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

local TOKEN_URL = 'https://oauth2.googleapis.com/token'
local AUTH_URL = 'https://accounts.google.com/o/oauth2/v2/auth'

local BUNDLED_CLIENT_ID = 'PLACEHOLDER'
local BUNDLED_CLIENT_SECRET = 'PLACEHOLDER'

---@class pending.OAuthCredentials
---@field client_id string
---@field client_secret string

---@class pending.OAuthTokens
---@field access_token string
---@field refresh_token string
---@field expires_in? integer
---@field obtained_at? integer

---@class pending.OAuthClient
---@field name string
---@field scope string
---@field port integer
---@field config_key string
local OAuthClient = {}
OAuthClient.__index = OAuthClient

---@class pending.oauth
local M = {}

---@param args string[]
---@param opts? table
---@return { code: integer, stdout: string, stderr: string }
function M.system(args, opts)
  local co = coroutine.running()
  if not co then
    return vim.system(args, opts or {}):wait() --[[@as { code: integer, stdout: string, stderr: string }]]
  end
  vim.system(args, opts or {}, function(result)
    vim.schedule(function()
      coroutine.resume(co, result)
    end)
  end)
  return coroutine.yield() --[[@as { code: integer, stdout: string, stderr: string }]]
end

---@param fn fun(): nil
function M.async(fn)
  coroutine.resume(coroutine.create(fn))
end

---@param str string
---@return string
function M.url_encode(str)
  return (
    str:gsub('([^%w%-%.%_%~])', function(c)
      return string.format('%%%02X', string.byte(c))
    end)
  )
end

---@param path string
---@return table?
function M.load_json_file(path)
  local f = io.open(path, 'r')
  if not f then
    return nil
  end
  local content = f:read('*a')
  f:close()
  if content == '' then
    return nil
  end
  local ok, decoded = pcall(vim.json.decode, content)
  if not ok then
    return nil
  end
  return decoded
end

---@param path string
---@param data table
---@return boolean
function M.save_json_file(path, data)
  local dir = vim.fn.fnamemodify(path, ':h')
  if vim.fn.isdirectory(dir) == 0 then
    vim.fn.mkdir(dir, 'p')
  end
  local f = io.open(path, 'w')
  if not f then
    return false
  end
  f:write(vim.json.encode(data))
  f:close()
  vim.fn.setfperm(path, 'rw-------')
  return true
end

---@param method string
---@param url string
---@param headers? string[]
---@param body? string
---@return table? result
---@return string? err
function M.curl_request(method, url, headers, body)
  local args = { 'curl', '-s', '-X', method }
  for _, h in ipairs(headers or {}) do
    table.insert(args, '-H')
    table.insert(args, h)
  end
  if body then
    table.insert(args, '-d')
    table.insert(args, body)
  end
  table.insert(args, url)
  local result = M.system(args, { text = true })
  if result.code ~= 0 then
    return nil, 'curl failed: ' .. (result.stderr or '')
  end
  if not result.stdout or result.stdout == '' then
    return {}, nil
  end
  local ok, decoded = pcall(vim.json.decode, result.stdout)
  if not ok then
    return nil, 'failed to parse response: ' .. result.stdout
  end
  if decoded.error then
    return nil, 'API error: ' .. (decoded.error.message or vim.json.encode(decoded.error))
  end
  return decoded, nil
end

---@param access_token string
---@return string[]
function M.auth_headers(access_token)
  return {
    'Authorization: Bearer ' .. access_token,
    'Content-Type: application/json',
  }
end

---@param backend_name string
---@return nil
function M.health(backend_name)
  if vim.fn.executable('curl') == 1 then
    vim.health.ok('curl found (required for ' .. backend_name .. ' sync)')
  else
    vim.health.warn('curl not found (needed for ' .. backend_name .. ' sync)')
  end
end

---@return string
function OAuthClient:token_path()
  return vim.fn.stdpath('data') .. '/pending/' .. self.name .. '_tokens.json'
end

---@return pending.OAuthCredentials
function OAuthClient:resolve_credentials()
  local cfg = config.get()
  local backend_cfg = (cfg.sync and cfg.sync[self.config_key]) or {}

  if backend_cfg.client_id and backend_cfg.client_secret then
    return {
      client_id = backend_cfg.client_id,
      client_secret = backend_cfg.client_secret,
    }
  end

  local data_dir = vim.fn.stdpath('data') .. '/pending/'
  local cred_paths = {}
  if backend_cfg.credentials_path then
    table.insert(cred_paths, backend_cfg.credentials_path)
  end
  table.insert(cred_paths, data_dir .. self.name .. '_credentials.json')
  table.insert(cred_paths, data_dir .. 'google_credentials.json')
  for _, cred_path in ipairs(cred_paths) do
    if cred_path then
      local creds = M.load_json_file(cred_path)
      if creds then
        if creds.installed then
          creds = creds.installed
        end
        if creds.client_id and creds.client_secret then
          return creds --[[@as pending.OAuthCredentials]]
        end
      end
    end
  end
  return {
    client_id = BUNDLED_CLIENT_ID,
    client_secret = BUNDLED_CLIENT_SECRET,
  }
end

---@return pending.OAuthTokens?
function OAuthClient:load_tokens()
  return M.load_json_file(self:token_path()) --[[@as pending.OAuthTokens?]]
end

---@param tokens pending.OAuthTokens
---@return boolean
function OAuthClient:save_tokens(tokens)
  return M.save_json_file(self:token_path(), tokens)
end

---@param creds pending.OAuthCredentials
---@param tokens pending.OAuthTokens
---@return pending.OAuthTokens?
function OAuthClient:refresh_access_token(creds, tokens)
  local body = 'client_id='
    .. M.url_encode(creds.client_id)
    .. '&client_secret='
    .. M.url_encode(creds.client_secret)
    .. '&grant_type=refresh_token'
    .. '&refresh_token='
    .. M.url_encode(tokens.refresh_token)
  local result = M.system({
    'curl',
    '-s',
    '-X',
    'POST',
    '-H',
    'Content-Type: application/x-www-form-urlencoded',
    '-d',
    body,
    TOKEN_URL,
  }, { text = true })
  if result.code ~= 0 then
    return nil
  end
  local ok, decoded = pcall(vim.json.decode, result.stdout or '')
  if not ok or not decoded.access_token then
    return nil
  end
  tokens.access_token = decoded.access_token --[[@as string]]
  tokens.expires_in = decoded.expires_in --[[@as integer?]]
  tokens.obtained_at = os.time()
  self:save_tokens(tokens)
  return tokens
end

---@return string?
function OAuthClient:get_access_token()
  local creds = self:resolve_credentials()
  local tokens = self:load_tokens()
  if not tokens or not tokens.refresh_token then
    return nil
  end
  local now = os.time()
  local obtained = tokens.obtained_at or 0
  local expires = tokens.expires_in or 3600
  if now - obtained > expires - 60 then
    tokens = self:refresh_access_token(creds, tokens)
    if not tokens then
      log.error(self.name .. ': token refresh failed — re-authenticating...')
      return nil
    end
  end
  return tokens.access_token
end

---@return nil
function OAuthClient:setup()
  local choice = vim.fn.inputlist({
    self.name .. ' setup:',
    '1. Enter client ID and secret',
    '2. Load from JSON file path',
  })
  vim.cmd.redraw()

  local id, secret

  if choice == 1 then
    while true do
      id = vim.trim(vim.fn.input(self.name .. ' client ID: '))
      if id == '' then
        return
      end
      if id:match('^%d+%-[%w_]+%.apps%.googleusercontent%.com$') then
        break
      end
      vim.cmd.redraw()
      vim.api.nvim_echo({
        {
          'invalid client ID — expected <numbers>-<hash>.apps.googleusercontent.com',
          'ErrorMsg',
        },
      }, false, {})
    end

    while true do
      secret = vim.trim(vim.fn.inputsecret(self.name .. ' client secret: '))
      if secret == '' then
        return
      end
      if secret:match('^GOCSPX%-') then
        break
      end
      vim.cmd.redraw()
      vim.api.nvim_echo(
        { { 'invalid client secret — expected GOCSPX-...', 'ErrorMsg' } },
        false,
        {}
      )
    end
  elseif choice == 2 then
    local fpath
    while true do
      fpath = vim.trim(vim.fn.input(self.name .. ' credentials file: ', '', 'file'))
      if fpath == '' then
        return
      end
      fpath = vim.fn.expand(fpath)
      local creds = M.load_json_file(fpath)
      if creds then
        if creds.installed then
          creds = creds.installed
        end
        if creds.client_id and creds.client_secret then
          id = creds.client_id
          secret = creds.client_secret
          break
        end
      end
      vim.cmd.redraw()
      vim.api.nvim_echo(
        { { 'could not read client_id/client_secret from ' .. fpath, 'ErrorMsg' } },
        false,
        {}
      )
    end
  else
    return
  end

  vim.schedule(function()
    local path = vim.fn.stdpath('data') .. '/pending/google_credentials.json'
    local ok = M.save_json_file(path, { client_id = id, client_secret = secret })
    if not ok then
      log.error(self.name .. ': failed to save credentials')
      return
    end
    log.info(self.name .. ': credentials saved, starting authorization...')
    self:auth()
  end)
end

---@param on_complete? fun(): nil
---@return nil
function OAuthClient:auth(on_complete)
  local creds = self:resolve_credentials()
  if creds.client_id == BUNDLED_CLIENT_ID then
    log.error(self.name .. ': no credentials configured — run :Pending ' .. self.name .. ' setup')
    return
  end
  local port = self.port

  local verifier_chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~'
  local verifier = {}
  math.randomseed(vim.uv.hrtime())
  for _ = 1, 64 do
    local idx = math.random(1, #verifier_chars)
    table.insert(verifier, verifier_chars:sub(idx, idx))
  end
  local code_verifier = table.concat(verifier)

  local hex = vim.fn.sha256(code_verifier)
  local binary = hex:gsub('..', function(h)
    return string.char(tonumber(h, 16))
  end)
  local code_challenge = vim.base64.encode(binary):gsub('+', '-'):gsub('/', '_'):gsub('=', '')

  local auth_url = AUTH_URL
    .. '?client_id='
    .. M.url_encode(creds.client_id)
    .. '&redirect_uri='
    .. M.url_encode('http://127.0.0.1:' .. port)
    .. '&response_type=code'
    .. '&scope='
    .. M.url_encode(self.scope)
    .. '&access_type=offline'
    .. '&prompt=consent'
    .. '&code_challenge='
    .. M.url_encode(code_challenge)
    .. '&code_challenge_method=S256'

  vim.ui.open(auth_url)
  log.info('Opening browser for Google authorization...')

  local server = vim.uv.new_tcp()
  local server_closed = false
  local function close_server()
    if server_closed then
      return
    end
    server_closed = true
    server:close()
  end

  server:bind('127.0.0.1', port)
  server:listen(1, function(err)
    if err then
      return
    end
    local conn = vim.uv.new_tcp()
    server:accept(conn)
    conn:read_start(function(read_err, data)
      if read_err or not data then
        conn:close()
        close_server()
        return
      end
      local code = data:match('[?&]code=([^&%s]+)')
      local response_body = code
          and '<html><body><h1>Authorization successful</h1><p>You can close this tab.</p></body></html>'
        or '<html><body><h1>Authorization failed</h1></body></html>'
      local http_response = 'HTTP/1.1 200 OK\r\nContent-Type: text/html\r\nConnection: close\r\n\r\n'
        .. response_body
      conn:write(http_response, function()
        conn:shutdown(function()
          conn:close()
        end)
      end)
      close_server()
      if code then
        vim.schedule(function()
          self:_exchange_code(creds, code, code_verifier, port, on_complete)
        end)
      end
    end)
  end)

  vim.defer_fn(function()
    if not server_closed then
      close_server()
      log.warn('OAuth callback timed out (120s).')
    end
  end, 120000)
end

---@param creds pending.OAuthCredentials
---@param code string
---@param code_verifier string
---@param port integer
---@param on_complete? fun(): nil
---@return nil
function OAuthClient:_exchange_code(creds, code, code_verifier, port, on_complete)
  local body = 'client_id='
    .. M.url_encode(creds.client_id)
    .. '&client_secret='
    .. M.url_encode(creds.client_secret)
    .. '&code='
    .. M.url_encode(code)
    .. '&code_verifier='
    .. M.url_encode(code_verifier)
    .. '&grant_type=authorization_code'
    .. '&redirect_uri='
    .. M.url_encode('http://127.0.0.1:' .. port)

  local result = M.system({
    'curl',
    '-s',
    '-X',
    'POST',
    '-H',
    'Content-Type: application/x-www-form-urlencoded',
    '-d',
    body,
    TOKEN_URL,
  }, { text = true })

  if result.code ~= 0 then
    self:_wipe()
    log.error('Token exchange failed.')
    return
  end

  local ok, decoded = pcall(vim.json.decode, result.stdout or '')
  if not ok or not decoded.access_token then
    self:_wipe()
    log.error('Invalid token response.')
    return
  end

  decoded.obtained_at = os.time()
  self:save_tokens(decoded)
  log.info(self.name .. ' authorized successfully.')
  if on_complete then
    on_complete()
  end
end

---@return nil
function OAuthClient:_wipe()
  os.remove(self:token_path())
  os.remove(vim.fn.stdpath('data') .. '/pending/google_credentials.json')
end

---@param opts { name: string, scope: string, port: integer, config_key: string }
---@return pending.OAuthClient
function M.new(opts)
  return setmetatable({
    name = opts.name,
    scope = opts.scope,
    port = opts.port,
    config_key = opts.config_key,
  }, OAuthClient)
end

M._BUNDLED_CLIENT_ID = BUNDLED_CLIENT_ID
M._BUNDLED_CLIENT_SECRET = BUNDLED_CLIENT_SECRET
M.BUNDLED_CLIENT_ID = BUNDLED_CLIENT_ID

M.google_client = M.new({
  name = 'google',
  scope = 'https://www.googleapis.com/auth/tasks' .. ' https://www.googleapis.com/auth/calendar',
  port = 18392,
  config_key = 'google',
})

return M