,7 @@ boolean updateValue(final XTYPE location) throws Exception
             throw new TimeoutException("Cannot update annotation, no lock on " + data);
         try
         {
-            final int index = search.findSampleLessOrEqual(data, location);
+            final int index = search.findClosestSample(data, location);
             if (index < 0)
                 return false;
             final PlotDataItem<XTYPE> sample = data.get(index);