move fingerprinting into polygons utility class

js/geo-polygons.js

@@ -37,7 +37,9 @@ var gs_base_polygons = exports;
         toClipper : toClipper,
         fromClipperNode : fromClipperNode,
         fromClipperTree : fromClipperTree,
-        cleanClipperTree : cleanClipperTree
+        cleanClipperTree : cleanClipperTree,
+        fingerprintCompare: fingerprintCompare,
+        fingerprint: fingerprint
     };
 
     /** ******************************************************************
@@ -803,4 +805,36 @@ var gs_base_polygons = exports;
         return points;
     }
 
+    function fingerprint(polys) {
+        let finger = [];
+        flatten(polys).sort((a,b) => {
+            return a.area() > b.area();
+        }).forEach(p => {
+            finger.push(p.area());
+            finger.push(p.perimeter());
+        });
+        return finger;
+    }
+
+    function fingerprintCompare(a, b) {
+        if (a === b) {
+            return true;
+        }
+        if (!a || !b) {
+            return false;
+        }
+        if (a.length !== b.length) {
+            return false;
+        }
+        for (let i=0; i<a.length; i += 2) {
+            if (Math.abs(a[i] - b[i]) > 0.001) {
+                return false;
+            }
+            if (Math.abs(a[i+1] - b[i+1]) > 0.0001) {
+                return false;
+            }
+        }
+        return true;
+    }
+
 })();