From aeb3aba951bca3307a1cbb7d1882437091e8070b Mon Sep 17 00:00:00 2001
From: Ulrik Sverdrup <bluss@users.noreply.github.com>
Date: Wed, 27 Jan 2016 00:35:03 +0100
Subject: [PATCH] collections: Use slices parts in PartialEq for VecDeque

This improves == for VecDeque by using the slice representation.

This will also improve further if codegen for slice comparison improves.

Benchmark run of 1000 u64 elements, comparing for equality (all equal).
Cpu time to compare the vecdeques is reduced to less than 50% of what it
was before.

```
test test_eq_u64       ... bench:  1,885 ns/iter (+/- 163) = 4244 MB/s
test test_eq_new_u64   ... bench:    802 ns/iter (+/- 100) = 9975 MB/s
```
---
 src/libcollections/vec_deque.rs     | 34 ++++++++++++++++++++++++++++-
 src/libcollectionstest/vec_deque.rs | 27 +++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/src/libcollections/vec_deque.rs b/src/libcollections/vec_deque.rs
index f434053390e54..f34fe2da7e84b 100644
--- a/src/libcollections/vec_deque.rs
+++ b/src/libcollections/vec_deque.rs
@@ -1968,7 +1968,39 @@ impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: PartialEq> PartialEq for VecDeque<A> {
     fn eq(&self, other: &VecDeque<A>) -> bool {
-        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a.eq(b))
+        if self.len() != other.len() {
+            return false;
+        }
+        let (sa, sb) = self.as_slices();
+        let (oa, ob) = other.as_slices();
+        if sa.len() == oa.len() {
+            sa == oa && sb == ob
+        } else if sa.len() < oa.len() {
+            // Always divisible in three sections, for example:
+            // self:  [a b c|d e f]
+            // other: [0 1 2 3|4 5]
+            // front = 3, mid = 1,
+            // [a b c] == [0 1 2] && [d] == [3] && [e f] == [4 5]
+            let front = sa.len();
+            let mid = oa.len() - front;
+
+            let (oa_front, oa_mid) = oa.split_at(front);
+            let (sb_mid, sb_back) = sb.split_at(mid);
+            debug_assert_eq!(sa.len(), oa_front.len());
+            debug_assert_eq!(sb_mid.len(), oa_mid.len());
+            debug_assert_eq!(sb_back.len(), ob.len());
+            sa == oa_front && sb_mid == oa_mid && sb_back == ob
+        } else {
+            let front = oa.len();
+            let mid = sa.len() - front;
+
+            let (sa_front, sa_mid) = sa.split_at(front);
+            let (ob_mid, ob_back) = ob.split_at(mid);
+            debug_assert_eq!(sa_front.len(), oa.len());
+            debug_assert_eq!(sa_mid.len(), ob_mid.len());
+            debug_assert_eq!(sb.len(), ob_back.len());
+            sa_front == oa && sa_mid == ob_mid && sb == ob_back
+        }
     }
 }
 
diff --git a/src/libcollectionstest/vec_deque.rs b/src/libcollectionstest/vec_deque.rs
index a50886bfdf303..742205df8d794 100644
--- a/src/libcollectionstest/vec_deque.rs
+++ b/src/libcollectionstest/vec_deque.rs
@@ -624,6 +624,33 @@ fn test_hash_after_rotation() {
     }
 }
 
+#[test]
+fn test_eq_after_rotation() {
+    // test that two deques are equal even if elements are laid out differently
+    let len = 28;
+    let mut ring: VecDeque<i32> = (0..len as i32).collect();
+    let mut shifted = ring.clone();
+    for _ in 0..10 {
+        // shift values 1 step to the right by pop, sub one, push
+        ring.pop_front();
+        for elt in &mut ring {
+            *elt -= 1;
+        }
+        ring.push_back(len - 1);
+    }
+
+    // try every shift
+    for _ in 0..shifted.capacity() {
+        shifted.pop_front();
+        for elt in &mut shifted {
+            *elt -= 1;
+        }
+        shifted.push_back(len - 1);
+        assert_eq!(shifted, ring);
+        assert_eq!(ring, shifted);
+    }
+}
+
 #[test]
 fn test_ord() {
     let x = VecDeque::new();