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A convenient way to organize asynchronous code. Promises are a faster and more efficient version of JavaScript's Promise and Java's CompletionStage's.

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/*
 * Copyright (C) 2020 ActiveJ LLC.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.activej.async.function;

import io.activej.async.process.AsyncExecutor;
import io.activej.async.process.AsyncExecutors;
import io.activej.promise.Promise;
import io.activej.promise.Promises;
import org.jetbrains.annotations.Contract;
import org.jetbrains.annotations.Nullable;

import java.util.ArrayDeque;

public final class AsyncSuppliers {

	@Contract(pure = true)
	public static  AsyncSupplier reuse(AsyncSupplier actual) {
		return new AsyncSupplier<>() {
			@Nullable Promise runningPromise;

			@SuppressWarnings("unchecked")
			@Override
			public Promise get() {
				if (runningPromise != null) return runningPromise;
				runningPromise = (Promise) actual.get();
				Promise runningPromise = this.runningPromise;
				runningPromise.whenComplete(() -> this.runningPromise = null);
				return runningPromise;
			}
		};
	}

	@Contract(pure = true)
	public static  AsyncSupplier coalesce(AsyncSupplier actual) {
		AsyncFunction fn = Promises.coalesce(() -> null, (a, v) -> {}, a -> actual.get());
		return () -> fn.apply(null);
	}

	@Contract(pure = true)
	public static  AsyncSupplier buffer(AsyncSupplier actual) {
		return buffer(1, Integer.MAX_VALUE, actual);
	}

	@Contract(pure = true)
	public static  AsyncSupplier buffer(int maxParallelCalls, int maxBufferedCalls, AsyncSupplier actualSupplier) {
		return ofExecutor(AsyncExecutors.buffered(maxParallelCalls, maxBufferedCalls), actualSupplier);
	}

	@Contract(pure = true)
	public static  AsyncSupplier ofExecutor(AsyncExecutor executor, AsyncSupplier supplier) {
		return () -> executor.execute(supplier);
	}

	@Contract(pure = true)
	public static  AsyncSupplier prefetch(int count, AsyncSupplier supplier) {
		return prefetch(count, supplier, supplier);
	}

	@Contract(pure = true)
	@SuppressWarnings("unchecked")
	public static  AsyncSupplier prefetch(
		int count, AsyncSupplier actualSupplier, AsyncSupplier prefetchSupplier
	) {
		if (count == 0) return (AsyncSupplier) actualSupplier;
		return new AsyncSupplier() {
			final ArrayDeque prefetched = new ArrayDeque<>();
			int prefetchCalls;

			@SuppressWarnings("unchecked")
			@Override
			public Promise get() {
				Promise result = prefetched.isEmpty() ?
					actualSupplier.get() :
					Promise.of(prefetched.pollFirst());
				prefetch();
				return (Promise) result;
			}

			void prefetch() {
				for (int i = 0; i < count - (prefetched.size() + prefetchCalls); i++) {
					prefetchCalls++;
					prefetchSupplier.get()
						.async()
						.subscribe((value, e) -> {
							prefetchCalls--;
							if (e == null) {
								prefetched.addLast(value);
							}
						});
				}
			}
		};
	}

}