refactor: reuse slots_diff calculation

Author: tejasbadadare

target_chains/solana/programs/pyth-solana-receiver/src/lib.rs

@@ -575,18 +575,14 @@ fn calculate_twap(start_msg: &TwapMessage, end_msg: &TwapMessage) -> Result<(i64
 
     // Calculate down_slots_ratio as an integer between 0 and 1_000_000
     // A value of 1_000_000 means all slots were missed and 0 means no slots were missed.
-    let total_slots = end_msg
-        .publish_slot
-        .checked_sub(start_msg.publish_slot)
-        .ok_or(ReceiverError::TwapCalculationOverflow)?;
     let total_down_slots = end_msg
         .num_down_slots
         .checked_sub(start_msg.num_down_slots)
         .ok_or(ReceiverError::TwapCalculationOverflow)?;
     let down_slots_ratio = total_down_slots
         .checked_mul(1_000_000)
         .ok_or(ReceiverError::TwapCalculationOverflow)?
-        .checked_div(total_slots)
+        .checked_div(slot_diff)
         .ok_or(ReceiverError::TwapCalculationOverflow)?;
     // down_slots_ratio is a number in [0, 1_000_000], so we only need 32 unsigned bits
     let down_slots_ratio =