package com.example.examplemod;

import com.google.common.base.Throwables;
import com.google.common.collect.ObjectArrays;
import cpw.mods.fml.common.Mod;
import cpw.mods.fml.common.Mod.EventHandler;
import cpw.mods.fml.common.event.FMLPreInitializationEvent;
import cpw.mods.fml.common.eventhandler.SubscribeEvent;
import net.minecraft.world.biome.BiomeGenBase;
import net.minecraft.world.gen.layer.GenLayer;
import net.minecraft.world.gen.layer.GenLayerBiome;
import net.minecraft.world.gen.layer.GenLayerRiverMix;
import net.minecraft.world.gen.layer.MyModGenLayerAccessor;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.event.terraingen.WorldTypeEvent;

import java.lang.reflect.Field;

@Mod(modid = ExampleMod.MODID, version = ExampleMod.VERSION)
public class ExampleMod
{
    public static final String MODID = "examplemod";
    public static final String VERSION = "1.0";

    public static BiomeGenBase myBiome;

    @EventHandler
    public void preInit(FMLPreInitializationEvent event) {
        MinecraftForge.TERRAIN_GEN_BUS.register(this);

        myBiome = new BiomeGenBase(245) {

        };
    }

    private static Field biomePatternGeneratorChainField;
    private static Field biomeArrayField;

    static {
        // TODO: SRG names for non-dev environment
        try {
            biomePatternGeneratorChainField = GenLayerRiverMix.class.getDeclaredField("biomePatternGeneratorChain");
            biomePatternGeneratorChainField.setAccessible(true);

            // this is just an example, there are 4 biome arrays in the class, i am not 100% what they do
            biomeArrayField = GenLayerBiome.class.getDeclaredField("field_151621_d");
            biomeArrayField.setAccessible(true);
        } catch (NoSuchFieldException e) {
            Throwables.propagate(e);
        }
    }

    @SubscribeEvent
    public void onGenLayerInit(WorldTypeEvent.InitBiomeGens event) throws IllegalAccessException {
        GenLayer biomeLayer = event.originalBiomeGens[1];
        do {
            if (biomeLayer instanceof GenLayerRiverMix) {
                biomeLayer = (GenLayer) biomePatternGeneratorChainField.get(biomeLayer);
            } else {
                biomeLayer = MyModGenLayerAccessor.getParent(biomeLayer);
            }
        } while (biomeLayer != null && !(biomeLayer instanceof GenLayerBiome));

        if (biomeLayer == null) {
            throw new IllegalStateException("couldn't locate GenLayerBiome!");
        }

        BiomeGenBase[] biomes = (BiomeGenBase[]) biomeArrayField.get(biomeLayer);
        biomes = ObjectArrays.concat(biomes, myBiome);
        biomeArrayField.set(biomeLayer, biomes);
    }

}

package net.minecraft.world.gen.layer;

/**
 * @author diesieben07
 */
public final class MyModGenLayerAccessor {

    public static GenLayer getParent(GenLayer genLayer) {
        return genLayer.parent;
    }

}