use super::super::common::initiate_validator_exit;
use super::Error;
use itertools::{Either, Itertools};
use types::*;

/// Performs a validator registry update, if required.
///
/// Spec v0.9.1
pub fn process_registry_updates<T: EthSpec>(
    state: &mut BeaconState<T>,
    spec: &ChainSpec,
) -> Result<(), Error> {
    // Process activation eligibility and ejections.
    // Collect eligible and exiting validators (we need to avoid mutating the state while iterating).
    // We assume it's safe to re-order the change in eligibility and `initiate_validator_exit`.
    // Rest assured exiting validators will still be exited in the same order as in the spec.
    let current_epoch = state.current_epoch();
    let is_eligible = |validator: &Validator| {
        validator.activation_eligibility_epoch == spec.far_future_epoch
            && validator.effective_balance == spec.max_effective_balance
    };
    let is_exiting_validator = |validator: &Validator| {
        validator.is_active_at(current_epoch)
            && validator.effective_balance <= spec.ejection_balance
    };
    let (eligible_validators, exiting_validators): (Vec<_>, Vec<_>) = state
        .validators
        .iter()
        .enumerate()
        .filter(|(_, validator)| is_eligible(validator) || is_exiting_validator(validator))
        .partition_map(|(index, validator)| {
            if is_eligible(validator) {
                Either::Left(index)
            } else {
                Either::Right(index)
            }
        });
    for index in eligible_validators {
        state.validators[index].activation_eligibility_epoch = current_epoch;
    }
    for index in exiting_validators {
        initiate_validator_exit(state, index, spec)?;
    }

    // Queue validators eligible for activation and not dequeued for activation prior to finalized epoch
    let activation_queue = state
        .validators
        .iter()
        .enumerate()
        .filter(|(_, validator)| {
            validator.activation_eligibility_epoch != spec.far_future_epoch
                && validator.activation_epoch
                    >= state.compute_activation_exit_epoch(state.finalized_checkpoint.epoch, spec)
        })
        .sorted_by_key(|(_, validator)| validator.activation_eligibility_epoch)
        .map(|(index, _)| index)
        .collect_vec();

    let churn_limit = state.get_churn_limit(spec)? as usize;
    let delayed_activation_epoch = state.compute_activation_exit_epoch(current_epoch, spec);
    for index in activation_queue.into_iter().take(churn_limit) {
        let validator = &mut state.validators[index];
        if validator.activation_epoch == spec.far_future_epoch {
            validator.activation_epoch = delayed_activation_epoch;
        }
    }

    Ok(())
}
