On an online learning site daily logins are gamified: we track the longest run of consecutive days a user logged in. Logged in today and tomorrow — streak of two; missed a day — streak resets and starts over. For every user compute the length of their longest streak of consecutive login days. Sort by user id. Hint: if you spread dates by their position in the login order and subtract the position number, rows of the same streak land on the same offset — that's the «streak anchor» you can group by.