Add epoch cache

consensus/state_processing/src/common/base.rs

@@ -1,19 +1,30 @@
 use integer_sqrt::IntegerSquareRoot;
-use safe_arith::SafeArith;
+use safe_arith::{ArithError, SafeArith};
 use types::*;
 
-/// Returns the base reward for some validator.
-pub fn get_base_reward<T: EthSpec>(
-    state: &BeaconState<T>,
-    index: usize,
-    // Should be == get_total_active_balance(state, spec)
-    total_active_balance: u64,
-    spec: &ChainSpec,
-) -> Result<u64, BeaconStateError> {
-    state
-        .get_effective_balance(index)?
-        .safe_mul(spec.base_reward_factor)?
-        .safe_div(total_active_balance.integer_sqrt())?
-        .safe_div(spec.base_rewards_per_epoch)
-        .map_err(Into::into)
+/// This type exists to avoid confusing `total_active_balance` with `sqrt_total_active_balance`,
+/// since they are used in close proximity and the same type (`u64`).
+#[derive(Copy, Clone)]
+pub struct SqrtTotalActiveBalance(u64);
+
+impl SqrtTotalActiveBalance {
+    pub fn new(total_active_balance: u64) -> Self {
+        Self(total_active_balance.integer_sqrt())
+    }
+
+    pub fn as_u64(&self) -> u64 {
+        self.0
+    }
+}
+
+/// Returns the base reward for some validator.
+pub fn get_base_reward(
+    validator_effective_balance: u64,
+    sqrt_total_active_balance: SqrtTotalActiveBalance,
+    spec: &ChainSpec,
+) -> Result<u64, ArithError> {
+    validator_effective_balance
+        .safe_mul(spec.base_reward_factor)?
+        .safe_div(sqrt_total_active_balance.as_u64())?
+        .safe_div(spec.base_rewards_per_epoch)
 }