Utility to get the longest straight chunk of a linestring

src/ol/geom/flat/straightchunk.js

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+goog.provide('ol.geom.flat.straightchunk');
+
+
+/**
+ * @param {number} maxAngle Maximum acceptable angle delta between segments.
+ * @param {Array.<number>} flatCoordinates Flat coordinates.
+ * @param {number} offset Offset.
+ * @param {number} end End.
+ * @param {number} stride Stride.
+ * @return {Array.<number>} Start and end of the first suitable chunk of the
+ * given `flatCoordinates`.
+ */
+ol.geom.flat.straightchunk.lineString = function(maxAngle, flatCoordinates, offset, end, stride) {
+  var chunkStart = offset;
+  var chunkEnd = offset;
+  var chunkM = 0;
+  var m = 0;
+  var start = offset;
+  var acos, i, m12, m23, x1, y1, x12, y12, x23, y23;
+  for (i = offset; i < end; i += stride) {
+    var x2 = flatCoordinates[i];
+    var y2 = flatCoordinates[i + 1];
+    if (x1 !== undefined) {
+      x23 = x2 - x1;
+      y23 = y2 - y1;
+      m23 = Math.sqrt(x23 * x23 + y23 * y23);
+      if (x12 !== undefined) {
+        m += m12;
+        acos = Math.acos((x12 * x23 + y12 * y23) / (m12 * m23));
+        if (acos > maxAngle) {
+          if (m > chunkM) {
+            chunkM = m;
+            chunkStart = start;
+            chunkEnd = i;
+          }
+          m = 0;
+          start = i - stride;
+        }
+      }
+      m12 = m23;
+      x12 = x23;
+      y12 = y23;
+    }
+    x1 = x2;
+    y1 = y2;
+  }
+  m += m23;
+  return m > chunkM ? [start, i] : [chunkStart, chunkEnd];
+};

test/spec/ol/geom/flat/straightchunk.test.js

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+goog.require('ol.geom.flat.straightchunk');
+
+
+describe('ol.geom.flat.straightchunk', function() {
+
+  describe('ol.geom.flat.straightchunk.lineString', function() {
+
+    describe('single segment with stride == 3', function() {
+      var flatCoords = [0, 0, 42, 1, 1, 42];
+      var stride = 3;
+
+      it('returns whole line with angle delta', function() {
+        var got = ol.geom.flat.straightchunk.lineString(Math.PI / 4, flatCoords, 0, 6, stride);
+        expect(got).to.eql([0, 6]);
+      });
+
+      it('returns whole line with zero angle delta', function() {
+        var got = ol.geom.flat.straightchunk.lineString(0, flatCoords, 0, 6, stride);
+        expect(got).to.eql([0, 6]);
+      });
+
+    });
+
+    describe('short line string', function() {
+      var flatCoords = [0, 0, 1, 0, 1, 1, 0, 1];
+      var stride = 2;
+
+      it('returns whole line if straight enough', function() {
+        var got = ol.geom.flat.straightchunk.lineString(Math.PI, flatCoords, 0, 8, stride);
+        expect(got).to.eql([0, 8]);
+      });
+
+      it('returns first matching chunk if all chunk lengths are the same', function() {
+        var got = ol.geom.flat.straightchunk.lineString(Math.PI / 4, flatCoords, 0, 8, stride);
+        expect(got).to.eql([0, 4]);
+      });
+
+    });
+
+    describe('longer line string', function() {
+      var flatCoords = [0, 0, 1, 0, 1, 1, 0, 1, 0, -1, -1, -1, -1, 0, -1, 2, -2, 4];
+      var stride = 2;
+
+      it('returns stright chunk from within the linestring', function() {
+        var got = ol.geom.flat.straightchunk.lineString(0, flatCoords, 0, 18, stride);
+        expect(got).to.eql([10, 16]);
+      });
+
+      it('returns long chunk at the end if angle and length within threshold', function() {
+        var got = ol.geom.flat.straightchunk.lineString(Math.PI / 4, flatCoords, 0, 18, stride);
+        expect(got).to.eql([10, 18]);
+      });
+
+    });
+
+  });
+
+});