New Itertools::tail

Note that all cases consume the iterator (even when n=0).

It could alternatively return a vector and not `VecIntoIter` but most of our methods do this (and immediately collect to a vector won't reallocate).

I don't think we should write a non-lazy `head` method (`.take(n)` is a lazy one) but it would be easy: `.take(n).collect_vec().into_iter()`.

Author: Philippe-Cholet

src/lib.rs

@@ -3133,6 +3133,53 @@ pub trait Itertools: Iterator {
         self.k_largest_by(k, k_smallest::key_to_cmp(key))
     }
 
+    /// Consumes the iterator and return an iterator of the last `n` elements.
+    ///
+    /// It allocates up to `n` elements.
+    /// The iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::{assert_equal, Itertools};
+    ///
+    /// let v = vec![5, 9, 8, 4, 2, 12, 0];
+    /// assert_equal(v.iter().tail(3), &[2, 12, 0]);
+    /// assert_equal(v.iter().tail(10), &v);
+    ///
+    /// assert_equal((0..100).tail(10), 90..100);
+    /// ```
+    ///
+    /// For double ended iterators without side-effects, you might prefer
+    /// `.rev().take(n).collect_vec().into_iter().rev()`
+    /// to have the same result without consuming the entire iterator.
+    #[cfg(feature = "use_alloc")]
+    fn tail(mut self, n: usize) -> VecIntoIter<Self::Item>
+    where
+        Self: Sized,
+    {
+        match n {
+            0 => {
+                self.last();
+                Vec::new()
+            }
+            1 => self.last().into_iter().collect(),
+            _ => {
+                let mut data: Vec<_> = self.by_ref().take(n).collect();
+                // If the iterator is not exhausted yet, update the data cyclically and
+                // finally rotate the data so that the order of the tail is respected.
+                if data.len() == n {
+                    let idx = self.fold(0, |i, val| {
+                        data[i] = val;
+                        (i + 1) % n
+                    });
+                    data.rotate_left(idx);
+                }
+                data
+            }
+        }
+        .into_iter()
+    }
+
     /// Collect all iterator elements into one of two
     /// partitions. Unlike [`Iterator::partition`], each partition may
     /// have a distinct type.

tests/quick.rs

@@ -1949,4 +1949,9 @@ quickcheck! {
             result_set.is_empty()
         }
     }
+
+    fn tail(v: Vec<i32>, n: u8) -> bool {
+        let n = n as usize;
+        itertools::equal(v.iter().tail(n), &v[v.len().saturating_sub(n)..])
+    }
 }