use eth2::BeaconNodeHttpClient;
use slog::{debug, error, warn, Logger};
use slot_clock::SlotClock;

/// A distance in slots.
const SYNC_TOLERANCE: u64 = 4;

/// Returns `true` if the beacon node is synced and ready for action.
///
/// Returns `false` if:
///
///  - The beacon node is unreachable.
///  - The beacon node indicates that it is syncing **AND** it is more than `SYNC_TOLERANCE` behind
///  the highest known slot.
///
///  The second condition means the even if the beacon node thinks that it's syncing, we'll still
///  try to use it if it's close enough to the head.
pub async fn is_synced<T: SlotClock>(
    beacon_node: &BeaconNodeHttpClient,
    slot_clock: &T,
    log_opt: Option<&Logger>,
) -> bool {
    let resp = match beacon_node.get_node_syncing().await {
        Ok(resp) => resp,
        Err(e) => {
            if let Some(log) = log_opt {
                error!(
                    log,
                    "Unable connect to beacon node";
                    "error" => e.to_string()
                )
            }

            return false;
        }
    };

    let is_synced = !resp.data.is_syncing || (resp.data.sync_distance.as_u64() < SYNC_TOLERANCE);

    if let Some(log) = log_opt {
        if !is_synced {
            debug!(
                log,
                "Beacon node sync status";
                "status" => format!("{:?}", resp),
            );

            warn!(
                log,
                "Beacon node is syncing";
                "msg" => "not receiving new duties",
                "sync_distance" => resp.data.sync_distance.as_u64(),
                "head_slot" => resp.data.head_slot.as_u64(),
            );
        }

        if let Some(local_slot) = slot_clock.now() {
            let remote_slot = resp.data.head_slot + resp.data.sync_distance;
            if remote_slot + 1 < local_slot || local_slot + 1 < remote_slot {
                error!(
                    log,
                    "Time discrepancy with beacon node";
                    "msg" => "check the system time on this host and the beacon node",
                    "beacon_node_slot" => remote_slot,
                    "local_slot" => local_slot,
                );
            }
        }
    }

    is_synced
}