Extending Codecs with Mixin

output.log

[15:18:46] [main/INFO] (Minecraft) [STDOUT]: Injected field: 47

TernarySurfaceConfigMixin.java

package robosky.uplands.mixin;

import java.util.function.Function;

import com.mojang.datafixers.kinds.App;
import com.mojang.serialization.Codec;
import com.mojang.serialization.MapCodec;
import com.mojang.serialization.codecs.RecordCodecBuilder;
import org.spongepowered.asm.mixin.Dynamic;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Unique;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.ModifyArg;

import net.minecraft.world.gen.surfacebuilder.TernarySurfaceConfig;

@Mixin(TernarySurfaceConfig.class)
public abstract class TernarySurfaceConfigMixin {

    // injected field(s)
    @Unique
    private int myInteger = 3;

    // codec modification
    // target the `RecordCodecBuilder.create(...)` call
    // generics are extra disgusting, see DFU docs for more info
    @Dynamic("Static init")
    @ModifyArg(
        method = "<clinit>",
        index = 0,
        at = @At(
            value = "INVOKE",
            target = "Lcom/mojang/serialization/codecs/RecordCodecBuilder;create(Ljava/util/function/Function;)Lcom/mojang/serialization/Codec;",
            remap = false
        )
    )
    private static Function<RecordCodecBuilder.Instance<TernarySurfaceConfig>, ? extends App<RecordCodecBuilder.Mu<TernarySurfaceConfig>, TernarySurfaceConfig>>
    extendCodec(Function<RecordCodecBuilder.Instance<TernarySurfaceConfig>, ? extends App<RecordCodecBuilder.Mu<TernarySurfaceConfig>, TernarySurfaceConfig>> fields) {
        MapCodec<TernarySurfaceConfig> mapCodec = RecordCodecBuilder.mapCodec(fields);
        return inst -> inst.group(
            // preserve the original fields
            mapCodec.forGetter(Function.identity()),
            // inject our new field
            // use optionalFieldOf so that vanilla JSON configs still deserialize properly
            Codec.INT.optionalFieldOf("myInt", 3).forGetter(cfg -> ((TernarySurfaceConfigMixin)(Object)cfg).myInteger)
        ).apply(inst, (cfg, i) -> {
            // set additional fields in cfg, then return it
            ((TernarySurfaceConfigMixin)(Object)cfg).myInteger = i;
            System.out.println("Injected field: " + i);
            return cfg;
        });
    }
}

uplands_surface_builder.json

{
  "type": "$modid:uplands_surface_builder",
  "config": {
    "top_material": {
      "Name": "$modid:uplander_grass"
    },
    "under_material": {
      "Name": "$modid:uplander_dirt"
    },
    "underwater_material": {
      "Name": "$modid:uplander_dirt"
    },
    "myInt": 47
  }
}