const cloud = require('wx-server-sdk')
cloud.init({ env: cloud.DYNAMIC_CURRENT_ENV })
const db = cloud.database()
const _ = db.command

const COLLECTION = 'plank_data'
const PAGE_SIZE = 100
const DEFAULT_MAX_RANK = 100  // fallback if client doesn't pass maxRank

const pad = (n) => String(n).padStart(2, '0')

// Fixed UTC+8 offset — the user base is China-only and `new Date()` in
// the cloud function runs in UTC, so without this shift "today" rolls
// over at 08:00 Beijing instead of 00:00. If you ever support users in
// other timezones, switch back to a client-provided date.
const TZ_OFFSET_MS = 8 * 60 * 60 * 1000

// In-instance cache of the latest doc per openid. The full-collection
// scan + dedup is the expensive part of building the board; cache it for
// a short TTL so repeated board opens (and day/month/year switches within
// a session) reuse one scan. Cold starts just re-scan. Not persistent
// across instances, which is fine - the board needn't be real-time.
const CACHE_TTL = 60 * 1000
let _docsCache = null  // { latestByOpenid: Map, ts }

const tsOf = (d) => {
  if (!d.updatedAt) return 0
  // CloudBase returns Date objects in the SDK, but be defensive in case
  // a stringified ISO timestamp sneaks through (older SDKs / migrations).
  if (d.updatedAt instanceof Date) return d.updatedAt.getTime()
  const t = new Date(d.updatedAt).getTime()
  return Number.isFinite(t) ? t : 0
}

// Page through the collection keeping only the most-recently-updated doc
// per openid. Uses cursor pagination (_id > lastId) instead of skip(N):
// skip is O(N) and degrades as the collection grows; a cursor is constant
// cost per page, and default get() order (_id ascending) won't drop/dup.
const _fetchLatestByOpenid = async () => {
  if (_docsCache && (Date.now() - _docsCache.ts) < CACHE_TTL) {
    return _docsCache.latestByOpenid
  }
  const latestByOpenid = new Map()
  // Exclude accounts that haven't synced in 90+ days to bound the scan.
  // Docs without an updatedAt field (pre-fix legacy) are included as well.
  const ninetyDaysAgo = new Date(Date.now() - 90 * 24 * 60 * 60 * 1000)
  let lastId = ''
  while (true) {
    const cond = lastId
      ? _.and([
          _.or([{ updatedAt: _.gte(ninetyDaysAgo) }, { updatedAt: _.exists(false) }]),
          { _id: _.gt(lastId) }
        ])
      : _.or([{ updatedAt: _.gte(ninetyDaysAgo) }, { updatedAt: _.exists(false) }])
    const res = await db.collection(COLLECTION).where(cond).limit(PAGE_SIZE).get()
    if (!res.data || res.data.length === 0) break
    for (const doc of res.data) {
      const openid = doc._openid || 'unknown'
      const current = latestByOpenid.get(openid)
      if (!current || tsOf(doc) > tsOf(current)) {
        latestByOpenid.set(openid, doc)
      }
    }
    if (res.data.length < PAGE_SIZE) break
    lastId = res.data[res.data.length - 1]._id
  }
  _docsCache = { latestByOpenid, ts: Date.now() }
  return latestByOpenid
}

exports.main = async (event) => {
  const { period, maxRank } = event || {}
  const limit = Math.max(1, Math.min(parseInt(maxRank) || DEFAULT_MAX_RANK, 500))
  if (!period) return { err: 'missing period' }

  const now = new Date()
  const local = new Date(now.getTime() + TZ_OFFSET_MS)
  const today = `${local.getUTCFullYear()}-${pad(local.getUTCMonth() + 1)}-${pad(local.getUTCDate())}`
  const thisMonth = today.substring(0, 7)
  // Year is computed from the same shifted date so year boundaries also
  // align with Beijing's midnight, not UTC's.
  const thisYear = String(local.getUTCFullYear())
  const myOpenid = cloud.getWXContext().OPENID

  let prefix, exact
  if (period === 'day')        { exact = today;   prefix = null }
  else if (period === 'month') { prefix = thisMonth; exact = null }
  else if (period === 'year')  { prefix = thisYear;  exact = null }
  else return { err: 'invalid period' }

  const userMap = new Map()

  // Latest doc per openid (cached + cursor-paginated in _fetchLatestByOpenid).
  // Dedup rationale: a user may have multiple docs from the pre-fix cold-
  // start add() bug; only the latest updatedAt counts, else we'd sum the
  // same records N times and inflate the board. Latest (not first) also
  // gives the freshest profile.nickname before admin-dedupe runs.
  const latestByOpenid = await _fetchLatestByOpenid()

  // Second pass: each openid appears exactly once, so the accumulation
  // logic doesn't need any dedup guards.
  for (const doc of latestByOpenid.values()) {
    const openid = doc._openid || 'unknown'
    const records = doc.records || {}
    let duration = 0
    let sessions = 0

    for (const monthKey of Object.keys(records)) {
      if (prefix && !monthKey.startsWith(prefix)) continue
      for (const r of records[monthKey]) {
        if (period === 'day') {
          // Use startsWith rather than === to tolerate date strings with
          // trailing time/zone info (e.g. "2026-06-11T09:11:24.587Z" if
          // some legacy path stored an ISO date) and to be timezone-
          // agnostic when client local date differs from server UTC date.
          if (r.date.startsWith(exact)) { duration += r.duration; sessions++ }
        } else {
          if (r.date.startsWith(prefix)) { duration += r.duration; sessions++ }
        }
      }
    }

    if (duration > 0) {
      const profile = doc.profile || {}
      userMap.set(openid, {
        openid,
        duration,
        sessions,
        nickname: profile.nickname || ''
      })
    }
  }

  // Full sorted list (for ranking)
  const allSorted = Array.from(userMap.values())
    .sort((a, b) => b.duration - a.duration)

  // My entry always included, even if outside top N
  const myEntry = userMap.get(myOpenid)
  const myRank = myEntry ? allSorted.findIndex(e => e.openid === myOpenid) + 1 : 0

  // Resolve display name: prefer the user's chosen nickname, fall back to
  // a masked openid for users who haven't set one.
  const _displayName = (entry) =>
    (entry && entry.nickname && entry.nickname.trim()) ||
    maskOpenid(entry && entry.openid)

  // Top N for the board
  const ranked = allSorted.slice(0, limit).map((item, i) => ({
    rank: i + 1,
    openid: item.openid,
    nickname: item.nickname || '',
    name: _displayName(item),
    duration: item.duration,
    sessions: item.sessions
  }))

  return {
    period,
    ranked,
    myOpenid,
    myEntry: myEntry ? {
      rank: myRank,
      openid: myEntry.openid,
      nickname: myEntry.nickname || '',
      name: _displayName(myEntry),
      duration: myEntry.duration,
      sessions: myEntry.sessions
    } : null,
    updatedAt: now.toISOString()
  }
}

function maskOpenid(openid) {
  if (!openid || openid === 'unknown') return '未知用户'
  if (openid.length <= 4) return '****' + openid
  return '****' + openid.slice(-4)
}