Damage step API improvements

Author: Yeregorix

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

@@ -24,18 +24,87 @@
  */
 package org.spongepowered.api.event.cause.entity.damage;
 
+import org.spongepowered.api.Sponge;
+import org.spongepowered.api.event.CauseStackManager;
+
+import java.util.Optional;
+import java.util.function.Consumer;
+
 /**
- * A damage modifier that will be applied before or after a {@link DamageStep}.
+ * A damage modifier that will create a {@link DamageStep}.
  */
-@FunctionalInterface
 public interface DamageModifier {
 
     /**
-     * Modifies the damage.
+     * Gets the {@link DamageStepType} of this modifier.
+     *
+     * @return the step type
+     */
+    DamageStepType type();
+
+    /**
+     * Gets the consumer that will modify the cause frame.
      *
-     * @param step The damage step this modifier is associated with.
-     * @param damage The current damage value.
-     * @return The next damage value
+     * @return The cause frame modifier
      */
-    double modify(DamageStep step, double damage);
+    Optional<Consumer<CauseStackManager.StackFrame>> frame();
+
+    /**
+     * Gets the function that will modify the damage.
+     * The function may be absent if the sole purpose of this modifier is to apply children steps.
+     *
+     * @return the damage modifier
+     */
+    Optional<Function> damage();
+
+    @FunctionalInterface
+    interface Function {
+        /**
+         * Modifies the damage.
+         *
+         * @param step   The damage step this modifier is associated with.
+         * @param damage The current damage value.
+         * @return The next damage value
+         */
+        double modify(DamageStep step, double damage);
+    }
+
+    /**
+     * Creates a new {@link Builder} to create {@link DamageModifier}s.
+     *
+     * @return The new builder
+     */
+    static Builder builder() {
+        return Sponge.game().builderProvider().provide(Builder.class);
+    }
+
+    /**
+     * A builder to create {@link DamageModifier}s.
+     */
+    interface Builder extends org.spongepowered.api.util.Builder<DamageModifier, Builder> {
+
+        /**
+         * Sets the {@link DamageStepType} for this modifier.
+         *
+         * @param type The damage step type
+         * @return this builder for chaining
+         */
+        Builder type(DamageStepType type);
+
+        /**
+         * Sets the cause frame modifier.
+         *
+         * @param frameModifier The frame modifier
+         * @return this builder for chaining
+         */
+        Builder frame(Consumer<CauseStackManager.StackFrame> frameModifier);
+
+        /**
+         * Sets the {@link Function} for this modifier.
+         *
+         * @param function The damage function
+         * @return this builder for chaining
+         */
+        Builder damage(Function function);
+    }
 }

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

@@ -27,30 +27,35 @@
 import org.spongepowered.api.event.Cause;
 
 import java.util.List;
+import java.util.Optional;
+import java.util.OptionalDouble;
 
 /**
- * Represents a step in the damage calculation.
+ * A step represent an operation made by the platform (vanilla and mods) or modifiers added by plugins.
+ * Steps are structured as trees where children modify the input or output of the parent step.
+ * A damage calculation is made of multiple trees of steps.
  */
 public interface DamageStep {
 
     /**
-     * Gets the {@link DamageStepType} for this {@link DamageStep}.
+     * Gets the {@link DamageStepType} of this step.
      *
-     * @return The damage step type
+     * @return the step type
      */
     DamageStepType type();
 
     /**
-     * Gets the cause of this {@link DamageStep}.
+     * Gets the {@link Cause} of this step.
      *
      * @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 +80,77 @@ 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 the {@link DamageStepHistory} this step belongs to.
+     *
+     * @return The history containing this step.
+     */
+    DamageStepHistory history();
+
+    /**
+     * Gets the parent of this step.
+     * Returns empty if this step is the root of its tree.
+     *
+     * @return The parent of this step
+     */
+    Optional<DamageStep> parent();
+
+    /**
+     * Gets the root of this step.
+     *
+     * @return The root of this step
+     */
+    default DamageStep root() {
+        DamageStep step = this;
+        Optional<DamageStep> parent;
+        while ((parent = step.parent()).isPresent()) {
+            step = parent.get();
+        }
+        return step;
+    }
 
     /**
-     * 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> 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> childrenAfter();
 }

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

@@ -0,0 +1,43 @@
+/*
+ * 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 java.util.List;
+
+/**
+ * Captures the root steps occurring during a damage calculation.
+ */
+public interface DamageStepHistory {
+
+    /**
+     * Gets the list of the captured root steps during the damage calculation in the order they have been applied.
+     * Note that this list is not an exhaustive representation of all the operations applied,
+     * especially in a modded environment.
+     * The list is unmodifiable and will gradually grow during the damage calculation.
+     *
+     * @return The root steps.
+     */
+    List<DamageStep> rootSteps();
+}

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

@@ -24,18 +24,35 @@
  */
 package org.spongepowered.api.event.cause.entity.damage;
 
-import org.spongepowered.api.event.Cause;
+import org.spongepowered.api.Sponge;
 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}.
  */
 @CatalogedBy(DamageStepTypes.class)
 public interface DamageStepType extends DefaultedRegistryValue {
 
+    /**
+     * Creates a new {@link DamageStepType}.
+     *
+     * @return the new step type
+     */
+    static DamageStepType create() {
+        return Sponge.game().factoryProvider().provide(Factory.class).create();
+    }
+
+    /**
+     * A factory to create {@link DamageStepType}s.
+     */
+    interface Factory {
+
+        /**
+         * Creates a new {@link DamageStepType}.
+         *
+         * @return the new step type
+         */
+        DamageStepType create();
+    }
 }

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

@@ -108,6 +108,13 @@ public final class DamageStepTypes {
      */
     public static final DefaultedRegistryReference<DamageStepType> ENCHANTMENT_COOLDOWN = DamageStepTypes.key(ResourceKey.sponge("enchantment_cooldown"));
 
+    /**
+     * Represents the last {@link DamageStep} applied during a damage calculation.
+     * This step happens just before the {@link org.spongepowered.api.event.entity.DamageCalculationEvent.Post}.
+     * This step does nothing but can be used to add modifiers to the final damage.
+     */
+    public static final DefaultedRegistryReference<DamageStepType> END = DamageStepTypes.key(ResourceKey.sponge("end"));
+
     /**
      * Represents a {@link DamageStep} that will modify freezing damage.
      * E.g. {@link org.spongepowered.api.entity.living.monster.Blaze} take more damage from freezing sources.
@@ -152,6 +159,13 @@ public final class DamageStepTypes {
      */
     public static final DefaultedRegistryReference<DamageStepType> SHIELD = DamageStepTypes.key(ResourceKey.sponge("shield"));
 
+    /**
+     * Represents the first {@link DamageStep} applied during a damage calculation.
+     * This step happens just after the {@link org.spongepowered.api.event.entity.DamageCalculationEvent.Pre}.
+     * This step does nothing but can be used to add modifiers to the base damage.
+     */
+    public static final DefaultedRegistryReference<DamageStepType> START = DamageStepTypes.key(ResourceKey.sponge("start"));
+
     /**
      * Represents a {@link DamageStep} that is applied for a sweeping attack.
      */