Source code of the configurable XP mod, available under a Creative Commons Attribution-Share Alike License.

ModMain.java

package sj224.mods.xp;

import java.util.HashMap;
import java.util.Map;

import net.minecraft.entity.EntityList;
import net.minecraft.entity.EntityLiving;
import net.minecraft.entity.boss.EntityDragon;
import net.minecraft.entity.monster.EntitySlime;
import net.minecraft.entity.monster.EntityZombie;
import net.minecraft.entity.passive.AbstractHorse;
import net.minecraft.entity.passive.EntityAnimal;
import net.minecraft.entity.passive.EntitySkeletonHorse;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.common.config.Configuration;
import net.minecraftforge.event.entity.living.LivingExperienceDropEvent;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.fml.common.Mod.EventHandler;
import net.minecraftforge.fml.common.event.FMLInitializationEvent;
import net.minecraftforge.fml.common.event.FMLPreInitializationEvent;
import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;

@Mod(modid = ModMain.MODID, version = ModMain.VERSION, acceptedMinecraftVersions = "[1.12,1.13)")
public class ModMain
{
    public static final String MODID = "customxp";
    public static final String VERSION = "1.2";
    
    static Map<String,Integer>xp=new HashMap<String, Integer>();
    
    static Configuration cfg;
    @EventHandler
    public void preInit(FMLPreInitializationEvent event)
    {
    	Configuration cfg=new Configuration(event.getSuggestedConfigurationFile());
        xp.put("bat",cfg.getInt("bat","mobs",0,0,9999,""));
        xp.put("chicken",cfg.getInt("chicken","mobs",2,0,9999,""));
        xp.put("cow",cfg.getInt("cow","mobs",2,0,9999,""));
        xp.put("mooshroom",cfg.getInt("mooshroom","mobs",2,0,9999,""));
        xp.put("pig",cfg.getInt("pig","mobs",2,0,9999,""));
        xp.put("rabbit",cfg.getInt("rabbit","mobs",2,0,9999,""));
        xp.put("sheep",cfg.getInt("sheep","mobs",2,0,9999,""));
        xp.put("horse",cfg.getInt("horse","mobs",2,0,9999,""));
        xp.put("skeleton_horse",cfg.getInt("horse","mobs",2,0,9999,""));
        xp.put("llama",cfg.getInt("llama","mobs",2,0,9999,""));
        xp.put("ocelot",cfg.getInt("ocelot","mobs",2,0,9999,""));
        xp.put("wolf",cfg.getInt("wolf","mobs",2,0,9999,""));
        xp.put("squid",cfg.getInt("squid","mobs",2,0,9999,""));
        xp.put("cave_spider",cfg.getInt("cave_spider","mobs",5,0,9999,""));
        xp.put("enderman",cfg.getInt("enderman","mobs",5,0,9999,""));
        xp.put("cave_spider",cfg.getInt("cave_spider","mobs",5,0,9999,""));
        xp.put("polar_bear",cfg.getInt("polar_bear","mobs",2,0,9999,""));
        xp.put("cave_spider",cfg.getInt("cave_spider","mobs",5,0,9999,""));
        xp.put("zombie_pigman",cfg.getInt("pigman","mobs",5,0,9999,""));
        xp.put("zombie_pigman_baby",cfg.getInt("baby_pigman","mobs",12,0,9999,""));
        xp.put("zombie",cfg.getInt("zombie","mobs",5,0,9999,""));
        xp.put("zombie_baby",cfg.getInt("baby_zombie","mobs",12,0,9999,""));
        xp.put("zombie_villager",cfg.getInt("zombie","mobs",5,0,9999,""));
        xp.put("zombie__villager_baby",cfg.getInt("baby_zombie","mobs",12,0,9999,""));
        xp.put("husk",cfg.getInt("husk","mobs",5,0,9999,""));
        xp.put("husk_baby",cfg.getInt("baby_husk","mobs",12,0,9999,""));
        xp.put("blaze",cfg.getInt("blaze","mobs",10,0,9999,""));
        xp.put("creeper",cfg.getInt("creeper","mobs",5,0,9999,""));
        xp.put("elder_guardian",cfg.getInt("elder_guardian","mobs",10,0,9999,""));
        xp.put("endermite",cfg.getInt("endermite","mobs",3,0,9999,""));
        xp.put("evocation_illager",cfg.getInt("evoker","mobs",10,0,9999,""));
        xp.put("vex",cfg.getInt("vex","mobs",3,0,9999,""));
        xp.put("vindication_illager",cfg.getInt("vindicator","mobs",5,0,9999,""));
        xp.put("ghast",cfg.getInt("ghast","mobs",5,0,9999,""));
        xp.put("guardian",cfg.getInt("guardian","mobs",10,0,9999,""));
        xp.put("big_slime",cfg.getInt("big_slime","mobs",4,0,9999,""));
        xp.put("big_magma_cube",cfg.getInt("big_magma_cube","mobs",4,0,9999,""));
        xp.put("shulker",cfg.getInt("shulker","mobs",5,0,9999,""));
        xp.put("silverfish",cfg.getInt("silverfish","mobs",5,0,9999,""));
        xp.put("skeleton",cfg.getInt("skeleton","mobs",5,0,9999,""));
        xp.put("wither_skeleton",cfg.getInt("wither_skeleton","mobs",5,0,9999,""));
        xp.put("stray",cfg.getInt("stray","mobs",5,0,9999,""));
        xp.put("witch",cfg.getInt("witch","mobs",5,0,9999,""));
        xp.put("wither",cfg.getInt("wither","mobs",50,0,9999,""));
        xp.put("dragon",cfg.getInt("ender_dragon","mobs",500,0,9999,""));
        xp.put("villager_golem",cfg.getInt("iron_golem","mobs",0,0,9999,""));
        xp.put("snowman",cfg.getInt("snow_golem","mobs",0,0,9999,""));
        cfg.save();
    }
    
    @EventHandler
    public void init(FMLInitializationEvent event){
    	MinecraftForge.EVENT_BUS.register(this);
    }
    @SubscribeEvent
    public void dropXP(LivingExperienceDropEvent e){
    	if(!(e.getEntityLiving() instanceof EntityLiving))return;
    	EntityLiving en=(EntityLiving)e.getEntityLiving();
    	String i=EntityList.getKey(en).getResourcePath();
    	System.out.println(i);
    	if(EntityList.getKey(e.getEntityLiving()).getResourceDomain().equals("minecraft")){
    		if(en instanceof EntitySlime){
    			int siz=((EntitySlime)en).getSlimeSize();
    			int xpc=xp.get("big_"+i);
    			System.out.println(xpc);
    			if(siz==0)xpc/=3;
    			if(siz==1)xpc/=1.8;
    			e.setDroppedExperience(xpc);return;
    		}else if(en instanceof EntityZombie){
    			int xpc=xp.get(i);
        		if(e.getOriginalExperience()==12){
        			xpc=xp.get(i+"_baby");        			
        		}
        		e.setDroppedExperience(xpc);return;
        	}else if((en instanceof AbstractHorse)&&!(en instanceof EntitySkeletonHorse)){
        		i="horse";
        	}else if(en instanceof EntityAnimal){
        		if(((EntityAnimal)en).getGrowingAge()<0){
        			return;
        		}
        	}else if(en instanceof EntityDragon){
        		int xpc=xp.get("dragon");
        		if(e.getOriginalExperience()==12000){
        			xpc*=20;
        		}
        		e.setDroppedExperience(xpc);return;
        	}
    		e.setDroppedExperience(xp.getOrDefault(i,e.getOriginalExperience()));
    	}
    	
    }
    
    
}

SuperJedi224 what does G,3,1,3, G,2,2,3, and G,2,1,4 equal to in your Graham's Sequence? How do you make the 4th entry go up? On your website almost infinite. I'm having trouble understanding the 4th entry.