it is done

core/src/mindustry/game/SpawnGroup.java

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+package mindustry.game;
+
+import arc.util.serialization.Json;
+import arc.util.serialization.Json.Serializable;
+import arc.util.serialization.JsonValue;
+import mindustry.content.*;
+import mindustry.ctype.ContentType;
+import mindustry.entities.type.BaseUnit;
+import mindustry.type.*;
+
+import static mindustry.Vars.content;
+
+/**
+ * A spawn group defines spawn information for a specific type of unit, with optional extra information like
+ * weapon equipped, ammo used, and status effects.
+ * Each spawn group can have multiple sub-groups spawned in different areas of the map.
+ */
+public class SpawnGroup implements Serializable{
+    public static final int never = Integer.MAX_VALUE;
+
+    /** The unit type spawned */
+    public UnitType type;
+    /** When this spawn should end */
+    public int end = never;
+    /** When this spawn should start */
+    public int begin;
+    /** The spacing, in waves, of spawns. For example, 2 = spawns every other wave */
+    public int spacing = 1;
+    /** Maximum amount of units that spawn */
+    public int max = 100;
+    /** How many waves need to pass before the amount of units spawned increases by 1 */
+    public float unitScaling = never;
+    /** Amount of enemies spawned initially, with no scaling */
+    public int unitAmount = 1;
+    /** Status effect applied to the spawned unit. Null to disable. */
+    public StatusEffect effect;
+    /** Items this unit spawns with. Null to disable. */
+    public ItemStack items;
+
+    public SpawnGroup(UnitType type){
+        this.type = type;
+    }
+
+    public SpawnGroup(){
+        //serialization use only
+    }
+
+    /** Returns the amount of units spawned on a specific wave. */
+    public int getUnitsSpawned(int wave){
+        if(wave < begin || wave > end || (wave - begin) % spacing != 0){
+            return 0;
+        }
+        return Math.min(unitAmount + (int)(((wave - begin) / spacing) / unitScaling), max);
+    }
+
+    /**
+     * Creates a unit, and assigns correct values based on this group's data.
+     * This method does not add() the unit.
+     */
+    public BaseUnit createUnit(Team team){
+        BaseUnit unit = type.create(team);
+
+        if(effect != null){
+            unit.applyEffect(effect, 999999f);
+        }
+
+        if(items != null){
+            unit.addItem(items.item, items.amount);
+        }
+
+        return unit;
+    }
+
+    @Override
+    public void write(Json json){
+        json.writeValue("type", type.name);
+        if(begin != 0) json.writeValue("begin", begin);
+        if(end != never) json.writeValue("end", end);
+        if(spacing != 1) json.writeValue("spacing", spacing);
+        //if(max != 40) json.writeValue("max", max);
+        if(unitScaling != never) json.writeValue("scaling", unitScaling);
+        if(unitAmount != 1) json.writeValue("amount", unitAmount);
+        if(effect != null) json.writeValue("effect", effect.id);
+    }
+
+    @Override
+    public void read(Json json, JsonValue data){
+        type = content.getByName(ContentType.unit, data.getString("type", "dagger"));
+        if(type == null) type = UnitTypes.dagger;
+        begin = data.getInt("begin", 0);
+        end = data.getInt("end", never);
+        spacing = data.getInt("spacing", 1);
+        //max = data.getInt("max", 40);
+        unitScaling = data.getFloat("scaling", never);
+        unitAmount = data.getInt("amount", 1);
+        effect = content.getByID(ContentType.status, data.getInt("effect", -1));
+    }
+
+    @Override
+    public String toString(){
+        return "SpawnGroup{" +
+        "type=" + type +
+        ", end=" + end +
+        ", begin=" + begin +
+        ", spacing=" + spacing +
+        ", max=" + max +
+        ", unitScaling=" + unitScaling +
+        ", unitAmount=" + unitAmount +
+        ", effect=" + effect +
+        ", items=" + items +
+        '}';
+    }
+}