// frontend/utils/preloadApi/config.js
// 配置加载器：合并用户配置与内置默认值，提取 fetcher 映射表

/**
 * 内置默认值（与设计文档 §3 defaults 一致）
 */
const BUILTIN_DEFAULTS = {
	ttl: 5 * 60 * 1000,
	persistence: 'memory',
	concurrency: 4,
	timeout: 10000,
	silent: true,
	limits: {
		maxEntrySizeKB: 1024,
		maxMemoryEntries: 100,
		maxFileCacheMB: 50
	}
}

/**
 * 加载并合并配置
 * @param {object} userConfig - 用户提供的 preload.config.js export
 * @returns {{ config: object, fetchers: object }}
 */
export function loadConfig(userConfig) {
	if (!userConfig) {
		throw new Error('[preload] config is required')
	}

	// 合并 defaults
	const mergedDefaults = {
		...BUILTIN_DEFAULTS,
		...(userConfig.defaults || {}),
		limits: {
			...BUILTIN_DEFAULTS.limits,
			...((userConfig.defaults && userConfig.defaults.limits) || {})
		}
	}

	const config = {
		defaults: mergedDefaults,
		startup: userConfig.startup || [],
		idle: userConfig.idle || [],
		pages: userConfig.pages || {}
	}

	// 提取 fetcher 映射表
	const fetchers = {}
	const allEntries = [
		...(config.startup || []),
		...(config.idle || [])
	]
	for (const entries of Object.values(config.pages || {})) {
		if (Array.isArray(entries)) allEntries.push(...entries)
	}

	for (const entry of allEntries) {
		if (entry.key && typeof entry.fetcher === 'function') {
			fetchers[entry.key] = entry.fetcher
		}
	}

	return { config, fetchers }
}