From a49b1d01ead98f91b7237b3d24e482efed5746c0 Mon Sep 17 00:00:00 2001
From: tshemsedinov <timur.shemsedinov@gmail.com>
Date: Tue, 21 Nov 2017 04:09:56 +0200
Subject: [PATCH] Async memoize first implementation

Refs: https://github.com/metarhia/metasync/issues/268
---
 lib/memoize.js  | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 metasync.js     |  3 ++-
 test/memoize.js | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 lib/memoize.js
 create mode 100644 test/memoize.js

diff --git a/lib/memoize.js b/lib/memoize.js
new file mode 100644
index 00000000..823cac0c
--- /dev/null
+++ b/lib/memoize.js
@@ -0,0 +1,49 @@
+'use strict';
+
+const util = require('util');
+
+function Memoized() {
+}
+
+util.inherits(Memoized, Function);
+
+const memoize = (
+  // Create memoized function
+  fn // function, sync or async
+  // Returns: function, memoized
+) => {
+  const cache = new Map();
+
+  const memoized = function(...args) {
+    const callback = args.pop();
+    const key = args[0];
+    const record = cache.get(key);
+    if (record) {
+      callback(record.err, record.data);
+      return;
+    }
+    fn(...args, (err, data) => {
+      cache.set(key, { err, data });
+      callback(err, data);
+    });
+  };
+
+  const fields = {
+    cache,
+    timeout: 0,
+    limit: 0,
+    size: 0,
+    maxSize: 0
+  };
+
+  Object.setPrototypeOf(memoized, Memoized.prototype);
+  return Object.assign(memoized, fields);
+};
+
+Memoized.prototype.clear = function() {
+  this.cache.clear();
+};
+
+module.exports = {
+  memoize,
+};
diff --git a/metasync.js b/metasync.js
index 725e8a87..624b9050 100644
--- a/metasync.js
+++ b/metasync.js
@@ -9,7 +9,8 @@ const submodules = [
   'array', // Array utilities
   'chain', // Process arrays sync and async array in chain
   'collector', // DataCollector and KeyCollector
-  'queue', // Concurrency
+  'queue', // Concurrent queue
+  'memoize', // Async memoization
 ].map(path => require('./lib/' + path));
 
 const flow = submodules[0].flow;
diff --git a/test/memoize.js b/test/memoize.js
new file mode 100644
index 00000000..cc61c189
--- /dev/null
+++ b/test/memoize.js
@@ -0,0 +1,39 @@
+'use strict';
+
+const tap = require('tap');
+const metasync = require('..');
+
+tap.test('memoize', (test) => {
+  const storage = {
+    file1: Buffer.from('file1'),
+    file2: Buffer.from('file2'),
+  };
+
+  const getData = (file, callback) => {
+    process.nextTick(() => {
+      const result = storage[file];
+      if (result) callback(null, result);
+      else callback(new Error('File not found'));
+    });
+  };
+
+  const memoizedGetData = metasync.memoize(getData);
+
+  memoizedGetData('file1', (err, data) => {
+    test.error(err);
+    test.strictSame(data, storage.file1);
+    memoizedGetData('file2', (err, data) => {
+      test.error(err);
+      test.strictSame(data, storage.file2);
+      memoizedGetData('file1', (err, data) => {
+        test.error(err);
+        test.strictSame(data, storage.file1);
+        memoizedGetData('file2', (err, data) => {
+          test.error(err);
+          test.strictSame(data, storage.file2);
+          test.end();
+        });
+      });
+    });
+  });
+});