From 6306a507e488dfbec6ea472104dddd14b43f59a4 Mon Sep 17 00:00:00 2001
From: philipahlberg <philipahlberg@gmail.com>
Date: Tue, 4 Aug 2020 19:33:57 +0200
Subject: [PATCH] Add implementation

---
 src/index.ts    | 77 +++++++++++++++++++++++++++++++++++++++++++++++--
 tests/index.mjs | 32 ++++++++++++++++++--
 2 files changed, 104 insertions(+), 5 deletions(-)

diff --git a/src/index.ts b/src/index.ts
index 8d9b8a2..57afcee 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,3 +1,76 @@
-export function add(a: number, b: number): number {
-  return a + b;
+/**
+ * Create an AsyncIterable that yields the values
+ * of the given Promises in the order they resolve.
+ * 
+ * @param promises An iterable of Promises.
+ */
+export const createRaceIterable = <T>(promises: Iterable<Promise<T>>): AsyncIterable<T> => {
+  return new RaceIterable(promises);
+};
+
+/**
+ * A simple wrapper to go from AsyncIterable to AsyncIterator.
+ */
+class RaceIterable<T> implements AsyncIterable<T> {
+  private iterable: Iterable<Promise<T>>;
+
+  constructor(iterable: Iterable<Promise<T>>) {
+    this.iterable = iterable;
+  }
+
+  [Symbol.asyncIterator](): AsyncIterator<T> {
+    return new RaceIterator(this.iterable);
+  }
 }
+
+class RaceIterator<T> implements AsyncIterator<T> {
+  private promises: Map<number, Promise<[number, T]>>;
+
+  constructor(iterable: Iterable<Promise<T>>) {
+    // Create a Map from key to Promise
+    const map = new Map<number, Promise<[number, T]>>();
+    // Assign a unique `key` to every Promise
+    for (const [key, promise] of enumerate(iterable)) {
+      // Create a wrapped Promise that resolves to [key, value]
+      const keyedPromise = createKeyedPromise(key, promise);
+      // Insert the wrapped Promise by its key
+      map.set(key, keyedPromise);
+    }
+    this.promises = map;
+  }
+
+  async next(): Promise<IteratorResult<T>> {
+    // If the pool is empty, so is the iterator.
+    if (this.promises.size === 0) {
+      return {
+        done: true,
+        value: undefined,
+      };
+    }
+
+    // Wait for the first Promise to resolve.
+    // Note: `Promise.race` creates a *new* Promise; it is not
+    // actually the Promise that resolved first.
+    const [key, value] = await Promise.race(this.promises.values());
+    // Remove the resolved Promise from the pool.
+    this.promises.delete(key);
+    // Resolve with the resolved value of the original Promise.
+    return {
+      done: false,
+      value,
+    };
+  }
+}
+
+function* enumerate<T>(iterable: Iterable<T>): Iterable<[number, T]> {
+  let i = 0;
+  for (const value of iterable) {
+    yield [i, value];
+    i = i + 1;
+  }
+}
+
+const createKeyedPromise = async <T>(key: number, promise: Promise<T>): Promise<[number, T]> => {
+  const value = await promise;
+  return [key, value];
+};
diff --git a/tests/index.mjs b/tests/index.mjs
index 668c07a..2a21644 100644
--- a/tests/index.mjs
+++ b/tests/index.mjs
@@ -1,6 +1,32 @@
 import { assertEqual } from '@windtunnel/assert';
-import { add } from '../dist/index.mjs';
+import { createRaceIterable } from '../dist/index.mjs';
 
-export function testAdd() {
-  assertEqual(add(1, 2), 3, '1 + 2 = 3');
+const timeout = (ms) => new Promise((resolve) => {
+  setTimeout(resolve, ms);
+});
+
+const collect = async (iterable) => {
+  const values = [];
+  for await (const value of iterable) {
+    values.push(value);
+  }
+  return values;
+};
+
+export async function testRaceIterable() {
+  const d = timeout(400).then(() => 'd');
+  const c = timeout(300).then(() => 'c');
+  const b = timeout(200).then(() => 'b');
+  const a = timeout(100).then(() => 'a');
+
+  const iterable = createRaceIterable([
+    c,
+    a,
+    d,
+    b,
+  ]);
+
+  const values = await collect(iterable);
+
+  assertEqual(values, ['a', 'b', 'c', 'd'], 'should resolve in order from fastest to slowest');
 }