arithmetic: fix PerThing pow (paritytech#9030)

* arithmetic: add failing test for pow

* arithmetic: fix PerThing::pow

* Revert back to previous optimisations

Co-authored-by: Gav Wood <gavin@parity.io>

primitives/arithmetic/src/per_things.rs

@@ -639,20 +639,20 @@ macro_rules! implement_per_thing {
 		impl Pow<usize> for $name {
 			type Output = Self;
 
-			fn pow(self, exp: usize) -> Self::Output {
+			fn pow(mut self, exp: usize) -> Self::Output {
 				if exp == 0 || self.is_one() {
 					return Self::one()
 				}
+
 				let mut result = self;
 				let mut exp = exp - 1;
 				while exp > 0 && !result.is_zero() {
-					if exp % 2 == 0 {
-						result = result.square();
-						exp /= 2;
-					} else {
+					if exp % 2 != 0 {
 						result = result * self;
 						exp -= 1;
 					}
+					self = self.square();
+					exp /= 2;
 				}
 				result
 			}
@@ -1107,11 +1107,13 @@ macro_rules! implement_per_thing {
 					$name::from_parts($max / 2).square(),
 				);
 
-				// x^3
-				assert_eq!(
-					$name::from_parts($max / 2).saturating_pow(3),
-					$name::from_parts($max / 8),
-				);
+				// x^2 .. x^16
+				for n in 1..=16 {
+					assert_eq!(
+						$name::from_parts($max / 2).saturating_pow(n),
+						$name::from_parts(($max as u128 / 2u128.pow(n as u32)) as $type),
+					);
+				}
 
 				// 0^n == 0
 				assert_eq!(