use std::{
    collections::{HashMap, VecDeque},
    net::SocketAddr,
    path::Path,
    sync::Arc,
};

use futures::{StreamExt, stream::FuturesUnordered};
use tokio_util::sync::CancellationToken;

use super::{
    planning::{ChunkDownloadResult, DownloadChunk, PeerDownloadPlan, resolve_file_peers},
    progress::DownloadProgressTracker,
    transport::download_from_peer,
    version_ini::VersionIniBuffer,
};
use crate::config::MAX_RETRY_COUNT;

/// Selects a peer for retrying a failed chunk.
fn select_retry_peer(peers: &[SocketAddr], last_peer: Option<SocketAddr>) -> Option<SocketAddr> {
    if peers.is_empty() {
        return None;
    }

    if peers.len() > 1
        && let Some(last) = last_peer
        && let Some(pos) = peers.iter().position(|addr| *addr == last)
    {
        let next_index = (pos + 1) % peers.len();
        return Some(peers[next_index]);
    }

    peers.first().copied()
}

/// Returns a fallback peer address for error reporting.
fn fallback_peer_addr(peers: &[SocketAddr], last_peer: Option<SocketAddr>) -> SocketAddr {
    last_peer
        .or_else(|| peers.first().copied())
        .unwrap_or_else(|| SocketAddr::from(([0, 0, 0, 0], 0)))
}

fn ensure_not_cancelled(cancel_token: &CancellationToken, game_id: &str) -> eyre::Result<()> {
    if cancel_token.is_cancelled() {
        eyre::bail!("download cancelled for game {game_id}");
    }
    Ok(())
}

struct RetryAttempt {
    peer_addr: SocketAddr,
    chunks: Vec<DownloadChunk>,
    result: eyre::Result<Vec<ChunkDownloadResult>>,
}

pub(super) struct RetryContext<'a> {
    pub(super) peers: &'a [SocketAddr],
    pub(super) base_dir: &'a Path,
    pub(super) game_id: &'a str,
    pub(super) file_peer_map: &'a HashMap<String, Vec<SocketAddr>>,
    pub(super) cancel_token: &'a CancellationToken,
    pub(super) version_buffer: Option<Arc<VersionIniBuffer>>,
    pub(super) progress_tracker: Arc<DownloadProgressTracker>,
}

fn plan_retry_batch(
    queue: &mut VecDeque<DownloadChunk>,
    peers: &[SocketAddr],
    file_peer_map: &HashMap<String, Vec<SocketAddr>>,
    final_results: &mut Vec<ChunkDownloadResult>,
) -> HashMap<SocketAddr, PeerDownloadPlan> {
    let mut retry_plans: HashMap<SocketAddr, PeerDownloadPlan> = HashMap::new();

    while let Some(mut chunk) = queue.pop_front() {
        let eligible_peers = resolve_file_peers(&chunk.relative_path, file_peer_map, peers);

        if chunk.retry_count >= MAX_RETRY_COUNT {
            final_results.push(ChunkDownloadResult {
                chunk: chunk.clone(),
                result: Err(eyre::eyre!(
                    "Retry budget exhausted for chunk: {}",
                    chunk.relative_path
                )),
                peer_addr: fallback_peer_addr(eligible_peers, chunk.last_peer),
            });
            continue;
        }

        let Some(peer_addr) = select_retry_peer(eligible_peers, chunk.last_peer) else {
            final_results.push(ChunkDownloadResult {
                chunk: chunk.clone(),
                result: Err(eyre::eyre!(
                    "No peers available to retry chunk: {}",
                    chunk.relative_path
                )),
                peer_addr: fallback_peer_addr(eligible_peers, chunk.last_peer),
            });
            continue;
        };

        chunk.last_peer = Some(peer_addr);
        retry_plans.entry(peer_addr).or_default().chunks.push(chunk);
    }

    retry_plans
}

async fn run_retry_batch(
    retry_plans: HashMap<SocketAddr, PeerDownloadPlan>,
    ctx: &RetryContext<'_>,
) -> eyre::Result<Vec<RetryAttempt>> {
    let mut attempts = FuturesUnordered::new();

    for (peer_addr, plan) in retry_plans {
        let retry_chunks = plan.chunks.clone();
        let base_dir = ctx.base_dir.to_path_buf();
        let game_id = ctx.game_id.to_string();
        let cancel_token = ctx.cancel_token.clone();
        let version_buffer = ctx.version_buffer.clone();
        let progress_tracker = ctx.progress_tracker.clone();

        attempts.push(async move {
            let result = download_from_peer(
                peer_addr,
                &game_id,
                plan,
                base_dir,
                &cancel_token,
                version_buffer,
                progress_tracker,
            )
            .await;
            RetryAttempt {
                peer_addr,
                chunks: retry_chunks,
                result,
            }
        });
    }

    let mut results = Vec::new();
    while !attempts.is_empty() {
        let result = tokio::select! {
            () = ctx.cancel_token.cancelled() => {
                eyre::bail!("download cancelled for game {}", ctx.game_id);
            }
            result = attempts.next() => result.expect("retry attempt should exist"),
        };
        results.push(result);
    }

    Ok(results)
}

fn handle_retry_chunk_result(
    result: ChunkDownloadResult,
    queue: &mut VecDeque<DownloadChunk>,
    final_results: &mut Vec<ChunkDownloadResult>,
) {
    let ChunkDownloadResult {
        mut chunk,
        result,
        peer_addr,
    } = result;

    match result {
        Ok(()) => final_results.push(ChunkDownloadResult {
            chunk,
            result: Ok(()),
            peer_addr,
        }),
        Err(err) => {
            chunk.retry_count += 1;
            chunk.last_peer = Some(peer_addr);

            if chunk.retry_count >= MAX_RETRY_COUNT {
                let context = format!("Retry budget exhausted for chunk: {}", chunk.relative_path);
                final_results.push(ChunkDownloadResult {
                    chunk,
                    result: Err(err.wrap_err(context)),
                    peer_addr,
                });
            } else {
                queue.push_back(chunk);
            }
        }
    }
}

fn handle_retry_attempt_error(
    peer_addr: SocketAddr,
    chunks: Vec<DownloadChunk>,
    err: &eyre::Report,
    queue: &mut VecDeque<DownloadChunk>,
    final_results: &mut Vec<ChunkDownloadResult>,
) {
    let error = err.to_string();

    for mut chunk in chunks {
        chunk.retry_count += 1;
        chunk.last_peer = Some(peer_addr);

        if chunk.retry_count >= MAX_RETRY_COUNT {
            final_results.push(ChunkDownloadResult {
                chunk: chunk.clone(),
                result: Err(eyre::eyre!(
                    "Retry budget exhausted for chunk after connection failure: {}: {error}",
                    chunk.relative_path
                )),
                peer_addr,
            });
        } else {
            queue.push_back(chunk);
        }
    }
}

/// Retries downloading failed chunks.
pub(super) async fn retry_failed_chunks(
    failed_chunks: Vec<DownloadChunk>,
    ctx: &RetryContext<'_>,
) -> eyre::Result<Vec<ChunkDownloadResult>> {
    let mut final_results = Vec::new();
    let mut queue: VecDeque<DownloadChunk> = failed_chunks.into_iter().collect();

    while !queue.is_empty() {
        ensure_not_cancelled(ctx.cancel_token, ctx.game_id)?;

        let retry_plans =
            plan_retry_batch(&mut queue, ctx.peers, ctx.file_peer_map, &mut final_results);
        if retry_plans.is_empty() {
            continue;
        }

        let attempts = run_retry_batch(retry_plans, ctx).await?;

        for attempt in attempts {
            let RetryAttempt {
                peer_addr,
                chunks,
                result,
            } = attempt;

            match result {
                Ok(results) => {
                    for result in results {
                        handle_retry_chunk_result(result, &mut queue, &mut final_results);
                    }
                }
                Err(err) => {
                    handle_retry_attempt_error(
                        peer_addr,
                        chunks,
                        &err,
                        &mut queue,
                        &mut final_results,
                    );
                }
            }
        }
    }

    Ok(final_results)
}

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

    fn loopback_addr(port: u16) -> SocketAddr {
        SocketAddr::from(([127, 0, 0, 1], port))
    }

    #[test]
    fn retry_peer_selection_cycles_after_last_failed_peer() {
        let peers = vec![
            loopback_addr(12000),
            loopback_addr(12001),
            loopback_addr(12002),
        ];

        assert_eq!(select_retry_peer(&peers, Some(peers[0])), Some(peers[1]));
        assert_eq!(select_retry_peer(&peers, Some(peers[1])), Some(peers[2]));
        assert_eq!(select_retry_peer(&peers, Some(peers[2])), Some(peers[0]));
    }

    #[test]
    fn retry_peer_selection_uses_first_peer_without_prior_failure() {
        let peers = vec![loopback_addr(12000), loopback_addr(12001)];

        assert_eq!(select_retry_peer(&peers, None), Some(peers[0]));
    }

    #[test]
    fn retry_peer_selection_wraps_between_two_peers() {
        let peers = vec![loopback_addr(12000), loopback_addr(12001)];

        assert_eq!(select_retry_peer(&peers, Some(peers[0])), Some(peers[1]));
        assert_eq!(select_retry_peer(&peers, Some(peers[1])), Some(peers[0]));
    }
}