From cfff40e900f5878e62d818985d8049357eccf099 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Thu, 29 Aug 2019 19:24:53 -0700
Subject: [PATCH] Optimize HashRing to avoid hashing for the common
 single-location case

Change-Id: I3423ac8bd5c56db60630257ac04706838a832d36
---
 includes/libs/HashRing.php                   |  6 +++++
 tests/phpunit/includes/libs/HashRingTest.php | 26 ++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/includes/libs/HashRing.php b/includes/libs/HashRing.php
index f8ab6a3d89..94413c2be5 100644
--- a/includes/libs/HashRing.php
+++ b/includes/libs/HashRing.php
@@ -129,6 +129,12 @@ class HashRing implements Serializable {
 			throw new InvalidArgumentException( "Invalid ring source specified." );
 		}
 
+		// Short-circuit for the common single-location case. Note that if there was only one
+		// location and it was ejected from the live ring, getLiveRing() would have error out.
+		if ( count( $this->weightByLocation ) == 1 ) {
+			return ( $limit > 0 ) ? [ $ring[0][self::KEY_LOCATION] ] : [];
+		}
+
 		// Locate the node index for this item's position on the hash ring
 		$itemIndex = $this->findNodeIndexForPosition( $this->getItemPosition( $item ), $ring );
 
diff --git a/tests/phpunit/includes/libs/HashRingTest.php b/tests/phpunit/includes/libs/HashRingTest.php
index 4afe3b56d5..8ddb7c9960 100644
--- a/tests/phpunit/includes/libs/HashRingTest.php
+++ b/tests/phpunit/includes/libs/HashRingTest.php
@@ -24,6 +24,32 @@ class HashRingTest extends PHPUnit\Framework\TestCase {
 		}
 	}
 
+	public function testHashRingSingleLocation() {
+		// SHA-1 based and weighted
+		$ring = new HashRing( [ 's1' => 1 ], 'sha1' );
+
+		$this->assertEquals(
+			[ 's1' => 1 ],
+			$ring->getLocationWeights(),
+			'Normalized location weights'
+		);
+
+		for ( $i = 0; $i < 5; $i++ ) {
+			$this->assertEquals(
+				's1',
+				$ring->getLocation( "hello$i" ),
+				'Items placed at proper locations'
+			);
+			$this->assertEquals(
+				[ 's1' ],
+				$ring->getLocations( "hello$i", 2 ),
+				'Items placed at proper locations'
+			);
+		}
+
+		$this->assertEquals( [], $ring->getLocations( "helloX", 0 ), "Limit of 0" );
+	}
+
 	public function testHashRingMapping() {
 		// SHA-1 based and weighted
 		$ring = new HashRing(
-- 
2.20.1