fix validator voting period computation

crates/governance/src/utils.rs

@@ -8,6 +8,7 @@ use namada_core::chain::Epoch;
 use namada_core::collections::HashMap;
 use namada_core::dec::Dec;
 use namada_core::token;
+use namada_core::uint::Uint;
 use namada_macros::BorshDeserializer;
 #[cfg(feature = "migrations")]
 use namada_migrations::*;
@@ -422,22 +423,29 @@ pub fn compute_proposal_result(
 }
 
 /// Calculate the valid voting window for a validator given proposal epoch
-/// details. The valid window is within 2/3 of the voting period.
-/// NOTE: technically the window can be more generous than 2/3 since the end
-/// epoch is a valid epoch for voting too.
-/// Returns `false` if any arithmetic fails.
+/// details. The valid window is approximately within 2/3 of the voting period.
+/// NOTE: technically the window can be more generous than 2/3, especially for
+/// low `voting_end_epoch - voting_start_epoch`, but it will never be less than
+/// 2/3. Returns `false` if any arithmetic fails.
 pub fn is_valid_validator_voting_period(
     current_epoch: Epoch,
     voting_start_epoch: Epoch,
     voting_end_epoch: Epoch,
 ) -> bool {
     if voting_start_epoch >= voting_end_epoch {
         false
+    } else if voting_end_epoch
+        .0
+        .checked_sub(voting_start_epoch.0)
+        .unwrap() // safe unwrap
+        <= 2
+    {
+        current_epoch == voting_start_epoch
     } else {
         (|| {
-            // From e_cur <= e_start + 2/3 * (e_end - e_start)
+            // From e_cur < e_start + 2/3 * (e_end - e_start)
             let is_within_two_thirds = checked!(
-                current_epoch * 3 <= voting_start_epoch + voting_end_epoch * 2
+                current_epoch * 3 < voting_start_epoch + voting_end_epoch * 2
             )
             .ok()?;
 
@@ -455,13 +463,26 @@ pub fn last_validator_voting_epoch(
     voting_end_epoch: Epoch,
 ) -> Result<Option<Epoch>, arith::Error> {
     if voting_start_epoch >= voting_end_epoch {
-        Ok(None)
+        return Ok(None);
+    }
+
+    #[allow(clippy::arithmetic_side_effects)]
+    let diff_epochs = voting_end_epoch.0 - voting_start_epoch.0;
+
+    if diff_epochs <= 2 {
+        Ok(Some(voting_start_epoch))
     } else {
-        let latest = checked!(
-            voting_start_epoch.0
-                + 2u64 * (voting_end_epoch.0 - voting_start_epoch.0) / 3u64
-        )?;
-        Ok(Some(Epoch(latest)))
+        let diff_epochs = Uint::from_u64(diff_epochs);
+        // ceiling multiplication by 2/3
+        let valid_period =
+            u64::try_from(diff_epochs.frac_mul_ceil(2.into(), 3.into())?).ok();
+
+        if let Some(period) = valid_period {
+            let latest = checked!(voting_start_epoch + period - 1u64)?;
+            Ok(Some(latest))
+        } else {
+            Ok(None)
+        }
     }
 }
 
@@ -1438,6 +1459,7 @@ mod test {
 
     #[test]
     fn test_validator_voting_period() {
+        // Voting period of 2 epochs
         assert!(!is_valid_validator_voting_period(
             0.into(),
             2.into(),
@@ -1448,7 +1470,7 @@ mod test {
             2.into(),
             4.into()
         ));
-        assert!(is_valid_validator_voting_period(
+        assert!(!is_valid_validator_voting_period(
             3.into(),
             2.into(),
             4.into()
@@ -1462,15 +1484,16 @@ mod test {
             last_validator_voting_epoch(2.into(), 4.into())
                 .unwrap()
                 .unwrap(),
-            3.into()
+            2.into()
         );
 
+        // Voting period of 3 epochs
         assert!(is_valid_validator_voting_period(
             3.into(),
             2.into(),
             5.into()
         ));
-        assert!(is_valid_validator_voting_period(
+        assert!(!is_valid_validator_voting_period(
             4.into(),
             2.into(),
             5.into()
@@ -1484,7 +1507,7 @@ mod test {
             last_validator_voting_epoch(2.into(), 5.into())
                 .unwrap()
                 .unwrap(),
-            4.into()
+            3.into()
         );
 
         for end_epoch in 1u64..=20 {