【高级技巧】拉链表：状态变更历史的两段式记录

local function closeCurrent(hist, key, now)
    local rows = hist[key] or {}
    for _, row in ipairs(rows) do
        if not row.endTs then
            row.endTs = now
        end
    end
end
local function appendState(hist, key, value, now)
    closeCurrent(hist, key, now)
    hist[key] = hist[key] or {}
    hist[key][#hist[key] + 1] = { value = value, startTs = now }
end

local function queryAt(hist, key, at)
    for _, row in ipairs(hist[key] or {}) do
        if row.startTs <= at and (not row.endTs or at < row.endTs) then
            return row.value
        end
    end
    return nil
end