use chrono::Utc;

use std::sync::{Arc, Mutex};

use super::event_store::EventStore;
use super::models::*;

pub struct StateMachine {
    store: Arc<Mutex<EventStore>>,
}

impl StateMachine {
    pub fn new(store: Arc<Mutex<EventStore>>) -> Self {
        Self { store }
    }

    /// C1 + C2: Single lock scope, spawn_blocking, transactional transition.
    pub async fn transition(
        &self,
        task_id: &str,
        new_status: TaskStatus,
        agent_id: Option<&str>,
        reason: &str,
    ) -> Result<Task, StateError> {
        let task_id = task_id.to_string();
        let reason = reason.to_string();
        let agent_id_owned = agent_id.map(String::from);
        let store = self.store.clone();

        tokio::task::spawn_blocking(move || -> Result<Task, StateError> {
            let mut store = store.lock().map_err(|e| StateError::Poisoned(e.to_string()))?;

            let task = store
                .read_task(&task_id)?
                .ok_or_else(|| StateError::TaskNotFound(task_id.clone()))?;

            Self::validate_transition(&task.status, &new_status)?;

            let now = Utc::now();
            let event = TaskEvent {
                event_id: uuid::Uuid::new_v4().to_string(),
                task_id: task_id.clone(),
                event_type: format!("task.{}", new_status.as_str()),
                agent_id: agent_id_owned.clone(),
                timestamp: now,
                payload: serde_json::json!({
                    "from_status": task.status.as_str(),
                    "to_status": new_status.as_str(),
                    "reason": reason,
                }),
            };

            Ok(store.transition_task(
                &task_id,
                new_status.as_str(),
                agent_id_owned.as_deref(),
                if new_status == TaskStatus::Assigned {
                    Some(now.to_rfc3339())
                } else {
                    None
                },
                if new_status == TaskStatus::Running {
                    Some(now.to_rfc3339())
                } else {
                    None
                },
                if matches!(
                    new_status,
                    TaskStatus::Completed | TaskStatus::Failed | TaskStatus::Cancelled
                ) {
                    Some(now.to_rfc3339())
                } else {
                    None
                },
                &event,
            )?)
        })
        .await
        .map_err(StateError::Join)?
    }

    pub async fn create_task(&self, task: &Task) -> Result<Task, StateError> {
        let task = task.clone();
        let store = self.store.clone();

        tokio::task::spawn_blocking(move || -> Result<Task, StateError> {
            let store = store.lock().map_err(|e| StateError::Poisoned(e.to_string()))?;

            store.insert_task(&task)?;

            let event = TaskEvent {
                event_id: uuid::Uuid::new_v4().to_string(),
                task_id: task.task_id.clone(),
                event_type: "task.created".into(),
                agent_id: None,
                timestamp: Utc::now(),
                payload: serde_json::json!({ "source": task.source }),
            };
            store.append_event_direct(&event)?;

            Ok(task)
        })
        .await
        .map_err(StateError::Join)?
    }

    fn validate_transition(from: &TaskStatus, to: &TaskStatus) -> Result<(), StateError> {
        let valid = match from {
            TaskStatus::Created => matches!(to, TaskStatus::Assigned | TaskStatus::Cancelled),
            TaskStatus::Assigned => matches!(to, TaskStatus::Running | TaskStatus::Cancelled),
            TaskStatus::Running => matches!(
                to,
                TaskStatus::Completed
                    | TaskStatus::Failed
                    | TaskStatus::AgentLost
                    | TaskStatus::Cancelled
            ),
            TaskStatus::Failed | TaskStatus::AgentLost => {
                matches!(to, TaskStatus::Assigned | TaskStatus::Cancelled)
            }
            TaskStatus::Completed | TaskStatus::Cancelled => false,
        };
        if !valid {
            return Err(StateError::InvalidTransition(
                from.as_str().to_string(),
                to.as_str().to_string(),
            ));
        }
        Ok(())
    }

    pub fn parse_status(s: &str) -> TaskStatus {
        match s {
            "created" => TaskStatus::Created,
            "assigned" => TaskStatus::Assigned,
            "running" => TaskStatus::Running,
            "completed" => TaskStatus::Completed,
            "failed" => TaskStatus::Failed,
            "agent_lost" => TaskStatus::AgentLost,
            "cancelled" => TaskStatus::Cancelled,
            _ => TaskStatus::Created,
        }
    }
}

#[derive(Debug, thiserror::Error)]
pub enum StateError {
    #[error("task not found: {0}")]
    TaskNotFound(String),
    #[error("invalid transition: {0} -> {1}")]
    InvalidTransition(String, String),
    #[error("database error: {0}")]
    Database(#[from] rusqlite::Error),
    #[error("task join error: {0}")]
    Join(#[from] tokio::task::JoinError),
    #[error("mutex poisoned: {0}")]
    Poisoned(String),
}