【高级技巧】倒排索引：按物品反查持有者的列表结构

local inverted = {}
local function indexAdd(itemName, owner)
    local set = inverted[itemName]
    if set == nil then
        set = {}
        inverted[itemName] = set
    end
    set[owner] = true
end
local function indexRemove(itemName, owner)
    local set = inverted[itemName]
    if set ~= nil then
        set[owner] = nil
    end
end
local function whoHas(itemName)
    local set = inverted[itemName]
    if set == nil then
        return {}
    end
    local owners = {}
    for owner in pairs(set) do
        owners[#owners + 1] = owner
    end
    return owners
end

local function auditSword(actor)
    actor = getplayerbyname(actor)
    local owners = whoHas("裁决之杖")
    local n = #owners
    sendmsg(actor, 1, "裁决之杖当前流通 " .. n .. " 把，名单已入审计日志。")
end