auto merge of rust-lang#7414 : gifnksm/rust/max_by, r=catamorphism

`max_by` method returns the element that gives the maximum value from the specfied function.
`max_by`/`min_by` are convenient when you want to get the value which has greatest/smallest scores.

Inspired by [ruby's Enumerable module](http://ruby-doc.org/core-2.0/Enumerable.html).

src/libstd/iterator.rs

@@ -351,6 +351,26 @@ pub trait IteratorUtil<A> {
 
     /// Count the number of elements satisfying the specified predicate
     fn count(&mut self, predicate: &fn(A) -> bool) -> uint;
+
+    /// Return the element that gives the maximum value from the specfied function
+    ///
+    /// # Example
+    ///
+    /// --- {.rust}
+    /// let xs = [-3, 0, 1, 5, -10];
+    /// assert_eq!(*xs.iter().max_by(|x| x.abs()).unwrap(), -10);
+    /// ---
+    fn max_by<B: Ord>(&mut self, f: &fn(&A) -> B) -> Option<A>;
+
+    /// Return the element that gives the minimum value from the specfied function
+    ///
+    /// # Example
+    ///
+    /// --- {.rust}
+    /// let xs = [-3, 0, 1, 5, -10];
+    /// assert_eq!(*xs.iter().min_by(|x| x.abs()).unwrap(), 0);
+    /// ---
+    fn min_by<B: Ord>(&mut self, f: &fn(&A) -> B) -> Option<A>;
 }
 
 /// Iterator adaptors provided for every `Iterator` implementation. The adaptor objects are also
@@ -519,6 +539,36 @@ impl<A, T: Iterator<A>> IteratorUtil<A> for T {
         }
         i
     }
+
+    #[inline]
+    fn max_by<B: Ord>(&mut self, f: &fn(&A) -> B) -> Option<A> {
+        self.fold(None, |max: Option<(A, B)>, x| {
+            let x_val = f(&x);
+            match max {
+                None             => Some((x, x_val)),
+                Some((y, y_val)) => if x_val > y_val {
+                    Some((x, x_val))
+                } else {
+                    Some((y, y_val))
+                }
+            }
+        }).map_consume(|(x, _)| x)
+    }
+
+    #[inline]
+    fn min_by<B: Ord>(&mut self, f: &fn(&A) -> B) -> Option<A> {
+        self.fold(None, |min: Option<(A, B)>, x| {
+            let x_val = f(&x);
+            match min {
+                None             => Some((x, x_val)),
+                Some((y, y_val)) => if x_val < y_val {
+                    Some((x, x_val))
+                } else {
+                    Some((y, y_val))
+                }
+            }
+        }).map_consume(|(x, _)| x)
+    }
 }
 
 /// A trait for iterators over elements which can be added together
@@ -1237,4 +1287,16 @@ mod tests {
         assert_eq!(xs.iter().count(|x| *x == 5), 1);
         assert_eq!(xs.iter().count(|x| *x == 95), 0);
     }
+
+    #[test]
+    fn test_max_by() {
+        let xs = [-3, 0, 1, 5, -10];
+        assert_eq!(*xs.iter().max_by(|x| x.abs()).unwrap(), -10);
+    }
+
+    #[test]
+    fn test_min_by() {
+        let xs = [-3, 0, 1, 5, -10];
+        assert_eq!(*xs.iter().min_by(|x| x.abs()).unwrap(), 0);
+    }
 }