Improve support for multiple protocol versions. Add tests.

src/test/java/com/velocitypowered/proxy/protocol/PacketRegistryTest.java

@@ -0,0 +1,48 @@
+package com.velocitypowered.proxy.protocol;
+
+import com.velocitypowered.proxy.protocol.packets.Handshake;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class PacketRegistryTest {
+    private StateRegistry.PacketRegistry setupRegistry() {
+        StateRegistry.PacketRegistry registry = new StateRegistry.PacketRegistry(ProtocolConstants.Direction.TO_CLIENT, StateRegistry.HANDSHAKE);
+        registry.register(Handshake.class, Handshake::new, new StateRegistry.PacketMapping(0x00, 1));
+        return registry;
+    }
+
+    @Test
+    void packetRegistryWorks() {
+        StateRegistry.PacketRegistry registry = setupRegistry();
+        MinecraftPacket packet = registry.createPacket(0, 1);
+        assertNotNull(packet, "Packet was not found in registry");
+        assertEquals(Handshake.class, packet.getClass(), "Registry returned wrong class");
+
+        assertEquals(0, registry.getId(packet, 1), "Registry did not return the correct packet ID");
+    }
+
+    @Test
+    void packetRegistryRevertsToBestOldVersion() {
+        StateRegistry.PacketRegistry registry = setupRegistry();
+        MinecraftPacket packet = registry.createPacket(0, 2);
+        assertNotNull(packet, "Packet was not found in registry");
+        assertEquals(Handshake.class, packet.getClass(), "Registry returned wrong class");
+
+        assertEquals(0, registry.getId(packet, 2), "Registry did not return the correct packet ID");
+    }
+
+    @Test
+    void packetRegistryDoesntProvideNewPacketsForOld() {
+        StateRegistry.PacketRegistry registry = setupRegistry();
+        assertNull(registry.createPacket(0, 0), "Packet was found in registry despite being too new");
+
+        assertThrows(IllegalArgumentException.class, () -> registry.getId(new Handshake(), 0), "Registry provided new packets for an old protocol version");
+    }
+
+    @Test
+    void failOnNoMappings() {
+        StateRegistry.PacketRegistry registry = new StateRegistry.PacketRegistry(ProtocolConstants.Direction.TO_CLIENT, StateRegistry.HANDSHAKE);
+        assertThrows(IllegalArgumentException.class, () -> registry.register(Handshake.class, Handshake::new));
+    }
+}