From ed9348ddb2394b53612157183147d1373e6aedb0 Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Mon, 29 Jun 2020 10:32:04 -0500
Subject: [PATCH 1/3] Fix removeLast(_:) performance for non-random-access
 collections

This replaces the `count` comparison precondition with a limited index
offset, which converts the method from O(n) to O(k).
---
 .../public/core/BidirectionalCollection.swift |  9 +++++---
 .../core/RangeReplaceableCollection.swift     | 22 +++++++++++++++----
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/stdlib/public/core/BidirectionalCollection.swift b/stdlib/public/core/BidirectionalCollection.swift
index 98b6cc89029b5..c702e3c5d5530 100644
--- a/stdlib/public/core/BidirectionalCollection.swift
+++ b/stdlib/public/core/BidirectionalCollection.swift
@@ -342,9 +342,12 @@ extension BidirectionalCollection where SubSequence == Self {
   public mutating func removeLast(_ k: Int) {
     if k == 0 { return }
     _precondition(k >= 0, "Number of elements to remove should be non-negative")
-    _precondition(count >= k,
-      "Can't remove more items from a collection than it contains")
-    self = self[startIndex..<index(endIndex, offsetBy: -k)]
+    guard let end = index(endIndex, offsetBy: -k, limitedBy: startIndex)
+    else {
+      _preconditionFailure(
+        "Can't remove more items from a collection than it contains")
+    }
+    self = self[startIndex..<end]
   }
 }
 
diff --git a/stdlib/public/core/RangeReplaceableCollection.swift b/stdlib/public/core/RangeReplaceableCollection.swift
index b2555ad7f107b..add9cb50f8b81 100644
--- a/stdlib/public/core/RangeReplaceableCollection.swift
+++ b/stdlib/public/core/RangeReplaceableCollection.swift
@@ -285,12 +285,17 @@ public protocol RangeReplaceableCollection: Collection
   /// Customization point for `removeLast()`.  Implement this function if you
   /// want to replace the default implementation.
   ///
+  /// The collection must not be empty.
+  ///
   /// - Returns: A non-nil value if the operation was performed.
   mutating func _customRemoveLast() -> Element?
 
   /// Customization point for `removeLast(_:)`.  Implement this function if you
   /// want to replace the default implementation.
   ///
+  /// - Parameter n: The number of elements to remove from the collection.
+  ///   `n` must be greater than or equal to zero and must not exceed the
+  ///   number of elements in the collection.
   /// - Returns: `true` if the operation was performed.
   mutating func _customRemoveLast(_ n: Int) -> Bool
 
@@ -802,7 +807,12 @@ extension RangeReplaceableCollection
 
   @inlinable
   public mutating func _customRemoveLast(_ n: Int) -> Bool {
-    self = self[startIndex..<index(endIndex, offsetBy: -n)]
+    guard let end = index(endIndex, offsetBy: -n, limitedBy: startIndex)
+    else {
+      _preconditionFailure(
+        "Can't remove more items from a collection than it contains")
+    }
+    self = self[startIndex..<end]
     return true
   }
 }
@@ -866,13 +876,17 @@ extension RangeReplaceableCollection where Self: BidirectionalCollection {
   public mutating func removeLast(_ k: Int) {
     if k == 0 { return }
     _precondition(k >= 0, "Number of elements to remove should be non-negative")
-    _precondition(count >= k,
-      "Can't remove more items from a collection than it contains")
     if _customRemoveLast(k) {
       return
     }
     let end = endIndex
-    removeSubrange(index(end, offsetBy: -k)..<end)
+    guard let start = index(end, offsetBy: -k, limitedBy: startIndex)
+    else {
+      _preconditionFailure(
+        "Can't remove more items from a collection than it contains")
+    }
+
+    removeSubrange(start..<end)
   }
 }
 

From 8e0f0ab55e64e5d25f2bb58b305bfabac9cfd06f Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Mon, 29 Jun 2020 13:50:14 -0500
Subject: [PATCH 2/3] Update the other removeLast implementation

---
 stdlib/public/core/RangeReplaceableCollection.swift | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/stdlib/public/core/RangeReplaceableCollection.swift b/stdlib/public/core/RangeReplaceableCollection.swift
index add9cb50f8b81..24e6e4bc91bf5 100644
--- a/stdlib/public/core/RangeReplaceableCollection.swift
+++ b/stdlib/public/core/RangeReplaceableCollection.swift
@@ -950,12 +950,15 @@ where Self: BidirectionalCollection, SubSequence == Self {
   public mutating func removeLast(_ k: Int) {
     if k == 0 { return }
     _precondition(k >= 0, "Number of elements to remove should be non-negative")
-    _precondition(count >= k,
-      "Can't remove more items from a collection than it contains")
     if _customRemoveLast(k) {
       return
     }
     let end = endIndex
+    guard let start = index(end, offsetBy: -k, limitedBy: startIndex)
+    else {
+      _preconditionFailure(
+        "Can't remove more items from a collection than it contains")
+    }
     removeSubrange(index(end, offsetBy: -k)..<end)
   }
 }

From d6a9f66787b22f6c29a958eafd8f8b6918a5788c Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Wed, 1 Jul 2020 00:18:16 -0500
Subject: [PATCH 3/3] Update
 stdlib/public/core/RangeReplaceableCollection.swift

Co-authored-by: Karoy Lorentey <klorentey@apple.com>
---
 stdlib/public/core/RangeReplaceableCollection.swift | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/public/core/RangeReplaceableCollection.swift b/stdlib/public/core/RangeReplaceableCollection.swift
index 24e6e4bc91bf5..6ec6afaea0416 100644
--- a/stdlib/public/core/RangeReplaceableCollection.swift
+++ b/stdlib/public/core/RangeReplaceableCollection.swift
@@ -959,7 +959,7 @@ where Self: BidirectionalCollection, SubSequence == Self {
       _preconditionFailure(
         "Can't remove more items from a collection than it contains")
     }
-    removeSubrange(index(end, offsetBy: -k)..<end)
+    removeSubrange(start..<end)
   }
 }