reduce memory usage for computation of cumulative distribution

Before: To compute the cumulative distribution we added every duration
into a LongList. This requires O(n) memory, where n is the number of
values.

Now: We store the durations + the number of occurrences in a
LongLongHashMap. This has the potential to reduce the memory
requirements if durations occur multiple times. There are a lot of
durations with 0, 1, 2 milliseconds. In the worst case every duration
is different. In that case the memory usage doubled with this solution.

Future: We are currently storing durations with milli seconds precision.
We don't have to do that. We cannot draw 100 million different values
on the y-axis in an images with only 1000px.

pdb-plotting/src/main/java/org/lucares/pdb/plot/api/CumulativeDistributionCustomAggregator.java

@@ -8,14 +8,68 @@ import java.io.OutputStreamWriter;
 import java.io.Writer;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Path;
+import java.util.LinkedHashMap;
 
-import org.lucares.collections.LongList;
+import org.lucares.collections.LongLongConsumer;
+import org.lucares.collections.LongLongHashMap;
 
 public class CumulativeDistributionCustomAggregator implements CustomAggregator {
 
 	private final static int POINTS = 500;
 
-	private final LongList values = new LongList();
+	private static final class ToPercentiles implements LongLongConsumer {
+
+		private long cumulativeCount = 0;
+
+		private long maxValue = 0;
+
+		private final LinkedHashMap<Double, Long> percentiles = new LinkedHashMap<>(POINTS);
+
+		private final double stepSize;
+
+		private double lastPercentile;
+		private double nextPercentile;
+
+		private final long totalValues;
+
+		public ToPercentiles(final long totalValues) {
+			this.totalValues = totalValues;
+			stepSize = 100.0 / POINTS;
+			nextPercentile = stepSize;
+		}
+
+		@Override
+		public void accept(final long duration, final long count) {
+			maxValue = duration;
+
+			cumulativeCount += count;
+			final double newPercentile = cumulativeCount * 100.0 / totalValues;
+
+			if (newPercentile >= nextPercentile) {
+				double currentPercentile = lastPercentile + stepSize;
+				while (currentPercentile <= newPercentile) {
+					percentiles.put(currentPercentile, duration);
+					currentPercentile += stepSize;
+				}
+				nextPercentile = currentPercentile;
+				lastPercentile = currentPercentile - stepSize;
+			}
+		}
+
+		public long getMaxValue() {
+			return maxValue;
+		}
+
+		public LinkedHashMap<Double, Long> getPercentiles() {
+			return percentiles;
+		}
+
+	}
+
+	// the rather large initial capacity should prevent too many grow&re-hash phases
+	private final LongLongHashMap map = new LongLongHashMap(5_000, 0.75);
+
+	private long totalValues = 0;
 
 	private final Path tmpDir;
 
@@ -25,7 +79,8 @@ public class CumulativeDistributionCustomAggregator implements CustomAggregator
 
 	@Override
 	public void addValue(final long epochMilli, final long value) {
-		values.add((int) value);
+		map.compute(value, 0, l -> l + 1);
+		totalValues++;
 	}
 
 	@Override
@@ -33,26 +88,25 @@ public class CumulativeDistributionCustomAggregator implements CustomAggregator
 		final char separator = ',';
 		final char newline = '\n';
 
-		values.parallelSort();
+		final ToPercentiles toPercentiles = new ToPercentiles(totalValues);
+		map.forEachOrdered(toPercentiles);
 
-		final LongList percentiles = new LongList(POINTS);
 		final File dataFile = File.createTempFile("data", ".dat", tmpDir.toFile());
 		try (final Writer output = new BufferedWriter(
 				new OutputStreamWriter(new FileOutputStream(dataFile), StandardCharsets.US_ASCII));) {
 
 			final StringBuilder data = new StringBuilder();
-			if (values.size() > 0) {
+			if (map.size() > 0) {
 				// compute the percentiles
-				for (int i = 0; i < POINTS; i++) {
-					data.append(i * (100 / (double) POINTS));
-					data.append(separator);
-					final long percentile = values.get((int) Math.floor(values.size() / ((double) POINTS) * i));
-					data.append(percentile);
-					data.append(newline);
+				toPercentiles.getPercentiles().forEach((percentile, value) -> {
 
-					percentiles.add(percentile);
-				}
-				final long maxValue = values.get(values.size() - 1);
+					data.append(percentile);
+					data.append(separator);
+					data.append(value);
+					data.append(newline);
+				});
+
+				final long maxValue = toPercentiles.getMaxValue();
 				data.append(100);
 				data.append(separator);
 				data.append(maxValue);
@@ -62,7 +116,7 @@ public class CumulativeDistributionCustomAggregator implements CustomAggregator
 
 		}
 
-		final String title = String.format("percentiles");
+		final String title = String.format("cumulative distribution");
 		return new AggregatedData(title, dataFile);
 	}
 

pdb-plotting/src/main/java/org/lucares/pdb/plot/api/PlotSettings.java

@@ -119,7 +119,7 @@ public class PlotSettings {
 	public DateTimeRange dateRange() {
 
 		final String[] startEnd = dateRangeAsString.split(Pattern.quote(" - "));
-		Preconditions.checkEqual(startEnd, 2, "invalid date range: ''{0}''", dateRangeAsString);
+		Preconditions.checkEqual(startEnd.length, 2, "invalid date range: ''{0}''", dateRangeAsString);
 
 		final OffsetDateTime startDate = LocalDateTime.parse(startEnd[0], DATE_FORMAT).atOffset(ZoneOffset.UTC);
 		final OffsetDateTime endDate = LocalDateTime.parse(startEnd[1], DATE_FORMAT).atOffset(ZoneOffset.UTC);