get Optifine in the dev environment

build.gradle

@@ -64,6 +64,7 @@ jar {
             exclude 'META-INF', 'META-INF/**'
         }
     }
+    exclude "optifine"
 }
 
 reobf {

src/main/kotlin/optifine/OptifineTweakerDevWrapper.kt

@@ -0,0 +1,33 @@
+package optifine
+
+import net.minecraft.launchwrapper.IClassTransformer
+import net.minecraft.launchwrapper.ITweaker
+import net.minecraft.launchwrapper.LaunchClassLoader
+import java.io.File
+
+class OptifineTweakerDevWrapper : ITweaker {
+    override fun acceptOptions(p0: MutableList<String>?, p1: File?, p2: File?, p3: String?) { }
+    override fun getLaunchArguments(): Array<out String>? = Array<String>(0) {""}
+    override fun getLaunchTarget() = "net.minecraft.client.main.Main"
+    override fun injectIntoClassLoader(classLoader: LaunchClassLoader) {
+        classLoader.registerTransformer("optifine.OptifineTransformerDevWrapper")
+    }
+}
+
+/**
+ * Wrapper around Optifine's class transformer.
+ *
+ * This class is only used in development to debug cross-mod issues with Optifine, and
+ * is not part of the release!
+ */
+class OptifineTransformerDevWrapper : IClassTransformer {
+
+    val ofTransformer = Class.forName("optifine.OptiFineClassTransformer").newInstance() as IClassTransformer
+
+    /**
+     * Call the Optifine transformer, but change dots to slashes in class names.
+     * This enables the Optifine transformer to load replacements from non-root locations in the jar file.
+     */
+    override fun transform(name: String?, transformedName: String?, classData: ByteArray?) =
+        ofTransformer.transform(name?.replace(".", "/"), transformedName, classData)
+}