GCRA Rate limiter implementation with Lua

gcra.lua

redis.replicate_commands()

local rate_limit_key = KEYS[1]
local burst = ARGV[1]
local emission_interval = tonumber(ARGV[2])

-- calculating time using this idea (https://github.com/rwz/redis-gcra/blob/master/vendor/perform_gcra_ratelimit.lua)
local jan_1_2017 = 1483228800
local now = redis.call("TIME")
now = (now[1] - jan_1_2017) + (now[2] / 1000000)

-- if tat does not exists, creates a new one 
local tat = redis.call("GET", rate_limit_key)
if not tat then
    tat = now
else
    tat = tonumber(tat)
end

tat = math.max(tat, now)

local burst_offset = emission_interval * burst
local allow_at = tat - burst_offset
local diff = now - allow_at
if diff < 0 then
    return {
        0, -- allowed
        0, -- remaining
        tostring(-diff) -- wait for
    }
end

local new_tat = tat + emission_interval
local key_expiration = new_tat - now

redis.call("SET", rate_limit_key, new_tat, "EX", math.ceil(key_expiration))

return {
    1, -- allowed
    tostring(math.ceil(diff / emission_interval)), -- remaining
    0,
}