1 files changed, 61 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/diff.rs b/third_party/rust/itertools/src/diff.rs
new file mode 100644
index 0000000000..1731f0639f
--- /dev/null
+++ b/third_party/rust/itertools/src/diff.rs
@@ -0,0 +1,61 @@
+//! "Diff"ing iterators for caching elements to sequential collections without requiring the new
+//! elements' iterator to be `Clone`.
+//!
+//! - [`Diff`] (produced by the [`diff_with`] function)
+//! describes the difference between two non-`Clone` iterators `I` and `J` after breaking ASAP from
+//! a lock-step comparison.
+
+use crate::free::put_back;
+use crate::structs::PutBack;
+
+/// A type returned by the [`diff_with`] function.
+///
+/// `Diff` represents the way in which the elements yielded by the iterator `I` differ to some
+/// iterator `J`.
+pub enum Diff<I, J>
+    where I: Iterator,
+          J: Iterator
+{
+    /// The index of the first non-matching element along with both iterator's remaining elements
+    /// starting with the first mis-match.
+    FirstMismatch(usize, PutBack<I>, PutBack<J>),
+    /// The total number of elements that were in `J` along with the remaining elements of `I`.
+    Shorter(usize, PutBack<I>),
+    /// The total number of elements that were in `I` along with the remaining elements of `J`.
+    Longer(usize, PutBack<J>),
+}
+
+/// Compares every element yielded by both `i` and `j` with the given function in lock-step and
+/// returns a [`Diff`] which describes how `j` differs from `i`.
+///
+/// If the number of elements yielded by `j` is less than the number of elements yielded by `i`,
+/// the number of `j` elements yielded will be returned along with `i`'s remaining elements as
+/// `Diff::Shorter`.
+///
+/// If the two elements of a step differ, the index of those elements along with the remaining
+/// elements of both `i` and `j` are returned as `Diff::FirstMismatch`.
+///
+/// If `i` becomes exhausted before `j` becomes exhausted, the number of elements in `i` along with
+/// the remaining `j` elements will be returned as `Diff::Longer`.
+pub fn diff_with<I, J, F>(i: I, j: J, is_equal: F)
+    -> Option<Diff<I::IntoIter, J::IntoIter>>
+    where I: IntoIterator,
+          J: IntoIterator,
+          F: Fn(&I::Item, &J::Item) -> bool
+{
+    let mut i = i.into_iter();
+    let mut j = j.into_iter();
+    let mut idx = 0;
+    while let Some(i_elem) = i.next() {
+        match j.next() {
+            None => return Some(Diff::Shorter(idx, put_back(i).with_value(i_elem))),
+            Some(j_elem) => if !is_equal(&i_elem, &j_elem) {
+                let remaining_i = put_back(i).with_value(i_elem);
+                let remaining_j = put_back(j).with_value(j_elem);
+                return Some(Diff::FirstMismatch(idx, remaining_i, remaining_j));
+            },
+        }
+        idx += 1;
+    }
+    j.next().map(|j_elem| Diff::Longer(idx, put_back(j).with_value(j_elem)))
+}