PluralKit/services/api/src/middleware/ratelimit.lua

-- this script has side-effects, so it requires replicate commands mode
-- redis.replicate_commands()

local rate_limit_key = KEYS[1]
local rate = ARGV[1]
local period = ARGV[2]
local cost = tonumber(ARGV[3])

local burst = rate

local emission_interval = period / rate
local increment = emission_interval * cost
local burst_offset = emission_interval * burst

-- redis returns time as an array containing two integers: seconds of the epoch
-- time (10 digits) and microseconds (6 digits). for convenience we need to
-- convert them to a floating point number. the resulting number is 16 digits,
-- bordering on the limits of a 64-bit double-precision floating point number.
-- adjust the epoch to be relative to Jan 1, 2017 00:00:00 GMT to avoid floating
-- point problems. this approach is good until "now" is 2,483,228,799 (Wed, 09
-- Sep 2048 01:46:39 GMT), when the adjusted value is 16 digits.
local jan_1_2017 = 1483228800
local now = redis.call("TIME")
now = (now[1] - jan_1_2017) + (now[2] / 1000000)

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 new_tat = tat + increment
local allow_at = new_tat - burst_offset

local diff = now - allow_at
local remaining = diff / emission_interval

if remaining < 0 then
    local reset_after = tat - now
    local retry_after = diff * -1
    return {
        0, -- remaining
        tostring(retry_after),
        reset_after,
    }
end

local reset_after = new_tat - now
if reset_after > 0 then
    redis.call("SET", rate_limit_key, new_tat, "EX", math.ceil(reset_after))
end
local retry_after = -1
return {
    remaining,
    tostring(retry_after),
    reset_after
}