Add optional web jukebox: vote for arena music tracks via a small HTTP API

src/main/java/us/tss3/blockparty/jukebox/VoteCodeManager.java

@@ -0,0 +1,48 @@
+package us.tss3.blockparty.jukebox;
+
+import java.security.SecureRandom;
+import java.util.Map;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Issues short-lived, easy-to-type numeric codes that link a web voting session back to a
+ * specific logged-in player, without requiring any real account/password auth on the web
+ * page. A code is only ever handed out via the in-game {@code /blockparty musicvote}
+ * command, so possessing one proves you're the player who ran it (or someone they showed it
+ * to) for as long as it's valid.
+ */
+public class VoteCodeManager {
+
+    private record CodeEntry(UUID playerId, long expiresAtMs) {
+    }
+
+    private final Map<String, CodeEntry> codes = new ConcurrentHashMap<>();
+    private final SecureRandom random = new SecureRandom();
+
+    public String issueCode(UUID playerId, int ttlMinutes) {
+        codes.values().removeIf(e -> e.playerId().equals(playerId));
+        String code;
+        do {
+            code = String.valueOf(100000 + random.nextInt(900000));
+        } while (codes.containsKey(code));
+        codes.put(code, new CodeEntry(playerId, System.currentTimeMillis() + ttlMinutes * 60_000L));
+        return code;
+    }
+
+    /** Resolves a code to the player who issued it, or null if unknown/expired. */
+    public UUID resolve(String code) {
+        if (code == null) {
+            return null;
+        }
+        CodeEntry entry = codes.get(code);
+        if (entry == null) {
+            return null;
+        }
+        if (System.currentTimeMillis() > entry.expiresAtMs()) {
+            codes.remove(code);
+            return null;
+        }
+        return entry.playerId();
+    }
+}