libcore: add an UnfoldrIterator like Haskell's unfoldr

src/libcore/iterator.rs

@@ -261,6 +261,28 @@ impl<A, T: Iterator<A>> Iterator<A> for TakeIterator<T> {
     }
 }
 
+pub struct UnfoldrIterator<'self, A, St> {
+    priv f: &'self fn(&mut St) -> Option<A>,
+    priv state: St
+}
+
+pub impl<'self, A, St> UnfoldrIterator<'self, A, St> {
+    #[inline]
+    fn new(f: &'self fn(&mut St) -> Option<A>, initial_state: St) -> UnfoldrIterator<'self, A, St> {
+        UnfoldrIterator {
+            f: f,
+            state: initial_state
+        }
+    }
+}
+
+impl<'self, A, St> Iterator<A> for UnfoldrIterator<'self, A, St> {
+    #[inline]
+    fn next(&mut self) -> Option<A> {
+        (self.f)(&mut self.state)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -326,4 +348,25 @@ mod tests {
         }
         assert_eq!(i, ys.len());
     }
+
+    #[test]
+    fn test_unfoldr() {
+        fn count(st: &mut uint) -> Option<uint> {
+            if *st < 10 {
+                let ret = Some(*st);
+                *st += 1;
+                ret
+            } else {
+                None
+            }
+        }
+
+        let mut it = UnfoldrIterator::new(count, 0);
+        let mut i = 0;
+        for it.advance |counted| {
+            assert_eq!(counted, i);
+            i += 1;
+        }
+        assert_eq!(i, 10);
+    }
 }