module FacebookAppToken

# File lib/facebook_app_token.rb, line 13
def self.fetch_token(email, password, app_id)
  # Login to the page
  logged_in = login(email, password, app_id)

  # Click OK
  confirmed = confirm(logged_in)

  # Retrieve token
  confirmed.content.match(/access_token=(\w+)&expires_in=/)[1]
end

# File lib/facebook_app_token.rb, line 9
def self.login_uri(app_id)
  "https://m.facebook.com/v2.10/dialog/oauth?client_id=#{URI::encode(app_id)}&scope=public_profile%2Cuser_education_history%2Cuser_friends%2Cemail%2Cuser_likes%2Cuser_photos%2Cuser_relationships%2Cuser_work_history%2Cuser_birthday&default_audience=friends&redirect_uri=fbconnect%3A%2F%2Fsuccess&auth_type=rerequest&display=touch&response_type=token%2Csigned_request&return_scopes=true"
end

# File lib/facebook_app_token.rb, line 52
def self.confirm(logged_in)
  confirm = logged_in.forms[0]
  confirm.click_button(confirm.button_with(name: '__CONFIRM__'))
end

# File lib/facebook_app_token.rb, line 30
def self.login(email, password, app_id)
  agent = prepare_agent
  uri = self.login_uri(app_id)
  page = agent.get(uri)

  if page.title == 'Error'
    raise FacebookAppToken::FacebookAuthenticationError, 'Facebook Authentication failed. Invalid App ID specified'
  end

  f = page.forms[0]
  f.field_with(name: 'email').value = email
  f.field_with(name: 'pass').value = password

  res = agent.submit f

  if res.uri.path.start_with? '/login'
    raise FacebookAppToken::FacebookAuthenticationError, 'Facebook Authentication failed. Check if you passed correct email and password'
  end

  res
end

# File lib/facebook_app_token.rb, line 24
def self.prepare_agent
  agent = Mechanize.new
  agent.user_agent = USER_AGENT
  agent
end