smooth weighted round-robin balancing

sb.lua

local mt = {}
mt.__index = mt

function mt:get()
    local max = 0
    local c = 0
    local cur = self.cur
    for i = 1, self.num do
        cur[i] = (cur[i] or 0) + self.wl[i]
        if cur[i] > max then
            max = cur[i]
            c = i
        end
    end
    cur[c] = cur[c] - self.sum
    return c
end

function mt:add(w)
    assert(w > 0, "weight must > 0")
    self.num = self.num + 1
    self.sum = self.sum + w
    table.insert(self.wl, w)
    self.cur = {}
    return self.num
end

function mt:del(idx)
    if idx > 0 and idx <= self.num then
        self.num = self.num - 1
        self.sum = self.sum - table.remove(self.wl, idx)
        self.cur = {}
        return true
    end
end

local M = {}
function M.new(num, weighted_list)
    assert(num > 0, "num must > 0" .. num)
    local sum = 0
    local wl = {}
    if not(weighted_list) then
        sum = num
        for i = 1, num do
            wl[i] = 1
        end
    else
        local w = 1
        for i = 1, num do
            w = weighted_list[i] or 1
            assert(w > 0, "weight must > 0")
            wl[i] = w
            sum = sum + w
        end
    end
    return setmetatable({
        num = num,
        wl = wl,
        sum = sum,
        cur = {},
    }, mt)
end

--[[test
local t = M.new(3, {10,5,1})
for i = 1, 30 do
    print(t:get())
end
print('---', t:add(10))
for i = 1, 30 do
    print(t:get())
end
print(t:del(1))
for i = 1, 30 do
    print(t:get())
end
]]
return M