Add future default methods

api/src/main/java/me/lokka30/treasury/api/common/misc/FutureHelper.java

@@ -0,0 +1,67 @@
+package me.lokka30.treasury.api.common.misc;
+
+import java.util.AbstractMap;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.CompletableFuture;
+import java.util.function.Function;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * A utility class to help with {@link CompletableFuture} stuff
+ *
+ * @author MrIvanPlays
+ * @since 1.2.1
+ */
+public class FutureHelper {
+
+    /**
+     * Maps, joins, and filters futures.
+     * Maps all the futures holding data type A to return a collection of data type B
+     * via the specified {@code mapper} {@link Function} and filters them via the specified
+     * filter function.
+     *
+     * @param mapper  mapper
+     * @param futures futures of data type A
+     * @param <A>     type parameter A
+     * @param <B>     type parameter B
+     * @return future with collection of data type B
+     */
+    @NotNull
+    public static <A, B> CompletableFuture<Collection<B>> mapJoinFilter(
+            @NotNull Function<A, CompletableFuture<Boolean>> filter,
+            @NotNull Function<A, B> mapper,
+            @NotNull Collection<CompletableFuture<A>> futures
+    ) {
+        Objects.requireNonNull(filter, "filter");
+        Objects.requireNonNull(mapper, "mapper");
+        Objects.requireNonNull(futures, "futures");
+
+        List<CompletableFuture<Map.Entry<A, Boolean>>> filteredFutures =
+                new ArrayList<>(futures.size());
+        for (CompletableFuture<A> future : futures) {
+            filteredFutures.add(future.thenCompose((a) -> {
+                return filter.apply(a)
+                        .thenApply((allowed) -> new AbstractMap.SimpleImmutableEntry<>(a, allowed));
+            }));
+        }
+        return CompletableFuture.allOf(
+                filteredFutures.toArray(new CompletableFuture[0])
+        ).thenApply((ignore) -> {
+
+            List<B> results = new ArrayList<>(filteredFutures.size());
+            for (CompletableFuture<Map.Entry<A, Boolean>> filteredFuture : filteredFutures) {
+                // By now, all futures are complete -- join() will not block
+                Map.Entry<A, Boolean> entry = filteredFuture.join();
+                if (entry.getValue()) {
+                    results.add(mapper.apply(entry.getKey()));
+                }
+            }
+            return results;
+        });
+    }
+
+}