scatter-proxy 0.1.0

use std::collections::HashSet;
use std::collections::VecDeque;
use std::future::Future;
use std::pin::Pin;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::Mutex;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};

use bytes::Bytes;
use http::{HeaderMap, StatusCode};
use tokio::sync::{oneshot, Notify};

use crate::error::ScatterProxyError;

/// Response from a successful proxied request.
#[derive(Debug)]
pub struct ScatterResponse {
    pub status: StatusCode,
    pub headers: HeaderMap,
    pub body: Bytes,
}

/// Handle returned to the caller when a task is submitted.
/// Implements `Future` so the caller can `.await` the proxied result.
#[derive(Debug)]
pub struct TaskHandle {
    rx: oneshot::Receiver<Result<ScatterResponse, ScatterProxyError>>,
}

impl Future for TaskHandle {
    type Output = Result<ScatterResponse, ScatterProxyError>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        Pin::new(&mut self.rx).poll(cx).map(|result| {
            result.unwrap_or_else(|_| Err(ScatterProxyError::Init("task channel closed".into())))
        })
    }
}

/// Internal task entry stored in the pool queue.
pub(crate) struct TaskEntry {
    #[allow(dead_code)]
    pub id: u64,
    pub request: reqwest::Request,
    pub host: String,
    pub attempts: usize,
    pub max_attempts: usize,
    pub submitted_at: Instant,
    pub task_timeout: Duration,
    pub result_tx: Option<oneshot::Sender<Result<ScatterResponse, ScatterProxyError>>>,
    pub last_error: String,
}

/// Thread-safe task pool with FIFO ordering and bounded capacity.
pub struct TaskPool {
    queue: Mutex<VecDeque<TaskEntry>>,
    capacity: usize,
    next_id: AtomicU64,
    notify: Notify,
    completed: AtomicU64,
    failed: AtomicU64,
}

impl TaskPool {
    /// Create a new task pool with the given maximum capacity.
    pub fn new(capacity: usize) -> Self {
        Self {
            queue: Mutex::new(VecDeque::new()),
            capacity,
            next_id: AtomicU64::new(1),
            notify: Notify::new(),
            completed: AtomicU64::new(0),
            failed: AtomicU64::new(0),
        }
    }

    /// Submit a single task. Returns a `TaskHandle` for await-ing the result.
    ///
    /// Returns `Err(PoolFull)` when the queue is already at capacity.
    pub fn submit(
        &self,
        request: reqwest::Request,
        max_attempts: usize,
        task_timeout: Duration,
    ) -> Result<TaskHandle, ScatterProxyError> {
        let host = request.url().host_str().unwrap_or("unknown").to_string();

        let (tx, rx) = oneshot::channel();
        let id = self.next_id.fetch_add(1, Ordering::Relaxed);

        let entry = TaskEntry {
            id,
            request,
            host,
            attempts: 0,
            max_attempts,
            submitted_at: Instant::now(),
            task_timeout,
            result_tx: Some(tx),
            last_error: String::new(),
        };

        {
            let mut queue = self.queue.lock().unwrap();
            if queue.len() >= self.capacity {
                return Err(ScatterProxyError::PoolFull {
                    capacity: self.capacity,
                });
            }
            queue.push_back(entry);
        }

        self.notify.notify_one();

        Ok(TaskHandle { rx })
    }

    /// Submit multiple tasks at once. Returns a `TaskHandle` per request.
    ///
    /// If the pool doesn't have room for the entire batch, no tasks are added
    /// and `Err(PoolFull)` is returned.
    pub fn submit_batch(
        &self,
        requests: Vec<reqwest::Request>,
        max_attempts: usize,
        task_timeout: Duration,
    ) -> Result<Vec<TaskHandle>, ScatterProxyError> {
        let count = requests.len();

        // Pre-check capacity so the batch is atomic.
        {
            let queue = self.queue.lock().unwrap();
            if queue.len() + count > self.capacity {
                return Err(ScatterProxyError::PoolFull {
                    capacity: self.capacity,
                });
            }
        }

        let mut handles = Vec::with_capacity(count);

        {
            let mut queue = self.queue.lock().unwrap();

            // Double-check after re-acquiring the lock.
            if queue.len() + count > self.capacity {
                return Err(ScatterProxyError::PoolFull {
                    capacity: self.capacity,
                });
            }

            for request in requests {
                let host = request.url().host_str().unwrap_or("unknown").to_string();
                let (tx, rx) = oneshot::channel();
                let id = self.next_id.fetch_add(1, Ordering::Relaxed);

                let entry = TaskEntry {
                    id,
                    request,
                    host,
                    attempts: 0,
                    max_attempts,
                    submitted_at: Instant::now(),
                    task_timeout,
                    result_tx: Some(tx),
                    last_error: String::new(),
                };
                queue.push_back(entry);
                handles.push(TaskHandle { rx });
            }
        }

        // Wake the scheduler — one notification per task added.
        for _ in 0..count {
            self.notify.notify_one();
        }

        Ok(handles)
    }

    /// Pick the next eligible task from the front of the queue.
    ///
    /// Tasks whose host appears in `skip_hosts` (e.g. circuit-broken hosts) are
    /// left in the queue. Returns `None` when no eligible task is found.
    pub(crate) fn pick_next(&self, skip_hosts: &HashSet<String>) -> Option<TaskEntry> {
        let mut queue = self.queue.lock().unwrap();
        let len = queue.len();

        for i in 0..len {
            if let Some(entry) = queue.get(i) {
                if !skip_hosts.contains(&entry.host) {
                    return queue.remove(i);
                }
            }
        }

        None
    }

    /// Push a failed task back to the tail of the queue for retry.
    pub(crate) fn push_back(&self, entry: TaskEntry) {
        {
            let mut queue = self.queue.lock().unwrap();
            queue.push_back(entry);
        }
        self.notify.notify_one();
    }

    /// Number of tasks currently waiting in the queue.
    pub fn pending_count(&self) -> usize {
        let queue = self.queue.lock().unwrap();
        queue.len()
    }

    /// Total number of tasks that completed successfully.
    pub fn completed_count(&self) -> u64 {
        self.completed.load(Ordering::Relaxed)
    }

    /// Total number of tasks that failed permanently.
    pub fn failed_count(&self) -> u64 {
        self.failed.load(Ordering::Relaxed)
    }

    /// Increment the completed counter.
    pub(crate) fn mark_completed(&self) {
        self.completed.fetch_add(1, Ordering::Relaxed);
    }

    /// Increment the failed counter.
    pub(crate) fn mark_failed(&self) {
        self.failed.fetch_add(1, Ordering::Relaxed);
    }

    /// Wait until a task becomes available (a task is submitted or pushed back).
    #[allow(dead_code)]
    pub(crate) async fn notified(&self) {
        self.notify.notified().await;
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::collections::HashSet;
    use std::time::Duration;

    /// Helper: build a GET request to the given URL.
    fn test_request(url: &str) -> reqwest::Request {
        reqwest::Client::new().get(url).build().unwrap()
    }

    // -----------------------------------------------------------------------
    // TaskPool::new
    // -----------------------------------------------------------------------

    #[test]
    fn new_pool_has_zero_pending() {
        let pool = TaskPool::new(100);
        assert_eq!(pool.pending_count(), 0);
        assert_eq!(pool.completed_count(), 0);
        assert_eq!(pool.failed_count(), 0);
    }

    // -----------------------------------------------------------------------
    // submit
    // -----------------------------------------------------------------------

    #[test]
    fn submit_increments_pending_count() {
        let pool = TaskPool::new(10);
        let _h = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();
        assert_eq!(pool.pending_count(), 1);
    }

    #[test]
    fn submit_returns_pool_full_when_at_capacity() {
        let pool = TaskPool::new(1);
        let _h1 = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let result = pool.submit(
            test_request("http://example.com/2"),
            3,
            Duration::from_secs(10),
        );
        assert!(result.is_err());
        match result.unwrap_err() {
            ScatterProxyError::PoolFull { capacity } => assert_eq!(capacity, 1),
            other => panic!("expected PoolFull, got: {other:?}"),
        }
    }

    #[test]
    fn submit_assigns_incrementing_ids() {
        let pool = TaskPool::new(10);
        let _h1 = pool
            .submit(test_request("http://a.com"), 3, Duration::from_secs(10))
            .unwrap();
        let _h2 = pool
            .submit(test_request("http://b.com"), 3, Duration::from_secs(10))
            .unwrap();

        let skip = HashSet::new();
        let t1 = pool.pick_next(&skip).unwrap();
        let t2 = pool.pick_next(&skip).unwrap();
        assert!(t2.id > t1.id);
    }

    #[test]
    fn submit_extracts_host_from_url() {
        let pool = TaskPool::new(10);
        let _h = pool
            .submit(
                test_request("http://myhost.example.com/path?q=1"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();
        assert_eq!(entry.host, "myhost.example.com");
    }

    // -----------------------------------------------------------------------
    // submit_batch
    // -----------------------------------------------------------------------

    #[test]
    fn submit_batch_adds_all_tasks() {
        let pool = TaskPool::new(10);
        let reqs = vec![
            test_request("http://a.com"),
            test_request("http://b.com"),
            test_request("http://c.com"),
        ];
        let handles = pool.submit_batch(reqs, 3, Duration::from_secs(10)).unwrap();
        assert_eq!(handles.len(), 3);
        assert_eq!(pool.pending_count(), 3);
    }

    #[test]
    fn submit_batch_atomic_rejection_when_pool_full() {
        let pool = TaskPool::new(2);
        let _h = pool
            .submit(test_request("http://x.com"), 3, Duration::from_secs(10))
            .unwrap();

        // 1 already in pool, capacity 2, trying to add 2 more → should fail
        let reqs = vec![test_request("http://a.com"), test_request("http://b.com")];
        let result = pool.submit_batch(reqs, 3, Duration::from_secs(10));
        assert!(result.is_err());
        // Original task should still be there
        assert_eq!(pool.pending_count(), 1);
    }

    #[test]
    fn submit_batch_empty_vec_is_ok() {
        let pool = TaskPool::new(10);
        let handles = pool
            .submit_batch(vec![], 3, Duration::from_secs(10))
            .unwrap();
        assert!(handles.is_empty());
        assert_eq!(pool.pending_count(), 0);
    }

    // -----------------------------------------------------------------------
    // pick_next
    // -----------------------------------------------------------------------

    #[test]
    fn pick_next_returns_fifo_order() {
        let pool = TaskPool::new(10);
        let _h1 = pool
            .submit(test_request("http://first.com"), 3, Duration::from_secs(10))
            .unwrap();
        let _h2 = pool
            .submit(
                test_request("http://second.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let skip = HashSet::new();
        let t = pool.pick_next(&skip).unwrap();
        assert_eq!(t.host, "first.com");
        assert_eq!(pool.pending_count(), 1);
    }

    #[test]
    fn pick_next_skips_circuit_broken_hosts() {
        let pool = TaskPool::new(10);
        let _h1 = pool
            .submit(
                test_request("http://broken.com/a"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();
        let _h2 = pool
            .submit(test_request("http://ok.com/b"), 3, Duration::from_secs(10))
            .unwrap();

        let mut skip = HashSet::new();
        skip.insert("broken.com".to_string());

        let t = pool.pick_next(&skip).unwrap();
        assert_eq!(t.host, "ok.com");
        // broken.com task is still in the queue
        assert_eq!(pool.pending_count(), 1);
    }

    #[test]
    fn pick_next_returns_none_when_all_hosts_skipped() {
        let pool = TaskPool::new(10);
        let _h = pool
            .submit(
                test_request("http://broken.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let mut skip = HashSet::new();
        skip.insert("broken.com".to_string());

        assert!(pool.pick_next(&skip).is_none());
        assert_eq!(pool.pending_count(), 1);
    }

    #[test]
    fn pick_next_returns_none_when_empty() {
        let pool = TaskPool::new(10);
        let skip = HashSet::new();
        assert!(pool.pick_next(&skip).is_none());
    }

    // -----------------------------------------------------------------------
    // push_back
    // -----------------------------------------------------------------------

    #[test]
    fn push_back_requeues_to_tail() {
        let pool = TaskPool::new(10);
        let _h1 = pool
            .submit(test_request("http://first.com"), 3, Duration::from_secs(10))
            .unwrap();
        let _h2 = pool
            .submit(
                test_request("http://second.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let skip = HashSet::new();
        let mut entry = pool.pick_next(&skip).unwrap();
        assert_eq!(entry.host, "first.com");
        entry.attempts += 1;
        entry.last_error = "connection refused".into();

        pool.push_back(entry);
        assert_eq!(pool.pending_count(), 2);

        // second.com should come first now
        let t = pool.pick_next(&skip).unwrap();
        assert_eq!(t.host, "second.com");

        // then re-queued first.com
        let t = pool.pick_next(&skip).unwrap();
        assert_eq!(t.host, "first.com");
        assert_eq!(t.attempts, 1);
        assert_eq!(t.last_error, "connection refused");
    }

    // -----------------------------------------------------------------------
    // mark_completed / mark_failed
    // -----------------------------------------------------------------------

    #[test]
    fn mark_completed_increments_counter() {
        let pool = TaskPool::new(10);
        pool.mark_completed();
        pool.mark_completed();
        assert_eq!(pool.completed_count(), 2);
    }

    #[test]
    fn mark_failed_increments_counter() {
        let pool = TaskPool::new(10);
        pool.mark_failed();
        assert_eq!(pool.failed_count(), 1);
    }

    // -----------------------------------------------------------------------
    // TaskHandle as Future
    // -----------------------------------------------------------------------

    #[tokio::test]
    async fn task_handle_receives_success() {
        let pool = TaskPool::new(10);
        let handle = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();

        let response = ScatterResponse {
            status: StatusCode::OK,
            headers: HeaderMap::new(),
            body: Bytes::from("hello"),
        };

        entry.result_tx.unwrap().send(Ok(response)).unwrap();

        let result = handle.await;
        assert!(result.is_ok());
        let resp = result.unwrap();
        assert_eq!(resp.status, StatusCode::OK);
        assert_eq!(resp.body, Bytes::from("hello"));
    }

    #[tokio::test]
    async fn task_handle_receives_error() {
        let pool = TaskPool::new(10);
        let handle = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();

        entry
            .result_tx
            .unwrap()
            .send(Err(ScatterProxyError::MaxAttemptsExhausted {
                host: "example.com".into(),
                attempts: 3,
                last_error: "timeout".into(),
            }))
            .unwrap();

        let result = handle.await;
        assert!(result.is_err());
    }

    #[tokio::test]
    async fn task_handle_returns_error_when_sender_dropped() {
        let pool = TaskPool::new(10);
        let handle = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        // Pick and drop the entry (which drops the sender).
        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();
        drop(entry);

        let result = handle.await;
        assert!(result.is_err());
        match result.unwrap_err() {
            ScatterProxyError::Init(msg) => assert!(msg.contains("channel closed")),
            other => panic!("expected Init, got: {other:?}"),
        }
    }

    // -----------------------------------------------------------------------
    // notified
    // -----------------------------------------------------------------------

    #[tokio::test]
    async fn notified_wakes_on_submit() {
        let pool = std::sync::Arc::new(TaskPool::new(10));
        let pool2 = pool.clone();

        let waiter = tokio::spawn(async move {
            pool2.notified().await;
            true
        });

        // Give the waiter a moment to park.
        tokio::time::sleep(Duration::from_millis(10)).await;

        let _h = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();

        let woke = tokio::time::timeout(Duration::from_secs(1), waiter)
            .await
            .unwrap()
            .unwrap();
        assert!(woke);
    }

    #[tokio::test]
    async fn notified_wakes_on_push_back() {
        let pool = std::sync::Arc::new(TaskPool::new(10));

        // Submit and pick a task to get an entry we can push_back.
        let _h = pool
            .submit(
                test_request("http://example.com"),
                3,
                Duration::from_secs(10),
            )
            .unwrap();
        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();

        let pool2 = pool.clone();
        let waiter = tokio::spawn(async move {
            pool2.notified().await;
            true
        });

        tokio::time::sleep(Duration::from_millis(10)).await;

        pool.push_back(entry);

        let woke = tokio::time::timeout(Duration::from_secs(1), waiter)
            .await
            .unwrap()
            .unwrap();
        assert!(woke);
    }

    // -----------------------------------------------------------------------
    // Capacity edge cases
    // -----------------------------------------------------------------------

    #[test]
    fn pool_with_zero_capacity_rejects_everything() {
        let pool = TaskPool::new(0);
        let result = pool.submit(test_request("http://a.com"), 1, Duration::from_secs(5));
        assert!(matches!(
            result,
            Err(ScatterProxyError::PoolFull { capacity: 0 })
        ));
    }

    #[test]
    fn pool_allows_submit_after_pick_frees_space() {
        let pool = TaskPool::new(1);
        let _h1 = pool
            .submit(test_request("http://a.com"), 1, Duration::from_secs(5))
            .unwrap();

        // Pool full now.
        assert!(pool
            .submit(test_request("http://b.com"), 1, Duration::from_secs(5))
            .is_err());

        // Pick the task, freeing a slot.
        let skip = HashSet::new();
        let _entry = pool.pick_next(&skip).unwrap();

        // Now we can submit again.
        let _h2 = pool
            .submit(test_request("http://c.com"), 1, Duration::from_secs(5))
            .unwrap();
        assert_eq!(pool.pending_count(), 1);
    }

    // -----------------------------------------------------------------------
    // TaskEntry fields
    // -----------------------------------------------------------------------

    #[test]
    fn task_entry_has_correct_defaults_on_submit() {
        let pool = TaskPool::new(10);
        let timeout = Duration::from_secs(42);
        let _h = pool
            .submit(test_request("http://host.example.com/path"), 7, timeout)
            .unwrap();

        let skip = HashSet::new();
        let entry = pool.pick_next(&skip).unwrap();

        assert_eq!(entry.host, "host.example.com");
        assert_eq!(entry.attempts, 0);
        assert_eq!(entry.max_attempts, 7);
        assert_eq!(entry.task_timeout, timeout);
        assert!(entry.last_error.is_empty());
        assert!(entry.result_tx.is_some());
    }

    // -----------------------------------------------------------------------
    // ScatterResponse basics
    // -----------------------------------------------------------------------

    #[test]
    fn scatter_response_debug() {
        let resp = ScatterResponse {
            status: StatusCode::NOT_FOUND,
            headers: HeaderMap::new(),
            body: Bytes::from("not found"),
        };
        let dbg = format!("{resp:?}");
        assert!(dbg.contains("404"));
    }

    // -----------------------------------------------------------------------
    // Multiple hosts with partial skipping
    // -----------------------------------------------------------------------

    #[test]
    fn pick_next_selects_first_non_skipped_preserves_order() {
        let pool = TaskPool::new(10);
        let _h1 = pool
            .submit(test_request("http://a.com/1"), 1, Duration::from_secs(5))
            .unwrap();
        let _h2 = pool
            .submit(test_request("http://b.com/2"), 1, Duration::from_secs(5))
            .unwrap();
        let _h3 = pool
            .submit(test_request("http://a.com/3"), 1, Duration::from_secs(5))
            .unwrap();
        let _h4 = pool
            .submit(test_request("http://c.com/4"), 1, Duration::from_secs(5))
            .unwrap();

        let mut skip = HashSet::new();
        skip.insert("a.com".to_string());

        // Should skip both a.com entries, return b.com first
        let t1 = pool.pick_next(&skip).unwrap();
        assert_eq!(t1.host, "b.com");

        let t2 = pool.pick_next(&skip).unwrap();
        assert_eq!(t2.host, "c.com");

        // Only a.com entries remain
        assert_eq!(pool.pending_count(), 2);
        assert!(pool.pick_next(&skip).is_none());
    }
}