Implement gacha banners (not newbie)

src/main/java/emu/nebula/util/WeightedList.java

@@ -0,0 +1,35 @@
+package emu.nebula.util;
+
+import java.util.NavigableMap;
+import java.util.TreeMap;
+import java.util.concurrent.ThreadLocalRandom;
+
+public class WeightedList<E> {
+    private final NavigableMap<Double, E> map = new TreeMap<>();
+    private double total = 0;
+
+    public WeightedList() {
+
+    }
+
+    public WeightedList<E> add(double weight, E result) {
+        if (weight <= 0) return this;
+        total += weight;
+        map.put(total, result);
+        return this;
+    }
+
+    public E next() {
+        double value = ThreadLocalRandom.current().nextDouble() * total;
+        return map.higherEntry(value).getValue();
+    }
+
+    public int size() {
+        return map.size();
+    }
+
+    public void clear() {
+        this.map.clear();
+        this.total = 0;
+    }
+}