check lakes before depressiong fill

modules/river-generator.js

@@ -27,13 +27,50 @@
     TIME && console.timeEnd("generateRivers");
 
     function prepareLakeData() {
+      const ELEVATION_LIMIT = 10;
+
       features.forEach(f => {
         if (f.type !== "lake") return;
         delete f.flux;
         delete f.inlets;
         delete f.outlet;
         delete f.height;
+        delete f.closed;
         !f.shoreline && Lakes.getShoreline(f);
+
+        // lake surface height is as lowest land cells around
+        const min = f.shoreline.sort((a, b) => h[a] - h[b])[0];
+        f.height = h[min] - 0.1;
+
+        // check if lake can be open (not in deep depression)
+        let deep = true;
+        const treshold = f.height + ELEVATION_LIMIT;
+        const queue = [min];
+        const checked = [];
+        checked[min] = true;
+
+        // check if elevated lake can potentially pour to another water body
+        while (deep && queue.length) {
+          const q = queue.pop();
+
+          for (const n of cells.c[q]) {
+            if (checked[n]) continue;
+            if (h[n] >= treshold) continue;
+
+            if (h[n] < 20) {
+              const nFeature = features[cells.f[n]];
+              if (nFeature.type === "ocean" || f.height > nFeature.height) {
+                deep = false;
+                break;
+              }
+            }
+
+            checked[n] = true;
+            queue.push(n);
+          }
+        }
+
+        f.closed = deep;
       });
     }