Constant FIXED_WINDOW_SCRIPT 

const FIXED_WINDOW_SCRIPT: &str = r#"
local current = redis.call('INCR', KEYS[1])
if current == 1 then
  redis.call('PEXPIRE', KEYS[1], ARGV[1])
end
local ttl = redis.call('PTTL', KEYS[1])
return {current, ttl}
"#;

INCR + first-increment PEXPIRE + PTTL, atomically.