Represents damage steps as trees

This allows capturing and inspecting damage values before and after each modifier.

Author: Yeregorix

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

@@ -27,30 +27,19 @@
 import org.spongepowered.api.event.Cause;
 
 import java.util.List;
+import java.util.OptionalDouble;
 
 /**
- * Represents a step in the damage calculation.
+ * Represents a tree of steps in the damage calculation.
+ * A damage calculation is made of multiple trees of steps.
  */
 public interface DamageStep {
 
-    /**
-     * Gets the {@link DamageStepType} for this {@link DamageStep}.
-     *
-     * @return The damage step type
-     */
-    DamageStepType type();
-
-    /**
-     * Gets the cause of this {@link DamageStep}.
-     *
-     * @return The cause of this step
-     */
-    Cause cause();
-
     /**
      * Gets whether this step is skipped.
-     * When skipped, only the step and its side effects are ignored, modifiers are still applied.
-     * A modifier willing to ignore every previous modifiers should revert the damage to {@link #damageBeforeModifiers()}.
+     * When skipped, only the step itself and its side effects are ignored, children are still applied.
+     * A modifier willing to ignore every previous children should revert the damage to {@link #damageBeforeChildren()},
+     * or call {@link #skip} on each child.
      *
      * @return Whether this step is skipped
      */
@@ -75,47 +64,88 @@ default void skip() {
     }
 
     /**
-     * The damage just before the modifiers of this step.
+     * The damage just before the children of this step.
+     * Returns empty if the value is not known yet.
      *
-     * @return The damage before this step
+     * @return The damage before the children of this step
      */
-    double damageBeforeModifiers();
+    OptionalDouble damageBeforeChildren();
 
     /**
      * The damage just before this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage before this step
-     * @throws IllegalStateException if called before the "before" modifiers have finished.
      */
-    double damageBeforeStep();
+    OptionalDouble damageBeforeSelf();
 
     /**
      * The damage just after this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage after this step
-     * @throws IllegalStateException if called before this step has finished
      */
-    double damageAfterStep();
+    OptionalDouble damageAfterSelf();
 
     /**
-     * The damage just after the modifiers of this step.
+     * The damage just after the children of this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage after this step
-     * @throws IllegalStateException if called before the modifiers have finished.
      */
-    double damageAfterModifiers();
+    OptionalDouble damageAfterChildren();
 
     /**
-     * Gets an immutable list of all modifiers that applies just before this step.
+     * Gets an immutable list of all children steps that applies just before this step.
      *
-     * @return The list of modifiers
+     * @return The list of children steps
      */
-    List<DamageModifier> modifiersBefore();
+    List<DamageStep.Child> childrenBefore();
 
     /**
-     * Gets an immutable list of all modifiers that applies just after this step.
+     * Gets an immutable list of all children steps that applies just after this step.
      *
-     * @return The list of modifiers
+     * @return The list of children steps
      */
-    List<DamageModifier> modifiersAfter();
+    List<DamageStep.Child> childrenAfter();
+
+    /**
+     * Represents a step made by the platform (vanilla and mods).
+     */
+    interface Root extends DamageStep {
+
+        /**
+         * Gets the {@link DamageStepType} for this step.
+         *
+         * @return The damage step type
+         */
+        DamageStepType type();
+
+        /**
+         * Gets the {@link Cause} of this step.
+         *
+         * @return The cause of this step
+         */
+        Cause cause();
+    }
+
+    /**
+     * Represents a step made by a plugin (a modifier).
+     */
+    interface Child extends DamageStep {
+
+        /**
+         * Gets the parent of this step.
+         *
+         * @return The parent of this step
+         */
+        DamageStep parent();
+
+        /**
+         * The {@link DamageModifier} function of this step.
+         *
+         * @return The damage modifier
+         */
+        DamageModifier modifier();
+    }
 }

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

@@ -0,0 +1,128 @@
+/*
+ * This file is part of SpongeAPI, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) SpongePowered <https://www.spongepowered.org>
+ * Copyright (c) contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.spongepowered.api.event.cause.entity.damage;
+
+import org.spongepowered.api.Sponge;
+import org.spongepowered.api.event.entity.DamageCalculationEvent;
+
+import java.util.List;
+import java.util.Optional;
+
+/**
+ * Represents the mutable equivalent of a {@link DamageStep}.
+ * This allows plugins to add and remove modifiers during the {@link DamageCalculationEvent.Pre}.
+ */
+public interface DamageStepSpec {
+
+    /**
+     * Gets a mutable list of all children steps that applies just before this step.
+     * Adding a step to this list will raise an error if the step is already the child of another step.
+     *
+     * @return The list of children steps
+     */
+    List<DamageStepSpec.Child> childrenBefore();
+
+    /**
+     * Gets a mutable list of all children steps that applies just after this step.
+     * Adding a step to this list will raise an error if the step is already the child of another step.
+     *
+     * @return The list of children steps
+     */
+    List<DamageStepSpec.Child> childrenAfter();
+
+    /**
+     * Represents a step made by the platform (vanilla and mods).
+     */
+    interface Root extends DamageStepSpec {
+
+        /**
+         * Gets the {@link DamageStepType} for this step.
+         *
+         * @return The damage step type
+         */
+        DamageStepType type();
+
+        static DamageStepSpec.Root of(DamageStepType type) {
+            return Sponge.game().factoryProvider().provide(Factory.class).of(type);
+        }
+
+        interface Factory {
+            DamageStepSpec.Root of(DamageStepType type);
+        }
+    }
+
+    /**
+     * Represents a step made by a plugin (a modifier).
+     */
+    interface Child extends DamageStepSpec {
+
+        /**
+         * Gets the parent of this step.
+         *
+         * @return The parent of this step
+         */
+        Optional<DamageStepSpec> parent();
+
+        /**
+         * The {@link DamageModifier} function of this step.
+         *
+         * @return The damage modifier
+         */
+        DamageModifier modifier();
+
+        /**
+         * Creates a new {@link DamageStepSpec.Child} from the given damage modifier.
+         *
+         * @param modifier The damage modifier
+         * @return The child step
+         */
+        static DamageStepSpec.Child of(DamageModifier modifier) {
+            return DamageStepSpec.Child.builder().modifier(modifier).build();
+        }
+
+        /**
+         * Creates a new {@link Builder} to create {@link DamageStepSpec.Child}s.
+         *
+         * @return The new builder
+         */
+        static Builder builder() {
+            return Sponge.game().builderProvider().provide(Builder.class);
+        }
+
+        /**
+         * A builder to create {@link DamageStepSpec.Child}s.
+         */
+        interface Builder extends org.spongepowered.api.util.Builder<DamageStepSpec.Child, Builder> {
+
+            /**
+             * Sets the {@link DamageModifier} function for this step.
+             *
+             * @param modifier The damage modifier
+             * @return this builder for chaining
+             */
+            Builder modifier(DamageModifier modifier);
+        }
+    }
+}

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

@@ -24,16 +24,11 @@
  */
 package org.spongepowered.api.event.cause.entity.damage;
 
-import org.spongepowered.api.event.Cause;
 import org.spongepowered.api.registry.DefaultedRegistryValue;
 import org.spongepowered.api.util.annotation.CatalogedBy;
 
 /**
- * A type of {@link DamageStep} that can apply a "grouping" so to speak
- * for the damage modifier. The use case is being able to differentiate between
- * various {@link DamageStep}s based on the {@link DamageStepType}
- * without digging through the {@link Cause} provided by
- * {@link DamageStep#cause()}.
+ * Represents a type of {@link DamageStep.Root}.
  */
 @CatalogedBy(DamageStepTypes.class)
 public interface DamageStepType extends DefaultedRegistryValue {