1 files changed, 58 insertions, 0 deletions
diff --git a/src/exactly_one_err.rs b/src/exactly_one_err.rs
new file mode 100644
index 0000000..e4925ff
--- /dev/null
+++ b/src/exactly_one_err.rs
@@ -0,0 +1,58 @@
+use std::iter::ExactSizeIterator;
+
+use crate::size_hint;
+
+/// Iterator returned for the error case of `IterTools::exactly_one()`
+/// This iterator yields exactly the same elements as the input iterator.
+///
+/// During the execution of exactly_one the iterator must be mutated.  This wrapper
+/// effectively "restores" the state of the input iterator when it's handed back.
+///
+/// This is very similar to PutBackN except this iterator only supports 0-2 elements and does not
+/// use a `Vec`.
+#[derive(Debug, Clone)]
+pub struct ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    first_two: (Option<I::Item>, Option<I::Item>),
+    inner: I,
+}
+
+impl<I> ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    /// Creates a new `ExactlyOneErr` iterator.
+    pub(crate) fn new(first_two: (Option<I::Item>, Option<I::Item>), inner: I) -> Self {
+        Self { first_two, inner }
+    }
+}
+
+impl<I> Iterator for ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.first_two
+            .0
+            .take()
+            .or_else(|| self.first_two.1.take())
+            .or_else(|| self.inner.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let mut additional_len = 0;
+        if self.first_two.0.is_some() {
+            additional_len += 1;
+        }
+        if self.first_two.1.is_some() {
+            additional_len += 1;
+        }
+        size_hint::add_scalar(self.inner.size_hint(), additional_len)
+    }
+}
+
+impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}