Itertools::tail with VecDeque rather than Vec

Author: Philippe-Cholet

src/lib.rs

@@ -55,7 +55,7 @@ extern crate core as std;
 extern crate alloc;
 
 #[cfg(feature = "use_alloc")]
-use alloc::{string::String, vec::Vec};
+use alloc::{collections::VecDeque, string::String, vec::Vec};
 
 pub use either::Either;
 
@@ -72,6 +72,8 @@ use std::fmt::Write;
 use std::hash::Hash;
 use std::iter::{once, IntoIterator};
 #[cfg(feature = "use_alloc")]
+type VecDequeIntoIter<T> = alloc::collections::vec_deque::IntoIter<T>;
+#[cfg(feature = "use_alloc")]
 type VecIntoIter<T> = alloc::vec::IntoIter<T>;
 use std::iter::FromIterator;
 
@@ -3153,32 +3155,25 @@ pub trait Itertools: Iterator {
     /// `.rev().take(n).rev()` to have a similar result (lazy and non-allocating)
     /// without consuming the entire iterator.
     #[cfg(feature = "use_alloc")]
-    fn tail(self, n: usize) -> VecIntoIter<Self::Item>
+    fn tail(self, n: usize) -> VecDequeIntoIter<Self::Item>
     where
         Self: Sized,
     {
         match n {
             0 => {
                 self.last();
-                Vec::new()
+                VecDeque::new()
             }
             1 => self.last().into_iter().collect(),
             _ => {
                 // Skip the starting part of iterator if possible.
                 let (low, _) = self.size_hint();
                 let mut iter = self.fuse().skip(low.saturating_sub(n));
-                let mut data: Vec<_> = iter.by_ref().take(n).collect();
-                // Update `data` cyclically.
-                let idx = iter.fold(0, |i, val| {
-                    data[i] = val;
-                    if i + 1 == n {
-                        0
-                    } else {
-                        i + 1
-                    }
+                let mut data: VecDeque<_> = iter.by_ref().take(n).collect();
+                iter.for_each(|value| {
+                    data.pop_front();
+                    data.push_back(value);
                 });
-                // Respect the insertion order.
-                data.rotate_left(idx);
                 data
             }
         }