From bc62437fb6832e57b96f76b66307cc4332c241d1 Mon Sep 17 00:00:00 2001
From: Anna Henningsen <anna@addaleax.net>
Date: Fri, 5 May 2017 11:04:20 +0200
Subject: [PATCH] assert: improve deepEqual perf for large input
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Use a Map instead of an array for checking previously found
cyclic references.

This reduces complexity for an array-of-objects case from
O(n²) to O(n·log n).

Fixes: https://github.com/nodejs/node/issues/12842
PR-URL: https://github.com/nodejs/node/pull/12849
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
---
 lib/assert.js | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/lib/assert.js b/lib/assert.js
index 28eb2fcf20cd8f..a1631ff2e6f428 100644
--- a/lib/assert.js
+++ b/lib/assert.js
@@ -285,15 +285,24 @@ function _deepEqual(actual, expected, strict, memos) {
   // Note: this accounts for both named and indexed properties on Arrays.
 
   // Use memos to handle cycles.
-  memos = memos || { actual: [], expected: [] };
-  const actualIndex = memos.actual.indexOf(actual);
-  if (actualIndex !== -1) {
-    if (actualIndex === memos.expected.indexOf(expected)) {
+  if (!memos) {
+    memos = {
+      actual: { map: new Map(), position: 0 },
+      expected: { map: new Map(), position: 0 }
+    };
+  }
+
+  const actualPosition = memos.actual.map.get(actual);
+  if (actualPosition !== undefined) {
+    if (actualPosition === memos.expected.map.get(expected)) {
       return true;
     }
+  } else {
+    memos.actual.map.set(actual, memos.actual.position++);
+  }
+  if (!memos.expected.map.has(expected)) {
+    memos.expected.map.set(expected, memos.expected.position++);
   }
-  memos.actual.push(actual);
-  memos.expected.push(expected);
 
   return objEquiv(actual, expected, strict, memos);
 }