use super::{PgPool, Uuid};

/// Track a thread (upsert).
#[tracing::instrument(skip_all)]
pub async fn track_thread(
    pool: &PgPool,
    user_id: Uuid,
    thread_id: Uuid,
) -> Result<(), sqlx::Error> {
    sqlx::query!(
        "INSERT INTO tracked_threads (user_id, thread_id)
         VALUES ($1, $2)
         ON CONFLICT (user_id, thread_id) DO NOTHING",
        user_id,
        thread_id,
    )
    .execute(pool)
    .await?;
    Ok(())
}

/// Untrack a thread.
#[tracing::instrument(skip_all)]
pub async fn untrack_thread(
    pool: &PgPool,
    user_id: Uuid,
    thread_id: Uuid,
) -> Result<(), sqlx::Error> {
    sqlx::query!(
        "DELETE FROM tracked_threads WHERE user_id = $1 AND thread_id = $2",
        user_id,
        thread_id,
    )
    .execute(pool)
    .await?;
    Ok(())
}

/// Stop tracking all threads for a user.
#[tracing::instrument(skip_all)]
pub async fn untrack_all(pool: &PgPool, user_id: Uuid) -> Result<(), sqlx::Error> {
    sqlx::query!("DELETE FROM tracked_threads WHERE user_id = $1", user_id)
        .execute(pool)
        .await?;
    Ok(())
}

/// Advance the read position for a tracked thread to `last_post_id`.
///
/// Monotonic: the position only ever moves forward in thread order. The caller
/// (`routes::forum::thread`) bumps to the last post *on the page being viewed*,
/// so an unguarded write would drag the position backward whenever a reader
/// revisits an earlier page, and `list_tracked_threads` would then re-count
/// already-read posts as unread. The two writes are also spawned off the
/// response path, so concurrent views of different pages race; keeping the
/// comparison in SQL means Postgres re-evaluates it against the winning row
/// under read-committed rather than letting the last writer win.
///
/// A `last_post_id` that names no row leaves the position untouched: the
/// scalar subquery yields NULL, the comparison is not true, and the guard
/// fails closed.
#[tracing::instrument(skip_all)]
pub async fn update_read_position(
    pool: &PgPool,
    user_id: Uuid,
    thread_id: Uuid,
    last_post_id: Uuid,
) -> Result<(), sqlx::Error> {
    sqlx::query!(
        "UPDATE tracked_threads SET last_read_post_id = $3
         WHERE user_id = $1 AND thread_id = $2
           AND (last_read_post_id IS NULL
                OR (SELECT np.created_at FROM posts np WHERE np.id = $3)
                   > (SELECT cp.created_at FROM posts cp WHERE cp.id = last_read_post_id))",
        user_id,
        thread_id,
        last_post_id,
    )
    .execute(pool)
    .await?;
    Ok(())
}