fix: possible overflow in difficulty calculation (fixes #3923) compan…

…ion (#4097)

Description
Quick fix to overflow problem in difficulty. This problem had already been settled for `big_endian_difficulty`.

Motivation and Context
In #4090, @CjS77 point it out that the `little_endian_difficulty` method had the same problem. This PR is an attempt to tackle it. 

How Has This Been Tested?
Adapted tests from previous PR (#4090)

base_layer/core/src/proof_of_work/difficulty.rs

@@ -132,6 +132,7 @@ pub mod util {
     pub(crate) fn little_endian_difficulty(hash: &[u8]) -> Difficulty {
         let scalar = U256::from_little_endian(hash); // Little endian so the hash has trailing zeroes
         let result = U256::MAX / scalar;
+        let result = result.min(u64::MAX.into());
         result.low_u64().into()
     }
 
@@ -140,7 +141,7 @@ pub mod util {
         use super::*;
 
         #[test]
-        fn high_target() {
+        fn be_high_target() {
             let target: &[u8] = &[
                 0xff, 0xff, 0xff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
                 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
@@ -150,19 +151,47 @@ pub mod util {
         }
 
         #[test]
-        fn max_difficulty() {
+        fn be_max_difficulty() {
             let target = U256::MAX / U256::from(u64::MAX);
             let mut bytes = [0u8; 32];
             target.to_big_endian(&mut bytes);
             assert_eq!(big_endian_difficulty(&bytes), Difficulty::from(u64::MAX));
         }
 
         #[test]
-        fn stop_overflow() {
+        fn be_stop_overflow() {
             let target: u64 = 64;
             let expected = u64::MAX;
             assert_eq!(big_endian_difficulty(&target.to_be_bytes()), Difficulty::from(expected));
         }
+
+        #[test]
+        fn le_high_target() {
+            let target: &[u8] = &[
+                0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+                0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xff, 0xff, 0xff,
+            ];
+            let expected = Difficulty::from(1);
+            assert_eq!(little_endian_difficulty(target), expected);
+        }
+
+        #[test]
+        fn le_max_difficulty() {
+            let target = U256::MAX / U256::from(u64::MAX);
+            let mut bytes = [0u8; 32];
+            target.to_little_endian(&mut bytes);
+            assert_eq!(little_endian_difficulty(&bytes), Difficulty::from(u64::MAX));
+        }
+
+        #[test]
+        fn le_stop_overflow() {
+            let target: u64 = 64;
+            let expected = u64::MAX;
+            assert_eq!(
+                little_endian_difficulty(&target.to_be_bytes()),
+                Difficulty::from(expected)
+            );
+        }
     }
 }