server/AyaNova/util/DataUtil.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace AyaNova.Util
+{
+    internal static class DataUtil
+    {
+        public static IEnumerable<Tuple<double, double>> LargestTriangleThreeBuckets(List<Tuple<double, double>> data, int threshold)
+        {
+            int dataLength = data.Count;
+            if (threshold >= dataLength || threshold == 0)
+                return data; // Nothing to do
+
+            List<Tuple<double, double>> sampled = new List<Tuple<double, double>>(threshold);
+
+            // Bucket size. Leave room for start and end data points
+            double every = (double)(dataLength - 2) / (threshold - 2);
+
+            int a = 0;
+            Tuple<double, double> maxAreaPoint = new Tuple<double, double>(0, 0);
+            int nextA = 0;
+
+            sampled.Add(data[a]); // Always add the first point
+
+            for (int i = 0; i < threshold - 2; i++)
+            {
+                // Calculate point average for next bucket (containing c)
+                double avgX = 0;
+                double avgY = 0;
+                int avgRangeStart = (int)(Math.Floor((i + 1) * every) + 1);
+                int avgRangeEnd = (int)(Math.Floor((i + 2) * every) + 1);
+                avgRangeEnd = avgRangeEnd < dataLength ? avgRangeEnd : dataLength;
+
+                int avgRangeLength = avgRangeEnd - avgRangeStart;
+
+                for (; avgRangeStart < avgRangeEnd; avgRangeStart++)
+                {
+                    avgX += data[avgRangeStart].Item1; // * 1 enforces Number (value may be Date)
+                    avgY += data[avgRangeStart].Item2;
+                }
+                avgX /= avgRangeLength;
+
+                avgY /= avgRangeLength;
+
+                // Get the range for this bucket
+                int rangeOffs = (int)(Math.Floor((i + 0) * every) + 1);
+                int rangeTo = (int)(Math.Floor((i + 1) * every) + 1);
+
+                // Point a
+                double pointAx = data[a].Item1; // enforce Number (value may be Date)
+                double pointAy = data[a].Item2;
+
+                double maxArea = -1;
+
+                for (; rangeOffs < rangeTo; rangeOffs++)
+                {
+                    // Calculate triangle area over three buckets
+                    double area = Math.Abs((pointAx - avgX) * (data[rangeOffs].Item2 - pointAy) -
+                                           (pointAx - data[rangeOffs].Item1) * (avgY - pointAy)
+                                      ) * 0.5;
+                    if (area > maxArea)
+                    {
+                        maxArea = area;
+                        maxAreaPoint = data[rangeOffs];
+                        nextA = rangeOffs; // Next a is this b
+                    }
+                }
+
+                sampled.Add(maxAreaPoint); // Pick this point from the bucket
+                a = nextA; // This a is the next a (chosen b)
+            }
+
+            sampled.Add(data[dataLength - 1]); // Always add last
+
+            return sampled;
+        }
+
+    }//eoc
+
+}//eons