//! 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.
//!
//! Nothing in this file runs a job. [`crate::runner`] is the other half — the
//! one that claims from this table, does the work through a handler, and
//! reports back. Worth knowing because for a long time it did not exist: every
//! producer called [`enqueue`] and nothing ever called [`claim_next`], so the
//! table only ever grew.

use rusqlite::{Connection, OptionalExtension};

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. **Retired** — see [`JobKind::RETIRED`].
    ///
    /// Kept so the number stays taken: a catalog written by 0.16.0 or earlier
    /// holds rows of kind 2, and reusing it would hand them to whatever took
    /// its place.
    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,
    /// Detect and embed the faces in one image (FR-CULL-8).
    ///
    /// One job does both, rather than splitting them: the proxy is already
    /// decoded and in memory, and the natural unit of resumable work is one
    /// photograph. Splitting would double the queue's row count for nothing.
    ///
    /// Runs against the proxy tier, never a full decode — a library that has
    /// been browsed has already paid for its proxies, so face indexing adds no
    /// RAW decodes that were not already happening.
    DetectFaces = 8,
}

impl JobKind {
    /// Every kind, so code that has to enumerate them cannot quietly miss one
    /// that was added later. A `match` would catch that; a hand-written array
    /// at each call site would not.
    pub const ALL: [JobKind; 9] = [
        JobKind::ScanFolder,
        JobKind::ExtractMetadata,
        JobKind::Thumbnail,
        JobKind::ReadSidecar,
        JobKind::WriteSidecar,
        JobKind::ContentHash,
        JobKind::FetchPreview,
        JobKind::FetchOriginal,
        JobKind::DetectFaces,
    ];

    /// Kinds that are no longer queued by anything, whose rows are deleted on
    /// sight by [`drop_retired`].
    ///
    /// `Thumbnail` is here because thumbnails are owed by the store, not by
    /// the queue. The grid's worker and the thumbnail sweep both find their
    /// work by asking `ThumbStore` what it lacks, and the store is shared
    /// between devices, so it is the only thing that can say another device
    /// already made one. Up to 0.16.0 every scan enqueued a job per
    /// photograph anyway and no handler ever claimed one: the reference
    /// catalog held 23,582 of them (#73; catalog.md §6.1).
    pub const RETIRED: [JobKind; 1] = [JobKind::Thumbnail];

    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,
            8 => JobKind::DetectFaces,
            _ => 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)
    }

    /// Whether `subject_id` names a row in `images`.
    ///
    /// Every kind but one is per-photograph. `ScanFolder`'s subject is a
    /// *folder*, and the two id spaces are unrelated — so anything that joins
    /// `subject_id` against `images` has to exclude it, or it will read one
    /// table's ids as another's and act on the answer.
    pub fn subject_is_image(self) -> bool {
        !matches!(self, JobKind::ScanFolder)
    }
}

/// 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,
}

impl Priority {
    /// Read back from the stored column.
    ///
    /// An unrecognised value reads as `Background` rather than failing: a
    /// priority is a hint about ordering, and refusing to run a job because
    /// its urgency is spelled oddly would be a worse answer than running it
    /// last.
    fn from_i64(v: i64) -> Self {
        match v {
            2 => Priority::Interactive,
            1 => Priority::Prefetch,
            _ => Priority::Background,
        }
    }
}

/// 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> {
    // Cached: a scan enqueues one per photograph it lists.
    conn.prepare_cached(
        "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",
    )?
    .execute(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.
pub fn claim_next(conn: &Connection, now: i64) -> Result<Option<Job>, CatalogError> {
    claim(conn, now, None)
}

/// Claim the next runnable job of one of `kinds`.
///
/// What lets a runner take only the work it can actually do. A device with no
/// connector must leave `FetchOriginal` rows alone rather than claim them and
/// fail them five times each with backoff; a runner gated onto an unmetered
/// network (FR-NC-6) passes the local kinds only and leaves the transfers
/// where they are. Neither is expressible by filtering *after* a claim,
/// because the claim has already marked the row `Running`.
///
/// An empty list claims nothing, which is the honest reading of "there is
/// nothing this worker can do".
pub fn claim_next_matching(
    conn: &Connection,
    now: i64,
    kinds: &[JobKind],
) -> Result<Option<Job>, CatalogError> {
    if kinds.is_empty() {
        return Ok(None);
    }
    claim(conn, now, Some(kinds))
}

/// The claim, as one statement.
///
/// # Why this is not a transaction around a read and a write
///
/// It used to be, and under two connections that is not safe in the way it
/// looks. A deferred transaction takes a read lock for the `SELECT` and only
/// tries to upgrade at the `UPDATE`; with WAL, a second worker that read the
/// same snapshot gets `SQLITE_BUSY_SNAPSHOT` on its write — an error a busy
/// handler cannot retry away, because the fix is to roll back and start over.
/// So the queue was correct only in the sense that the loser failed loudly.
///
/// `UPDATE ... WHERE id = (SELECT ...) RETURNING` is one statement, so it is
/// one implicit transaction that takes the write lock immediately. Two workers
/// serialise, the loser waits out its busy timeout rather than erroring, and
/// neither can see a row the other is already holding.
fn claim(
    conn: &Connection,
    now: i64,
    kinds: Option<&[JobKind]>,
) -> Result<Option<Job>, CatalogError> {
    // `now` first, then the kinds, matching the order the placeholders appear
    // in the text below.
    let mut args: Vec<i64> = vec![now];
    let filter = match kinds {
        None => String::new(),
        Some(kinds) => {
            // Built from the kind *count*, never from anything a user typed —
            // the same discipline `collections::descendants` uses, since
            // `carray` is not compiled in.
            let placeholders = std::iter::repeat_n("?", kinds.len())
                .collect::<Vec<_>>()
                .join(",");
            args.extend(kinds.iter().map(|k| *k as i64));
            format!(" AND kind IN ({placeholders})")
        }
    };

    let sql = format!(
        "UPDATE jobs
            SET state = 1, attempts = attempts + 1
          WHERE id = (SELECT id
                        FROM jobs
                       WHERE state = 0 AND not_before <= ?{filter}
                       ORDER BY priority DESC, id ASC
                       LIMIT 1)
      RETURNING id, kind, subject_id, priority, attempts, payload"
    );

    let claimed = conn
        .query_row(&sql, rusqlite::params_from_iter(args.iter()), |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)?,
            ))
        })
        .optional()?;

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

    let Some(kind) = JobKind::from_i64(kind) else {
        // A row written by a build that knows a kind this one does not — a
        // downgrade, or a catalog synced from a newer device. Running it as
        // some other kind would be worse than not running it, so it is parked
        // where the next claim will not see it again.
        //
        // Answering `None` understates what is queued for one pass. The
        // alternative is a loop that keeps claiming the same unreadable row.
        abandon(conn, id, &format!("unknown job kind {kind}"))?;
        return Ok(None);
    };

    Ok(Some(Job {
        id,
        kind,
        subject_id,
        priority: Priority::from_i64(priority),
        attempts,
        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 {
        abandon(conn, 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(())
    }
}

/// Give up on a job now, with no further retries.
///
/// For failures a retry cannot fix — the subject is gone, the payload is
/// unreadable, the format is one this build does not know. Walking the whole
/// retry ladder to reach a conclusion the first attempt already reached costs
/// five wakeups and five backoffs per photograph, which on a phone is the
/// difference the user notices.
///
/// The row is kept rather than deleted, because "this file failed and here is
/// why" is something the user is entitled to see (NFR-ARCH-4).
pub fn abandon(conn: &Connection, id: i64, err: &str) -> Result<(), CatalogError> {
    conn.execute(
        "UPDATE jobs SET state = 2, last_error = ?2 WHERE id = ?1",
        rusqlite::params![id, err],
    )?;
    Ok(())
}

/// Put a claimed job back exactly as it was found.
///
/// For a worker that is being stopped rather than a job that is going wrong:
/// the platform revoked the slot, the user left the screen. The attempt the
/// claim consumed is given back, because nothing was learned about the file —
/// without that, five backgroundings in a row would mark good work as failed.
///
/// Guarded on the claim still being *this* claim. There is no owner column, so
/// `attempts` stands in for one: it is bumped by every claim, so the row only
/// still reads `state = 1` with the caller's own attempt number while nobody
/// else has taken it since. A worker that comes back after the recovery pass
/// handed its job to someone else therefore changes nothing, rather than
/// releasing a job another worker is in the middle of.
pub fn release(conn: &Connection, job: &Job) -> Result<(), CatalogError> {
    conn.execute(
        "UPDATE jobs SET state = 0, attempts = max(0, attempts - 1)
          WHERE id = ?1 AND state = 1 AND attempts = ?2",
        rusqlite::params![job.id, job.attempts],
    )?;
    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).
///
/// The attempt the dead claim consumed is deliberately *not* refunded. A job
/// that takes the process down with it is indistinguishable from one that
/// fails, and the attempt counter is the only evidence that survives a death —
/// without it a poison-pill job is reclaimed and re-run forever.
pub fn recover_orphaned(conn: &Connection) -> Result<usize, CatalogError> {
    let n = conn.execute("UPDATE jobs SET state = 0 WHERE state = 1", [])?;
    Ok(n)
}

/// Delete jobs whose photograph is gone.
///
/// Coalescing keeps the table one row per unit of work, but nothing shrinks it
/// when the work stops existing: a library that has been culled carries a
/// job for every photograph deleted since the last time anything
/// looked. Each one would be claimed, run, and failed five times.
///
/// Only kinds whose subject really is an image ([`JobKind::subject_is_image`])
/// are considered — `ScanFolder`'s subject is a folder id, and joining it
/// against `images` would delete jobs by coincidence of numbering.
///
/// There is no foreign key to do this instead. `jobs.subject_id` deliberately
/// references nothing: it means different tables for different kinds, and a
/// constraint that is right for eight of nine kinds is not a constraint.
pub fn reap_orphan_subjects(conn: &Connection) -> Result<usize, CatalogError> {
    let kinds: Vec<i64> = JobKind::ALL
        .iter()
        .filter(|k| k.subject_is_image())
        .map(|k| *k as i64)
        .collect();
    let placeholders = std::iter::repeat_n("?", kinds.len())
        .collect::<Vec<_>>()
        .join(",");

    let n = conn.execute(
        &format!(
            "DELETE FROM jobs
              WHERE subject_id IS NOT NULL
                AND kind IN ({placeholders})
                AND NOT EXISTS (SELECT 1 FROM images WHERE images.id = jobs.subject_id)"
        ),
        rusqlite::params_from_iter(kinds.iter()),
    )?;
    Ok(n)
}

/// Delete every row of a [`JobKind::RETIRED`] kind.
///
/// Not a migration, deliberately. A schema bump makes an older build refuse
/// the synced catalog snapshot, and a device still on 0.16.0 would lose the
/// catalog to save a megabyte. So this runs where the queue is readied —
/// [`crate::runner::recover`], at every open — and has to be cheap when there
/// is nothing to do: `kind` leads the `UNIQUE(kind, subject_id)` index, so an
/// empty answer is one index probe, not a table scan.
///
/// Every open rather than once, because once is not enough: an older build
/// opening the same catalog enqueues them again on its next scan.
///
/// Rows in any state go. Nothing claims these kinds, so none can be running,
/// and a failed one would be a report about work nobody was going to do.
pub fn drop_retired(conn: &Connection) -> Result<usize, CatalogError> {
    let kinds: Vec<i64> = JobKind::RETIRED.iter().map(|k| *k as i64).collect();
    let placeholders = std::iter::repeat_n("?", kinds.len())
        .collect::<Vec<_>>()
        .join(",");
    let n = conn.execute(
        &format!("DELETE FROM jobs WHERE kind IN ({placeholders})"),
        rusqlite::params_from_iter(kinds.iter()),
    )?;
    Ok(n)
}

/// How much is left, by state.
///
/// One query rather than a listing, because the caller is a progress line: a
/// foreground service's notification has to say how much remains without
/// reading a hundred thousand rows to find out (FR-PLAT-AND-4).
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
pub struct Counts {
    /// Claimable now or after a backoff.
    pub pending: usize,
    /// Claimed by someone. After a clean start that is a live worker; before
    /// [`recover_orphaned`] it is a dead one.
    pub running: usize,
    /// Given up on, and kept so the user can see what failed and why.
    pub failed: usize,
}

impl Counts {
    /// Work that is still going to happen.
    pub fn outstanding(&self) -> usize {
        self.pending + self.running
    }
}

/// Count the queue by state.
pub fn counts(conn: &Connection) -> Result<Counts, CatalogError> {
    // `sum` over no rows is NULL, not 0 — an empty queue would otherwise fail
    // to convert rather than counting nothing.
    let (pending, running, failed) = conn.query_row(
        "SELECT sum(state = 0), sum(state = 1), sum(state = 2) FROM jobs",
        [],
        |r| {
            Ok((
                r.get::<_, Option<i64>>(0)?,
                r.get::<_, Option<i64>>(1)?,
                r.get::<_, Option<i64>>(2)?,
            ))
        },
    )?;
    Ok(Counts {
        pending: pending.unwrap_or(0) as usize,
        running: running.unwrap_or(0) as usize,
        failed: failed.unwrap_or(0) as usize,
    })
}

#[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);
    }

    /// An image row, so a job has a subject that exists.
    fn image(c: &Connection, id: i64) {
        c.execute(
            "INSERT INTO roots(id, kind, label) VALUES (1, 'local', '/lib')
             ON CONFLICT DO NOTHING",
            [],
        )
        .unwrap();
        c.execute(
            "INSERT INTO images(id, root_id, source_ref, added_at) VALUES (?1, 1, ?2, 0)",
            rusqlite::params![id, format!("/lib/{id}.CR3")],
        )
        .unwrap();
    }

    #[test]
    fn a_runner_claims_only_the_kinds_it_names() {
        // The property a filtered claim exists for: work this worker cannot do
        // is left untouched — not claimed, not attempted, not failed.
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        enqueue(
            &c,
            JobKind::FetchOriginal,
            Some(1),
            Priority::Interactive,
            None,
        )
        .unwrap();

        // `FetchOriginal` is the higher priority and would be claimed first by
        // an unfiltered claim. It is not this worker's to take.
        let job = claim_next_matching(&c, 0, &[JobKind::Thumbnail])
            .unwrap()
            .expect("the thumbnail is claimable");
        assert_eq!(job.kind, JobKind::Thumbnail);
        assert!(claim_next_matching(&c, 0, &[JobKind::Thumbnail])
            .unwrap()
            .is_none());

        let (state, attempts): (i64, i64) = c
            .query_row(
                "SELECT state, attempts FROM jobs WHERE kind = ?1",
                [JobKind::FetchOriginal as i64],
                |r| Ok((r.get(0)?, r.get(1)?)),
            )
            .unwrap();
        assert_eq!(state, JobState::Pending as i64);
        assert_eq!(attempts, 0);
    }

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

    #[test]
    fn releasing_a_claim_gives_the_attempt_back() {
        // A stopped worker has learned nothing about the file, so the claim it
        // is handing back must cost nothing.
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();

        let job = claim_next(&c, 0).unwrap().unwrap();
        assert_eq!(job.attempts, 1);
        release(&c, &job).unwrap();

        let again = claim_next(&c, 0).unwrap().expect("claimable again at once");
        assert_eq!(again.attempts, 1, "the release refunded the first attempt");
    }

    #[test]
    fn releasing_a_job_someone_else_has_reclaimed_does_nothing() {
        // `attempts` standing in for an owner column. A worker that comes back
        // after the recovery pass handed its job to someone else must not
        // release a claim that is no longer its to release — which would drop
        // the live worker's job back into the queue to be run twice.
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();

        let stale = claim_next(&c, 0).unwrap().unwrap();
        recover_orphaned(&c).unwrap();
        let live = claim_next(&c, 0).unwrap().unwrap();
        assert_eq!(live.attempts, 2);

        release(&c, &stale).unwrap();

        let (state, attempts): (i64, i64) = c
            .query_row("SELECT state, attempts FROM jobs", [], |r| {
                Ok((r.get(0)?, r.get(1)?))
            })
            .unwrap();
        assert_eq!(
            state,
            JobState::Running as i64,
            "the live claim still holds"
        );
        assert_eq!(attempts, 2, "and its attempt was not refunded for it");
    }

    #[test]
    fn abandoning_skips_the_whole_retry_ladder() {
        let c = db();
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        let job = claim_next(&c, 0).unwrap().unwrap();

        abandon(&c, job.id, "not an image this build can read").unwrap();

        assert!(claim_next(&c, 1_000_000).unwrap().is_none());
        let (state, attempts, err): (i64, i64, String) = c
            .query_row("SELECT state, attempts, last_error FROM jobs", [], |r| {
                Ok((r.get(0)?, r.get(1)?, r.get(2)?))
            })
            .unwrap();
        assert_eq!(state, JobState::Failed as i64);
        assert_eq!(attempts, 1, "one attempt, not MAX_ATTEMPTS");
        // Kept, not deleted: the user is entitled to see what failed and why.
        assert!(err.contains("this build can read"));
    }

    #[test]
    fn jobs_for_a_deleted_photograph_are_reaped() {
        // Coalescing keeps the table one row per unit of work; nothing shrank
        // it when the work stopped existing.
        let c = db();
        image(&c, 1);
        image(&c, 2);
        enqueue(&c, JobKind::Thumbnail, Some(1), Priority::Background, None).unwrap();
        enqueue(&c, JobKind::Thumbnail, Some(2), Priority::Background, None).unwrap();
        enqueue(
            &c,
            JobKind::ExtractMetadata,
            Some(2),
            Priority::Background,
            None,
        )
        .unwrap();

        c.execute("DELETE FROM images WHERE id = 2", []).unwrap();
        assert_eq!(reap_orphan_subjects(&c).unwrap(), 2);

        let left: i64 = c
            .query_row("SELECT subject_id FROM jobs", [], |r| r.get(0))
            .unwrap();
        assert_eq!(left, 1);
    }

    #[test]
    fn a_folder_scan_is_not_reaped_by_image_ids() {
        // `ScanFolder`'s subject is a folder. Joining it against `images`
        // would delete it whenever the numbering happened not to collide —
        // which, on a fresh library, is almost always.
        let c = db();
        enqueue(
            &c,
            JobKind::ScanFolder,
            Some(1),
            Priority::Background,
            Some("/lib/2024"),
        )
        .unwrap();

        assert_eq!(reap_orphan_subjects(&c).unwrap(), 0);
        assert!(claim_next(&c, 0).unwrap().is_some());
    }

    #[test]
    fn a_job_kind_from_a_newer_build_is_parked_rather_than_guessed_at() {
        // A catalog synced from a device running a later build. Running an
        // unknown kind as some arbitrary known one is worse than not running
        // it, and the old code silently read every unknown kind as
        // `ExtractMetadata`.
        let c = db();
        c.execute(
            "INSERT INTO jobs(kind, subject_id, priority, state) VALUES (99, 1, 0, 0)",
            [],
        )
        .unwrap();

        assert!(claim_next(&c, 0).unwrap().is_none());

        let (state, err): (i64, String) = c
            .query_row("SELECT state, last_error FROM jobs", [], |r| {
                Ok((r.get(0)?, r.get(1)?))
            })
            .unwrap();
        assert_eq!(state, JobState::Failed as i64);
        assert!(err.contains("99"), "{err}");
    }

    #[test]
    fn counts_say_what_is_left() {
        // What a foreground service's notification is built from: a number,
        // without reading a hundred thousand rows to find it.
        let c = db();
        assert_eq!(counts(&c).unwrap(), Counts::default());

        for id in 1..=3 {
            enqueue(&c, JobKind::Thumbnail, Some(id), Priority::Background, None).unwrap();
        }
        let job = claim_next(&c, 0).unwrap().unwrap();
        abandon(&c, job.id, "nope").unwrap();
        claim_next(&c, 0).unwrap().unwrap();

        let n = counts(&c).unwrap();
        assert_eq!(n.pending, 1);
        assert_eq!(n.running, 1);
        assert_eq!(n.failed, 1);
        assert_eq!(n.outstanding(), 2, "failed work is not outstanding work");
    }

    #[test]
    fn every_kind_is_in_all() {
        // `ALL` is what the reap builds its kind filter from, so a kind added
        // to the enum and forgotten here would quietly stop being reaped.
        for (i, kind) in JobKind::ALL.iter().enumerate() {
            assert_eq!(
                JobKind::from_i64(i as i64),
                Some(*kind),
                "ALL is out of step with the discriminants at {i}"
            );
        }
        assert!(JobKind::from_i64(JobKind::ALL.len() as i64).is_none());
    }

    #[test]
    fn only_a_folder_scan_has_a_non_image_subject() {
        assert!(!JobKind::ScanFolder.subject_is_image());
        for kind in JobKind::ALL.iter().filter(|k| **k != JobKind::ScanFolder) {
            assert!(kind.subject_is_image(), "{kind:?}");
        }
    }

    #[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());
    }
}