Fix rounding error between triangle area calculation and intersection calculation

src/signed_area.js

@@ -1,5 +1,7 @@
 'use strict';
 
+var intersection  = require('./segment_intersection');
+
 /**
  * Signed area of the triangle (p0, p1, p2)
  * @param  {Array.<Number>} p0
@@ -8,5 +10,8 @@
  * @return {Number}
  */
 module.exports = function signedArea(p0, p1, p2) {
+  // avoid rounding error b/t intersection calculation and triangle area calc
+  var inters = intersection(p0, p1, p0, p2)
+  if (inters.length === 2) return 0
   return (p0[0] - p2[0]) * (p1[1] - p2[1]) - (p1[0] - p2[0]) * (p0[1] - p2[1]);
 };

test/edge_cases.test.js

@@ -218,6 +218,31 @@ tap.test('Edge cases', function(main) {
     t.end();
   });
 
+  main.test('no rounding error between intersection calculation and triangle area', (t) => {
+    const p1 = [[
+      [-62.8, -41],
+      [-63.0001099, -41.1121599],
+      [-62.93564, -41.0940399],
+      [-62.8, -41]
+    ]];
+    const p2 =[[
+      [-62.8, -41.2],
+      [-62.8, -41],
+      [-62.964969880531925, -41.10228339712406],
+      [-63.0001099, -41.1121599],
+      [-62.8, -41.2]
+    ]];
+    const expected = [[[
+      [-63.0001099, -41.1121599],
+      [-62.964969880531925, -41.10228339712406],
+      [-62.8, -41],
+      [-63.0001099, -41.1121599]
+    ]]]
+
+    t.deepEqual(martinez.diff(p1, p2), expected)
+    t.end();
+  });
+
   main.test('collapsed edges removed', (t) => {
     const p1 = [[
       [355,139],