local m = {}

local function rext(a, i)
    i = i - 1
    local v = a[i]
    if v then
        return i, v
    end
end

function m.ipairs_rev(a)
    return rext, a, (#a + 1)
end

function m.between(x, a, b)
    return a <= x and x <= b
end

function m.inside(px, py, x, y, w, h)
    return m.between(px, x, x+w) and
    m.between(py, y, y+h)
end

function m.add_entity(a, x)
    table.insert(a, x)
    x.idx = #a
end

function m.del_entity(a, x)
    table.remove(a, x)

    for i, v in ipairs(a) do
        v.idx = i
    end
end

return m