Move tile priority function to the TileQueue module

src/ol/TileQueue.js

@@ -3,7 +3,7 @@
  */
 import TileState from './TileState.js';
 import EventType from './events/EventType.js';
-import PriorityQueue from './structs/PriorityQueue.js';
+import PriorityQueue, {DROP} from './structs/PriorityQueue.js';
 
 
 /**
@@ -119,3 +119,34 @@ class TileQueue extends PriorityQueue {
 
 
 export default TileQueue;
+
+
+/**
+ * @param {import('./PluggableMap.js').FrameState} frameState Frame state.
+ * @param {import("./Tile.js").default} tile Tile.
+ * @param {string} tileSourceKey Tile source key.
+ * @param {import("./coordinate.js").Coordinate} tileCenter Tile center.
+ * @param {number} tileResolution Tile resolution.
+ * @return {number} Tile priority.
+ */
+export function getTilePriority(frameState, tile, tileSourceKey, tileCenter, tileResolution) {
+  // Filter out tiles at higher zoom levels than the current zoom level, or that
+  // are outside the visible extent.
+  if (!frameState || !(tileSourceKey in frameState.wantedTiles)) {
+    return DROP;
+  }
+  if (!frameState.wantedTiles[tileSourceKey][tile.getKey()]) {
+    return DROP;
+  }
+  // Prioritize the highest zoom level tiles closest to the focus.
+  // Tiles at higher zoom levels are prioritized using Math.log(tileResolution).
+  // Within a zoom level, tiles are prioritized by the distance in pixels between
+  // the center of the tile and the center of the viewport.  The factor of 65536
+  // means that the prioritization should behave as desired for tiles up to
+  // 65536 * Math.log(2) = 45426 pixels from the focus.
+  const center = frameState.viewState.center;
+  const deltaX = tileCenter[0] - center[0];
+  const deltaY = tileCenter[1] - center[1];
+  return 65536 * Math.log(tileResolution) +
+      Math.sqrt(deltaX * deltaX + deltaY * deltaY) / tileResolution;
+}