Use 'select_nth_unstable' to get median. (#467)

* Use 'select_nth_unstable' to get median.

* fix

* Update comments doc.

oracle/src/default_combine_data.rs

@@ -27,13 +27,13 @@ where
 
 		let count = values.len() as u32;
 		let minimum_count = MinimumCount::get();
-		if count < minimum_count {
+		if count < minimum_count || count == 0 {
 			return prev_value;
 		}
 
-		values.sort_by(|a, b| a.value.cmp(&b.value));
-
-		let median_index = count / 2;
-		Some(values[median_index as usize].clone())
+		let mid_index = count / 2;
+		// Won't panic as `values` ensured not empty.
+		let (_, value, _) = values.select_nth_unstable_by(mid_index as usize, |a, b| a.value.cmp(&b.value));
+		Some(value.clone())
 	}
 }

traits/src/data_provider.rs

@@ -30,10 +30,9 @@ pub fn median<T: Ord + Clone>(mut items: Vec<T>) -> Option<T> {
 
 	let mid_index = items.len() / 2;
 
-	items.sort();
-
-	// Won't panic as guarded items not empty case.
-	Some(items[mid_index as usize].clone())
+	// Won't panic as `items` ensured not empty.
+	let (_, item, _) = items.select_nth_unstable(mid_index);
+	Some(item.clone())
 }
 
 #[macro_export]