Fixed Window algorithm: Redis key format and behavior

rate-limiter:
  use-algo: fixed-window-algo
  window-size: 60      # seconds per window
  max-requests: 5      # max requests per window

local windowSize = tonumber(ARGV[1])      -- e.g. 60 (seconds)
local limit = tonumber(ARGV[2])           -- e.g. 5 (max requests)

-- Get current server time from Redis (avoids client clock skew)
local now = redis.call('TIME')
local currentSeconds = tonumber(now[1])

-- Compute which window bucket this request falls into
local window = math.floor(currentSeconds / windowSize)

-- Build the full Redis key including the window bucket
local key = KEYS[1] .. ':' .. window

-- Atomically increment the counter for this window
local current = redis.call('INCR', key)

-- Set TTL only on the first request so the key expires at window end
if current == 1 then
  redis.call('EXPIRE', key, windowSize)
end

-- Read remaining TTL for the response
local ttl = redis.call('TTL', key)

-- Return {allowed, currentCount, ttl}
if current > limit then
  return {0, current, ttl}   -- denied
else
  return {1, current, ttl}   -- allowed
end