excellent.jl

using Compat

function julia1(k)
    for n = 10^(k-1):10^k-1
        front = n*(10^k + n)
        root = floor(Int,sqrt(front))

        for t = root-2:root+2
            back = t * (t - 1)
            length(string(t)) > k && break
            back > front && break
            if back == front
                println(STDERR,n,t)
                break
            end
        end
    end
end

function julia2(k)
    for a = 10^(k-1):10^k-1
        front = a * (10^k + a)
        root = floor(front^0.5)

        for b = root-1:root+1
            back = b * (b - 1);
            back > front && break
            log(10,b) > k && continue
            if front == back
                @printf STDERR "%d%d\n" a b
                # missing break?
            end
        end
    end
end

function julia3(k)
    for n = 10^(k-1):10^k-1
        front = n*(10^k + n)
        root = isqrt(front)

        for t = root-2:root+2
            back = t * (t - 1)
            ndigits(t) > k && break
            back > front && break
            if back == front
                println(STDERR,n,t)
                break
            end
        end
    end
end

digits = @compat parse(Int,ARGS[1])
digits > 0 && iseven(digits) || error("""
    Number of digits must be even and non-zero! You said [$digits]
    """)

k = div(digits,2)

for _ = 1:10
    println("julia1,$digits,", @elapsed julia1(k))
    println("julia2,$digits,", @elapsed julia2(k))
    println("julia3,$digits,", @elapsed julia3(k))
end