local config = require('pending.config')
local log = require('pending.log')
local oauth = require('pending.sync.oauth')

local M = {}

M.name = 'gcal'

local BASE_URL = 'https://www.googleapis.com/calendar/v3'

---@param access_token string
---@return table<string, string>? name_to_id
---@return string? err
local function get_all_calendars(access_token)
  local data, err = oauth.curl_request(
    'GET',
    BASE_URL .. '/users/me/calendarList',
    oauth.auth_headers(access_token)
  )
  if err then
    return nil, err
  end
  local result = {}
  for _, item in ipairs(data and data.items or {}) do
    if item.summary then
      result[item.summary] = item.id
    end
  end
  return result, nil
end

---@param access_token string
---@param name string
---@param existing table<string, string>
---@return string? calendar_id
---@return string? err
local function find_or_create_calendar(access_token, name, existing)
  if existing[name] then
    return existing[name], nil
  end
  local body = vim.json.encode({ summary = name })
  local created, err =
    oauth.curl_request('POST', BASE_URL .. '/calendars', oauth.auth_headers(access_token), body)
  if err then
    return nil, err
  end
  local id = created and created.id
  if id then
    existing[name] = id
  end
  return id, nil
end

---@param date_str string
---@return string
local function next_day(date_str)
  local y, m, d = date_str:match('^(%d%d%d%d)-(%d%d)-(%d%d)')
  local t = os.time({ year = tonumber(y) or 0, month = tonumber(m) or 0, day = tonumber(d) or 0 })
    + 86400
  return os.date('%Y-%m-%d', t) --[[@as string]]
end

---@param access_token string
---@param calendar_id string
---@param task pending.Task
---@return string? event_id
---@return string? err
local function create_event(access_token, calendar_id, task)
  local event = {
    summary = task.description,
    start = { date = task.due },
    ['end'] = { date = next_day(task.due or '') },
    transparency = 'transparent',
    extendedProperties = {
      private = { taskId = tostring(task.id) },
    },
  }
  local data, err = oauth.curl_request(
    'POST',
    BASE_URL .. '/calendars/' .. oauth.url_encode(calendar_id) .. '/events',
    oauth.auth_headers(access_token),
    vim.json.encode(event)
  )
  if err then
    return nil, err
  end
  return data and data.id, nil
end

---@param access_token string
---@param calendar_id string
---@param event_id string
---@param task pending.Task
---@return string? err
local function update_event(access_token, calendar_id, event_id, task)
  local event = {
    summary = task.description,
    start = { date = task.due },
    ['end'] = { date = next_day(task.due or '') },
    transparency = 'transparent',
  }
  local _, err = oauth.curl_request(
    'PATCH',
    BASE_URL
      .. '/calendars/'
      .. oauth.url_encode(calendar_id)
      .. '/events/'
      .. oauth.url_encode(event_id),
    oauth.auth_headers(access_token),
    vim.json.encode(event)
  )
  return err
end

---@param access_token string
---@param calendar_id string
---@param event_id string
---@return string? err
local function delete_event(access_token, calendar_id, event_id)
  local _, err = oauth.curl_request(
    'DELETE',
    BASE_URL
      .. '/calendars/'
      .. oauth.url_encode(calendar_id)
      .. '/events/'
      .. oauth.url_encode(event_id),
    oauth.auth_headers(access_token)
  )
  return err
end

---@param callback fun(access_token: string): nil
local function with_token(callback)
  oauth.async(function()
    local token = oauth.google_client:get_access_token()
    if not token then
      oauth.google_client:auth(function()
        oauth.async(function()
          local fresh = oauth.google_client:get_access_token()
          if fresh then
            callback(fresh)
          else
            log.error(oauth.google_client.name .. ': authorization failed or was cancelled')
          end
        end)
      end)
      return
    end
    callback(token)
  end)
end

function M.push()
  with_token(function(access_token)
    local calendars, cal_err = get_all_calendars(access_token)
    if cal_err or not calendars then
      log.error(cal_err or 'failed to fetch calendars')
      return
    end

    local s = require('pending').store()
    local created, updated, deleted = 0, 0, 0

    for _, task in ipairs(s:tasks()) do
      local extra = task._extra or {}
      local event_id = extra['_gcal_event_id'] --[[@as string?]]
      local cal_id = extra['_gcal_calendar_id'] --[[@as string?]]

      local should_delete = event_id ~= nil
        and cal_id ~= nil
        and (
          task.status == 'done'
          or task.status == 'deleted'
          or (task.status == 'pending' and not task.due)
        )

      if should_delete then
        local del_err =
          delete_event(access_token, cal_id --[[@as string]], event_id --[[@as string]])
        if del_err then
          log.warn('gcal delete failed: ' .. del_err)
        else
          extra['_gcal_event_id'] = nil
          extra['_gcal_calendar_id'] = nil
          if next(extra) == nil then
            task._extra = nil
          else
            task._extra = extra
          end
          task.modified = os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
          deleted = deleted + 1
        end
      elseif task.status == 'pending' and task.due then
        local cat = task.category or config.get().default_category
        if event_id and cal_id then
          local upd_err = update_event(access_token, cal_id, event_id, task)
          if upd_err then
            log.warn('gcal update failed: ' .. upd_err)
          else
            updated = updated + 1
          end
        else
          local lid, lid_err = find_or_create_calendar(access_token, cat, calendars)
          if lid_err or not lid then
            log.warn('gcal calendar failed: ' .. (lid_err or 'unknown'))
          else
            local new_id, create_err = create_event(access_token, lid, task)
            if create_err then
              log.warn('gcal create failed: ' .. create_err)
            elseif new_id then
              if not task._extra then
                task._extra = {}
              end
              task._extra['_gcal_event_id'] = new_id
              task._extra['_gcal_calendar_id'] = lid
              task.modified = os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
              created = created + 1
            end
          end
        end
      end
    end

    s:save()
    require('pending')._recompute_counts()
    local buffer = require('pending.buffer')
    if buffer.bufnr() and vim.api.nvim_buf_is_valid(buffer.bufnr()) then
      buffer.render(buffer.bufnr())
    end
    log.info(string.format('Google Calendar pushed — +%d ~%d -%d', created, updated, deleted))
  end)
end

---@return nil
function M.health()
  oauth.health(M.name)
end

return M