//! In-memory task registry for sub-agent task lifecycle management.
//!
//! Provides create, get, list, stop, update, and output operations
//! matching the upstream TaskCreate/TaskGet/TaskList/TaskStop/TaskUpdate/TaskOutput
//! tool surface.

use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::time::{SystemTime, UNIX_EPOCH};

use serde::{Deserialize, Serialize};

/// Current status of a managed task.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum TaskStatus {
    Created,
    Running,
    Completed,
    Failed,
    Stopped,
}

impl std::fmt::Display for TaskStatus {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Created => write!(f, "created"),
            Self::Running => write!(f, "running"),
            Self::Completed => write!(f, "completed"),
            Self::Failed => write!(f, "failed"),
            Self::Stopped => write!(f, "stopped"),
        }
    }
}

/// A single managed task entry.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Task {
    pub task_id: String,
    pub prompt: String,
    pub description: Option<String>,
    pub status: TaskStatus,
    pub created_at: u64,
    pub updated_at: u64,
    pub messages: Vec<TaskMessage>,
    pub output: String,
    pub team_id: Option<String>,
}

/// A message exchanged with a running task.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct TaskMessage {
    pub role: String,
    pub content: String,
    pub timestamp: u64,
}

/// Thread-safe task registry.
#[derive(Debug, Clone, Default)]
pub struct TaskRegistry {
    inner: Arc<Mutex<RegistryInner>>,
}

#[derive(Debug, Default)]
struct RegistryInner {
    tasks: HashMap<String, Task>,
    counter: u64,
}

fn now_secs() -> u64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_secs()
}

impl TaskRegistry {
    /// Create a new empty registry.
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    /// Create a new task and return its ID.
    pub fn create(&self, prompt: &str, description: Option<&str>) -> Task {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        inner.counter += 1;
        let ts = now_secs();
        let task_id = format!("task_{:08x}_{}", ts, inner.counter);
        let task = Task {
            task_id: task_id.clone(),
            prompt: prompt.to_owned(),
            description: description.map(str::to_owned),
            status: TaskStatus::Created,
            created_at: ts,
            updated_at: ts,
            messages: Vec::new(),
            output: String::new(),
            team_id: None,
        };
        inner.tasks.insert(task_id, task.clone());
        task
    }

    /// Look up a task by ID.
    #[must_use]
    pub fn get(&self, task_id: &str) -> Option<Task> {
        let inner = self.inner.lock().expect("registry lock poisoned");
        inner.tasks.get(task_id).cloned()
    }

    /// List all tasks, optionally filtered by status.
    #[must_use]
    pub fn list(&self, status_filter: Option<TaskStatus>) -> Vec<Task> {
        let inner = self.inner.lock().expect("registry lock poisoned");
        inner
            .tasks
            .values()
            .filter(|t| status_filter.is_none_or(|s| t.status == s))
            .cloned()
            .collect()
    }

    /// Mark a task as stopped.
    pub fn stop(&self, task_id: &str) -> Result<Task, String> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get_mut(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;

        match task.status {
            TaskStatus::Completed | TaskStatus::Failed | TaskStatus::Stopped => {
                return Err(format!(
                    "task {task_id} is already in terminal state: {}",
                    task.status
                ));
            }
            _ => {}
        }

        task.status = TaskStatus::Stopped;
        task.updated_at = now_secs();
        Ok(task.clone())
    }

    /// Send a message to a task, updating its state.
    pub fn update(&self, task_id: &str, message: &str) -> Result<Task, String> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get_mut(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;

        task.messages.push(TaskMessage {
            role: String::from("user"),
            content: message.to_owned(),
            timestamp: now_secs(),
        });
        task.updated_at = now_secs();
        Ok(task.clone())
    }

    /// Get the accumulated output of a task.
    pub fn output(&self, task_id: &str) -> Result<String, String> {
        let inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;
        Ok(task.output.clone())
    }

    /// Append output to a task (used by the task executor).
    pub fn append_output(&self, task_id: &str, output: &str) -> Result<(), String> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get_mut(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;
        task.output.push_str(output);
        task.updated_at = now_secs();
        Ok(())
    }

    /// Transition a task to a new status.
    pub fn set_status(&self, task_id: &str, status: TaskStatus) -> Result<(), String> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get_mut(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;
        task.status = status;
        task.updated_at = now_secs();
        Ok(())
    }

    /// Assign a task to a team.
    pub fn assign_team(&self, task_id: &str, team_id: &str) -> Result<(), String> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        let task = inner
            .tasks
            .get_mut(task_id)
            .ok_or_else(|| format!("task not found: {task_id}"))?;
        task.team_id = Some(team_id.to_owned());
        task.updated_at = now_secs();
        Ok(())
    }

    /// Remove a task from the registry.
    #[must_use]
    pub fn remove(&self, task_id: &str) -> Option<Task> {
        let mut inner = self.inner.lock().expect("registry lock poisoned");
        inner.tasks.remove(task_id)
    }

    /// Number of tasks in the registry.
    #[must_use]
    pub fn len(&self) -> usize {
        let inner = self.inner.lock().expect("registry lock poisoned");
        inner.tasks.len()
    }

    /// Whether the registry has no tasks.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

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

    #[test]
    fn creates_and_retrieves_tasks() {
        let registry = TaskRegistry::new();
        let task = registry.create("Do something", Some("A test task"));
        assert_eq!(task.status, TaskStatus::Created);
        assert_eq!(task.prompt, "Do something");
        assert_eq!(task.description.as_deref(), Some("A test task"));

        let fetched = registry.get(&task.task_id).expect("task should exist");
        assert_eq!(fetched.task_id, task.task_id);
    }

    #[test]
    fn lists_tasks_with_optional_filter() {
        let registry = TaskRegistry::new();
        registry.create("Task A", None);
        let task_b = registry.create("Task B", None);
        registry
            .set_status(&task_b.task_id, TaskStatus::Running)
            .expect("set status should succeed");

        let all = registry.list(None);
        assert_eq!(all.len(), 2);

        let running = registry.list(Some(TaskStatus::Running));
        assert_eq!(running.len(), 1);
        assert_eq!(running[0].task_id, task_b.task_id);

        let created = registry.list(Some(TaskStatus::Created));
        assert_eq!(created.len(), 1);
    }

    #[test]
    fn stops_running_task() {
        let registry = TaskRegistry::new();
        let task = registry.create("Stoppable", None);
        registry
            .set_status(&task.task_id, TaskStatus::Running)
            .unwrap();

        let stopped = registry.stop(&task.task_id).expect("stop should succeed");
        assert_eq!(stopped.status, TaskStatus::Stopped);

        // Stopping again should fail
        let result = registry.stop(&task.task_id);
        assert!(result.is_err());
    }

    #[test]
    fn updates_task_with_messages() {
        let registry = TaskRegistry::new();
        let task = registry.create("Messageable", None);
        let updated = registry
            .update(&task.task_id, "Here's more context")
            .expect("update should succeed");
        assert_eq!(updated.messages.len(), 1);
        assert_eq!(updated.messages[0].content, "Here's more context");
        assert_eq!(updated.messages[0].role, "user");
    }

    #[test]
    fn appends_and_retrieves_output() {
        let registry = TaskRegistry::new();
        let task = registry.create("Output task", None);
        registry
            .append_output(&task.task_id, "line 1\n")
            .expect("append should succeed");
        registry
            .append_output(&task.task_id, "line 2\n")
            .expect("append should succeed");

        let output = registry.output(&task.task_id).expect("output should exist");
        assert_eq!(output, "line 1\nline 2\n");
    }

    #[test]
    fn assigns_team_and_removes_task() {
        let registry = TaskRegistry::new();
        let task = registry.create("Team task", None);
        registry
            .assign_team(&task.task_id, "team_abc")
            .expect("assign should succeed");

        let fetched = registry.get(&task.task_id).unwrap();
        assert_eq!(fetched.team_id.as_deref(), Some("team_abc"));

        let removed = registry.remove(&task.task_id);
        assert!(removed.is_some());
        assert!(registry.get(&task.task_id).is_none());
        assert!(registry.is_empty());
    }

    #[test]
    fn rejects_operations_on_missing_task() {
        let registry = TaskRegistry::new();
        assert!(registry.stop("nonexistent").is_err());
        assert!(registry.update("nonexistent", "msg").is_err());
        assert!(registry.output("nonexistent").is_err());
        assert!(registry.append_output("nonexistent", "data").is_err());
        assert!(registry
            .set_status("nonexistent", TaskStatus::Running)
            .is_err());
    }
}