LuaJIT FFI array sort

ffisort.lua

local function sort(arr, len, elemtype, key)
    local INSERTION_THRESHOLD = 32

    local function insertionSort(arr, len)
        for i = 1, len-1 do
            local cur = elemtype(arr[i])
            local curKey = key(cur)
            local j = i
            while j > 0 and key(arr[j-1]) > curKey do
                arr[j] = arr[j-1]
                j = j - 1
            end
            arr[j] = cur
        end
    end

    local function swap(arr, a, b)
        local temp = elemtype(arr[a])
        arr[a] = arr[b]
        arr[b] = temp
    end

    local function partition(arr, len)
        local pivotKey = key(arr[0])
        local first = 1
        local last = len - 1
        while first < last do
            while first < last and key(arr[last]) >= pivotKey do
                last = last - 1
            end
            while first < last and key(arr[first]) <= pivotKey do
                first = first + 1
            end
            if first < last then
                swap(arr, first, last)
            end
        end
        if key(arr[last]) >= pivotKey then
            last = last - 1
        end
        swap(arr, 0, last)
        return last
    end

    local function quickSort(arr, len)
        if len <= INSERTION_THRESHOLD then
            return
        end
        local pivotIdx = partition(arr, len)
        quickSort(arr, pivotIdx)
        return quickSort(arr + pivotIdx + 1, len - pivotIdx - 1)
    end

    quickSort(arr, len)
    insertionSort(arr, len)
end