Add preliminary future combinators

lib/Aggregate.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Alert;
+
+class Aggregate {
+
+    /**
+     * Create a new Future that will resolve when all Futures in the array resolve
+     *
+     * @param array $futures
+     * @return Future
+     */
+    public static function all(array $futures) {
+        return (new PromiseGroup($futures))->getFuture();
+    }
+
+}

lib/PromiseGroup.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Alert;
+
+class PromiseGroup extends Promise {
+    private $futureGroup = [];
+    private $resolvedValues = [];
+    private $isComplete = FALSE;
+
+    public function __construct(array $futureGroup) {
+        if (empty($futureGroup)) {
+            throw new \RuntimeException(
+                sprintf('Array at %s Argument 1 must not be empty', __METHOD__)
+            );
+        }
+
+        parent::__construct();
+
+        $this->futureGroup = $futureGroup;
+
+        foreach ($futureGroup as $key => $future) {
+            if (!$future instanceof Future) {
+                throw new \InvalidArgumentException(
+                    sprintf(
+                        'Future array required at Argument 1: %s provided at index %s',
+                        gettype($future),
+                        $key
+                    )
+                );
+            }
+
+            $isComplete = $future->isComplete();
+
+            if ($isComplete && $this->resolveIndividualFuture($future, $key)) {
+                return;
+            } elseif (!$isComplete) {
+                $future->onComplete(function($future) use ($key) {
+                    $this->resolveIndividualFuture($future, $key);
+                });
+            }
+        }
+    }
+
+    private function resolveIndividualFuture($future, $key) {
+        unset($this->futureGroup[$key]);
+
+        if ($this->isComplete) {
+            return TRUE;
+        } elseif ($future->succeeded()) {
+            $this->resolvedValues[$key] = $future->getValue();
+            return ($this->isComplete = empty($this->futureGroup))
+                ? parent::succeed($this->resolvedValues)
+                : FALSE;
+        } else {
+            parent::fail($future->getError());
+            return ($this->isComplete = TRUE);
+        }
+    }
+}