// Filename: frontend/js/pages/chat/SessionManager.js

import { nanoid } from 'https://cdn.jsdelivr.net/npm/nanoid/nanoid.js';

const LOCAL_STORAGE_KEY = 'gemini_chat_state';

/**
 * Manages the state and persistence of chat sessions.
 * This class handles loading from/saving to localStorage,
 * and all operations like creating, switching, and deleting sessions.
 */
export class SessionManager {
    constructor() {
        this.state = null;
    }

    /**
     * Initializes the manager by loading state from localStorage or creating a default state.
     */
    init() {
        this._loadState();
    }

    // --- Public API for state access ---

    getSessions() {
        return this.state.sessions;
    }

    getCurrentSessionId() {
        return this.state.currentSessionId;
    }

    getCurrentSession() {
        return this.state.sessions.find(s => s.id === this.state.currentSessionId);
    }

    // --- Public API for state mutation ---

    /**
     * Creates a new, empty session and sets it as the current one.
     */
    createSession() {
        const newSessionId = nanoid();
        const newSession = {
            id: newSessionId,
            name: '新会话',
            systemPrompt: '',
            messages: [],
            modelConfig: { model: 'gemini-2.0-flash-lite' },
            params: { temperature: 0.7 }
        };
        this.state.sessions.unshift(newSession);
        this.state.currentSessionId = newSessionId;
        this._saveState();
    }

    /**
     * Switches the current session to the one with the given ID.
     * @param {string} sessionId The ID of the session to switch to.
     */
    switchSession(sessionId) {
        if (this.state.currentSessionId === sessionId) return;
        this.state.currentSessionId = sessionId;
        this._saveState();
    }

    /**
     * Deletes a session by its ID.
     * @param {string} sessionId The ID of the session to delete.
     */
    deleteSession(sessionId) {
        this.state.sessions = this.state.sessions.filter(s => s.id !== sessionId);

        if (this.state.currentSessionId === sessionId) {
            this.state.currentSessionId = this.state.sessions[0]?.id || null;
            if (!this.state.currentSessionId) {
                this._createInitialState(); // Create a new one if all are deleted
            }
        }
        
        this._saveState();
    }
    
    /**
     * [NEW] Clears all messages from the currently active session.
     */
    clearCurrentSession() {
        const currentSession = this.getCurrentSession();
        if (currentSession) {
            currentSession.messages = [];
            this._saveState();
        }
    }
    
    /**
     * Adds a message to the current session and updates the session name if it's the first message.
     * @param {object} message The message object to add.
     * @returns {object} The session that was updated.
     */
    addMessage(message) {
        const currentSession = this.getCurrentSession();
        if (currentSession) {
            if (currentSession.messages.length === 0 && message.role === 'user') {
                currentSession.name = message.content.substring(0, 30);
            }
            currentSession.messages.push(message);
            this._saveState();
            return currentSession;
        }
        return null;
    }

    deleteMessage(messageId) {
        const currentSession = this.getCurrentSession();
        if (currentSession) {
            const messageIndex = currentSession.messages.findIndex(m => m.id === messageId);
            if (messageIndex > -1) {
                currentSession.messages.splice(messageIndex, 1);
                this._saveState();
                console.log(`Message ${messageId} deleted.`);
            }
        }
    }

    truncateMessagesAfter(messageId) {
        const currentSession = this.getCurrentSession();
        if (currentSession) {
            const messageIndex = currentSession.messages.findIndex(m => m.id === messageId);
            // Ensure the message exists and it's not already the last one
            if (messageIndex > -1 && messageIndex < currentSession.messages.length - 1) {
                currentSession.messages.splice(messageIndex + 1);
                this._saveState();
                console.log(`Truncated messages after ${messageId}.`);
            }
        }
    }
    // --- Private persistence methods ---

    _saveState() {
        try {
            localStorage.setItem(LOCAL_STORAGE_KEY, JSON.stringify(this.state));
        } catch (error) {
            console.error("Failed to save session state:", error);
        }
    }

    _loadState() {
        try {
            const stateString = localStorage.getItem(LOCAL_STORAGE_KEY);
            if (stateString) {
                this.state = JSON.parse(stateString);
            } else {
                this._createInitialState();
            }
        } catch (error) {
            console.error("Failed to load session state, creating initial state:", error);
            this._createInitialState();
        }
    }
    
    _createInitialState() {
        const initialSessionId = nanoid();
        this.state = {
            sessions: [{
                id: initialSessionId,
                name: '新会话',
                systemPrompt: '',
                messages: [],
                modelConfig: { model: 'gemini-2.0-flash-lite' },
                params: { temperature: 0.7 }
            }],
            currentSessionId: initialSessionId,
            settings: {}
        };
        this._saveState();
    }
}