Add "roll downhill" logic

island.ts

@@ -104,14 +104,38 @@ export function IslandApplet() {
 
   const islands = new IslandGrid(WIDTH, HEIGHT, 128);
 
-  const basePos = islands.data.length >> 1;
+  const len = islands.data.length;
+  const basePos = len >> 1;
+  const width = islands.width;
+
+  function drop(pos: number) {
+    const lowerNeighbors: number[] = [];
+
+    function check(adjPos: number) {
+      if (islands.data[adjPos] < islands.data[pos]) {
+        lowerNeighbors.push(adjPos);
+      }
+    }
+
+    check((pos - width) % len);
+    check((pos - 1) % len);
+    check((pos + 1) % len);
+    check((pos + width) % len);
+
+    if (lowerNeighbors.length > 0) {
+      const downhill = lowerNeighbors[islands.rng() % lowerNeighbors.length];
+      drop(downhill);
+    } else {
+      islands.data[pos]++;
+    }
+  }
 
   function tick() {
     const islandTiles = islands.floodSearch(basePos, (tile) => tile > 0);
 
     const pos = islandTiles[islands.rng() % islandTiles.length];
 
-    islands.data[pos]++;
+    drop(pos);
 
     renderIslands(islands, cx);
   }