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

local M = {}

M.name = 'gtasks'

local BASE_URL = 'https://tasks.googleapis.com/tasks/v1'
local SCOPE = 'https://www.googleapis.com/auth/tasks'

local client = oauth.new({
  name = 'gtasks',
  scope = SCOPE,
  port = 18393,
  config_key = 'gtasks',
})

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

---@param access_token string
---@param name string
---@param existing table<string, string>
---@return string? list_id
---@return string? err
local function find_or_create_tasklist(access_token, name, existing)
  if existing[name] then
    return existing[name], nil
  end
  local body = vim.json.encode({ title = name })
  local created, err = oauth.curl_request(
    'POST',
    BASE_URL .. '/users/@me/lists',
    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 access_token string
---@param list_id string
---@return table[]? items
---@return string? err
local function list_gtasks(access_token, list_id)
  local url = BASE_URL
    .. '/lists/'
    .. oauth.url_encode(list_id)
    .. '/tasks?showCompleted=true&showHidden=true'
  local data, err = oauth.curl_request('GET', url, oauth.auth_headers(access_token))
  if err then
    return nil, err
  end
  return data and data.items or {}, nil
end

---@param access_token string
---@param list_id string
---@param body table
---@return string? task_id
---@return string? err
local function create_gtask(access_token, list_id, body)
  local data, err = oauth.curl_request(
    'POST',
    BASE_URL .. '/lists/' .. oauth.url_encode(list_id) .. '/tasks',
    oauth.auth_headers(access_token),
    vim.json.encode(body)
  )
  if err then
    return nil, err
  end
  return data and data.id, nil
end

---@param access_token string
---@param list_id string
---@param task_id string
---@param body table
---@return string? err
local function update_gtask(access_token, list_id, task_id, body)
  local _, err = oauth.curl_request(
    'PATCH',
    BASE_URL .. '/lists/' .. oauth.url_encode(list_id) .. '/tasks/' .. oauth.url_encode(task_id),
    oauth.auth_headers(access_token),
    vim.json.encode(body)
  )
  return err
end

---@param access_token string
---@param list_id string
---@param task_id string
---@return string? err
local function delete_gtask(access_token, list_id, task_id)
  local _, err = oauth.curl_request(
    'DELETE',
    BASE_URL .. '/lists/' .. oauth.url_encode(list_id) .. '/tasks/' .. oauth.url_encode(task_id),
    oauth.auth_headers(access_token)
  )
  return err
end

---@param due string YYYY-MM-DD or YYYY-MM-DDThh:mm
---@return string RFC 3339
local function due_to_rfc3339(due)
  local date = due:match('^(%d%d%d%d%-%d%d%-%d%d)')
  return (date or due) .. 'T00:00:00.000Z'
end

---@param rfc string RFC 3339 from GTasks
---@return string YYYY-MM-DD
local function rfc3339_to_date(rfc)
  return rfc:match('^(%d%d%d%d%-%d%d%-%d%d)') or rfc
end

---@param task pending.Task
---@return string?
local function build_notes(task)
  local parts = {}
  if task.priority and task.priority > 0 then
    table.insert(parts, 'pri:' .. task.priority)
  end
  if task.recur then
    local spec = task.recur
    if task.recur_mode == 'completion' then
      spec = '!' .. spec
    end
    table.insert(parts, 'rec:' .. spec)
  end
  if #parts == 0 then
    return nil
  end
  return table.concat(parts, ' ')
end

---@param notes string?
---@return integer priority
---@return string? recur
---@return string? recur_mode
local function parse_notes(notes)
  if not notes then
    return 0, nil, nil
  end
  local priority = 0
  local recur = nil
  local recur_mode = nil
  local pri = notes:match('pri:(%d+)')
  if pri then
    priority = tonumber(pri) or 0
  end
  local rec = notes:match('rec:(!?[%w]+)')
  if rec then
    if rec:sub(1, 1) == '!' then
      recur = rec:sub(2)
      recur_mode = 'completion'
    else
      recur = rec
    end
  end
  return priority, recur, recur_mode
end

---@param task pending.Task
---@return table
local function task_to_gtask(task)
  local body = {
    title = task.description,
    status = task.status == 'done' and 'completed' or 'needsAction',
  }
  if task.due then
    body.due = due_to_rfc3339(task.due)
  end
  local notes = build_notes(task)
  if notes then
    body.notes = notes
  end
  return body
end

---@param gtask table
---@param category string
---@return table fields for store:add / store:update
local function gtask_to_fields(gtask, category)
  local priority, recur, recur_mode = parse_notes(gtask.notes)
  local fields = {
    description = gtask.title or '',
    category = category,
    status = gtask.status == 'completed' and 'done' or 'pending',
    priority = priority,
    recur = recur,
    recur_mode = recur_mode,
  }
  if gtask.due then
    fields.due = rfc3339_to_date(gtask.due)
  end
  return fields
end

---@param s pending.Store
---@return table<string, pending.Task>
local function build_id_index(s)
  ---@type table<string, pending.Task>
  local index = {}
  for _, task in ipairs(s:tasks()) do
    local extra = task._extra or {}
    local gtid = extra['_gtasks_task_id'] --[[@as string?]]
    if gtid then
      index[gtid] = task
    end
  end
  return index
end

---@param access_token string
---@param tasklists table<string, string>
---@param s pending.Store
---@param now_ts string
---@param by_gtasks_id table<string, pending.Task>
---@return integer created
---@return integer updated
---@return integer deleted
local function push_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
  local created, updated, deleted = 0, 0, 0
  for _, task in ipairs(s:tasks()) do
    local extra = task._extra or {}
    local gtid = extra['_gtasks_task_id'] --[[@as string?]]
    local list_id = extra['_gtasks_list_id'] --[[@as string?]]

    if task.status == 'deleted' and gtid and list_id then
      local err = delete_gtask(access_token, list_id, gtid)
      if err then
        log.warn('gtasks delete failed: ' .. err)
      else
        if not task._extra then
          task._extra = {}
        end
        task._extra['_gtasks_task_id'] = nil
        task._extra['_gtasks_list_id'] = nil
        if next(task._extra) == nil then
          task._extra = nil
        end
        task.modified = now_ts
        deleted = deleted + 1
      end
    elseif task.status ~= 'deleted' then
      if gtid and list_id then
        local err = update_gtask(access_token, list_id, gtid, task_to_gtask(task))
        if err then
          log.warn('gtasks update failed: ' .. err)
        else
          updated = updated + 1
        end
      elseif task.status == 'pending' then
        local cat = task.category or config.get().default_category
        local lid, err = find_or_create_tasklist(access_token, cat, tasklists)
        if not err and lid then
          local new_id, create_err = create_gtask(access_token, lid, task_to_gtask(task))
          if create_err then
            log.warn('gtasks create failed: ' .. create_err)
          elseif new_id then
            if not task._extra then
              task._extra = {}
            end
            task._extra['_gtasks_task_id'] = new_id
            task._extra['_gtasks_list_id'] = lid
            task.modified = now_ts
            by_gtasks_id[new_id] = task
            created = created + 1
          end
        end
      end
    end
  end
  return created, updated, deleted
end

---@param access_token string
---@param tasklists table<string, string>
---@param s pending.Store
---@param now_ts string
---@param by_gtasks_id table<string, pending.Task>
---@return integer created
---@return integer updated
local function pull_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
  local created, updated = 0, 0
  for list_name, list_id in pairs(tasklists) do
    local items, err = list_gtasks(access_token, list_id)
    if err then
      log.warn('error fetching list ' .. list_name .. ': ' .. err)
    else
      for _, gtask in ipairs(items or {}) do
        local local_task = by_gtasks_id[gtask.id]
        if local_task then
          local gtask_updated = gtask.updated or ''
          local local_modified = local_task.modified or ''
          if gtask_updated > local_modified then
            local fields = gtask_to_fields(gtask, list_name)
            for k, v in pairs(fields) do
              local_task[k] = v
            end
            local_task.modified = now_ts
            updated = updated + 1
          end
        else
          local fields = gtask_to_fields(gtask, list_name)
          fields._extra = {
            _gtasks_task_id = gtask.id,
            _gtasks_list_id = list_id,
          }
          local new_task = s:add(fields)
          by_gtasks_id[gtask.id] = new_task
          created = created + 1
        end
      end
    end
  end
  return created, updated
end

---@param access_token string
---@return table<string, string>? tasklists
---@return pending.Store? s
---@return string? now_ts
local function sync_setup(access_token)
  local tasklists, tl_err = get_all_tasklists(access_token)
  if tl_err or not tasklists then
    log.error(tl_err or 'failed to fetch task lists')
    return nil, nil, nil
  end
  local s = require('pending').store()
  local now_ts = os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
  return tasklists, s, now_ts
end

---@param callback fun(access_token: string): nil
local function with_token(callback)
  oauth.async(function()
    local token = client:get_access_token()
    if not token then
      client:auth(function()
        oauth.async(function()
          local fresh = client:get_access_token()
          if fresh then
            callback(fresh)
          end
        end)
      end)
      return
    end
    callback(token)
  end)
end

function M.setup()
  client:setup()
end

function M.auth()
  client:auth()
end

function M.push()
  with_token(function(access_token)
    local tasklists, s, now_ts = sync_setup(access_token)
    if not tasklists then
      return
    end
    ---@cast s pending.Store
    ---@cast now_ts string
    local by_gtasks_id = build_id_index(s)
    local created, updated, deleted = push_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
    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 Tasks pushed — +%d ~%d -%d', created, updated, deleted))
  end)
end

function M.pull()
  with_token(function(access_token)
    local tasklists, s, now_ts = sync_setup(access_token)
    if not tasklists then
      return
    end
    ---@cast s pending.Store
    ---@cast now_ts string
    local by_gtasks_id = build_id_index(s)
    local created, updated = pull_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
    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 Tasks pulled — +%d ~%d', created, updated))
  end)
end

function M.sync()
  with_token(function(access_token)
    local tasklists, s, now_ts = sync_setup(access_token)
    if not tasklists then
      return
    end
    ---@cast s pending.Store
    ---@cast now_ts string
    local by_gtasks_id = build_id_index(s)
    local pushed_create, pushed_update, pushed_delete =
      push_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
    local pulled_create, pulled_update = pull_pass(access_token, tasklists, s, now_ts, by_gtasks_id)
    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 Tasks synced — push: +%d ~%d -%d, pull: +%d ~%d',
        pushed_create,
        pushed_update,
        pushed_delete,
        pulled_create,
        pulled_update
      )
    )
  end)
end

M._due_to_rfc3339 = due_to_rfc3339
M._rfc3339_to_date = rfc3339_to_date
M._build_notes = build_notes
M._parse_notes = parse_notes
M._task_to_gtask = task_to_gtask
M._gtask_to_fields = gtask_to_fields

---@return nil
function M.health()
  oauth.health(M.name)
  local tokens = client:load_tokens()
  if tokens and tokens.refresh_token then
    vim.health.ok('gtasks tokens found')
  else
    vim.health.info('no gtasks tokens — run :Pending gtasks auth')
  end
end

return M