test: characterize multipart compatibility

Pin the Scala Function4 callback surface and MCPC fallback behavior before the Java port.

src/test/java/codechicken/multipart/handler/MultipartCompatiblityCharacterizationTest.java

@@ -0,0 +1,103 @@
+package codechicken.multipart.handler;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import java.util.Set;
+import java.util.TreeSet;
+
+import net.minecraft.world.World;
+
+import org.junit.jupiter.api.Test;
+import org.objectweb.asm.Type;
+
+import scala.Function4;
+import scala.runtime.AbstractFunction4;
+
+class MultipartCompatiblityCharacterizationTest {
+
+    @Test
+    void keepsTheTwoStaticFacadesAndCompanionSingletons() throws Exception {
+        assertFacade(
+                MultipartCompatiblity.class,
+                "canAddPart()Lscala/Function4;",
+                "canAddPart_$eq(Lscala/Function4;)V",
+                "load()V");
+        assertCompanion(
+                MultipartCompatiblity$.class,
+                "canAddPart()Lscala/Function4;",
+                "canAddPart_$eq(Lscala/Function4;)V",
+                "load()V");
+        assertFacade(MCPCCompatModule.class, "load()V");
+        assertCompanion(MCPCCompatModule$.class, "load()V");
+
+        Field callback = MultipartCompatiblity$.class.getDeclaredField("canAddPart");
+        assertEquals(Function4.class, callback.getType());
+        assertTrue(Modifier.isPrivate(callback.getModifiers()));
+    }
+
+    @Test
+    void defaultPlacementCallbackAlwaysAllows() {
+        assertEquals(Boolean.TRUE, MultipartCompatiblity.canAddPart().apply(null, -1, 0, 1));
+    }
+
+    @Test
+    void facadeAndCompanionShareTheMutableCallback() {
+        Function4<World, Object, Object, Object, Object> original = MultipartCompatiblity.canAddPart();
+        Function4<World, Object, Object, Object, Object> replacement = new PlacementCallback();
+        try {
+            MultipartCompatiblity.canAddPart_$eq(replacement);
+            assertSame(replacement, MultipartCompatiblity.canAddPart());
+            assertSame(replacement, MultipartCompatiblity$.MODULE$.canAddPart());
+            assertEquals(Boolean.TRUE, replacement.apply(null, 4, 5, 6));
+            assertEquals(Boolean.FALSE, replacement.apply(null, 4, 5, 7));
+
+            MultipartCompatiblity$.MODULE$.canAddPart_$eq(original);
+            assertSame(original, MultipartCompatiblity.canAddPart());
+        } finally {
+            MultipartCompatiblity.canAddPart_$eq(original);
+        }
+    }
+
+    private static void assertFacade(Class<?> type, String... expectedMethods) {
+        assertTrue(Modifier.isPublic(type.getModifiers()));
+        assertTrue(Modifier.isFinal(type.getModifiers()));
+        assertEquals(new TreeSet<>(Arrays.asList(expectedMethods)), publicMethodSignatures(type));
+        assertEquals(0, type.getFields().length);
+    }
+
+    private static void assertCompanion(Class<?> type, String... expectedMethods) throws Exception {
+        assertTrue(Modifier.isPublic(type.getModifiers()));
+        assertTrue(Modifier.isFinal(type.getModifiers()));
+        assertEquals(new TreeSet<>(Arrays.asList(expectedMethods)), publicMethodSignatures(type));
+
+        Field module = type.getField("MODULE$");
+        assertSame(type, module.getType());
+        assertTrue(Modifier.isStatic(module.getModifiers()));
+        assertTrue(Modifier.isFinal(module.getModifiers()));
+        assertSame(module.get(null), module.get(null));
+    }
+
+    private static Set<String> publicMethodSignatures(Class<?> type) {
+        Set<String> signatures = new TreeSet<>();
+        for (Method method : type.getDeclaredMethods()) {
+            if (Modifier.isPublic(method.getModifiers())) {
+                signatures.add(method.getName() + Type.getMethodDescriptor(method));
+            }
+        }
+        return signatures;
+    }
+
+    private static final class PlacementCallback extends AbstractFunction4<World, Object, Object, Object, Object> {
+
+        @Override
+        public Object apply(World world, Object x, Object y, Object z) {
+            return ((Integer) x) + ((Integer) y) + ((Integer) z) == 15;
+        }
+    }
+}