From cad46e789edf86a7792b640dfc33302c4b8d503e Mon Sep 17 00:00:00 2001
From: Alexander Hirsch <alexander.hirsch@gin.de>
Date: Tue, 29 Oct 2019 13:55:53 +0100
Subject: [PATCH] Add Itertools::filter_map_results

---
 src/adaptors/mod.rs | 65 +++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs          | 19 +++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/src/adaptors/mod.rs b/src/adaptors/mod.rs
index 73d02a75d..9b01f8c82 100644
--- a/src/adaptors/mod.rs
+++ b/src/adaptors/mod.rs
@@ -1184,6 +1184,71 @@ impl<I, F, T, E> Iterator for FilterResults<I, F>
     }
 }
 
+/// An iterator adapter to filter and apply a transformation on values within a nested `Result`.
+///
+/// See [`.filter_map_results()`](../trait.Itertools.html#method.filter_map_results) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct FilterMapResults<I, F> {
+    iter: I,
+    f: F
+}
+
+/// Create a new `FilterResults` iterator.
+pub fn filter_map_results<I, F, T, U, E>(iter: I, f: F) -> FilterMapResults<I, F>
+    where I: Iterator<Item = Result<T, E>>,
+          F: FnMut(T) -> Option<U>,
+{
+    FilterMapResults {
+        iter: iter,
+        f: f,
+    }
+}
+
+impl<I, F, T, U, E> Iterator for FilterMapResults<I, F>
+    where I: Iterator<Item = Result<T, E>>,
+          F: FnMut(T) -> Option<U>,
+{
+    type Item = Result<U, E>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            match self.iter.next() {
+                Some(Ok(v)) => {
+                    if let Some(v) = (self.f)(v) {
+                        return Some(Ok(v));
+                    }
+                },
+                Some(Err(e)) => return Some(Err(e)),
+                None => return None,
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    fn fold<Acc, Fold>(self, init: Acc, mut fold_f: Fold) -> Acc
+        where Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut f = self.f;
+        self.iter.fold(init, move |acc, v| {
+            if let Some(v) = v.map(&mut f).transpose() {
+                fold_f(acc, v)
+            } else {
+                acc
+            }
+        })
+    }
+
+    fn collect<C>(self) -> C
+        where C: FromIterator<Self::Item>
+    {
+        let mut f = self.f;
+        self.iter.filter_map(move |v| v.map(&mut f).transpose()).collect()
+    }
+}
+
 /// An iterator adapter to get the positions of each element that matches a predicate.
 ///
 /// See [`.positions()`](../trait.Itertools.html#method.positions) for more information.
diff --git a/src/lib.rs b/src/lib.rs
index 6686832dc..2d548b9b0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -82,6 +82,7 @@ pub mod structs {
         DedupBy,
         Interleave,
         InterleaveShortest,
+        FilterMapResults,
         FilterResults,
         Product,
         PutBack,
@@ -731,6 +732,24 @@ pub trait Itertools : Iterator {
         adaptors::filter_results(self, f)
     }
 
+    /// Return an iterator adaptor that filters and transforms every
+    /// `Result::Ok` value with the provided closure. `Result::Err`
+    /// values are unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(22), Err(false), Ok(11)];
+    /// let it = input.into_iter().filter_map_results(|i| if i > 20 { Some(i * 2) } else { None });
+    /// itertools::assert_equal(it, vec![Ok(44), Err(false)]);
+    /// ```
+    fn filter_map_results<F, T, U, E>(self, f: F) -> FilterMapResults<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> Option<U>,
+    {
+        adaptors::filter_map_results(self, f)
+    }
+
     /// Return an iterator adaptor that merges the two base iterators in
     /// ascending order.  If both base iterators are sorted (ascending), the
     /// result is sorted.