fix elimination of empty triangles

first point of triangle that is eliminated should always be kept - we
want to always keep the first and last points of the simplified polyline

Author: Natalia Kowalczyk

.gitignore

@@ -1,3 +1,5 @@
 components
 build
 node_modules
+.project
+.externalToolBuilders

index.js

@@ -20,11 +20,12 @@ function calculate(poly) {
     ts = { heap: heap(areaCompare, true) },
     triangle,
     trianglePrev,
-    a = poly[0], b = poly[1], c,
+    a = poly[0], b, c = poly[1],
     list = [];
 
   // calculate areas
   for (i = 2; i < poly.length; i++) {
+    b = c;
     c = poly[i];
     triangle = {
       a: a,
@@ -35,11 +36,10 @@ function calculate(poly) {
       prev: trianglePrev,
       _heapIndex: 0
     };
-    a = b;
-    b = c;
     if (!triangle.area) {
       continue;
     }
+    a = b;
     if (trianglePrev) {
       trianglePrev.next = triangle;
     }

test/simplify.js

@@ -65,7 +65,16 @@ describe('simplify', function() {
       [-91.96509, 39.55763]
     ];
 
-    simplify(poly, 9).should.have.length(7);
+    simplify(poly, 9).should.eql([
+      [-91.9655,  39.55001],
+      [-91.96581, 39.55024],
+      [-91.96596, 39.55041],
+      [-91.96599, 39.55053],
+      [-91.96588, 39.55320],
+      [-91.96581, 39.55578],
+      [-91.96573, 39.55764],
+      [-91.96509, 39.55763]
+    ]);
   });
 
 
@@ -75,4 +84,9 @@ describe('simplify', function() {
 
     simplify(poly, 40).should.eql(simplified);
   });
+
+  it('should keep the first point', function () {
+     var poly = [[0, 0], [1, 0], [2, 0], [3, 1], [4, 2], [4, 4]];
+     simplify(poly, 4).should.eql([[0, 0], [2, 0], [4, 2], [4, 4]]);
+  });
 });