Merge pull request qupath#2041 from petebankhead/histogram

Create Histogram from edges and counts

Author: petebankhead

qupath-core/src/main/java/qupath/lib/analysis/stats/Histogram.java

@@ -23,10 +23,10 @@
 
 package qupath.lib.analysis.stats;
 
-import java.util.Collection;
-
 import qupath.lib.objects.PathObject;
 
+import java.util.Collection;
+
 
 /**
  * Class for storing histogram data & basic statistics.
@@ -350,6 +350,39 @@ public Histogram(ArrayWrappers.ArrayWrapper values, int nBins, double minEdge, d
 		buildHistogram(values, nBins, minEdge, maxEdge);
 	}
 
+    /**
+     * Create histogram from an existing edges and counts arrays.
+     * <p>
+     * This is useful when a histogram already exists in another format, and it would be too expensive to rebuild
+     * from the original values.
+     * <p>
+     * Note that the statistics (min, max, mean, std.dev. and variance) are estimated from the bin centers, and may not
+     * match the values from the data originally used to build the histogram.
+     *
+     * @param edges an array of edges, which should be of length {@code nBins + 1}.
+     * @param counts an array of counts, which should be of length {@code nBins}.
+     */
+    public Histogram(double[] edges, long[] counts) {
+        this.edgeMin = edges[0];
+        this.edgeMax = edges[edges.length-1];
+        this.edges = edges.clone();
+        this.counts = counts.clone();
+        boolean maybeInteger = true;
+        long countsSum = 0L;
+        var stats = new RunningStatistics();
+        for (int i = 0; i < edges.length-1; i++) {
+            double center = (edges[i] + edges[i+1]) / 2.0;
+            maybeInteger = maybeInteger && Math.round(center) == center;
+            for (long k = 0; k < counts[i]; k++) {
+                stats.addValue(center);
+                countsSum++;
+            }
+        }
+        this.isInteger = maybeInteger;
+        this.stats = stats;
+        this.countSum = countsSum;
+    }
+
 
 	private void buildHistogram(final ArrayWrappers.ArrayWrapper values, int nBins, double minEdge, double maxEdge) {