//! Bounded diagnostic-event history for the browser's live log view.

use std::{collections::VecDeque, time::SystemTime};

use kaptan_log::Level;
use kaptan_mavlink::generated::kaptan::LogLevel;
use serde::{Serialize, Serializer};

const MAX_LOG_ENTRIES: usize = 512;

fn serialize_level<S>(level: &Level, serializer: S) -> Result<S::Ok, S::Error>
where
    S: Serializer,
{
    serializer.serialize_str(level.as_str())
}

/// Decodes a wire event level, rejecting the filter-only Off value.
pub const fn event_level_from_wire(level: LogLevel) -> Option<Level> {
    match level {
        LogLevel::LOG_LEVEL_OFF => None,
        LogLevel::LOG_LEVEL_TRACE => Some(Level::Trace),
        LogLevel::LOG_LEVEL_DEBUG => Some(Level::Debug),
        LogLevel::LOG_LEVEL_INFO => Some(Level::Info),
        LogLevel::LOG_LEVEL_WARN => Some(Level::Warn),
        LogLevel::LOG_LEVEL_ERROR => Some(Level::Error),
    }
}

/// Human-readable packet-view representation.
pub const fn wire_level_label(level: LogLevel) -> &'static str {
    match event_level_from_wire(level) {
        Some(level) => level.as_str(),
        None => "OFF",
    }
}

/// One diagnostic event decoded from a CRC-valid Kaptan MAVLink frame.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct LogObservation {
    pub system_id: u8,
    pub component_id: u8,
    pub level: Level,
    pub truncated: bool,
    pub text: String,
}

/// Read-only browser projection of the current link session's log history.
#[derive(Clone, Debug, Serialize)]
pub struct LogSnapshot {
    pub session: u32,
    pub discarded: u64,
    pub entries: Vec<LogView>,
}

/// One retained diagnostic event, ordered by `id` within a link session.
#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
pub struct LogView {
    pub id: u32,
    pub received_at_ms: u64,
    pub system_id: u8,
    pub component_id: u8,
    #[serde(serialize_with = "serialize_level")]
    pub level: Level,
    pub truncated: bool,
    pub text: String,
}

#[derive(Default)]
pub struct LogMonitor {
    entries: VecDeque<LogView>,
    session: u32,
    next_id: u32,
    discarded: u64,
}

impl LogMonitor {
    /// Retains every event in receive order up to the fixed host-side bound.
    pub fn observe(
        &mut self,
        observations: impl IntoIterator<Item = LogObservation>,
        now: SystemTime,
    ) {
        let received_at_ms = now
            .duration_since(SystemTime::UNIX_EPOCH)
            .unwrap_or_default()
            .as_millis()
            .min(u128::from(u64::MAX)) as u64;
        for observation in observations {
            if self.next_id == u32::MAX {
                self.clear();
            }
            if self.entries.len() == MAX_LOG_ENTRIES {
                self.entries.pop_front();
                self.discarded = self.discarded.saturating_add(1);
            }
            self.entries.push_back(LogView {
                id: self.next_id,
                received_at_ms,
                system_id: observation.system_id,
                component_id: observation.component_id,
                level: observation.level,
                truncated: observation.truncated,
                text: observation.text,
            });
            self.next_id += 1;
        }
    }

    pub fn snapshot(&self) -> LogSnapshot {
        LogSnapshot {
            session: self.session,
            discarded: self.discarded,
            entries: self.entries.iter().cloned().collect(),
        }
    }

    /// Starts a fresh browser history for a new serial-link session.
    pub fn clear(&mut self) {
        self.entries.clear();
        self.session = self.session.wrapping_add(1);
        self.next_id = 0;
        self.discarded = 0;
    }
}

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

    fn observation(index: usize) -> LogObservation {
        LogObservation {
            system_id: 1,
            component_id: 2,
            level: Level::Info,
            truncated: false,
            text: format!("event {index}"),
        }
    }

    #[test]
    fn retains_distinct_events_in_receive_order() {
        let mut monitor = LogMonitor::default();
        let now = SystemTime::UNIX_EPOCH + Duration::from_millis(1_234);
        monitor.observe([observation(0), observation(1)], now);

        let snapshot = monitor.snapshot();
        assert_eq!(snapshot.entries.len(), 2);
        assert_eq!(snapshot.entries[0].id, 0);
        assert_eq!(snapshot.entries[0].received_at_ms, 1_234);
        assert!(!snapshot.entries[0].truncated);
        assert_eq!(snapshot.entries[0].text, "event 0");
        assert_eq!(snapshot.entries[1].id, 1);
        assert_eq!(snapshot.entries[1].text, "event 1");
        assert_eq!(snapshot.discarded, 0);
        assert_eq!(
            serde_json::to_value(&snapshot).unwrap()["entries"][0]["level"],
            "INFO"
        );
    }

    #[test]
    fn drops_only_the_oldest_entries_at_the_fixed_bound() {
        let mut monitor = LogMonitor::default();
        monitor.observe(
            (0..=MAX_LOG_ENTRIES).map(observation),
            SystemTime::UNIX_EPOCH,
        );

        let snapshot = monitor.snapshot();
        assert_eq!(snapshot.entries.len(), MAX_LOG_ENTRIES);
        assert_eq!(snapshot.entries[0].text, "event 1");
        assert_eq!(snapshot.entries[MAX_LOG_ENTRIES - 1].text, "event 512");
        assert_eq!(snapshot.discarded, 1);
    }

    #[test]
    fn clear_starts_a_new_link_session() {
        let mut monitor = LogMonitor::default();
        monitor.observe([observation(0)], SystemTime::UNIX_EPOCH);
        monitor.clear();

        let snapshot = monitor.snapshot();
        assert_eq!(snapshot.session, 1);
        assert!(snapshot.entries.is_empty());
        assert_eq!(snapshot.discarded, 0);
    }

    #[test]
    fn wire_level_accepts_only_event_values() {
        assert_eq!(event_level_from_wire(LogLevel::LOG_LEVEL_OFF), None);
        for (wire, expected) in [
            (LogLevel::LOG_LEVEL_TRACE, Level::Trace),
            (LogLevel::LOG_LEVEL_DEBUG, Level::Debug),
            (LogLevel::LOG_LEVEL_INFO, Level::Info),
            (LogLevel::LOG_LEVEL_WARN, Level::Warn),
            (LogLevel::LOG_LEVEL_ERROR, Level::Error),
        ] {
            assert_eq!(event_level_from_wire(wire), Some(expected));
        }
        assert_eq!(wire_level_label(LogLevel::LOG_LEVEL_TRACE), "TRACE");
        assert_eq!(wire_level_label(LogLevel::LOG_LEVEL_OFF), "OFF");
    }

    #[test]
    fn truncation_state_survives_the_browser_projection() {
        let mut event = observation(0);
        event.truncated = true;
        let mut monitor = LogMonitor::default();
        monitor.observe([event], SystemTime::UNIX_EPOCH);

        let snapshot = monitor.snapshot();
        assert!(snapshot.entries[0].truncated);
        assert_eq!(
            serde_json::to_value(&snapshot).unwrap()["entries"][0]["truncated"],
            true
        );
    }

    #[test]
    fn identifier_exhaustion_starts_a_fresh_rendering_session() {
        let mut monitor = LogMonitor {
            next_id: u32::MAX,
            ..LogMonitor::default()
        };
        monitor.observe([observation(0)], SystemTime::UNIX_EPOCH);

        let snapshot = monitor.snapshot();
        assert_eq!(snapshot.session, 1);
        assert_eq!(snapshot.entries.len(), 1);
        assert_eq!(snapshot.entries[0].id, 0);
        assert_eq!(snapshot.entries[0].text, "event 0");
    }
}