From be148d7c10bcf0f8e1490f7496a3cd79cd0fef8d Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Sat, 24 Jul 2021 18:19:32 +0200
Subject: [PATCH] Implement Itertools::multiunzip

---
 src/lib.rs        | 25 ++++++++++++++++++++
 src/unziptuple.rs | 58 +++++++++++++++++++++++++++++++++++++++++++++++
 tests/test_std.rs |  6 +++++
 3 files changed, 89 insertions(+)
 create mode 100644 src/unziptuple.rs

diff --git a/src/lib.rs b/src/lib.rs
index 7dd624135..935db388a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -179,6 +179,7 @@ pub use crate::repeatn::repeat_n;
 #[allow(deprecated)]
 pub use crate::sources::{repeat_call, unfold, iterate};
 pub use crate::with_position::Position;
+pub use crate::unziptuple::{multiunzip, MultiUnzip};
 pub use crate::ziptuple::multizip;
 mod adaptors;
 mod either_or_both;
@@ -237,6 +238,7 @@ mod tuple_impl;
 mod duplicates_impl;
 #[cfg(feature = "use_std")]
 mod unique_impl;
+mod unziptuple;
 mod with_position;
 mod zip_eq_impl;
 mod zip_longest;
@@ -3401,6 +3403,29 @@ pub trait Itertools : Iterator {
     {
         self.map(f).counts()
     }
+
+    /// Unzips an iterator over tuples into a tuple of containers.
+    ///
+    /// The first element of each tuple will be put into the first container, the second element into 
+    /// the second, ....
+    ///
+    /// It can be thought of as the reverse operation to [`Itertools::multiunzip`].
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let inputs = vec![(1, 2, 3), (4, 5, 6), (7, 8, 9)];
+    ///
+    /// let (a, b, c): (Vec<_>, Vec<_>, Vec<_>) = multiunzip(inputs);
+    ///
+    /// assert_eq!((a, b, c), (vec![1, 4, 7], vec![2, 7, 8], vec![3, 6, 9]));
+    /// ```
+    fn multiunzip<FromI>(self) -> FromI
+    where
+        Self: Sized + MultiUnzip<FromI>,
+    {
+        MultiUnzip::multiunzip(self)
+    }
 }
 
 impl<T: ?Sized> Itertools for T where T: Iterator { }
diff --git a/src/unziptuple.rs b/src/unziptuple.rs
new file mode 100644
index 000000000..047880bc7
--- /dev/null
+++ b/src/unziptuple.rs
@@ -0,0 +1,58 @@
+/// Unzips an iterator over tuples into a tuple of containers.
+///
+/// ```
+/// use itertools::multiunzip;
+///
+/// let inputs = vec![(1, 2, 3), (4, 5, 6), (7, 8, 9)];
+///
+/// let (a, b, c): (Vec<_>, Vec<_>, Vec<_>) = multiunzip(inputs);
+///
+/// assert_eq!((a, b, c), (vec![1, 4, 7], vec![2, 7, 8], vec![3, 6, 9]));
+/// ```
+pub fn multiunzip<FromI, I>(i: I) -> FromI
+where
+    I: IntoIterator,
+    I::IntoIter: MultiUnzip<FromI>,
+{
+    i.into_iter().multiunzip()
+}
+
+/// An iterator that can be unzipped into multiple collections.
+///
+/// See [`.multiunzip()`](crate::Itertools::multiunzip) for more information.
+pub trait MultiUnzip<FromI>: Iterator {
+    /// Unzip this iterator into multiple collections.
+    fn multiunzip(self) -> FromI;
+}
+
+macro_rules! impl_unzip_iter {
+    ($($T:ident => $FromT:ident),*) => (
+        impl_unzip_iter!(@rec $($T => $FromT,)*);
+    );
+    (@rec) => ();
+    (@rec $__:ident => $___:ident, $($T:ident => $FromT:ident,)*) => (
+        #[allow(non_snake_case)]
+        impl<IT: Iterator<Item = ($($T,)*)>, $($T, $FromT: Default + Extend<$T>),* > MultiUnzip<($($FromT,)*)> for IT {
+            fn multiunzip(self) -> ($($FromT,)*) {
+                let mut res = ($($FromT::default(),)*);
+                let ($($FromT,)*) = &mut res;
+
+                // Still unstable #72631
+                // let (lower_bound, _) = self.size_hint();
+                // if lower_bound > 0 {
+                //     $($FromT.extend_reserve(lower_bound);)*
+                // }
+
+                self.fold((), |(), ($($T,)*)| {
+                    // Still unstable #72631
+                    // $( $FromT.extend_one($T); )*
+                    $( $FromT.extend(std::iter::once($T)); )*
+                });
+                res
+            }
+        }
+        impl_unzip_iter!(@rec $($T => $FromT,)*);
+    );
+}
+
+impl_unzip_iter!(L => FromL, K => FromK, J => FromJ, I => FromI, H => FromH, G => FromG, F => FromF, E => FromE, D => FromD, C => FromC, B => FromB, A => FromA);
diff --git a/tests/test_std.rs b/tests/test_std.rs
index 7cda9b5e5..6230240d3 100644
--- a/tests/test_std.rs
+++ b/tests/test_std.rs
@@ -1080,3 +1080,9 @@ fn exactly_one_question_mark_return() -> Result<(), ExactlyOneError<std::slice::
     [].iter().exactly_one()?;
     Ok(())
 }
+
+#[test]
+fn multiunzip() {
+    let (a, b, c): (Vec<_>, Vec<_>, Vec<_>) = [(0, 1, 2), (3, 4, 5), (6, 7, 8)].iter().cloned().multiunzip();    
+    assert_eq!((a, b, c), (vec![1, 4, 7], vec![2, 7, 8], vec![3, 6, 9]));
+}
\ No newline at end of file