TODO unfinished attack/damage event overhaul

src/main/java/org/spongepowered/api/event/cause/entity/damage/DamageModifier.java

@@ -25,6 +25,7 @@
 package org.spongepowered.api.event.cause.entity.damage;
 
 import org.checkerframework.checker.nullness.qual.Nullable;
+import org.spongepowered.api.ResourceKey;
 import org.spongepowered.api.entity.Entity;
 import org.spongepowered.api.event.Cause;
 import org.spongepowered.api.item.ItemTypes;
@@ -63,6 +64,14 @@ static Builder builder() {
      */
     DamageModifierType type();
 
+    /**
+     * Returns the damage modifier group.
+     * <p>Grouped modifiers calculate their damage independently from each other</p>
+     *
+     * @return The damage modifier group
+     */
+    ResourceKey group();
+
     /**
      * Gets the cause of this {@link DamageModifier}.
      *
@@ -92,6 +101,7 @@ final class Builder implements org.spongepowered.api.util.Builder<DamageModifier
 
         @Nullable DamageModifierType type;
         @Nullable Cause cause;
+        @Nullable ResourceKey group;
         @Nullable ItemStackSnapshot snapshot;
 
         Builder() {
@@ -121,6 +131,41 @@ public Builder type(final DamageModifierType damageModifierType) {
             return this;
         }
 
+        /**
+         * The main attack damage calculated for an {@link org.spongepowered.api.event.entity.AttackEntityEvent}
+         *
+         * @return This builder, for chaining
+         */
+        public Builder attackDamageGroup() {
+            this.group = ResourceKey.minecraft("attack_damage");
+            return this;
+        }
+
+        /**
+         * The enchantment attack damage calculated for an {@link org.spongepowered.api.event.entity.AttackEntityEvent}
+         *
+         * @return This builder, for chaining
+         */
+        public Builder attackEnchantmentGroup() {
+            this.group = ResourceKey.minecraft("attack_enchantment");
+            return this;
+        }
+
+        /**
+         * The damage calculated for an {@link org.spongepowered.api.event.entity.DamageEntityEvent}
+         *
+         * @return This builder, for chaining
+         */
+        public Builder damageReductionGroup() {
+            this.group = ResourceKey.minecraft("damage_reduction");
+            return this;
+        }
+
+        public Builder group(final ResourceKey group) {
+            this.group = group;
+            return this;
+        }
+
         public Builder item(final ItemStack itemStack) {
             this.item(java.util.Objects.requireNonNull(itemStack, "ItemStack").createSnapshot());
             return this;
@@ -179,10 +224,12 @@ private static class ImplementedDamageModifier implements DamageModifier {
             private final DamageModifierType type;
             private final Cause cause;
             @Nullable private final ItemStackSnapshot snapshot;
+            private final ResourceKey group;
 
             ImplementedDamageModifier(final Builder builder) {
                 this.type = java.util.Objects.requireNonNull(builder.type, "DamageType is null!");
                 this.cause = java.util.Objects.requireNonNull(builder.cause, "Cause is null!");
+                this.group = java.util.Objects.requireNonNull(builder.group, "Group is null!");
                 this.snapshot = builder.snapshot;
             }
 
@@ -201,6 +248,10 @@ public Optional<ItemStackSnapshot> contributingItem() {
                 return Optional.ofNullable(this.snapshot);
             }
 
+            public ResourceKey group() {
+                return group;
+            }
+
             @Override
             public int hashCode() {
                 return Objects.hash(this.type, this.cause);

src/main/java/org/spongepowered/api/event/entity/AttackEntityEvent.java

@@ -122,6 +122,7 @@
  * return damage;
  * }</pre></blockquote>
  *
+ * TODO explain groups
  * <p>After which, the "final" damage is simply the summation of the
  * "base" damage and all "modified damage" for each {@link DamageModifier}
  * provided in this event.</p>

src/main/java/org/spongepowered/api/event/entity/DamageEntityEvent.java

@@ -119,6 +119,7 @@
  * return damage;
  * }</pre></blockquote>
  *
+ * TODO explain groups
  * <p>After which, the "final" damage is simply the summation of the
  * "base" damage and all "modified damage" for each {@link DamageModifier}
  * provided in this event.</p>
@@ -134,6 +135,7 @@
  * the provided pairing will be added at the
  * "end" of the list for "modifying" the "base" damage.</p>
  *
+ * TODO this is wrong?
  * <p>Note that this event is intended for processing incoming damage to
  * an {@link Entity} prior to any {@link DamageModifier}s associated with
  * the {@link #entity()}. The {@link DamageEntityEvent} is used

src/main/java/org/spongepowered/api/event/impl/entity/AbstractModifierEvent.java

@@ -24,6 +24,9 @@
  */
 package org.spongepowered.api.event.impl.entity;
 
+import org.jetbrains.annotations.NotNull;
+import org.spongepowered.api.ResourceKey;
+import org.spongepowered.api.event.cause.entity.damage.DamageModifier;
 import org.spongepowered.api.event.cause.entity.damage.ModifierFunction;
 import org.spongepowered.api.event.entity.DamageEntityEvent;
 import org.spongepowered.api.event.impl.AbstractEvent;
@@ -35,6 +38,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.function.DoubleUnaryOperator;
+import java.util.stream.DoubleStream;
 
 /**
  * An abstract base class for implementations of {@link DamageEntityEvent}.
@@ -92,11 +96,21 @@ protected void recalculateDamages(final double baseAmount) {
     }
 
     protected double finalAmount(final double baseAmount) {
-        double damage = baseAmount;
-        for (final T entry : this.modifierFunctions) {
-            damage += entry.function().applyAsDouble(damage);
+        final var amounts = finalAmounts(baseAmount, this.modifierFunctions);
+        return amounts.values().stream().mapToDouble(Double::doubleValue).sum();
+    }
+
+    public static <T extends ModifierFunction<M>, M> Map<ResourceKey, Double> finalAmounts(final double baseAmount, final List<T> modifiers) {
+        var defaultGroup = ResourceKey.sponge("default");
+        Map<ResourceKey, Double> amounts = new HashMap<>();
+        for (final T entry : modifiers) {
+            var group = defaultGroup;
+            if (entry.modifier() instanceof DamageModifier damageModifier) {
+                group = damageModifier.group();
+            }
+            amounts.compute(group, (k, v) -> entry.function().applyAsDouble(v == null ? baseAmount : v));
         }
-        return damage;
+        return amounts;
     }
 
     /**