//! Session and thread model for turn-based agent interactions.
//!
//! A Session contains one or more Threads. Each Thread represents a
//! conversation/interaction sequence with the agent. Threads contain
//! Turns, which are request/response pairs.
//!
//! This model supports:
//! - Undo: Roll back to a previous turn
//! - Interrupt: Cancel the current turn mid-execution
//! - Compaction: Summarize old turns to save context
//! - Resume: Continue from a saved checkpoint

use std::collections::HashMap;

use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
use uuid::Uuid;

use crate::llm::ChatMessage;

/// A session containing one or more threads.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Session {
    /// Unique session ID.
    pub id: Uuid,
    /// User ID that owns this session.
    pub user_id: String,
    /// Active thread ID.
    pub active_thread: Option<Uuid>,
    /// All threads in this session.
    pub threads: HashMap<Uuid, Thread>,
    /// When the session was created.
    pub created_at: DateTime<Utc>,
    /// When the session was last active.
    pub last_active_at: DateTime<Utc>,
    /// Session metadata.
    pub metadata: serde_json::Value,
}

impl Session {
    /// Create a new session.
    pub fn new(user_id: impl Into<String>) -> Self {
        let now = Utc::now();
        Self {
            id: Uuid::new_v4(),
            user_id: user_id.into(),
            active_thread: None,
            threads: HashMap::new(),
            created_at: now,
            last_active_at: now,
            metadata: serde_json::Value::Null,
        }
    }

    /// Create a new thread in this session.
    pub fn create_thread(&mut self) -> &mut Thread {
        let thread = Thread::new(self.id);
        let thread_id = thread.id;
        self.threads.insert(thread_id, thread);
        self.active_thread = Some(thread_id);
        self.last_active_at = Utc::now();
        self.threads.get_mut(&thread_id).expect("just inserted")
    }

    /// Get the active thread.
    pub fn active_thread(&self) -> Option<&Thread> {
        self.active_thread.and_then(|id| self.threads.get(&id))
    }

    /// Get the active thread mutably.
    pub fn active_thread_mut(&mut self) -> Option<&mut Thread> {
        self.active_thread.and_then(|id| self.threads.get_mut(&id))
    }

    /// Get or create the active thread.
    pub fn get_or_create_thread(&mut self) -> &mut Thread {
        if self.active_thread.is_none() {
            self.create_thread();
        }
        self.active_thread_mut().expect("just created")
    }

    /// Switch to a different thread.
    pub fn switch_thread(&mut self, thread_id: Uuid) -> bool {
        if self.threads.contains_key(&thread_id) {
            self.active_thread = Some(thread_id);
            self.last_active_at = Utc::now();
            true
        } else {
            false
        }
    }
}

/// State of a thread.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum ThreadState {
    /// Thread is idle, waiting for input.
    Idle,
    /// Thread is processing a turn.
    Processing,
    /// Thread is waiting for user approval.
    AwaitingApproval,
    /// Thread has completed (no more turns expected).
    Completed,
    /// Thread was interrupted.
    Interrupted,
}

/// A conversation thread within a session.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Thread {
    /// Unique thread ID.
    pub id: Uuid,
    /// Parent session ID.
    pub session_id: Uuid,
    /// Current state.
    pub state: ThreadState,
    /// Turns in this thread.
    pub turns: Vec<Turn>,
    /// When the thread was created.
    pub created_at: DateTime<Utc>,
    /// When the thread was last updated.
    pub updated_at: DateTime<Utc>,
    /// Thread metadata (e.g., title, tags).
    pub metadata: serde_json::Value,
}

impl Thread {
    /// Create a new thread.
    pub fn new(session_id: Uuid) -> Self {
        let now = Utc::now();
        Self {
            id: Uuid::new_v4(),
            session_id,
            state: ThreadState::Idle,
            turns: Vec::new(),
            created_at: now,
            updated_at: now,
            metadata: serde_json::Value::Null,
        }
    }

    /// Get the current turn number (1-indexed for display).
    pub fn turn_number(&self) -> usize {
        self.turns.len() + 1
    }

    /// Get the last turn.
    pub fn last_turn(&self) -> Option<&Turn> {
        self.turns.last()
    }

    /// Get the last turn mutably.
    pub fn last_turn_mut(&mut self) -> Option<&mut Turn> {
        self.turns.last_mut()
    }

    /// Start a new turn with user input.
    pub fn start_turn(&mut self, user_input: impl Into<String>) -> &mut Turn {
        let turn_number = self.turns.len();
        let turn = Turn::new(turn_number, user_input);
        self.turns.push(turn);
        self.state = ThreadState::Processing;
        self.updated_at = Utc::now();
        self.turns.last_mut().expect("just pushed")
    }

    /// Complete the current turn with a response.
    pub fn complete_turn(&mut self, response: impl Into<String>) {
        if let Some(turn) = self.turns.last_mut() {
            turn.complete(response);
        }
        self.state = ThreadState::Idle;
        self.updated_at = Utc::now();
    }

    /// Fail the current turn with an error.
    pub fn fail_turn(&mut self, error: impl Into<String>) {
        if let Some(turn) = self.turns.last_mut() {
            turn.fail(error);
        }
        self.state = ThreadState::Idle;
        self.updated_at = Utc::now();
    }

    /// Mark the thread as awaiting approval.
    pub fn await_approval(&mut self) {
        self.state = ThreadState::AwaitingApproval;
        self.updated_at = Utc::now();
    }

    /// Interrupt the current turn.
    pub fn interrupt(&mut self) {
        if let Some(turn) = self.turns.last_mut() {
            turn.interrupt();
        }
        self.state = ThreadState::Interrupted;
        self.updated_at = Utc::now();
    }

    /// Resume after interruption.
    pub fn resume(&mut self) {
        if self.state == ThreadState::Interrupted {
            self.state = ThreadState::Idle;
            self.updated_at = Utc::now();
        }
    }

    /// Get all messages for context building.
    pub fn messages(&self) -> Vec<ChatMessage> {
        let mut messages = Vec::new();
        for turn in &self.turns {
            messages.push(ChatMessage::user(&turn.user_input));
            if let Some(ref response) = turn.response {
                messages.push(ChatMessage::assistant(response));
            }
        }
        messages
    }

    /// Truncate turns to a specific count (keeping most recent).
    pub fn truncate_turns(&mut self, keep: usize) {
        if self.turns.len() > keep {
            let drain_count = self.turns.len() - keep;
            self.turns.drain(0..drain_count);
            // Re-number remaining turns
            for (i, turn) in self.turns.iter_mut().enumerate() {
                turn.turn_number = i;
            }
        }
    }

    /// Restore thread state from a checkpoint's messages.
    ///
    /// Clears existing turns and rebuilds from message pairs.
    /// Messages should alternate: user, assistant, user, assistant...
    pub fn restore_from_messages(&mut self, messages: Vec<ChatMessage>) {
        self.turns.clear();
        self.state = ThreadState::Idle;

        // Messages alternate: user, assistant, user, assistant...
        let mut iter = messages.into_iter().peekable();
        let mut turn_number = 0;

        while let Some(msg) = iter.next() {
            if msg.role == crate::llm::Role::User {
                let mut turn = Turn::new(turn_number, &msg.content);

                // Check if next is assistant response
                if let Some(next) = iter.peek() {
                    if next.role == crate::llm::Role::Assistant {
                        let response = iter.next().expect("peeked");
                        turn.complete(&response.content);
                    }
                }

                self.turns.push(turn);
                turn_number += 1;
            }
        }

        self.updated_at = Utc::now();
    }
}

/// State of a turn.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum TurnState {
    /// Turn is being processed.
    Processing,
    /// Turn completed successfully.
    Completed,
    /// Turn failed with an error.
    Failed,
    /// Turn was interrupted.
    Interrupted,
}

/// A single turn (request/response pair) in a thread.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Turn {
    /// Turn number (0-indexed).
    pub turn_number: usize,
    /// User input that started this turn.
    pub user_input: String,
    /// Agent response (if completed).
    pub response: Option<String>,
    /// Tool calls made during this turn.
    pub tool_calls: Vec<TurnToolCall>,
    /// Turn state.
    pub state: TurnState,
    /// When the turn started.
    pub started_at: DateTime<Utc>,
    /// When the turn completed.
    pub completed_at: Option<DateTime<Utc>>,
    /// Error message (if failed).
    pub error: Option<String>,
}

impl Turn {
    /// Create a new turn.
    pub fn new(turn_number: usize, user_input: impl Into<String>) -> Self {
        Self {
            turn_number,
            user_input: user_input.into(),
            response: None,
            tool_calls: Vec::new(),
            state: TurnState::Processing,
            started_at: Utc::now(),
            completed_at: None,
            error: None,
        }
    }

    /// Complete this turn.
    pub fn complete(&mut self, response: impl Into<String>) {
        self.response = Some(response.into());
        self.state = TurnState::Completed;
        self.completed_at = Some(Utc::now());
    }

    /// Fail this turn.
    pub fn fail(&mut self, error: impl Into<String>) {
        self.error = Some(error.into());
        self.state = TurnState::Failed;
        self.completed_at = Some(Utc::now());
    }

    /// Interrupt this turn.
    pub fn interrupt(&mut self) {
        self.state = TurnState::Interrupted;
        self.completed_at = Some(Utc::now());
    }

    /// Record a tool call.
    pub fn record_tool_call(&mut self, name: impl Into<String>, params: serde_json::Value) {
        self.tool_calls.push(TurnToolCall {
            name: name.into(),
            parameters: params,
            result: None,
            error: None,
        });
    }

    /// Record tool call result.
    pub fn record_tool_result(&mut self, result: serde_json::Value) {
        if let Some(call) = self.tool_calls.last_mut() {
            call.result = Some(result);
        }
    }

    /// Record tool call error.
    pub fn record_tool_error(&mut self, error: impl Into<String>) {
        if let Some(call) = self.tool_calls.last_mut() {
            call.error = Some(error.into());
        }
    }
}

/// Record of a tool call made during a turn.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct TurnToolCall {
    /// Tool name.
    pub name: String,
    /// Parameters passed to the tool.
    pub parameters: serde_json::Value,
    /// Result from the tool (if successful).
    pub result: Option<serde_json::Value>,
    /// Error from the tool (if failed).
    pub error: Option<String>,
}

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

    #[test]
    fn test_session_creation() {
        let mut session = Session::new("user-123");
        assert!(session.active_thread.is_none());

        session.create_thread();
        assert!(session.active_thread.is_some());
    }

    #[test]
    fn test_thread_turns() {
        let mut thread = Thread::new(Uuid::new_v4());

        thread.start_turn("Hello");
        assert_eq!(thread.state, ThreadState::Processing);
        assert_eq!(thread.turns.len(), 1);

        thread.complete_turn("Hi there!");
        assert_eq!(thread.state, ThreadState::Idle);
        assert_eq!(thread.turns[0].response, Some("Hi there!".to_string()));
    }

    #[test]
    fn test_thread_messages() {
        let mut thread = Thread::new(Uuid::new_v4());

        thread.start_turn("First message");
        thread.complete_turn("First response");
        thread.start_turn("Second message");
        thread.complete_turn("Second response");

        let messages = thread.messages();
        assert_eq!(messages.len(), 4);
    }

    #[test]
    fn test_turn_tool_calls() {
        let mut turn = Turn::new(0, "Test input");
        turn.record_tool_call("echo", serde_json::json!({"message": "test"}));
        turn.record_tool_result(serde_json::json!("test"));

        assert_eq!(turn.tool_calls.len(), 1);
        assert!(turn.tool_calls[0].result.is_some());
    }

    #[test]
    fn test_restore_from_messages() {
        let mut thread = Thread::new(Uuid::new_v4());

        // First add some turns
        thread.start_turn("Original message");
        thread.complete_turn("Original response");

        // Now restore from different messages
        let messages = vec![
            ChatMessage::user("Hello"),
            ChatMessage::assistant("Hi there!"),
            ChatMessage::user("How are you?"),
            ChatMessage::assistant("I'm good!"),
        ];

        thread.restore_from_messages(messages);

        assert_eq!(thread.turns.len(), 2);
        assert_eq!(thread.turns[0].user_input, "Hello");
        assert_eq!(thread.turns[0].response, Some("Hi there!".to_string()));
        assert_eq!(thread.turns[1].user_input, "How are you?");
        assert_eq!(thread.turns[1].response, Some("I'm good!".to_string()));
        assert_eq!(thread.state, ThreadState::Idle);
    }

    #[test]
    fn test_restore_from_messages_incomplete_turn() {
        let mut thread = Thread::new(Uuid::new_v4());

        // Messages with incomplete last turn (no assistant response)
        let messages = vec![
            ChatMessage::user("Hello"),
            ChatMessage::assistant("Hi there!"),
            ChatMessage::user("How are you?"),
        ];

        thread.restore_from_messages(messages);

        assert_eq!(thread.turns.len(), 2);
        assert_eq!(thread.turns[1].user_input, "How are you?");
        assert!(thread.turns[1].response.is_none());
    }
}