function factor{T<:Integer}(n::T)
    0 < n || error("number to be factored must be positive")
    h = Dict{T,Int}()
    n == 1 && return h
    local s::T, p::T
    s = isqrt(n)
    p = 2
    while p <= s
        if n % p == 0
            while n % p == 0
                h[p] = get(h,p,0) + 1
                n = div(n,p)
            end
            s = isqrt(n)
        end
        p += (p==2 ? 1 : 2)
    end
    return h
end