spec: initiate_validator_exit v0.6.1

Added a new field `exit_cache` to the BeaconState, which caches
the number of validators exiting at each epoch.

eth2/state_processing/src/common/exit.rs

@@ -0,0 +1,39 @@
+use std::cmp::max;
+use types::{BeaconStateError as Error, *};
+
+/// Initiate the exit of the validator of the given `index`.
+///
+/// Spec v0.6.1
+pub fn initiate_validator_exit(
+    state: &mut BeaconState,
+    index: usize,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    if index >= state.validator_registry.len() {
+        return Err(Error::UnknownValidator);
+    }
+
+    // Return if the validator already initiated exit
+    if state.validator_registry[index].exit_epoch != spec.far_future_epoch {
+        return Ok(());
+    }
+
+    // Compute exit queue epoch
+    let delayed_epoch = state.get_delayed_activation_exit_epoch(state.current_epoch(spec), spec);
+    let mut exit_queue_epoch = state
+        .exit_cache
+        .max_epoch()
+        .map_or(delayed_epoch, |epoch| max(epoch, delayed_epoch));
+    let exit_queue_churn = state.exit_cache.get_churn_at(exit_queue_epoch);
+
+    if exit_queue_churn >= state.get_churn_limit(spec)? {
+        exit_queue_epoch += 1;
+    }
+
+    state.exit_cache.record_validator_exit(exit_queue_epoch);
+    state.validator_registry[index].exit_epoch = exit_queue_epoch;
+    state.validator_registry[index].withdrawable_epoch =
+        exit_queue_epoch + spec.min_validator_withdrawability_delay;
+
+    Ok(())
+}