//! TRACES: FR-CAT-3 | NFR-ARCH-2 | FR-PLAT-AND-3
//! The background work queue.
//!
//! Jobs live in the catalog, so they survive process death — routine on
//! Android rather than exceptional (FR-PLAT-AND-3). Two properties carry the
//! design:
//!
//! - **Coalescing.** `UNIQUE(kind, subject_id)` makes enqueueing idempotent,
//!   so every code path that notices a change can just enqueue and let the
//!   table absorb the redundancy.
//! - **Priority shared with the GPU scheduler** (ARCH §5.3), so one notion of
//!   urgency governs the whole app and visible work always preempts bulk work.

use rusqlite::Connection;

use crate::error::CatalogError;

/// What a job does.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(i64)]
pub enum JobKind {
    /// Recursive incremental scan from a folder (§scan).
    ScanFolder = 0,
    /// Promote an image from stat-only to full EXIF.
    ExtractMetadata = 1,
    /// Build or rebuild a thumbnail.
    Thumbnail = 2,
    /// A sidecar on disk is newer than what the catalog read.
    ReadSidecar = 3,
    /// Flush a local edit to its sidecar. Debounced, never per slider tick.
    WriteSidecar = 4,
    /// Whole-file hash. On demand only — import dedup, reconnect-by-hash.
    ContentHash = 5,
    /// Range-extract an embedded preview from a remote file (FR-NC-3).
    FetchPreview = 6,
    /// Fetch a full original: pinned by rule, or explicitly asked for.
    FetchOriginal = 7,
}

impl JobKind {
    fn from_i64(v: i64) -> Option<Self> {
        Some(match v {
            0 => JobKind::ScanFolder,
            1 => JobKind::ExtractMetadata,
            2 => JobKind::Thumbnail,
            3 => JobKind::ReadSidecar,
            4 => JobKind::WriteSidecar,
            5 => JobKind::ContentHash,
            6 => JobKind::FetchPreview,
            7 => JobKind::FetchOriginal,
            _ => return None,
        })
    }

    /// Whether this job transfers over the network, and so is subject to the
    /// metered-connection and charging constraints in FR-NC-6.
    pub fn is_network(self) -> bool {
        matches!(self, JobKind::FetchPreview | JobKind::FetchOriginal)
    }
}

/// Scheduling class, matching the GPU tile scheduler (ARCH §5.3).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
#[repr(i64)]
pub enum Priority {
    /// Bulk work: metadata sweeps, rule-driven fetches, hashing.
    Background = 0,
    /// Just outside the viewport; the next image in culling.
    Prefetch = 1,
    /// Visible cells, and the image currently open.
    ///
    /// Strictly preempts background work. Without this, scrolling during a
    /// bulk thumbnail pass misses its frame budget — the common case, not an
    /// edge case (NFR-ARCH-2).
    Interactive = 2,
}

/// Lifecycle state.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(i64)]
pub enum JobState {
    Pending = 0,
    Running = 1,
    Failed = 2,
}

/// A job ready to run.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Job {
    pub id: i64,
    pub kind: JobKind,
    pub subject_id: Option<i64>,
    pub priority: Priority,
    pub attempts: i64,
    pub payload: Option<String>,
}

/// Give up after this many attempts and attach the error to the subject.
///
/// One corrupt file must not stall the queue behind endless retries
/// (FR-RAW-4).
pub const MAX_ATTEMPTS: i64 = 5;

/// Backoff before retrying a failed job, in seconds.
///
/// Exponential, capped — a server that is down for an hour should not be
/// retried every second, and a transient decode failure should not wait an
/// hour.
pub fn backoff_seconds(attempts: i64) -> i64 {
    const CAP: i64 = 300;
    match attempts {
        a if a <= 0 => 0,
        a if a >= 9 => CAP,
        a => (1i64 << (a - 1)).min(CAP),
    }
}

/// Enqueue work, coalescing with any identical pending job.
///
/// Re-requesting at a higher priority *promotes* the existing row rather than
/// duplicating it, which is what lets the grid shout "this one is visible now"
/// about a job already queued in the background.
pub fn enqueue(
    conn: &Connection,
    kind: JobKind,
    subject_id: Option<i64>,
    priority: Priority,
    payload: Option<&str>,
) -> Result<(), CatalogError> {
    conn.execute(
        "INSERT INTO jobs(kind, subject_id, priority, state, payload)
         VALUES (?1, ?2, ?3, 0, ?4)
         ON CONFLICT(kind, subject_id) DO UPDATE SET
             priority = max(jobs.priority, excluded.priority),
             -- A job that failed and is being re-requested deserves a fresh
             -- start: the file may well have changed since it failed.
             state      = CASE WHEN jobs.state = 2 THEN 0 ELSE jobs.state END,
             attempts   = CASE WHEN jobs.state = 2 THEN 0 ELSE jobs.attempts END,
             not_before = CASE WHEN jobs.state = 2 THEN 0 ELSE jobs.not_before END",
        rusqlite::params![kind as i64, subject_id, priority as i64, payload],
    )?;
    Ok(())
}

/// Claim the next runnable job, highest priority first.
///
/// `now` is passed rather than read from the clock so backoff is testable.
/// Claiming marks the row `Running` in the same transaction as the read, so
/// two workers cannot take the same job.
pub fn claim_next(conn: &Connection, now: i64) -> Result<Option<Job>, CatalogError> {
    let tx = conn.unchecked_transaction()?;

    let job = tx
        .query_row(
            "SELECT id, kind, subject_id, priority, attempts, payload
               FROM jobs
              WHERE state = 0 AND not_before <= ?1
              ORDER BY priority DESC, id ASC
              LIMIT 1",
            [now],
            |r| {
                Ok((
                    r.get::<_, i64>(0)?,
                    r.get::<_, i64>(1)?,
                    r.get::<_, Option<i64>>(2)?,
                    r.get::<_, i64>(3)?,
                    r.get::<_, i64>(4)?,
                    r.get::<_, Option<String>>(5)?,
                ))
            },
        )
        .ok();

    let Some((id, kind, subject_id, priority, attempts, payload)) = job else {
        return Ok(None);
    };

    tx.execute(
        "UPDATE jobs SET state = 1, attempts = attempts + 1 WHERE id = ?1",
        [id],
    )?;
    tx.commit()?;

    Ok(Some(Job {
        id,
        kind: JobKind::from_i64(kind).unwrap_or(JobKind::ExtractMetadata),
        subject_id,
        priority: match priority {
            2 => Priority::Interactive,
            1 => Priority::Prefetch,
            _ => Priority::Background,
        },
        attempts: attempts + 1,
        payload,
    }))
}

/// Job finished successfully.
pub fn complete(conn: &Connection, id: i64) -> Result<(), CatalogError> {
    conn.execute("DELETE FROM jobs WHERE id = ?1", [id])?;
    Ok(())
}

/// Job failed. Reschedules with backoff, or gives up past [`MAX_ATTEMPTS`].
pub fn fail(conn: &Connection, job: &Job, now: i64, err: &str) -> Result<(), CatalogError> {
    if job.attempts >= MAX_ATTEMPTS {
        conn.execute(
            "UPDATE jobs SET state = 2, last_error = ?2 WHERE id = ?1",
            rusqlite::params![job.id, err],
        )?;
    } else {
        conn.execute(
            "UPDATE jobs SET state = 0, not_before = ?2, last_error = ?3 WHERE id = ?1",
            rusqlite::params![job.id, now + backoff_seconds(job.attempts), err],
        )?;
    }
    Ok(())
}

/// Recover jobs orphaned by process death.
///
/// A row left `Running` has no owner — the process that claimed it is gone.
/// Called at startup, before any worker begins (FR-PLAT-AND-3).
pub fn recover_orphaned(conn: &Connection) -> Result<usize, CatalogError> {
    let n = conn.execute("UPDATE jobs SET state = 0 WHERE state = 1", [])?;
    Ok(n)
}

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

    fn db() -> Connection {
        let c = Connection::open_in_memory().unwrap();
        schema::configure(&c).unwrap();
        schema::migrate(&c).unwrap();
        c
    }

    #[test]
    fn repeated_enqueue_coalesces() {
        let c = db();
        for _ in 0..10 {
            enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        }
        let n: i64 = c
            .query_row("SELECT count(*) FROM jobs", [], |r| r.get(0))
            .unwrap();
        assert_eq!(n, 1);
    }

    #[test]
    fn re_enqueueing_at_higher_priority_promotes() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        // The grid scrolls this image into view.
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Interactive, None).unwrap();

        let p: i64 = c
            .query_row("SELECT priority FROM jobs", [], |r| r.get(0))
            .unwrap();
        assert_eq!(p, Priority::Interactive as i64);
    }

    #[test]
    fn priority_never_regresses() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Interactive, None).unwrap();
        // A background sweep must not demote work the user is waiting on.
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();

        let p: i64 = c
            .query_row("SELECT priority FROM jobs", [], |r| r.get(0))
            .unwrap();
        assert_eq!(p, Priority::Interactive as i64);
    }

    #[test]
    fn claim_takes_highest_priority_first() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        enqueue(&c, JobKind::Thumbnail, Some(2), Priority::Interactive, None).unwrap();
        enqueue(&c, JobKind::Thumbnail, Some(3), Priority::Prefetch, None).unwrap();

        let first = claim_next(&c, 0).unwrap().unwrap();
        assert_eq!(first.subject_id, Some(2));
        let second = claim_next(&c, 0).unwrap().unwrap();
        assert_eq!(second.subject_id, Some(3));
    }

    #[test]
    fn a_claimed_job_is_not_claimed_twice() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        assert!(claim_next(&c, 0).unwrap().is_some());
        assert!(claim_next(&c, 0).unwrap().is_none());
    }

    #[test]
    fn failure_backs_off_then_becomes_claimable_again() {
        let c = db();
        enqueue(
            &c,
            JobKind::FetchPreview,
            Some(1),
            Priority::Background,
            None,
        )
        .unwrap();
        let job = claim_next(&c, 100).unwrap().unwrap();
        fail(&c, &job, 100, "network down").unwrap();

        // Still backing off.
        assert!(claim_next(&c, 100).unwrap().is_none());
        // Past the backoff.
        assert!(claim_next(&c, 100 + backoff_seconds(job.attempts))
            .unwrap()
            .is_some());
    }

    #[test]
    fn a_persistently_failing_job_stops_retrying() {
        let c = db();
        enqueue(
            &c,
            JobKind::ExtractMetadata,
            Some(1),
            Priority::Background,
            None,
        )
        .unwrap();

        let mut now = 0;
        for _ in 0..MAX_ATTEMPTS {
            let job = claim_next(&c, now).unwrap().expect("should be claimable");
            fail(&c, &job, now, "corrupt file").unwrap();
            now += backoff_seconds(job.attempts);
        }

        // One corrupt file must not stall the queue forever (FR-RAW-4).
        assert!(claim_next(&c, now + 100_000).unwrap().is_none());
        let state: i64 = c
            .query_row("SELECT state FROM jobs", [], |r| r.get(0))
            .unwrap();
        assert_eq!(state, JobState::Failed as i64);
    }

    #[test]
    fn re_requesting_a_failed_job_gives_it_a_fresh_start() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        let mut now = 0;
        for _ in 0..MAX_ATTEMPTS {
            let job = claim_next(&c, now).unwrap().unwrap();
            fail(&c, &job, now, "boom").unwrap();
            now += backoff_seconds(job.attempts);
        }
        // The file changed on disk, so the old failure says nothing about it.
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Interactive, None).unwrap();
        let job = claim_next(&c, now).unwrap().expect("retryable again");
        assert_eq!(job.attempts, 1);
    }

    #[test]
    fn orphaned_jobs_return_to_pending_on_restart() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        claim_next(&c, 0).unwrap().unwrap();
        // Process dies here. Android does this routinely.
        assert_eq!(recover_orphaned(&c).unwrap(), 1);
        assert!(claim_next(&c, 0).unwrap().is_some());
    }

    #[test]
    fn backoff_grows_then_caps() {
        assert_eq!(backoff_seconds(0), 0);
        assert_eq!(backoff_seconds(1), 1);
        assert_eq!(backoff_seconds(3), 4);
        assert_eq!(backoff_seconds(100), 300);
    }

    #[test]
    fn network_jobs_are_identifiable_for_metered_gating() {
        // FR-NC-6: transfers respect unmetered-network and charging
        // constraints; local work must not be gated by them.
        assert!(JobKind::FetchOriginal.is_network());
        assert!(JobKind::FetchPreview.is_network());
        assert!(!JobKind::Thumbnail.is_network());
        assert!(!JobKind::ExtractMetadata.is_network());
    }
}