Flag Rollout

Task

A feature flag service decides who sees the new checkout while it rolls out to 20 percent of users. Support says some users see the new checkout on one visit and the old one on the next. The team's own tests pass. Read the ticket, find the cause in the rollout code, and fix it so that every user gets one stable answer.

Requirements

1. evaluate(flags, request) answers one request. flags maps a flag name to its config, such as {"percent": 20}. percent is an integer from 0 to 100 (ValueError containing percent otherwise). An unknown flag raises ValueError containing flag. Do not change the arguments.
2. {op: "bucket", flag, user} returns {bucket}: an integer from 0 to 99, computed from the flag name and the user ID as docs/bucketing.md defines. The same flag and user always give the same bucket. user is a non-empty string (ValueError containing user otherwise).
3. {op: "check", flag, user} returns {enabled, reason}. A flag at percent P is on for exactly P of the 100 buckets, the lowest numbered ones. reason is "rollout" when the flag is on and "outside_rollout" when it is off.
4. {op: "audience", flag, users} returns {enabled, count}: the users from the list who get the flag, in the order given, and how many there are. users is a list (ValueError containing users otherwise). Any other op raises ValueError containing op.