1 files changed, 78 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/tee.rs b/third_party/rust/itertools/src/tee.rs
new file mode 100644
index 0000000000..ea4752906f
--- /dev/null
+++ b/third_party/rust/itertools/src/tee.rs
@@ -0,0 +1,78 @@
+use super::size_hint;
+
+use std::cell::RefCell;
+use alloc::collections::VecDeque;
+use alloc::rc::Rc;
+
+/// Common buffer object for the two tee halves
+#[derive(Debug)]
+struct TeeBuffer<A, I> {
+    backlog: VecDeque<A>,
+    iter: I,
+    /// The owner field indicates which id should read from the backlog
+    owner: bool,
+}
+
+/// One half of an iterator pair where both return the same elements.
+///
+/// See [`.tee()`](crate::Itertools::tee) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+#[derive(Debug)]
+pub struct Tee<I>
+    where I: Iterator
+{
+    rcbuffer: Rc<RefCell<TeeBuffer<I::Item, I>>>,
+    id: bool,
+}
+
+pub fn new<I>(iter: I) -> (Tee<I>, Tee<I>)
+    where I: Iterator
+{
+    let buffer = TeeBuffer{backlog: VecDeque::new(), iter, owner: false};
+    let t1 = Tee{rcbuffer: Rc::new(RefCell::new(buffer)), id: true};
+    let t2 = Tee{rcbuffer: t1.rcbuffer.clone(), id: false};
+    (t1, t2)
+}
+
+impl<I> Iterator for Tee<I>
+    where I: Iterator,
+          I::Item: Clone
+{
+    type Item = I::Item;
+    fn next(&mut self) -> Option<Self::Item> {
+        // .borrow_mut may fail here -- but only if the user has tied some kind of weird
+        // knot where the iterator refers back to itself.
+        let mut buffer = self.rcbuffer.borrow_mut();
+        if buffer.owner == self.id {
+            match buffer.backlog.pop_front() {
+                None => {}
+                some_elt => return some_elt,
+            }
+        }
+        match buffer.iter.next() {
+            None => None,
+            Some(elt) => {
+                buffer.backlog.push_back(elt.clone());
+                buffer.owner = !self.id;
+                Some(elt)
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let buffer = self.rcbuffer.borrow();
+        let sh = buffer.iter.size_hint();
+
+        if buffer.owner == self.id {
+            let log_len = buffer.backlog.len();
+            size_hint::add_scalar(sh, log_len)
+        } else {
+            sh
+        }
+    }
+}
+
+impl<I> ExactSizeIterator for Tee<I>
+    where I: ExactSizeIterator,
+          I::Item: Clone
+{}