Add abstract declare commands handler

common/src/main/java/us/myles/ViaVersion/api/rewriters/CommandRewriter.java

@@ -0,0 +1,84 @@
+package us.myles.ViaVersion.api.rewriters;
+
+import org.jetbrains.annotations.Nullable;
+import us.myles.ViaVersion.api.PacketWrapper;
+import us.myles.ViaVersion.api.protocol.ClientboundPacketType;
+import us.myles.ViaVersion.api.protocol.Protocol;
+import us.myles.ViaVersion.api.remapper.PacketRemapper;
+import us.myles.ViaVersion.api.type.Type;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Abstract rewriter for the declare commands packet to handle argument type name and content changes.
+ */
+public abstract class CommandRewriter {
+    protected final Protocol protocol;
+    protected final Map<String, CommandArgumentConsumer> parserHandlers = new HashMap<>();
+
+    protected CommandRewriter(Protocol protocol) {
+        this.protocol = protocol;
+    }
+
+    public void handleArgument(PacketWrapper wrapper, String argumentType) throws Exception {
+        CommandArgumentConsumer handler = parserHandlers.get(argumentType);
+        if (handler != null) {
+            handler.accept(wrapper);
+        }
+    }
+
+    public void registerDeclareCommands(ClientboundPacketType packetType) {
+        protocol.registerOutgoing(packetType, new PacketRemapper() {
+            @Override
+            public void registerMap() {
+                handler(wrapper -> {
+                    int size = wrapper.passthrough(Type.VAR_INT);
+                    for (int i = 0; i < size; i++) {
+                        byte flags = wrapper.passthrough(Type.BYTE);
+                        wrapper.passthrough(Type.VAR_INT_ARRAY_PRIMITIVE); // Children indices
+                        if ((flags & 0x08) != 0) {
+                            wrapper.passthrough(Type.VAR_INT); // Redirect node index
+                        }
+
+                        byte nodeType = (byte) (flags & 0x03);
+                        if (nodeType == 1 || nodeType == 2) { // Literal/argument node
+                            wrapper.passthrough(Type.STRING); // Name
+                        }
+
+                        if (nodeType == 2) { // Argument node
+                            String argumentType = handleArgumentType(wrapper.read(Type.STRING));
+                            if (argumentType != null) {
+                                wrapper.write(Type.STRING, argumentType);
+                                handleArgument(wrapper, argumentType);
+                            }
+                        }
+
+                        if ((flags & 0x10) != 0) {
+                            wrapper.passthrough(Type.STRING); // Suggestion type
+                        }
+                    }
+
+                    wrapper.passthrough(Type.VAR_INT); // Root node index
+                });
+            }
+        });
+    }
+
+    /**
+     * Can be overridden if needed.
+     *
+     * @param argumentType argument type
+     * @return new argument type, or null if it should be removed
+     */
+    @Nullable
+    protected String handleArgumentType(String argumentType) {
+        return argumentType;
+    }
+
+    @FunctionalInterface
+    public interface CommandArgumentConsumer {
+
+        void accept(PacketWrapper wrapper) throws Exception;
+    }
+}