use anyhow::{Context, Result};
use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
use std::fs;
use std::path::PathBuf;

use crate::config::Config;

const SYSTEM_PROMPT: &str = "You are an AI assistant running in a terminal (CLI) environment. \
Optimise all answers for 80‑column readability, prefer plain text, \
ASCII art or concise bullet lists over heavy markup, and wrap code \
snippets in fenced blocks when helpful. Do not emit trailing spaces or \
control characters.";

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Message {
    pub role: String,
    pub content: String,
}

#[derive(Debug, Clone)]
pub struct ConversationStats {
    pub total_messages: usize,
    pub user_messages: usize,
    pub assistant_messages: usize,
    pub total_characters: usize,
    pub average_message_length: usize,
}

#[derive(Debug, Clone)]
pub struct SessionInfo {
    pub name: String,
    pub last_modified: DateTime<Utc>,
    pub model: Option<String>,
    pub message_count: Option<usize>,
    pub file_size: Option<u64>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SessionData {
    pub model: String,
    pub messages: Vec<Message>,
    pub enable_web_search: bool,
    pub enable_reasoning_summary: bool,
    #[serde(default = "default_reasoning_effort")]
    pub reasoning_effort: String,
    pub updated_at: DateTime<Utc>,
}

fn default_reasoning_effort() -> String {
    "medium".to_string()
}

#[derive(Debug, Clone)]
pub struct Session {
    pub name: String,
    pub model: String,
    pub messages: Vec<Message>,
    pub enable_web_search: bool,
    pub enable_reasoning_summary: bool,
    pub reasoning_effort: String,
}

impl Session {
    pub fn new(name: String, model: String) -> Self {
        let mut session = Self {
            name,
            model,
            messages: Vec::new(),
            enable_web_search: true,
            enable_reasoning_summary: false,
            reasoning_effort: "medium".to_string(),
        };
        
        // Add system prompt as first message
        session.messages.push(Message {
            role: "system".to_string(),
            content: SYSTEM_PROMPT.to_string(),
        });
        
        session
    }

    pub fn sessions_dir() -> Result<PathBuf> {
        let config = Config::load().unwrap_or_default();
        let home = dirs::home_dir().context("Could not find home directory")?;
        let sessions_dir = home.join(&config.session.sessions_dir_name);
        
        if !sessions_dir.exists() {
            fs::create_dir_all(&sessions_dir)
                .with_context(|| format!("Failed to create sessions directory: {:?}", sessions_dir))?;
        }
        
        Ok(sessions_dir)
    }

    pub fn session_path(name: &str) -> Result<PathBuf> {
        let config = Config::load().unwrap_or_default();
        Ok(Self::sessions_dir()?.join(format!("{}.{}", name, config.session.file_extension)))
    }

    pub fn save(&self) -> Result<()> {
        let data = SessionData {
            model: self.model.clone(),
            messages: self.messages.clone(),
            enable_web_search: self.enable_web_search,
            enable_reasoning_summary: self.enable_reasoning_summary,
            reasoning_effort: self.reasoning_effort.clone(),
            updated_at: Utc::now(),
        };

        let path = Self::session_path(&self.name)?;
        let tmp_path = path.with_extension("tmp");
        
        let json_data = serde_json::to_string_pretty(&data)
            .context("Failed to serialize session data")?;
        
        fs::write(&tmp_path, json_data)
            .with_context(|| format!("Failed to write session to {:?}", tmp_path))?;
        
        fs::rename(&tmp_path, &path)
            .with_context(|| format!("Failed to rename {:?} to {:?}", tmp_path, path))?;
        
        Ok(())
    }

    pub fn load(name: &str) -> Result<Self> {
        let path = Self::session_path(name)?;
        
        if !path.exists() {
            return Err(anyhow::anyhow!("Session '{}' does not exist", name));
        }

        let json_data = fs::read_to_string(&path)
            .with_context(|| format!("Failed to read session from {:?}", path))?;
        
        let data: SessionData = serde_json::from_str(&json_data)
            .with_context(|| format!("Failed to parse session data from {:?}", path))?;

        let mut session = Self {
            name: name.to_string(),
            model: data.model,
            messages: data.messages,
            enable_web_search: data.enable_web_search,
            enable_reasoning_summary: data.enable_reasoning_summary,
            reasoning_effort: data.reasoning_effort,
        };

        // Ensure system prompt is present
        if session.messages.is_empty() || session.messages[0].role != "system" {
            session.messages.insert(0, Message {
                role: "system".to_string(),
                content: SYSTEM_PROMPT.to_string(),
            });
        }

        Ok(session)
    }

    pub fn add_user_message(&mut self, content: String) {
        self.messages.push(Message {
            role: "user".to_string(),
            content,
        });
        self.truncate_history_if_needed();
    }

    pub fn add_assistant_message(&mut self, content: String) {
        self.messages.push(Message {
            role: "assistant".to_string(),
            content,
        });
        self.truncate_history_if_needed();
    }

    /// Truncates conversation history to stay within configured limits
    fn truncate_history_if_needed(&mut self) {
        let config = Config::load().unwrap_or_default();
        let max_history = config.limits.max_conversation_history;
        
        // Always preserve the system prompt (first message)
        if self.messages.len() > max_history + 1 {
            let system_prompt = self.messages[0].clone();
            let messages_to_keep = max_history;
            let start_index = self.messages.len() - messages_to_keep;
            
            // Keep the most recent messages
            let mut new_messages = vec![system_prompt];
            new_messages.extend_from_slice(&self.messages[start_index..]);
            
            self.messages = new_messages;
        }
    }

    /// Truncates individual messages that exceed reasonable length
    pub fn truncate_long_messages(&mut self) {
        const MAX_MESSAGE_LENGTH: usize = 10000; // 10k characters per message
        const TRUNCATION_NOTICE: &str = "\n\n[Message truncated for performance...]";
        
        for message in &mut self.messages {
            if message.content.len() > MAX_MESSAGE_LENGTH {
                message.content.truncate(MAX_MESSAGE_LENGTH - TRUNCATION_NOTICE.len());
                message.content.push_str(TRUNCATION_NOTICE);
            }
        }
    }

    /// Gets conversation statistics
    pub fn get_stats(&self) -> ConversationStats {
        let total_messages = self.messages.len();
        let user_messages = self.messages.iter().filter(|m| m.role == "user").count();
        let assistant_messages = self.messages.iter().filter(|m| m.role == "assistant").count();
        let total_chars: usize = self.messages.iter().map(|m| m.content.len()).sum();
        let avg_message_length = if total_messages > 0 { total_chars / total_messages } else { 0 };
        
        ConversationStats {
            total_messages,
            user_messages,
            assistant_messages,
            total_characters: total_chars,
            average_message_length: avg_message_length,
        }
    }

    pub fn clear_messages(&mut self) {
        self.messages.clear();
        // Re-add system prompt
        self.messages.push(Message {
            role: "system".to_string(),
            content: SYSTEM_PROMPT.to_string(),
        });
    }

    pub fn list_sessions() -> Result<Vec<(String, DateTime<Utc>)>> {
        let sessions = Self::list_sessions_lazy(false)?;
        Ok(sessions.into_iter().map(|s| (s.name, s.last_modified)).collect())
    }

    /// Lists sessions with lazy loading - only loads full data if detailed=true
    pub fn list_sessions_lazy(detailed: bool) -> Result<Vec<SessionInfo>> {
        let config = Config::load().unwrap_or_default();
        let sessions_dir = Self::sessions_dir()?;
        
        if !sessions_dir.exists() {
            return Ok(Vec::new());
        }

        let mut sessions = Vec::new();
        let mut count = 0;
        
        for entry in fs::read_dir(&sessions_dir)? {
            let entry = entry?;
            let path = entry.path();
            
            // Respect max sessions limit for performance
            if count >= config.limits.max_sessions_to_list {
                break;
            }
            
            if let Some(extension) = path.extension() {
                if extension == config.session.file_extension.as_str() {
                    if let Some(name) = path.file_stem().and_then(|s| s.to_str()) {
                        let metadata = entry.metadata()?;
                        let modified = metadata.modified()?;
                        let datetime = DateTime::<Utc>::from(modified);
                        let file_size = metadata.len();
                        
                        let (model, message_count) = if detailed {
                            // Only load session data if detailed info is requested
                            match Self::get_session_metadata(name) {
                                Ok((model, count)) => (Some(model), Some(count)),
                                Err(_) => (None, None)
                            }
                        } else {
                            (None, None)
                        };
                        
                        sessions.push(SessionInfo {
                            name: name.to_string(),
                            last_modified: datetime,
                            model,
                            message_count,
                            file_size: Some(file_size),
                        });
                        
                        count += 1;
                    }
                }
            }
        }
        
        sessions.sort_by(|a, b| b.last_modified.cmp(&a.last_modified)); // Sort by modification time, newest first
        Ok(sessions)
    }

    /// Gets metadata from a session file without loading all messages
    fn get_session_metadata(name: &str) -> Result<(String, usize)> {
        let path = Self::session_path(name)?;
        
        if !path.exists() {
            return Err(anyhow::anyhow!("Session '{}' does not exist", name));
        }

        let json_data = fs::read_to_string(&path)
            .with_context(|| format!("Failed to read session from {:?}", path))?;
        
        // Parse only the fields we need for metadata
        let data: SessionData = serde_json::from_str(&json_data)
            .with_context(|| format!("Failed to parse session data from {:?}", path))?;
        
        Ok((data.model, data.messages.len()))
    }

    /// Optimizes session data in memory by removing redundant information
    pub fn optimize_for_memory(&mut self) {
        // Truncate very long messages for memory efficiency
        self.truncate_long_messages();
        
        // Remove excessive whitespace from messages
        for message in &mut self.messages {
            message.content = message.content
                .lines()
                .map(|line| line.trim())
                .collect::<Vec<_>>()
                .join("\n")
                .trim()
                .to_string();
        }
    }

    /// Checks if session needs cleanup based on size and age
    pub fn needs_cleanup(&self) -> bool {
        let stats = self.get_stats();
        let config = Config::load().unwrap_or_default();
        
        // Check if conversation is too long
        if stats.total_messages > config.limits.max_conversation_history * 2 {
            return true;
        }
        
        // Check if total character count is excessive (>1MB)
        if stats.total_characters > 1_000_000 {
            return true;
        }
        
        false
    }

    /// Performs aggressive cleanup for memory optimization
    pub fn cleanup_for_memory(&mut self) {
        let config = Config::load().unwrap_or_default();
        let target_messages = config.limits.max_conversation_history / 2;
        
        if self.messages.len() > target_messages + 1 {
            let system_prompt = self.messages[0].clone();
            let start_index = self.messages.len() - target_messages;
            
            let mut new_messages = vec![system_prompt];
            new_messages.extend_from_slice(&self.messages[start_index..]);
            
            self.messages = new_messages;
        }
        
        self.optimize_for_memory();
    }

    pub fn delete_session(name: &str) -> Result<()> {
        let path = Self::session_path(name)?;
        
        if !path.exists() {
            return Err(anyhow::anyhow!("Session '{}' does not exist", name));
        }

        fs::remove_file(&path)
            .with_context(|| format!("Failed to delete session file: {:?}", path))?;
        
        Ok(())
    }
}