[core] Organize files that were in map folder better

Fixes #996

src/mbgl/tile/tile_cache.cpp

@@ -0,0 +1,65 @@
+#include <mbgl/tile/tile_cache.hpp>
+#include <mbgl/tile/tile_data.hpp>
+
+#include <cassert>
+
+namespace mbgl {
+
+void TileCache::setSize(size_t size_) {
+    size = size_;
+
+    while (orderedKeys.size() > size) {
+        auto key = orderedKeys.front();
+        orderedKeys.pop_front();
+        tiles.erase(key);
+    }
+
+    assert(orderedKeys.size() <= size);
+
+    tiles.reserve(size);
+}
+
+void TileCache::add(uint64_t key, std::shared_ptr<TileData> data) {
+
+    // insert new or query existing data
+    if (tiles.emplace(key, data).second) {
+        // remove existing data key
+        orderedKeys.remove(key);
+    }
+
+    // (re-)insert data key as newest
+    orderedKeys.push_back(key);
+
+    // purge oldest key/data if necessary
+    if (orderedKeys.size() > size) {
+        get(orderedKeys.front());
+    }
+
+    assert(orderedKeys.size() <= size);
+};
+
+std::shared_ptr<TileData> TileCache::get(uint64_t key) {
+
+    std::shared_ptr<TileData> data;
+
+    auto it = tiles.find(key);
+    if (it != tiles.end()) {
+        data = it->second;
+        tiles.erase(it);
+        orderedKeys.remove(key);
+        assert(data->isReady());
+    }
+
+    return data;
+};
+
+bool TileCache::has(uint64_t key) {
+    return tiles.find(key) != tiles.end();
+}
+
+void TileCache::clear() {
+    orderedKeys.clear();
+    tiles.clear();
+}
+
+} // namespace mbgl