//! Database access.
//!
//! §8 imposes two structural requirements that this module exists to satisfy:
//!
//! 1. **A single writer connection, serialized through one owner**, with a read
//!    pool alongside. SQLite permits only one writer at a time even in WAL mode;
//!    pointing a multi-connection pool at writes and relying on `busy_timeout`
//!    to sort it out is explicitly rejected by the spec. Here the writer lives
//!    behind a `Mutex`, so contention queues in Rust rather than surfacing as
//!    `SQLITE_BUSY`.
//! 2. **All access behind a thin repository layer** rather than queries
//!    scattered through handlers — this is what keeps the Turso/Postgres options
//!    cheap and localises the serialization in one place.
//!
//! rusqlite is synchronous, so every call is wrapped in `spawn_blocking`: a
//! write that waits on the mutex must never block a Tokio worker thread.

pub mod repo;

use std::sync::{Arc, Mutex};

use anyhow::Context;
use rusqlite::Connection;

/// The whole schema, as portable SQL — it runs unchanged on Postgres, so
/// SQLite-specific forms (`INSERT OR REPLACE`) are avoided in favour of the
/// standard `INSERT ... ON CONFLICT` (§8). Keeping it in one `.sql` file rather
/// than scattered through the repository is what makes that reviewable.
///
/// TRACES: DR-014 | PR-004
const SCHEMA: &str = include_str!("schema.sql");

/// Handle to the database: one serialized writer, plus read connections.
///
/// Cloning is cheap and shares the same underlying connections.
/// TRACES: DR-003 | PR-004
#[derive(Clone)]
pub struct Db {
    writer: Arc<Mutex<Connection>>,
    readers: Arc<ReadPool>,
}

struct ReadPool {
    conns: Mutex<Vec<Connection>>,
    path: String,
}

impl ReadPool {
    fn acquire(&self) -> anyhow::Result<Connection> {
        if let Some(c) = self.conns.lock().expect("read pool poisoned").pop() {
            return Ok(c);
        }
        open_conn(&self.path, false)
    }

    fn release(&self, conn: Connection) {
        let mut conns = self.conns.lock().expect("read pool poisoned");
        // Bounded: excess connections are dropped rather than accumulating.
        if conns.len() < 8 {
            conns.push(conn);
        }
    }
}

fn open_conn(path: &str, writer: bool) -> anyhow::Result<Connection> {
    let conn = Connection::open(path).with_context(|| format!("opening database {path}"))?;

    // WAL gives concurrent readers alongside the single writer, which suits a
    // read-dominated workload; `synchronous = NORMAL` is safe under WAL, and
    // `busy_timeout` makes contention wait rather than error (§8).
    conn.pragma_update(None, "journal_mode", "WAL")?;
    conn.pragma_update(None, "synchronous", "NORMAL")?;
    conn.pragma_update(None, "busy_timeout", 5_000)?;
    conn.pragma_update(None, "foreign_keys", true)?;
    if !writer {
        conn.pragma_update(None, "query_only", true)?;
    }
    Ok(conn)
}

impl Db {
    /// Opens the database, applying the schema. Idempotent — every statement in
    /// `schema.sql` is `IF NOT EXISTS`.
    pub fn open(path: &str) -> anyhow::Result<Self> {
        let writer = open_conn(path, true)?;
        writer.execute_batch(SCHEMA).context("applying schema")?;

        Ok(Self {
            writer: Arc::new(Mutex::new(writer)),
            readers: Arc::new(ReadPool { conns: Mutex::new(Vec::new()), path: path.to_string() }),
        })
    }

    /// Runs `f` against the serialized writer connection on a blocking thread.
    ///
    /// `f` receives a `Transaction`, so a manifest's scene rows go in as one
    /// transaction rather than one per row (§8), and a failure rolls back.
    pub async fn write<T, F>(&self, f: F) -> anyhow::Result<T>
    where
        T: Send + 'static,
        F: FnOnce(&rusqlite::Transaction<'_>) -> anyhow::Result<T> + Send + 'static,
    {
        let writer = self.writer.clone();
        tokio::task::spawn_blocking(move || {
            let mut conn = writer.lock().expect("writer poisoned");
            let tx = conn.transaction()?;
            let out = f(&tx)?;
            tx.commit()?;
            Ok(out)
        })
        .await
        .context("writer task panicked")?
    }

    /// Runs `f` against a read connection on a blocking thread.
    pub async fn read<T, F>(&self, f: F) -> anyhow::Result<T>
    where
        T: Send + 'static,
        F: FnOnce(&Connection) -> anyhow::Result<T> + Send + 'static,
    {
        let readers = self.readers.clone();
        tokio::task::spawn_blocking(move || {
            let conn = readers.acquire()?;
            let out = f(&conn);
            readers.release(conn);
            out
        })
        .await
        .context("reader task panicked")?
    }
}

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

    #[tokio::test]
    async fn schema_applies_and_roundtrips() {
        let db = Db::open(":memory:").unwrap();
        // In-memory databases are per-connection, so only exercise the writer.
        let n = db
            .write(|tx| {
                tx.execute(
                    "INSERT INTO contributors (id, token_hash, created_at) VALUES (?1, ?2, ?3)",
                    rusqlite::params!["c1", "hash", "2026-01-01T00:00:00Z"],
                )?;
                Ok(tx.query_row("SELECT COUNT(*) FROM contributors", [], |r| r.get::<_, i64>(0))?)
            })
            .await
            .unwrap();
        assert_eq!(n, 1);
    }

    #[tokio::test]
    async fn write_rolls_back_on_error() {
        let db = Db::open(":memory:").unwrap();
        let res: anyhow::Result<()> = db
            .write(|tx| {
                tx.execute(
                    "INSERT INTO contributors (id, token_hash, created_at) VALUES ('c1','h','t')",
                    [],
                )?;
                anyhow::bail!("deliberate failure")
            })
            .await;
        assert!(res.is_err());
        let n = db
            .write(|tx| {
                Ok(tx.query_row("SELECT COUNT(*) FROM contributors", [], |r| r.get::<_, i64>(0))?)
            })
            .await
            .unwrap();
        assert_eq!(n, 0, "failed transaction must not persist rows");
    }
}