optimizations, more comfortable PromiseFactory api and support virtual threaded executors

futur-api/src/main/java/dev/tommyjs/futur/joiner/PromiseJoiner.java

@@ -0,0 +1,66 @@
+package dev.tommyjs.futur.joiner;
+
+import dev.tommyjs.futur.promise.*;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Iterator;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+
+abstract class PromiseJoiner<V, K, T, R> {
+
+    private final CompletablePromise<R> joined;
+
+    protected PromiseJoiner(@NotNull PromiseFactory factory) {
+        this.joined = factory.unresolved();
+    }
+
+    public @NotNull Promise<R> joined() {
+        return joined;
+    }
+
+    protected abstract K getKey(V value);
+
+    protected abstract @NotNull Promise<T> getPromise(V value);
+
+    protected abstract @Nullable Throwable onFinish(int index, K key, @NotNull PromiseCompletion<T> completion);
+
+    protected abstract R getResult();
+
+    protected void join(@NotNull Iterator<V> promises, boolean link) {
+        AtomicBoolean waiting = new AtomicBoolean();
+        AtomicInteger count = new AtomicInteger();
+
+        int i = 0;
+        do {
+            V value = promises.next();
+            Promise<T> p = getPromise(value);
+            if (link) {
+                AbstractPromise.cancelOnFinish(p, joined);
+            }
+
+            if (!joined.isCompleted()) {
+                count.incrementAndGet();
+                K key = getKey(value);
+                int index = i++;
+
+                p.addListener((res) -> {
+                    Throwable e = onFinish(index, key, res);
+                    if (e != null) {
+                        joined.completeExceptionally(e);
+                    } else if (count.decrementAndGet() == 0 && waiting.get()) {
+                        joined.complete(getResult());
+                    }
+                });
+            }
+        } while (promises.hasNext());
+
+        count.updateAndGet((v) -> {
+            if (v == 0) joined.complete(getResult());
+            else waiting.set(true);
+            return v;
+        });
+    }
+
+}