Allow for polygonArea to return signed areas

HarryStevens · Oct 19, 2019 · 0a1d2ff · 0a1d2ff
1 parent ab99d27
commit 0a1d2ff
Show file tree

Hide file tree

Showing 6 changed files with 76 additions and 67 deletions.
diff --git a/README.md b/README.md
diff --git a/build/geometric.js b/build/geometric.js
@@ -68,6 +68,7 @@
 
   // Calculates the area of a polygon.
   function polygonArea(vertices) {
+    var signed = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : false;
     var a = 0;
 
     for (var i = 0, l = vertices.length; i < l; i++) {
@@ -77,7 +78,7 @@
       a -= v1[0] * v0[1];
     }
 
-    return Math.abs(a / 2);
+    return signed ? a / 2 : Math.abs(a / 2);
   }
 
   // Calculates the bounds of a polygon.

diff --git a/build/geometric.min.js b/build/geometric.min.js
diff --git a/build/geometric.zip b/build/geometric.zip
diff --git a/src/polygons/polygonArea.js b/src/polygons/polygonArea.js
@@ -1,5 +1,5 @@
 // Calculates the area of a polygon.
-export function polygonArea(vertices){
+export function polygonArea(vertices, signed = false){
   let a = 0;
 
   for (let i = 0, l = vertices.length; i < l; i++) {
@@ -10,5 +10,5 @@ export function polygonArea(vertices){
     a -= v1[0] * v0[1];
   }
 
-  return Math.abs(a / 2);
+  return signed ? a / 2 : Math.abs(a / 2);
 }
diff --git a/test/polygonArea-test.js b/test/polygonArea-test.js
@@ -1,12 +1,21 @@
-var tape = require("tape"),
+const tape = require("tape"),
     geometric = require("../");
 
-tape("polygonArea(polygon) calculates the area of a polygon", function(test) {
+tape("polygonArea(polygon) calculates the area of a polygon", test => {
   test.equal(geometric.polygonArea([[0, 0], [1, 0], [1, 1], [0, 1]]), 1);
   test.equal(geometric.polygonArea([[0, 0], [2, 0], [2, 2], [0, 2]]), 4);
   test.equal(geometric.polygonArea([[0, 0], [3, 0], [3, 3], [0, 3]]), 9);
   test.equal(geometric.polygonArea([[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]), 1);
   test.equal(geometric.polygonArea([[0, 0], [2, 0], [2, 2], [0, 2], [0, 0]]), 4);
   test.equal(geometric.polygonArea([[0, 0], [3, 0], [3, 3], [0, 3], [0, 0]]), 9);
   test.end();
-});
+});
+
+tape("If the polygon's winding order is counter-clockwise and signed is set to true, returns a negative area", test => {
+  const p = [[119, 87], [61, 150], [131, 197], [206, 135]];
+  test.equal(geometric.polygonArea(p, true), -8065);
+  test.equal(geometric.polygonArea(p, false), 8065);
+  test.equal(geometric.polygonArea(p), 8065);
+  test.end();
+});
+