libcore: Add IteratorUtil::filter_map method

src/libcore/iterator.rs

@@ -34,6 +34,7 @@ pub trait IteratorUtil<A> {
     // FIXME: #5898: should be called map
     fn transform<'r, B>(self, f: &'r fn(A) -> B) -> MapIterator<'r, A, B, Self>;
     fn filter<'r>(self, predicate: &'r fn(&A) -> bool) -> FilterIterator<'r, A, Self>;
+    fn filter_map<'r,  B>(self, f: &'r fn(A) -> Option<B>) -> FilterMapIterator<'r, A, B, Self>;
     fn enumerate(self) -> EnumerateIterator<Self>;
     fn skip_while<'r>(self, predicate: &'r fn(&A) -> bool) -> SkipWhileIterator<'r, A, Self>;
     fn take_while<'r>(self, predicate: &'r fn(&A) -> bool) -> TakeWhileIterator<'r, A, Self>;
@@ -74,6 +75,11 @@ impl<A, T: Iterator<A>> IteratorUtil<A> for T {
         FilterIterator{iter: self, predicate: predicate}
     }
 
+    #[inline(always)]
+    fn filter_map<'r, B>(self, f: &'r fn(A) -> Option<B>) -> FilterMapIterator<'r, A, B, T> {
+        FilterMapIterator { iter: self, f: f }
+    }
+
     #[inline(always)]
     fn enumerate(self) -> EnumerateIterator<T> {
         EnumerateIterator{iter: self, count: 0}
@@ -213,6 +219,28 @@ impl<'self, A, T: Iterator<A>> Iterator<A> for FilterIterator<'self, A, T> {
     }
 }
 
+pub struct FilterMapIterator<'self, A, B, T> {
+    priv iter: T,
+    priv f: &'self fn(A) -> Option<B>
+}
+
+impl<'self, A, B, T: Iterator<A>> Iterator<B> for FilterMapIterator<'self, A, B, T> {
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        loop {
+            match self.iter.next() {
+                None    => { return None; }
+                Some(a) => {
+                    match (self.f)(a) {
+                        Some(b) => { return Some(b); }
+                        None    => { loop; }
+                    }
+                }
+            }
+        }
+    }
+}
+
 pub struct EnumerateIterator<T> {
     priv iter: T,
     priv count: uint
@@ -432,6 +460,13 @@ mod tests {
         assert_eq!(i, expected.len());
     }
 
+    #[test]
+    fn test_filter_map() {
+        let it  = Counter::new(0u, 1u).take(10)
+            .filter_map(|x: uint| if x.is_even() { Some(x*x) } else { None });
+        assert_eq!(it.to_vec(), ~[0*0, 2*2, 4*4, 6*6, 8*8]);
+    }
+
     #[test]
     fn test_iterator_enumerate() {
         let xs = [0u, 1, 2, 3, 4, 5];