'use client'
import { useEffect } from 'react'

import type { AnalyticsConfig } from './types.js'

import { gtag } from '../consent/googleConsent.js'
import { CONSENT_COOKIE, DEFAULT_CONSENT, parseConsent, setDefaultConsent  } from '../consent/index.js'

declare global {
  interface Window {
    dataLayer?: unknown[]
  }
}

const GTM_SCRIPT = (id: string) => `https://www.googletagmanager.com/gtm.js?id=${id}`
const GA4_SCRIPT = (id: string) => `https://www.googletagmanager.com/gtag/js?id=${id}`

/** Reads the current consent state from cookie, or defaults if none. */
function readConsent() {
  if (typeof document === 'undefined') {return DEFAULT_CONSENT}
  const raw = document.cookie
    .split('; ')
    .find((c) => c.startsWith(`${CONSENT_COOKIE}=`))
    ?.split('=')[1]
  return parseConsent(raw) ?? DEFAULT_CONSENT
}

function injectScript(src: string) {
  if (document.querySelector(`script[src="${src}"]`)) {return}
  const s = document.createElement('script')
  s.src = src
  s.async = true
  document.head.appendChild(s)
}

/**
 * Loads Google Analytics — GTM if a container ID is set, otherwise GA4 gtag.
 *
 * Consent Mode is initialised to the visitor's stored consent BEFORE the tag
 * loads (via setDefaultConsent from the consent module), so tags respect the
 * choice from the first hit; useConsent sends an update the moment the visitor
 * decides. IDs are public and come from SiteIntegrations, passed in as props
 * (the client project reads them server-side).
 *
 * Renders nothing — it only injects the scripts. Mount once, high in the tree
 * (e.g. root layout), inside ConsentProvider.
 */
export function Analytics({ ga4MeasurementId, gtmContainerId }: AnalyticsConfig) {
  useEffect(() => {
    if (!ga4MeasurementId && !gtmContainerId) {return}

    // 1. Consent Mode default = the visitor's stored choice, before tags load.
    setDefaultConsent(readConsent())

    // 2. Load the tag.
    if (gtmContainerId) {
      window.dataLayer = window.dataLayer || []
      window.dataLayer.push({ event: 'gtm.js', 'gtm.start': Date.now() })
      injectScript(GTM_SCRIPT(gtmContainerId))
    } else if (ga4MeasurementId) {
      injectScript(GA4_SCRIPT(ga4MeasurementId))
      // Uses the shared gtag (pushes `arguments`, as Google's tags expect);
      // real gtag.js wires itself up once the script loads.
      gtag('js', new Date())
      gtag('config', ga4MeasurementId)
    }
  }, [ga4MeasurementId, gtmContainerId])

  return null
}