Fast tileCount with help from @mapbox/sphericalmercator module

2 files changed, 24 insertions, 0 deletions

diff --git a/src/mbgl/util/tile_cover.cpp b/src/mbgl/util/tile_cover.cpp
index b53e91162c..c06634c9b2 100644
--- a/src/mbgl/util/tile_cover.cpp
+++ b/src/mbgl/util/tile_cover.cpp
@@ -169,5 +169,26 @@ std::vector<UnwrappedTileID> tileCover(const TransformState& state, int32_t z) {
         z);
 }
 
+// Taken from https://github.com/mapbox/sphericalmercator#xyzbbox-zoom-tms_style-srs
+// Computes the projected tiles for the lower left and uppoer right points of the bounds
+// and uses that to compute the tile cover count
+unsigned long tileCount(const LatLngBounds& bounds, uint8_t zoom, uint16_t tileSize_){
+
+    auto sw = Projection::project(bounds.southwest().wrapped(), zoom, tileSize_);
+    auto ne = Projection::project(bounds.northeast().wrapped(), zoom, tileSize_);
+
+    auto x1 = floor(sw.x/ tileSize_);
+    auto x2 = floor((ne.x - 1) / tileSize_);
+    auto y1 = floor(sw.y/ tileSize_);
+    auto y2 = floor((ne.y - 1) / tileSize_);
+
+    auto minX = std::fmax(std::min(x1, x2), 0);
+    auto maxX = std::max(x1, x2);
+    auto minY = (std::pow(2, zoom) - 1) - std::max(y1, y2);
+    auto maxY = (std::pow(2, zoom) - 1) - std::fmax(std::min(y1, y2), 0);
+    
+    return (maxX - minX + 1) * (maxY - minY + 1);
+}
+
 } // namespace util
 } // namespace mbgl
diff --git a/src/mbgl/util/tile_cover.hpp b/src/mbgl/util/tile_cover.hpp
index 2d32d8bf41..405e6a48e6 100644
--- a/src/mbgl/util/tile_cover.hpp
+++ b/src/mbgl/util/tile_cover.hpp
@@ -18,5 +18,8 @@ int32_t coveringZoomLevel(double z, SourceType type, uint16_t tileSize);
 std::vector<UnwrappedTileID> tileCover(const TransformState&, int32_t z);
 std::vector<UnwrappedTileID> tileCover(const LatLngBounds&, int32_t z);
 
+// Compute only the count of tiles needed for tileCover
+unsigned long tileCount(const LatLngBounds&, uint8_t z, uint16_t tileSize);
+
 } // namespace util
 } // namespace mbgl