Add PeekMap and adaptors::peek_map

src/adaptors/map.rs

@@ -1,3 +1,4 @@
+use core::iter::Peekable;
 use std::iter::FromIterator;
 use std::marker::PhantomData;
 
@@ -122,3 +123,41 @@ pub fn map_into<I, R>(iter: I) -> MapInto<I, R> {
         f: MapSpecialCaseFnInto(PhantomData),
     }
 }
+
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct PeekMap<I, F, U>
+where
+    I: Iterator,
+    F: FnMut(I::Item, Option<&I::Item>) -> U,
+{
+    iter: Peekable<I>,
+    f: F,
+}
+
+impl<I, F, U> Iterator for PeekMap<I, F, U>
+where
+    I: Iterator,
+    F: FnMut(I::Item, Option<&I::Item>) -> U,
+{
+    type Item = U;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        Some((self.f)(self.iter.next()?, self.iter.peek()))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+/// Create a new `PeekMap` iterator.
+pub fn peek_map<I, F, U>(iter: I, f: F) -> PeekMap<I, F, U>
+where
+    I: Iterator,
+    F: FnMut(I::Item, Option<&I::Item>) -> U,
+{
+    PeekMap {
+        iter: iter.peekable(),
+        f,
+    }
+}

src/adaptors/mod.rs

@@ -10,7 +10,7 @@ mod multi_product;
 pub use self::coalesce::*;
 #[allow(deprecated)]
 pub use self::map::MapResults;
-pub use self::map::{map_into, map_ok, MapInto, MapOk};
+pub use self::map::{map_into, map_ok, peek_map, MapInto, MapOk, PeekMap};
 #[cfg(feature = "use_alloc")]
 pub use self::multi_product::*;
 

src/lib.rs

@@ -84,8 +84,8 @@ pub mod structs {
     pub use crate::adaptors::MultiProduct;
     pub use crate::adaptors::{
         Batching, Coalesce, Dedup, DedupBy, DedupByWithCount, DedupWithCount, FilterMapOk,
-        FilterOk, Interleave, InterleaveShortest, MapInto, MapOk, Positions, Product, PutBack,
-        TakeWhileRef, TupleCombinations, Update, WhileSome,
+        FilterOk, Interleave, InterleaveShortest, MapInto, MapOk, PeekMap, Positions, Product,
+        PutBack, TakeWhileRef, TupleCombinations, Update, WhileSome,
     };
     #[allow(deprecated)]
     pub use crate::adaptors::{MapResults, Step};
@@ -3964,6 +3964,14 @@ pub trait Itertools: Iterator {
             _ => Err(sh),
         }
     }
+
+    fn peek_map<B, F>(self, f: F) -> PeekMap<Self, F, B>
+    where
+        Self: Sized,
+        F: FnMut(Self::Item, Option<&Self::Item>) -> B,
+    {
+        adaptors::peek_map(self, f)
+    }
 }
 
 impl<T: ?Sized> Itertools for T where T: Iterator {}