Merge pull request Turfjs#1387 from pirxpilot:area

AminoffZ · Oct 22, 2023 · 4c4e197 · 4c4e197
1 parent 14a2ac5
commit 4c4e197
Showing 1 changed file with 47 additions and 35 deletions.
diff --git a/packages/turf-area/index.ts b/packages/turf-area/index.ts
@@ -67,6 +67,15 @@ function polygonArea(coords: any) {
   return total;
 }
 
+/**
+ * @private
+ * A constant factor used to compute the area of a polygon.
+ * It's derived from the square of the Earth's radius divided by 2.
+ *
+ * @type {number}
+ */
+const FACTOR = (earthRadius * earthRadius) / 2;
+
 /**
  * @private
  * Calculate the approximate area of the polygon were it projected onto the earth.
@@ -81,43 +90,46 @@ function polygonArea(coords: any) {
  * @returns {number} The approximate signed geodesic area of the polygon in square meters.
  */
 function ringArea(coords: number[][]) {
-  let p1;
-  let p2;
-  let p3;
-  let lowerIndex;
-  let middleIndex;
-  let upperIndex;
-  let i;
-  let total = 0;
-  const coordsLength = coords.length;
-
-  if (coordsLength > 2) {
-    for (i = 0; i < coordsLength; i++) {
-      if (i === coordsLength - 2) {
-        // i = N-2
-        lowerIndex = coordsLength - 2;
-        middleIndex = coordsLength - 1;
-        upperIndex = 0;
-      } else if (i === coordsLength - 1) {
-        // i = N-1
-        lowerIndex = coordsLength - 1;
-        middleIndex = 0;
-        upperIndex = 1;
-      } else {
-        // i = 0 to N-3
-        lowerIndex = i;
-        middleIndex = i + 1;
-        upperIndex = i + 2;
-      }
-      p1 = coords[lowerIndex];
-      p2 = coords[middleIndex];
-      p3 = coords[upperIndex];
-      total += (rad(p3[0]) - rad(p1[0])) * Math.sin(rad(p2[1]));
+    // it's a ring - ignore the last one
+    const coordsLength = coords.length - 1;
+
+    if (coordsLength < 3) {
+        return 0;
     }
 
-    total = (total * earthRadius * earthRadius) / 2;
-  }
-  return total;
+    let total = 0;
+
+    const f_x = rad(coords[0][0]), f_y = rad(coords[0][1]);
+    const s_x = rad(coords[1][0]), s_y = rad(coords[1][1]);
+
+    let l_x = f_x, l_y = f_y;
+    let m_x = s_x, m_y = s_y;
+
+    let u_x = 0, u_y = 0;
+
+    for (let i = 2; i < coordsLength; i++) {
+        u_x = rad(coords[i][0]); u_y = rad(coords[i][1]);
+
+        total += (u_x - l_x) * Math.sin(m_y);
+
+        l_x = m_x; l_y = m_y;
+        m_x = u_x; m_y = u_y;
+    }
+
+    // handle 2 extra triangles (since we started from i = 2)
+
+    u_x = f_x; u_y = f_y;
+
+    total += (u_x - l_x) * Math.sin(m_y);
+
+    l_x = m_x; l_y = m_y;
+    m_x = u_x; m_y = u_y;
+
+    u_x = s_x; u_y = s_y;
+
+    total += (u_x - l_x) * Math.sin(m_y);
+
+    return total * FACTOR;
 }
 
 function rad(num: number) {