//! The task that tells the cloud what this gateway can see.
//!
//! Its reason for existing is small and specific: adding a printer in the web
//! console should not begin with someone walking to a machine to read its IP
//! address off the screen. It cannot finish the job - the credential is on
//! that screen and no probe can read it - but it removes the address, the
//! serial number and the guesswork about which vendor the thing is.
//!
//! # It does nothing unless asked
//!
//! `discovery.enabled` is false by default, and this task then parks forever
//! rather than looping over a disabled check. The reason is in
//! `DiscoverySettings`: a sweep is unsolicited traffic on someone else's
//! network, and a gateway that did it by default would be scanning networks
//! its operator never meant to point it at.
//!
//! Note which way the switch faces. It is read from the gateway's own config
//! and the cloud cannot set it, because a setting whose entire purpose is to
//! bound what this process does to the local network has to be decided where
//! that network is understood.

use std::time::Duration;

use tokio::sync::mpsc::Sender;

use crate::config::DiscoverySettings;
use crate::uplink::ClientMessage;

/// Scans on a timer and reports each pass, forever.
///
/// Never returns, so it sits in `main`'s `select!` alongside the other
/// long-running tasks. With discovery off it parks on a future that never
/// completes, which costs nothing and keeps the "every arm runs forever"
/// property of that `select!` true.
pub async fn run(settings: DiscoverySettings, printers_path: String, outbound: Sender<ClientMessage>) {
    if !settings.enabled {
        tracing::info!("printer discovery is off (discovery.enabled); not scanning this network");
        std::future::pending::<()>().await;
        return;
    }
    tracing::info!(
        subnets = ?settings.subnets,
        ssdp = settings.ssdp,
        serial = settings.serial,
        every = ?settings.interval,
        "printer discovery is ON; this gateway will probe the network above"
    );

    let mut ticker = tokio::time::interval(settings.interval);
    // A pass takes tens of seconds - an SSDP window alone is 32 - so a tick
    // can come due while one is still running. `Delay` drops the ones that
    // came due rather than firing them back to back afterwards, which is what
    // `Burst` (the default) would do and would turn a slow scan into a
    // continuous one.
    ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);

    loop {
        // Fires immediately the first time, so a gateway reports what it can
        // see shortly after boot rather than one interval later.
        ticker.tick().await;

        // Re-read every pass, not once at startup. Nothing re-reads the fleet
        // file on a running daemon yet, so adding a printer still means a
        // restart - but someone who has *edited* the file and not restarted
        // should not keep being offered the printer they just configured.
        let configured = match crate::fleet::configured(std::path::Path::new(&printers_path)) {
            Ok(configured) => configured,
            // Fail the pass rather than report against an empty set. The
            // alternative offers every printer the farm already runs as a
            // fresh find, which is worse than no report: it is a list of
            // things to add that are already added.
            Err(err) => {
                tracing::warn!(%err, "cannot tell which printers are already configured; skipping this discovery pass");
                continue;
            }
        };

        let scan = super::scan::scan(&settings, &configured).await;
        if !scan.looked() {
            // Every enabled source failed. An empty report would assert that
            // this network has no unconfigured printers, which is a claim
            // about the farm we have no evidence for - see `Scan::looked`.
            tracing::warn!(
                attempted = scan.attempted,
                "every discovery source failed; not reporting, because an empty report would mean 'nothing is there'"
            );
            continue;
        }

        let printers: Vec<_> = scan.candidates.iter().map(super::Candidate::to_wire).collect();
        tracing::info!(count = printers.len(), "reporting discovered printers");
        let message = ClientMessage::DiscoveredPrinters {
            printers,
            scanned_at: now_rfc3339(),
        };

        // `send`, not `try_send`: the queue outlives any one connection, so
        // waiting here is waiting for the cloud to come back, which is
        // exactly the right thing to do with a scan result. If the channel is
        // ever closed the uplink is gone and so is the process.
        if outbound.send(message).await.is_err() {
            tracing::warn!("uplink queue closed; stopping discovery reports");
            return;
        }
    }
}

/// RFC 3339 in UTC, which is what the schema's `date-time` means.
///
/// Public so `examples/discover_printers.rs` can print a frame with a real
/// timestamp in it - a capture is meant to be feedable straight to the
/// backend's validator, and a placeholder there would not be.
pub fn now_rfc3339() -> String {
    let secs = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .unwrap_or(Duration::ZERO)
        .as_secs();
    format_rfc3339(secs)
}

/// Formats a Unix second count, with the clock passed in so it is testable.
///
/// Hand-rolled because this crate has no date library and one field does not
/// justify adding one - `scanned_at` is read by a human looking at a "found
/// 20 minutes ago" label, not by arithmetic.
fn format_rfc3339(secs: u64) -> String {
    let (days, time) = (secs / 86_400, secs % 86_400);
    let (hour, minute, second) = (time / 3600, (time % 3600) / 60, time % 60);

    // Civil date from a day count, by the algorithm in Howard Hinnant's
    // `civil_from_days`: shift the epoch to 0000-03-01 so a leap day lands at
    // the end of the cycle and the month-length pattern repeats cleanly.
    let z = days as i64 + 719_468;
    let era = z.div_euclid(146_097);
    let doe = z.rem_euclid(146_097);
    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
    let mp = (5 * doy + 2) / 153;
    let day = doy - (153 * mp + 2) / 5 + 1;
    let month = if mp < 10 { mp + 3 } else { mp - 9 };
    let year = yoe + era * 400 + i64::from(month <= 2);

    format!("{year:04}-{month:02}-{day:02}T{hour:02}:{minute:02}:{second:02}Z")
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn timestamps_match_what_date_u_prints() {
        // Pinned against `date -u -d @N`: an off-by-one in the civil date
        // arithmetic is invisible until it is a day or a year out.
        assert_eq!(format_rfc3339(0), "1970-01-01T00:00:00Z");
        // A leap day, which is the whole reason this arithmetic needs care.
        assert_eq!(format_rfc3339(1_709_164_800), "2024-02-29T00:00:00Z");
        assert_eq!(format_rfc3339(1_792_413_296), "2026-10-19T12:34:56Z");
        // Past 2100, where the "divisible by 100 is not a leap year" rule
        // bites and a naive every-four-years version drifts.
        assert_eq!(format_rfc3339(4_102_444_800), "2100-01-01T00:00:00Z");
        // The last second of a year, which catches a December/January slip.
        assert_eq!(format_rfc3339(1_767_225_599), "2025-12-31T23:59:59Z");
    }
}