/**
 * @mizan/runtime — The client state kernel.
 *
 * Zero framework dependencies. React, Vue, Svelte — all import from here.
 *
 * Four concerns:
 * 1. Configuration — baseUrl, auth headers, CSRF
 * 2. Context registry — mounted providers register for invalidation
 * 3. Invalidation — microtask-batched, scoped or broad
 * 4. Fetch — mizanFetch (GET context bundles) + mizanCall (POST mutations)
 */

// === Error ===

export class MizanError extends Error {
    constructor(public status: number, public body: string) {
        super(`Mizan call failed (${status})`)
    }
}

// === Configuration ===

interface MizanConfig {
    baseUrl: string
    getHeaders: () => Record<string, string> | Promise<Record<string, string>>
    csrfCookieName: string
    csrfHeaderName: string
}

const config: MizanConfig = {
    baseUrl: '/api/mizan',
    getHeaders: () => ({}),
    csrfCookieName: 'csrftoken',
    csrfHeaderName: 'X-CSRFToken',
}

export function configure(opts: Partial<MizanConfig>): void {
    Object.assign(config, opts)
}

export function getConfig(): Readonly<MizanConfig> {
    return config
}

// === CSRF ===

function getCSRFToken(): string | null {
    if (typeof document === 'undefined') return null
    const match = document.cookie.match(new RegExp(`${config.csrfCookieName}=([^;]+)`))
    return match?.[1] ?? null
}

// === Session Init ===

let _sessionReady: Promise<void> | null = null

/**
 * Initialize a session (fetches CSRF cookie from GET /session/).
 * Called automatically on first fetch if not called explicitly.
 * No-op if a CSRF cookie already exists.
 */
export function initSession(): Promise<void> {
    if (_sessionReady) return _sessionReady

    _sessionReady = (async () => {
        // If we already have a CSRF token, skip
        if (getCSRFToken()) return

        try {
            await fetch(`${config.baseUrl}/session/`, { credentials: 'include' })
        } catch (e) {
            console.error('[mizan] Session init failed:', e)
        }
    })()

    return _sessionReady
}

// === Context Registry ===

export type RefetchFn = () => void
type ParamKey = string

interface ContextEntry {
    params: Record<string, any>
    refetch: RefetchFn
}

const contexts: Map<string, Map<ParamKey, ContextEntry>> = new Map()

export function registerContext(
    name: string,
    params: Record<string, any>,
    refetch: RefetchFn,
): () => void {
    if (!contexts.has(name)) contexts.set(name, new Map())
    const key = JSON.stringify(params)
    contexts.get(name)!.set(key, { params, refetch })
    return () => contexts.get(name)!.delete(key)
}

// === Invalidation ===

const pending: Set<string> = new Set()
const pendingScoped: Map<string, Record<string, any>> = new Map()
let scheduled = false

export function invalidate(context: string, params?: Record<string, any>): void {
    if (params) {
        pendingScoped.set(context, params)
    } else {
        pending.add(context)
    }
    if (!scheduled) {
        scheduled = true
        queueMicrotask(flush)
    }
}

function flush(): void {
    for (const name of pending) {
        const entries = contexts.get(name)
        if (entries) entries.forEach(entry => entry.refetch())
    }

    for (const [name, params] of pendingScoped) {
        if (pending.has(name)) continue
        const entries = contexts.get(name)
        if (!entries) continue
        const key = JSON.stringify(params)
        const entry = entries.get(key)
        if (entry) entry.refetch()
    }

    pending.clear()
    pendingScoped.clear()
    scheduled = false
}

// === Fetch ===

async function resolveHeaders(): Promise<Record<string, string>> {
    await initSession()

    const custom = await config.getHeaders()
    const csrf = getCSRFToken()

    return {
        ...custom,
        ...(csrf ? { [config.csrfHeaderName]: csrf } : {}),
        'Accept': 'application/json',
    }
}

export async function mizanFetch(
    contextName: string,
    params?: Record<string, any>,
): Promise<any> {
    const url = new URL(
        `${config.baseUrl}/ctx/${contextName}/`,
        typeof globalThis.location !== 'undefined' ? globalThis.location.origin : 'http://localhost',
    )
    if (params) {
        for (const [k, v] of Object.entries(params)) {
            url.searchParams.set(k, String(v))
        }
    }

    const headers = await resolveHeaders()
    const res = await fetch(url.toString(), { headers, credentials: 'same-origin' })
    if (!res.ok) throw new MizanError(res.status, await res.text())
    return res.json()
}

export async function mizanCall(
    functionName: string,
    args: Record<string, any>,
): Promise<any> {
    const headers = await resolveHeaders()
    headers['Content-Type'] = 'application/json'

    const res = await fetch(`${config.baseUrl}/call/`, {
        method: 'POST',
        headers,
        credentials: 'same-origin',
        body: JSON.stringify({ fn: functionName, args }),
    })
    if (!res.ok) throw new MizanError(res.status, await res.text())

    const data = await res.json()

    // Server-driven invalidation
    if (data.invalidate) {
        for (const entry of data.invalidate) {
            if (typeof entry === 'string') {
                invalidate(entry)
            } else {
                invalidate(entry.context, entry.params)
            }
        }
    }

    return data.result
}