quick draft of an array/vector library for lua (mostly as an example of metatables)

array.lua

local NONE = setmetatable({}, {['__tostring'] = function() return "NONE" end})

local Array = {
    ['__newindex'] = function(a, k, v)
                         if type(k) ~= "number" then error("bad index") end
                         if k < 1 then error("out of bounds") end
                         if a._lim ~= NONE and k <= a._lim then
                             a._v[k] = v
                         elseif a._lim == NONE then
                             if k > a._len then
                                 for i=a._len+1,2*a._len do
                                     a._v[i] = NONE
                                 end
                                 a._len = 2*a._len
                             end
                             a._v[k] = v
                         else
                             error("out of bounds")
                         end
                     end,
    ['__index'] = function(a, k)
                      return a._v[k] or NONE
                  end,
}

-- Make a new array. If an explicit limit is given, the length
-- will be capped, otherwise the array will be doubled on demand
-- and filled with NONEs.
function new(len)
    local a = {}
    local v = {}
    if len then                 -- const length
        len = tonumber(len)
        if not len then error("bad length") end
        for i=1,len do v[i] = NONE end
        a._len = len
        a._lim = len
    else -- grow on demand
        a._len = 1
        v[1] = NONE
        a._lim = nil
    end
    a._v=v

    return setmetatable(a, Array)
end

function test()
    local a = new(3)
    for i=1,3 do print(i, a[i]) end
    a[2] = 23
    print ""
    for i=1,3 do print(i, a[i]) end
    print ""

    local a2 = new()
    a2[3] = "badger"
    for i=1,5 do print(i, a2[i]) end

    a2[9] = "mushroom"
    for i=1,10 do print(i, a2[i]) end
end

return { new=new, test=test, NONE=NONE, }