devdocs/tools.txt

@@ -604,3 +604,90 @@ select
   58.43239007949476
 
 from generate_series(1, 525600) s(i)
+
+
+
+JAVASCRIPT DOWNSAMPLING TOOL
+//====================
+//https://github.com/joshcarr/largest-triangle-three-buckets.js/blob/master/lib/largest-triangle-three-buckets.js
+ 'use strict';
+
+  function largestTriangleThreeBuckets( data, threshold, xAccessor, yAccessor ) {
+
+    var floor = Math.floor,
+      abs = Math.abs,
+      dataLength = data.length,
+      sampled = [],
+      sampledIndex = 0,
+      every = ( dataLength - 2 ) / ( threshold - 2 ), // Bucket size. Leave room for start and end data points
+      a = 0, // Initially a is the first point in the triangle
+      maxAreaPoint,
+      maxArea,
+      area,
+      nextA,
+      i,
+      avgX = 0,
+      avgY = 0,
+      avgRangeStart,
+      avgRangeEnd,
+      avgRangeLength,
+      rangeOffs,
+      rangeTo,
+      pointAX,
+      pointAY;
+
+    if ( threshold >= dataLength || threshold === 0 ) {
+      return data; // Nothing to do
+    }
+
+    sampled[ sampledIndex++ ] = data[ a ]; // Always add the first point
+
+    for ( i = 0; i < threshold - 2; i++ ) {
+
+      // Calculate point average for next bucket (containing c)
+      avgX = 0;
+      avgY = 0;
+      avgRangeStart = floor( ( i + 1 ) * every ) + 1;
+      avgRangeEnd = floor( ( i + 2 ) * every ) + 1;
+      avgRangeEnd = avgRangeEnd < dataLength ? avgRangeEnd : dataLength;
+
+      avgRangeLength = avgRangeEnd - avgRangeStart;
+
+      for ( ; avgRangeStart < avgRangeEnd; avgRangeStart++ ) {
+        avgX += data[ avgRangeStart ][ xAccessor ] * 1; // * 1 enforces Number (value may be Date)
+        avgY += data[ avgRangeStart ][ yAccessor ] * 1;
+      }
+      avgX /= avgRangeLength;
+      avgY /= avgRangeLength;
+
+      // Get the range for this bucket
+      rangeOffs = floor( ( i + 0 ) * every ) + 1;
+      rangeTo   = floor( ( i + 1 ) * every ) + 1;
+
+      // Point a
+      pointAX = data[ a ][ xAccessor ] * 1; // enforce Number (value may be Date)
+      pointAY = data[ a ][ yAccessor ] * 1;
+
+      maxArea = area = -1;
+
+      for ( ; rangeOffs < rangeTo; rangeOffs++ ) {
+        // Calculate triangle area over three buckets
+        area = abs( ( pointAX - avgX ) * ( data[ rangeOffs ][ yAccessor ] - pointAY ) -
+              ( pointAX - data[ rangeOffs ][ xAccessor ] ) * ( avgY - pointAY )
+              ) * 0.5;
+        if ( area > maxArea ) {
+          maxArea = area;
+          maxAreaPoint = data[ rangeOffs ];
+          nextA = rangeOffs; // Next a is this b
+        }
+      }
+
+      sampled[ sampledIndex++ ] = maxAreaPoint; // Pick this point from the bucket
+      a = nextA; // This a is the next a (chosen b)
+    }
+
+    sampled[ sampledIndex++ ] = data[ dataLength - 1 ]; // Always add last
+
+    return sampled;
+  }
+  //=============================================