Lua树状数组：单点更新与区间求和

local tree, n = {}, 8
local function lowbit(i)
    local l = 1
    while i % 2 == 0 do
        i = i / 2
        l = l * 2
    end
    return l
end
local function add(i, v)
    while i <= n do
        tree[i] = (tree[i] or 0) + v
        i = i + lowbit(i)
    end
end
local function prefix(i)
    local s = 0
    while i > 0 do
        s = s + (tree[i] or 0)
        i = i - lowbit(i)
    end
    return s
end

for i = 1, 8 do add(i, i) end   -- 初始 1..8
add(3, 5)                        -- 第 3 笔追加 5
local p = getplayerbyname("fenwick")
sendmsg(p, 1, "前 5 笔合计 " .. prefix(5))
-- 初始 15，改后 20