const ADSENSE_TOKEN_URL = 'https://oauth2.googleapis.com/token'
const ADSENSE_API_BASE_URL = 'https://adsense.googleapis.com/v2'
const ADSENSE_REPORT_METRICS = [
  'ESTIMATED_EARNINGS',
  'PAGE_VIEWS',
  'IMPRESSIONS',
  'CLICKS',
  'PAGE_VIEWS_RPM'
]

let cachedAccessToken = ''
let cachedAccessTokenExpiresAt = 0

/**
 * AdSense API 연동 설정을 반환한다.
 * @returns {{ clientId: string, clientSecret: string, refreshToken: string, accountName: string, currencyCode: string }} AdSense 설정
 */
const getAdsenseConfig = () => ({
  clientId: process.env.GOOGLE_ADSENSE_CLIENT_ID || '',
  clientSecret: process.env.GOOGLE_ADSENSE_CLIENT_SECRET || '',
  refreshToken: process.env.GOOGLE_ADSENSE_REFRESH_TOKEN || '',
  accountName: process.env.GOOGLE_ADSENSE_ACCOUNT_NAME || '',
  currencyCode: process.env.GOOGLE_ADSENSE_CURRENCY_CODE || ''
})

/**
 * AdSense API 호출에 필요한 설정이 모두 있는지 확인한다.
 * @param {ReturnType<typeof getAdsenseConfig>} config - AdSense 설정
 * @returns {boolean} 설정 완료 여부
 */
const isAdsenseConfigured = (config) => {
  return Boolean(config.clientId && config.clientSecret && config.refreshToken && config.accountName)
}

/**
 * Date 객체를 Google API Date 필드로 변환한다.
 * @param {Date} date - 날짜
 * @returns {{ year: string, month: string, day: string }} Google API Date 필드
 */
const toGoogleDateFields = (date) => ({
  year: String(date.getFullYear()),
  month: String(date.getMonth() + 1),
  day: String(date.getDate())
})

/**
 * 로컬 시간 기준 날짜 시작점을 만든다.
 * @param {Date} date - 기준 날짜
 * @returns {Date} 날짜 시작점
 */
const startOfLocalDay = (date) => new Date(date.getFullYear(), date.getMonth(), date.getDate())

/**
 * 날짜에 일 단위를 더한다.
 * @param {Date} date - 기준 날짜
 * @param {number} days - 더할 일수
 * @returns {Date} 계산된 날짜
 */
const addDays = (date, days) => {
  const nextDate = new Date(date)
  nextDate.setDate(nextDate.getDate() + days)
  return nextDate
}

/**
 * 이번 달 시작 날짜를 만든다.
 * @param {Date} date - 기준 날짜
 * @returns {Date} 이번 달 시작 날짜
 */
const startOfLocalMonth = (date) => new Date(date.getFullYear(), date.getMonth(), 1)

/**
 * 빈 AdSense 지표를 만든다.
 * @param {string} label - 기간 라벨
 * @returns {Object} 빈 지표
 */
const createEmptyAdsenseMetric = (label) => ({
  label,
  estimatedEarnings: 0,
  pageViews: 0,
  impressions: 0,
  clicks: 0,
  pageViewsRpm: 0
})

/**
 * 숫자 셀 값을 안전하게 변환한다.
 * @param {unknown} value - 원본 값
 * @returns {number} 숫자 값
 */
const toMetricNumber = (value) => {
  const number = Number(value || 0)
  return Number.isFinite(number) ? number : 0
}

/**
 * Google OAuth refresh token으로 access token을 발급한다.
 * @param {ReturnType<typeof getAdsenseConfig>} config - AdSense 설정
 * @returns {Promise<string>} access token
 */
const getAdsenseAccessToken = async (config) => {
  const now = Date.now()

  if (cachedAccessToken && cachedAccessTokenExpiresAt > now + 60000) {
    return cachedAccessToken
  }

  const response = await fetch(ADSENSE_TOKEN_URL, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded'
    },
    body: new URLSearchParams({
      client_id: config.clientId,
      client_secret: config.clientSecret,
      refresh_token: config.refreshToken,
      grant_type: 'refresh_token'
    })
  })
  const data = await response.json().catch(() => ({}))

  if (!response.ok) {
    throw new Error(data.error_description || data.error || 'AdSense access token 발급 실패')
  }

  cachedAccessToken = data.access_token || ''
  cachedAccessTokenExpiresAt = now + (Number(data.expires_in || 0) * 1000)

  if (!cachedAccessToken) {
    throw new Error('AdSense access token 응답이 비어 있습니다.')
  }

  return cachedAccessToken
}

/**
 * AdSense 보고서 URL을 만든다.
 * @param {Object} options - 보고서 옵션
 * @param {string} options.accountName - AdSense 계정 이름
 * @param {Date} options.startDate - 시작일
 * @param {Date} options.endDate - 종료일
 * @param {string} options.currencyCode - 통화 코드
 * @returns {URL} 보고서 URL
 */
const createAdsenseReportUrl = ({ accountName, startDate, endDate, currencyCode }) => {
  const url = new URL(`${ADSENSE_API_BASE_URL}/${accountName}/reports:generate`)
  const start = toGoogleDateFields(startDate)
  const end = toGoogleDateFields(endDate)

  for (const metric of ADSENSE_REPORT_METRICS) {
    url.searchParams.append('metrics', metric)
  }

  url.searchParams.set('startDate.year', start.year)
  url.searchParams.set('startDate.month', start.month)
  url.searchParams.set('startDate.day', start.day)
  url.searchParams.set('endDate.year', end.year)
  url.searchParams.set('endDate.month', end.month)
  url.searchParams.set('endDate.day', end.day)
  url.searchParams.set('languageCode', 'ko-KR')

  if (currencyCode) {
    url.searchParams.set('currencyCode', currencyCode)
  }

  return url
}

/**
 * AdSense 단일 기간 보고서를 조회한다.
 * @param {Object} options - 조회 옵션
 * @param {string} options.label - 기간 라벨
 * @param {string} options.accessToken - access token
 * @param {ReturnType<typeof getAdsenseConfig>} options.config - AdSense 설정
 * @param {Date} options.startDate - 시작일
 * @param {Date} options.endDate - 종료일
 * @returns {Promise<Object>} 기간 지표
 */
const fetchAdsensePeriodReport = async ({ label, accessToken, config, startDate, endDate }) => {
  const url = createAdsenseReportUrl({
    accountName: config.accountName,
    startDate,
    endDate,
    currencyCode: config.currencyCode
  })
  const response = await fetch(url, {
    headers: {
      Authorization: `Bearer ${accessToken}`
    }
  })
  const data = await response.json().catch(() => ({}))

  if (!response.ok) {
    throw new Error(data.error?.message || `AdSense 보고서 조회 실패: ${response.status}`)
  }

  const row = Array.isArray(data.rows) ? data.rows[0] : null
  const cells = row && Array.isArray(row.cells) ? row.cells : []
  const headers = Array.isArray(data.headers) ? data.headers : []
  const metric = createEmptyAdsenseMetric(label)

  headers.forEach((header, index) => {
    const value = toMetricNumber(cells[index]?.value)

    if (header.name === 'ESTIMATED_EARNINGS') {
      metric.estimatedEarnings = value
      return
    }

    if (header.name === 'PAGE_VIEWS') {
      metric.pageViews = value
      return
    }

    if (header.name === 'IMPRESSIONS') {
      metric.impressions = value
      return
    }

    if (header.name === 'CLICKS') {
      metric.clicks = value
      return
    }

    if (header.name === 'PAGE_VIEWS_RPM') {
      metric.pageViewsRpm = value
    }
  })

  return metric
}

/**
 * 관리자 대시보드용 AdSense 요약을 조회한다.
 * @returns {Promise<Object>} AdSense 요약
 */
export const getAdsenseDashboardSummary = async () => {
  const config = getAdsenseConfig()

  if (!isAdsenseConfigured(config)) {
    return {
      configured: false,
      ok: false,
      message: 'AdSense API 환경 변수가 아직 설정되지 않았습니다.',
      currencyCode: config.currencyCode || 'KRW',
      generatedAt: new Date().toISOString(),
      today: createEmptyAdsenseMetric('오늘'),
      yesterday: createEmptyAdsenseMetric('어제'),
      month: createEmptyAdsenseMetric('이번 달')
    }
  }

  try {
    const today = startOfLocalDay(new Date())
    const yesterday = addDays(today, -1)
    const monthStart = startOfLocalMonth(today)
    const accessToken = await getAdsenseAccessToken(config)
    const [todayMetric, yesterdayMetric, monthMetric] = await Promise.all([
      fetchAdsensePeriodReport({
        label: '오늘',
        accessToken,
        config,
        startDate: today,
        endDate: today
      }),
      fetchAdsensePeriodReport({
        label: '어제',
        accessToken,
        config,
        startDate: yesterday,
        endDate: yesterday
      }),
      fetchAdsensePeriodReport({
        label: '이번 달',
        accessToken,
        config,
        startDate: monthStart,
        endDate: today
      })
    ])

    return {
      configured: true,
      ok: true,
      message: '',
      currencyCode: config.currencyCode || 'KRW',
      generatedAt: new Date().toISOString(),
      today: todayMetric,
      yesterday: yesterdayMetric,
      month: monthMetric
    }
  } catch (error) {
    return {
      configured: true,
      ok: false,
      message: error instanceof Error ? error.message : 'AdSense 통계를 불러오지 못했습니다.',
      currencyCode: config.currencyCode || 'KRW',
      generatedAt: new Date().toISOString(),
      today: createEmptyAdsenseMetric('오늘'),
      yesterday: createEmptyAdsenseMetric('어제'),
      month: createEmptyAdsenseMetric('이번 달')
    }
  }
}