better tile drill-down strategy, closes #21

when we drill-down to a specific tile, we now always cut each tile on
the path to 4 tiles instead of cutting out only one child; this makes
batch on-demand tiling faster in dynamic applications, avoids
unnecessary clipping, and makes sure we never keep references to
geometry we no longer need, which leads to better memory footprint

debug/viz.js

@@ -187,3 +187,21 @@ backButton.onclick = function () {
     drawTile();
     if (z === 0) backButton.style.display = 'none';
 };
+
+/*eslint-disable no-unused-vars */
+function drillDown() {
+    var i, j;
+    console.time('drill down');
+    for (i = 0; i < 10; i++) {
+        for (j = 0; j < 10; j++) {
+            tileIndex.getTile(7, 30 + i, 45 + j);
+        }
+    }
+    for (i = 0; i < 10; i++) {
+        for (j = 0; j < 10; j++) {
+            tileIndex.getTile(8, 70 + i, 100 + j);
+        }
+    }
+    console.timeEnd('drill down');
+}
+/*eslint-enable no-unused-vars */

src/index.js

@@ -87,14 +87,29 @@ GeoJSONVT.prototype.splitTile = function (features, z, x, y, cz, cx, cy) {
             }
         }
 
-        if (!cz && (z === options.maxZoom || tile.numPoints <= options.maxPoints ||
-                isClippedSquare(tile.features, extent, buffer)) || z === options.baseZoom || z === cz) {
-            tile.source = features;
-            continue; // stop tiling
+        // save reference to original geometry in tile so that we can drill down later if we stop now
+        tile.source = features;
+
+        // stop tiling if the tile is degenerate
+        if (isClippedSquare(tile.features, extent, buffer)) continue;
+
+        // if it's the first-pass tiling
+        if (!cz) {
+            // stop tiling if we reached max zoom, or if the tile is too simple
+            if (z === options.maxZoom || tile.numPoints <= options.maxPoints) continue;
+
+        // if a drilldown to a specific tile
+        } else {
+            // stop tiling if we reached base zoom or our target tile zoom
+            if (z === options.baseZoom || z === cz) continue;
+
+            // stop tiling if it's not an ancestor of the target tile
+            var m = 1 << (cz - z);
+            if (x !== Math.floor(cx / m) && y !== Math.floor(cy / m)) continue;
         }
 
-        if (cz) tile.source = features;
-        else tile.source = null;
+        // if we slice further down, no need to keep source geometry
+        tile.source = null;
 
         if (debug > 1) console.time('clipping');
 
@@ -103,29 +118,21 @@ GeoJSONVT.prototype.splitTile = function (features, z, x, y, cz, cx, cy) {
             k2 = 0.5 - k1,
             k3 = 0.5 + k1,
             k4 = 1 + k1,
+            tl, bl, tr, br, left, right;
 
-            tl, bl, tr, br, left, right,
-            m, goLeft, goTop;
-
-        if (cz) { // if we have a specific tile to drill down to, calculate where to go
-            m = 1 << (cz - z);
-            goLeft = cx / m - x < 0.5;
-            goTop = cy / m - y < 0.5;
-        }
-
-        tl = bl = tr = br = left = right = null;
+        tl = bl = tr = br = null;
 
-        if (!cz ||  goLeft) left  = clip(features, z2, x - k1, x + k3, 0, intersectX);
-        if (!cz || !goLeft) right = clip(features, z2, x + k2, x + k4, 0, intersectX);
+        left  = clip(features, z2, x - k1, x + k3, 0, intersectX);
+        right = clip(features, z2, x + k2, x + k4, 0, intersectX);
 
         if (left) {
-            if (!cz ||  goTop) tl = clip(left, z2, y - k1, y + k3, 1, intersectY);
-            if (!cz || !goTop) bl = clip(left, z2, y + k2, y + k4, 1, intersectY);
+            tl = clip(left, z2, y - k1, y + k3, 1, intersectY);
+            bl = clip(left, z2, y + k2, y + k4, 1, intersectY);
         }
 
         if (right) {
-            if (!cz ||  goTop) tr = clip(right, z2, y - k1, y + k3, 1, intersectY);
-            if (!cz || !goTop) br = clip(right, z2, y + k2, y + k4, 1, intersectY);
+            tr = clip(right, z2, y - k1, y + k3, 1, intersectY);
+            br = clip(right, z2, y + k2, y + k4, 1, intersectY);
         }
 
         if (debug > 1) console.timeEnd('clipping');

src/simplify.js

@@ -33,7 +33,8 @@ function simplify(points, tolerance) {
 
         if (maxSqDist > sqTolerance) {
             points[index][2] = maxSqDist; // save the point importance in squared pixels as a z coordinate
-            stack.push(first, index);
+            stack.push(first);
+            stack.push(index);
             first = index;
 
         } else {