From c33a0d0a9098f8766cd34f524ec4ee79a84f8af4 Mon Sep 17 00:00:00 2001
From: David Barri <japgolly@gmail.com>
Date: Mon, 27 Apr 2020 11:39:38 +1000
Subject: [PATCH] Optimise IndexedSeqOps.fold{Left,Right}

Inherited foldRight implementation calls reverse.foldLeft
---
 src/library/scala/collection/IndexedSeq.scala | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/src/library/scala/collection/IndexedSeq.scala b/src/library/scala/collection/IndexedSeq.scala
index bfd85aec6262..a52eba3e3d2f 100644
--- a/src/library/scala/collection/IndexedSeq.scala
+++ b/src/library/scala/collection/IndexedSeq.scala
@@ -56,6 +56,32 @@ trait IndexedSeqOps[+A, +CC[_], +C] extends Any with SeqOps[A, CC, C] { self =>
       } else Iterator.empty.next()
   }
 
+  override def foldLeft[B](z: B)(f: (B, A) => B): B = {
+    // For Vectors with sizes of [100, 1000, 10000] this is [1.2, 1.6, 1.0]x as fast
+    // as using while+iterator.
+    var b = z
+    var i = 0
+    while (i < length) {
+      val a = self(i)
+      b = f(b, a)
+      i += 1
+    }
+    b
+  }
+
+  override def foldRight[B](z: B)(f: (A, B) => B): B = {
+    // This is 0.28% slower at size=100, 0.8% faster at size=1000, and 2.4% faster at size=10000,
+    // than using while+reverseIterator.
+    var b = z
+    var i = length
+    while (i > 0) {
+      i -= 1
+      val a = self(i)
+      b = f(a, b)
+    }
+    b
+  }
+
   override def view: IndexedSeqView[A] = new IndexedSeqView.Id[A](this)
 
   @deprecated("Use .view.slice(from, until) instead of .view(from, until)", "2.13.0")