use std::collections::HashMap;
use std::path::PathBuf;
use std::sync::{Arc, Mutex};
use std::time::Instant;

use tokio::sync::RwLock;

use crate::core::project_hash;
use crate::core::session::SessionState;

const MAX_CACHED_SESSIONS: usize = 8;

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct SharedSessionKey {
    pub project_hash: String,
    pub workspace_id: String,
    pub channel_id: String,
}

impl SharedSessionKey {
    pub fn new(project_root: &str, workspace_id: &str, channel_id: &str) -> Self {
        Self {
            project_hash: project_hash::hash_project_root(project_root),
            workspace_id: normalize_id(workspace_id, "default"),
            channel_id: normalize_id(channel_id, "default"),
        }
    }
}

struct SessionEntry {
    session: Arc<RwLock<SessionState>>,
    project_root: String,
    last_accessed: Instant,
}

pub struct SharedSessionStore {
    sessions: Mutex<HashMap<SharedSessionKey, SessionEntry>>,
}

impl Default for SharedSessionStore {
    fn default() -> Self {
        Self {
            sessions: Mutex::new(HashMap::new()),
        }
    }
}

impl SharedSessionStore {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn get_or_load(
        &self,
        project_root: &str,
        workspace_id: &str,
        channel_id: &str,
    ) -> Arc<RwLock<SessionState>> {
        let key = SharedSessionKey::new(project_root, workspace_id, channel_id);
        let disk_key = key.clone();
        let root = project_root.to_string();
        let mut map = self
            .sessions
            .lock()
            .unwrap_or_else(std::sync::PoisonError::into_inner);

        if let Some(entry) = map.get_mut(&key) {
            entry.last_accessed = Instant::now();
            return entry.session.clone();
        }

        Self::evict_lru_if_needed(&mut map);

        let mut loaded = load_session_from_disk(&root, &disk_key)
            .or_else(|| SessionState::load_latest_for_project_root(&root))
            .unwrap_or_default();
        loaded.project_root = Some(root.clone());
        let session = Arc::new(RwLock::new(loaded));

        map.insert(
            key,
            SessionEntry {
                session: session.clone(),
                project_root: root,
                last_accessed: Instant::now(),
            },
        );

        session
    }

    fn evict_lru_if_needed(map: &mut HashMap<SharedSessionKey, SessionEntry>) {
        if map.len() < MAX_CACHED_SESSIONS {
            return;
        }

        let lru_key = map
            .iter()
            .min_by_key(|(_, e)| e.last_accessed)
            .map(|(k, _)| k.clone());

        if let Some(key) = lru_key {
            if let Some(entry) = map.remove(&key) {
                if let Ok(session) = entry.session.try_read() {
                    persist_session_to_disk(&key, &entry.project_root, &session);
                }
            }
        }
    }

    pub fn active_count(&self) -> usize {
        self.sessions
            .lock()
            .unwrap_or_else(std::sync::PoisonError::into_inner)
            .len()
    }

    /// Returns the max sessions limit for diagnostics.
    pub fn max_sessions() -> usize {
        MAX_CACHED_SESSIONS
    }

    pub fn persist_best_effort(
        &self,
        project_root: &str,
        workspace_id: &str,
        channel_id: &str,
        session: &SessionState,
    ) {
        let key = SharedSessionKey::new(project_root, workspace_id, channel_id);
        persist_session_to_disk(&key, project_root, session);
    }
}

fn persist_session_to_disk(key: &SharedSessionKey, _project_root: &str, session: &SessionState) {
    let Some(dir) = shared_session_dir(key) else {
        return;
    };
    let _ = std::fs::create_dir_all(&dir);
    let state_path = dir.join("session.json");
    let tmp = dir.join("session.json.tmp");

    if let Ok(json) = serde_json::to_string_pretty(session) {
        let _ = std::fs::write(&tmp, json);
        let _ = std::fs::rename(&tmp, &state_path);
    }

    if session.task.is_some() {
        let snapshot = session.build_compaction_snapshot();
        let _ = std::fs::write(dir.join("snapshot.txt"), snapshot);
    }
}

fn normalize_id(s: &str, fallback: &str) -> String {
    let t = s.trim();
    if t.is_empty() {
        fallback.to_string()
    } else {
        // Keep IDs URL/header safe.
        t.chars()
            .filter(|c| c.is_ascii_alphanumeric() || *c == '-' || *c == '_' || *c == '.')
            .collect::<String>()
    }
}

fn shared_session_dir(key: &SharedSessionKey) -> Option<PathBuf> {
    let data = crate::core::data_dir::lean_ctx_data_dir().ok()?;
    Some(
        data.join("context-os")
            .join("sessions")
            .join(&key.project_hash)
            .join(&key.workspace_id)
            .join(&key.channel_id),
    )
}

fn load_session_from_disk(project_root: &str, key: &SharedSessionKey) -> Option<SessionState> {
    let dir = shared_session_dir(key)?;
    let state_path = dir.join("session.json");
    let json = std::fs::read_to_string(&state_path).ok()?;
    let mut session: SessionState = serde_json::from_str(&json).ok()?;
    // Safety: enforce project_root from the current server.
    session.project_root = Some(project_root.to_string());
    Some(session)
}

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

    #[test]
    fn normalize_id_filters_weird_chars() {
        assert_eq!(normalize_id("  ", "x"), "x");
        assert_eq!(normalize_id("abc-123_DEF", "x"), "abc-123_DEF");
        assert_eq!(normalize_id("a b$c", "x"), "abc");
    }

    #[test]
    fn key_is_stable() {
        let k1 = SharedSessionKey::new("/tmp/proj", "ws", "ch");
        let k2 = SharedSessionKey::new("/tmp/proj", "ws", "ch");
        assert_eq!(k1, k2);
    }

    #[tokio::test]
    async fn concurrent_session_access_no_data_race() {
        let store = Arc::new(SharedSessionStore::new());
        let n_tasks: usize = 8;

        let mut handles = vec![];
        for task_idx in 0..n_tasks {
            let store = Arc::clone(&store);
            handles.push(tokio::spawn(async move {
                let project_root = "/tmp/test-concurrent";
                for i in 0..10 {
                    let session_arc = store.get_or_load(project_root, "ws-shared", "ch-shared");
                    let mut s = session_arc.write().await;
                    s.files_touched.push(crate::core::session::FileTouched {
                        path: format!("file-{task_idx}-{i}.rs"),
                        file_ref: None,
                        read_count: 1,
                        modified: false,
                        last_mode: "full".to_string(),
                        tokens: 0,
                        stale: false,
                        context_item_id: None,
                    });
                }
            }));
        }

        for h in handles {
            h.await.unwrap();
        }

        let final_arc = store.get_or_load("/tmp/test-concurrent", "ws-shared", "ch-shared");
        let final_session = final_arc.read().await;
        assert_eq!(
            final_session.files_touched.len(),
            n_tasks * 10,
            "all concurrent mutations must be persisted"
        );
    }

    #[tokio::test]
    async fn different_workspace_channels_are_isolated() {
        let store = SharedSessionStore::new();

        {
            let arc_a = store.get_or_load("/tmp/proj-iso", "ws-a", "ch-1");
            arc_a
                .write()
                .await
                .files_touched
                .push(crate::core::session::FileTouched {
                    path: "fileA.rs".to_string(),
                    file_ref: None,
                    read_count: 1,
                    modified: false,
                    last_mode: "full".to_string(),
                    tokens: 0,
                    stale: false,
                    context_item_id: None,
                });
        }
        {
            let arc_b = store.get_or_load("/tmp/proj-iso", "ws-b", "ch-1");
            arc_b
                .write()
                .await
                .files_touched
                .push(crate::core::session::FileTouched {
                    path: "fileB.rs".to_string(),
                    file_ref: None,
                    read_count: 1,
                    modified: false,
                    last_mode: "full".to_string(),
                    tokens: 0,
                    stale: false,
                    context_item_id: None,
                });
        }

        let session_a = store.get_or_load("/tmp/proj-iso", "ws-a", "ch-1");
        let session_b = store.get_or_load("/tmp/proj-iso", "ws-b", "ch-1");

        assert_eq!(session_a.read().await.files_touched.len(), 1);
        assert_eq!(session_a.read().await.files_touched[0].path, "fileA.rs");
        assert_eq!(session_b.read().await.files_touched.len(), 1);
        assert_eq!(session_b.read().await.files_touched[0].path, "fileB.rs");
    }
}
