remap obsfucated mod to named. I should make a gui that automatically downloads mappings and minecraft jar for this.

build.gradle

plugins {
    id 'java'
}

group = 'org.example'
version = '1.0-SNAPSHOT'

repositories {
    mavenCentral()
    maven { url "https://maven.fabricmc.net" }
    maven { url "https://maven.legacyfabric.net" }
    maven { url "https://jitpack.io" }
}

dependencies {
    implementation "net.fabricmc:tiny-remapper:0.11.0"
    implementation "net.fabricmc:mapping-io:0.7.1"
    implementation "net.legacyfabric:intermediary:<version>"
}

Main.java

import net.fabricmc.mappingio.MappingReader;
import net.fabricmc.mappingio.tree.MemoryMappingTree;
import net.fabricmc.tinyremapper.*;

import java.io.*;
import java.nio.file.Paths;

public class Main {
    public static void main(String... args) throws IOException {
        var tree = new MemoryMappingTree();
        try (InputStream mappings = Main.class.getResourceAsStream("mappings/mappings.tiny")) {
            // you've got bigger problems if you don't have a mappings set
            assert mappings != null;
            MappingReader.read(new InputStreamReader(mappings), tree);
        }
        TinyRemapper remapper = TinyRemapper.newRemapper()
                .withMappings(TinyRemapperMappingsHelper.create(tree, "official", "named"))
                .renameInvalidLocals(true)
                .rebuildSourceFilenames(true)
                .checkPackageAccess(true)
                .fixPackageAccess(true)
                .build();
        remapper.readInputs(Paths.get("OptiFine_<version>_MOD.jar"));
        remapper.readClassPath(Paths.get("minecraft-client-<version>.jar"));
        OutputConsumerPath outputConsumer = new OutputConsumerPath.Builder(Paths.get("out.jar")).build();
        remapper.apply(outputConsumer);
        remapper.finish();
        outputConsumer.close();
    }
}

TinyRemapperMappingsHelper.java

/*
 * Copyright 2016 FabricMC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import net.fabricmc.mappingio.tree.MappingTree;
import net.fabricmc.tinyremapper.IMappingProvider;

public class TinyRemapperMappingsHelper {
    private TinyRemapperMappingsHelper() {
    }

    private static IMappingProvider.Member memberOf(String className, String memberName, String descriptor) {
        return new IMappingProvider.Member(className, memberName, descriptor);
    }

    public static IMappingProvider create(MappingTree mappings, String from, String to) {
        return (acceptor) -> {
            final int fromId = mappings.getNamespaceId(from);
            final int toId = mappings.getNamespaceId(to);

            for (MappingTree.ClassMapping classDef : mappings.getClasses()) {
                final String className = classDef.getName(fromId);
                String dstName = classDef.getName(toId);

                if (dstName == null) {
                    dstName = className;
                }

                acceptor.acceptClass(className, dstName);

                for (MappingTree.FieldMapping field : classDef.getFields()) {
                    acceptor.acceptField(memberOf(className, field.getName(fromId), field.getDesc(fromId)), field.getName(toId));
                }

                for (MappingTree.MethodMapping method : classDef.getMethods()) {
                    IMappingProvider.Member methodIdentifier = memberOf(className, method.getName(fromId), method.getDesc(fromId));
                    acceptor.acceptMethod(methodIdentifier, method.getName(toId));
                }
            }
            acceptor.acceptField(new IMappingProvider.Member("bqm", "renderDistance", "I"), "renderDistance_OF");
            acceptor.acceptMethod(new IMappingProvider.Member("cbi", "rotate", "(Lcv;I)I"), "rotate_OF");
            acceptor.acceptMethod(new IMappingProvider.Member("cbi", "rotate", "(Lcv;)Lcv;"), "rotate_OF");
        };
    }
}