use std::fmt;
use std::time::Duration;

use sea_orm::DatabaseConnection;

use shanty_db::entities::download_queue::DownloadStatus;
use shanty_db::entities::wanted_item::WantedStatus;
use shanty_db::queries;

use crate::backend::{BackendConfig, DownloadBackend, DownloadTarget};
use crate::error::{DlError, DlResult};

/// Statistics from a queue processing run.
#[derive(Debug, Default, Clone)]
pub struct DlStats {
    pub downloads_attempted: u64,
    pub downloads_completed: u64,
    pub downloads_failed: u64,
    pub downloads_skipped: u64,
}

impl fmt::Display for DlStats {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "attempted: {}, completed: {}, failed: {}, skipped: {}",
            self.downloads_attempted,
            self.downloads_completed,
            self.downloads_failed,
            self.downloads_skipped,
        )
    }
}

const MAX_RETRIES: i32 = 3;
const RETRY_DELAYS: [Duration; 3] = [
    Duration::from_secs(30),
    Duration::from_secs(120),
    Duration::from_secs(600),
];

/// Process all pending items in the download queue.
pub async fn run_queue(
    conn: &DatabaseConnection,
    backend: &impl DownloadBackend,
    config: &BackendConfig,
    dry_run: bool,
) -> DlResult<DlStats> {
    let mut stats = DlStats::default();

    loop {
        let item = match queries::downloads::get_next_pending(conn).await? {
            Some(item) => item,
            None => break,
        };

        stats.downloads_attempted += 1;

        tracing::info!(
            id = item.id,
            query = %item.query,
            retry = item.retry_count,
            "processing download"
        );

        if dry_run {
            tracing::info!(id = item.id, query = %item.query, "DRY RUN: would download");
            stats.downloads_skipped += 1;
            // Mark as failed temporarily so we don't loop forever on the same item
            queries::downloads::update_status(
                conn,
                item.id,
                DownloadStatus::Failed,
                Some("dry run"),
            )
            .await?;
            continue;
        }

        // Mark as downloading
        queries::downloads::update_status(conn, item.id, DownloadStatus::Downloading, None)
            .await?;

        // Determine download target
        let target = if let Some(ref url) = item.source_url {
            DownloadTarget::Url(url.clone())
        } else {
            DownloadTarget::Query(item.query.clone())
        };

        // Attempt download
        match backend.download(&target, config).await {
            Ok(result) => {
                tracing::info!(
                    id = item.id,
                    path = %result.file_path.display(),
                    title = %result.title,
                    "download completed"
                );

                queries::downloads::update_status(
                    conn,
                    item.id,
                    DownloadStatus::Completed,
                    None,
                )
                .await?;

                // Update wanted item status if linked
                if let Some(wanted_id) = item.wanted_item_id {
                    if let Err(e) = queries::wanted::update_status(
                        conn,
                        wanted_id,
                        WantedStatus::Downloaded,
                    )
                    .await
                    {
                        tracing::warn!(
                            wanted_id = wanted_id,
                            error = %e,
                            "failed to update wanted item status"
                        );
                    }
                }

                stats.downloads_completed += 1;
            }
            Err(e) => {
                let error_msg = e.to_string();
                tracing::error!(
                    id = item.id,
                    query = %item.query,
                    error = %error_msg,
                    "download failed"
                );

                queries::downloads::update_status(
                    conn,
                    item.id,
                    DownloadStatus::Failed,
                    Some(&error_msg),
                )
                .await?;

                // Auto-retry transient errors
                if e.is_transient() && item.retry_count < MAX_RETRIES {
                    let delay_idx = item.retry_count.min(RETRY_DELAYS.len() as i32 - 1) as usize;
                    let delay = RETRY_DELAYS[delay_idx];
                    tracing::info!(
                        id = item.id,
                        retry = item.retry_count + 1,
                        delay_secs = delay.as_secs(),
                        "scheduling retry"
                    );
                    queries::downloads::retry_failed(conn, item.id).await?;

                    // If rate limited, pause before continuing
                    if matches!(e, DlError::RateLimited(_)) {
                        tracing::warn!("rate limited — pausing queue processing");
                        tokio::time::sleep(delay).await;
                    }
                }

                stats.downloads_failed += 1;
            }
        }
    }

    tracing::info!(%stats, "queue processing complete");
    Ok(stats)
}

/// Download a single item directly (not from queue).
pub async fn download_single(
    backend: &impl DownloadBackend,
    target: DownloadTarget,
    config: &BackendConfig,
    dry_run: bool,
) -> DlResult<()> {
    if dry_run {
        match &target {
            DownloadTarget::Url(url) => {
                tracing::info!(url = %url, "DRY RUN: would download URL");
            }
            DownloadTarget::Query(q) => {
                tracing::info!(query = %q, "DRY RUN: would search and download");

                // Still run the search to show what would be downloaded
                let results = backend.search(q).await?;
                if let Some(best) = results.first() {
                    tracing::info!(
                        title = %best.title,
                        artist = ?best.artist,
                        url = %best.url,
                        "would download"
                    );
                } else {
                    tracing::warn!(query = %q, "no results found");
                }
            }
        }
        return Ok(());
    }

    let result = backend.download(&target, config).await?;
    println!(
        "Downloaded: {} → {}",
        result.title,
        result.file_path.display()
    );
    Ok(())
}