redis zpopmin / zpopmax in lua for redis < 5.0.0

zpopmin-zpopmax.lua

-- redis-priority-queue
-- Author: Ensky Lin
-- Version: 1.0.0
-- (can only be used in 3.2+)

-- Get mandatory vars
local action = ARGV[1];
local key = ARGV[2];
local count = ARGV[3];

-- returns true if empty or null
-- http://stackoverflow.com/a/19667498/50501
local function isempty(s)
  return s == nil or s == '' or type(s) == 'userdata'
end

-- Making sure required fields are not nil
assert(not isempty(action), 'ERR1: Action is missing')
assert(not isempty(key), 'ERR2: Queue name is missing')
assert(not isempty(count), 'ERR3: Count is missing')

if action == 'zpopmin' or action == 'zpopmax'
then
    -- debug
    redis.debug(action..' executed with count '..count);

    if action == 'zpopmin' -- asc sorting
    then
        rangeMethod = 'ZRANGEBYSCORE';
        fromMin = '-inf';
        toMax ='+inf';
    else -- desc sorting
        rangeMethod = 'ZREVRANGEBYSCORE';
        fromMin = '+inf';
        toMax ='-inf';
    end

    -- Retrieve items
    local popped = redis.call(rangeMethod, key, fromMin, toMax, 'LIMIT', 0, count)

    -- If items are popped from the list
        -- Rotate thru popped items
    if popped then
        for _,item in ipairs(popped) do
            -- debug
            redis.debug('...popped item -> '..item);

            -- Remove item
            redis.call('ZREM', key, item)
        end
    end

    return popped;
else
    error('ERR3: Invalid action.')
end