Fix mean on empty arrays (#2612)

* Fix mean on empty arrays

* Update code style

* Add empty slice tests

common/src/calc.rs

@@ -9,6 +9,10 @@ pub fn mean<T>(values: &[T]) -> U256
 where
     T: Into<U256> + Copy,
 {
+    if values.is_empty() {
+        return U256::zero()
+    }
+
     values.iter().copied().fold(U256::zero(), |sum, val| sum + val.into()) / values.len()
 }
 
@@ -18,30 +22,44 @@ pub fn median_sorted<T>(values: &[T]) -> T
 where
     T: Add<Output = T> + Div<u64, Output = T> + From<u64> + Copy,
 {
+    if values.is_empty() {
+        return 0u64.into()
+    }
+
     let len = values.len();
-    if len > 0 {
-        let mid = len / 2;
-        if len % 2 == 0 {
-            (values[mid - 1] + values[mid]) / 2u64
-        } else {
-            values[mid]
-        }
+    let mid = len / 2;
+    if len % 2 == 0 {
+        (values[mid - 1] + values[mid]) / 2u64
     } else {
-        0u64.into()
+        values[mid]
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn calc_mean_empty() {
+        let values: [u64; 0] = [];
+        let m = mean(&values);
+        assert_eq!(m, U256::zero());
+    }
+
     #[test]
     fn calc_mean() {
         let values = [0u64, 1u64, 2u64, 3u64, 4u64, 5u64, 6u64];
         let m = mean(&values);
         assert_eq!(m, 3u64.into());
     }
 
+    #[test]
+    fn calc_median_empty() {
+        let values: Vec<u64> = vec![];
+        let m = median_sorted(&values);
+        assert_eq!(m, 0);
+    }
+
     #[test]
     fn calc_median() {
         let mut values = vec![29, 30, 31, 40, 59, 61, 71];