/**
 * Background Data Service
 * Handles preemptive loading of tree data for better performance
 */
export class BackgroundDataService {
    constructor() {
        this.treeDataCache = null;
        this.isLoading = false;
        this.loadPromise = null;
        this.authToken = null;
        this.batchSize = 1000;
    }

    setAuthToken(token) {
        this.authToken = token;
    }

    async authenticatedFetch(url, options = {}) {
        if (!this.authToken) {
            throw new Error('No auth token available');
        }

        const headers = {
            'Content-Type': 'application/json',
            'Authorization': `Bearer ${this.authToken}`,
            ...options.headers
        };
        
        const response = await fetch(url, {
            ...options,
            headers
        });
        
        if (response.status === 401) {
            // Token expired or invalid
            localStorage.removeItem('auth_token');
            localStorage.removeItem('user_info');
            window.location.href = '/login';
            return null;
        }
        
        return response;
    }

    /**
     * Start preemptive loading of all tree data
     */
    async startPreemptiveLoading() {
        if (this.isLoading || this.treeDataCache) {
            return this.loadPromise;
        }

        console.log('🚀 Starting preemptive tree data loading...');
        this.isLoading = true;

        this.loadPromise = this.loadAllTreesInBackground();
        return this.loadPromise;
    }

    /**
     * Load all trees in background with batching
     */
    async loadAllTreesInBackground() {
        try {
            let allTrees = [];
            let offset = 0;
            let hasMoreTrees = true;
            
            console.log('📦 Loading trees in background batches...');
            
            while (hasMoreTrees && allTrees.length < 3000) { // Safety limit
                console.log(`📥 Background batch: offset=${offset}, limit=${this.batchSize}`);
                
                const response = await this.authenticatedFetch(`/api/trees?limit=${this.batchSize}&offset=${offset}`);
                if (!response) {
                    console.error('❌ Failed to fetch background batch');
                    break;
                }
                
                const batchTrees = await response.json();
                console.log(`✅ Background batch loaded: ${batchTrees.length} trees`);
                
                if (batchTrees.length === 0) {
                    hasMoreTrees = false;
                    break;
                }
                
                allTrees = allTrees.concat(batchTrees);
                offset += this.batchSize;
                
                // If we got less than the batch size, we've reached the end
                if (batchTrees.length < this.batchSize) {
                    hasMoreTrees = false;
                }
                
                // Small delay to prevent overwhelming the server
                await this.delay(100);
            }
            
            // Filter out problematic trees
            const filteredTrees = allTrees.filter(tree => {
                // Exclude tree ID 18
                if (tree.id === 18) {
                    console.log('🔄 Background: Excluding tree ID 18 from cache');
                    return false;
                }
                return true;
            });
            
            this.treeDataCache = filteredTrees;
            this.isLoading = false;
            
            console.log(`🎉 Background loading complete: ${filteredTrees.length} trees cached`);
            
            // Dispatch custom event to notify that data is ready
            window.dispatchEvent(new CustomEvent('treeDataReady', { 
                detail: { count: filteredTrees.length } 
            }));
            
            return filteredTrees;
            
        } catch (error) {
            console.error('❌ Background tree loading failed:', error);
            this.isLoading = false;
            throw error;
        }
    }

    /**
     * Get cached tree data (returns immediately if available)
     */
    getCachedTreeData() {
        return this.treeDataCache;
    }

    /**
     * Check if data is ready
     */
    isDataReady() {
        return this.treeDataCache !== null;
    }

    /**
     * Get tree data - uses cache if available, otherwise loads
     */
    async getTreeData() {
        if (this.treeDataCache) {
            console.log('⚡ Using cached tree data');
            return this.treeDataCache;
        }
        
        if (this.isLoading) {
            console.log('⏳ Waiting for background loading to complete...');
            return await this.loadPromise;
        }
        
        console.log('🔄 Cache miss, loading trees now...');
        return await this.startPreemptiveLoading();
    }

    /**
     * Clear cached data (useful for refresh scenarios)
     */
    clearCache() {
        this.treeDataCache = null;
        this.isLoading = false;
        this.loadPromise = null;
        console.log('🗑️ Tree data cache cleared');
    }

    /**
     * Simple delay utility
     */
    delay(ms) {
        return new Promise(resolve => setTimeout(resolve, ms));
    }

    /**
     * Get loading status for UI feedback
     */
    getLoadingStatus() {
        return {
            isLoading: this.isLoading,
            isReady: this.isDataReady(),
            cacheSize: this.treeDataCache ? this.treeDataCache.length : 0
        };
    }
}

// Export singleton instance
export const backgroundDataService = new BackgroundDataService();