Add option to deduplicate prefix/suffix stacks (#1285)

api/src/main/java/me/lucko/luckperms/api/metastacking/DuplicateRemovalFunction.java

@@ -0,0 +1,111 @@
+/*
+ * This file is part of LuckPerms, licensed under the MIT License.
+ *
+ *  Copyright (c) lucko (Luck) <luck@lucko.me>
+ *  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 me.lucko.luckperms.api.metastacking;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+
+import java.util.HashSet;
+import java.util.List;
+import java.util.ListIterator;
+import java.util.Set;
+
+/**
+ * Functional interface which removes duplicate entries from a list.
+ *
+ * <p>Used by LuckPerms to remove duplicate entries from a MetaStack.</p>
+ *
+ * @since 4.4
+ */
+public interface DuplicateRemovalFunction {
+
+    /**
+     * Removes duplicates from the given list, according to the behaviour
+     * of the function.
+     *
+     * @param list the entries
+     * @param <T> the type of entries
+     */
+    <T> void processDuplicates(@NonNull List<T> list);
+
+    /**
+     * A {@link DuplicateRemovalFunction} that does not remove duplicates.
+     */
+    DuplicateRemovalFunction RETAIN_ALL = new DuplicateRemovalFunction() {
+        @Override
+        public <T> void processDuplicates(@NonNull List<T> list) {
+
+        }
+
+        @Override
+        public String toString() {
+            return "DuplicateRemovalFunction#RETAIN_ALL";
+        }
+    };
+
+    /**
+     * A {@link DuplicateRemovalFunction} that retains only the first duplicate.
+     */
+    DuplicateRemovalFunction FIRST_ONLY = new DuplicateRemovalFunction() {
+        @SuppressWarnings("Java8CollectionRemoveIf")
+        @Override
+        public <T> void processDuplicates(@NonNull List<T> list) {
+            Set<T> seen = new HashSet<>();
+            for (ListIterator<T> it = list.listIterator(); it.hasNext(); ) {
+                T next = it.next();
+                if (!seen.add(next)) {
+                    it.remove();
+                }
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "DuplicateRemovalFunction#FIRST_ONLY";
+        }
+    };
+
+    /**
+     * A {@link DuplicateRemovalFunction} that retains only the first duplicate.
+     */
+    DuplicateRemovalFunction LAST_ONLY = new DuplicateRemovalFunction() {
+        @Override
+        public <T> void processDuplicates(@NonNull List<T> list) {
+            Set<T> seen = new HashSet<>();
+            for (ListIterator<T> it = list.listIterator(list.size()); it.hasPrevious(); ) {
+                T next = it.previous();
+                if (!seen.add(next)) {
+                    it.remove();
+                }
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "DuplicateRemovalFunction#LAST_ONLY";
+        }
+    };
+
+}

api/src/main/java/me/lucko/luckperms/api/metastacking/MetaStackDefinition.java

@@ -49,6 +49,15 @@ public interface MetaStackDefinition {
      */
     @NonNull List<MetaStackElement> getElements();
 
+    /**
+     * Gets the duplicate removal function, applied to the entries before
+     * formatting takes place.
+     *
+     * @return the duplicate removal function
+     * @since 4.4
+     */
+    @NonNull DuplicateRemovalFunction getDuplicateRemovalFunction();
+
     /**
      * Gets the spacer string added before any stack elements
      *

api/src/main/java/me/lucko/luckperms/api/metastacking/MetaStackFactory.java

@@ -64,6 +64,21 @@ public interface MetaStackFactory {
      * @param endSpacer    the spacer to be included at the end of the stacks output
      * @return the new stack definition instance
      */
-    @NonNull MetaStackDefinition createDefinition(@NonNull List<MetaStackElement> elements, @NonNull String startSpacer, @NonNull String middleSpacer, @NonNull String endSpacer);
+    default @NonNull MetaStackDefinition createDefinition(@NonNull List<MetaStackElement> elements, @NonNull String startSpacer, @NonNull String middleSpacer, @NonNull String endSpacer) {
+        return createDefinition(elements, DuplicateRemovalFunction.RETAIN_ALL, startSpacer, middleSpacer, endSpacer);
+    }
+
+    /**
+     * Creates a new {@link MetaStackDefinition} with the given properties.
+     *
+     * @param elements                 the elements to be included in the stack.
+     * @param duplicateRemovalFunction the duplicate removal function
+     * @param startSpacer              the spacer to be included at the start of the stacks output
+     * @param middleSpacer             the spacer to be included between stack elements
+     * @param endSpacer                the spacer to be included at the end of the stacks output
+     * @return the new stack definition instance
+     */
+    @NonNull MetaStackDefinition createDefinition(@NonNull List<MetaStackElement> elements, @NonNull DuplicateRemovalFunction duplicateRemovalFunction, @NonNull String startSpacer, @NonNull String middleSpacer, @NonNull String endSpacer);
+
 
 }