Magic square generator

Question

Create the shortest function to print a magic square of given size.

A magic square is a NxN matrix of distinct numbers and the sum of each row, column and diagonal is equal to a constant.

A magic square of size 4 is:

07 12 01 14
02 13 08 11
16 03 10 05
09 06 15 04

And the constant is 34.

The function should be able to handle inputs up to N = 30 and doesn't need to show leading zeroes.

Using Matlab's magic function or similar built-in function is not allowed.

More useful information: http://mathworld.wolfram.com/MagicSquare.html

::Edit::

The output doesn't need to be pretty printed and may contain [](){}, or List/Array delimiters of your language. Although, it should use one line for each row

E.g.

1, 2,
3, 4,

((9,8), 
 (6,7))

[10 2]
[3 456]

PS: The corner case, when N = 2 is undefined behavior as there's no magic square of that size.

Also, the function should complete in less than 1 minute to avoid brute force.

Answer 1

Ruby, 344 characters

q=->n{r=0...n;n%2>0?(m=r.map{[]*n};x=n/2;y=i=0;r.map{r.map{m[y%=n][x%=n]=i+=1;x+=1;y-=1};x-=1;y+=2};m):n<5?[[16,3,2,13],[5,10,11,8],[9,6,7,12],[4,15,14,1]]:(d=q[n/2];r.map{|y|r.map{|x|4*d[b=y/2][a=x/2]-[0,3,2,1,3,0,2,1,3,0,1,2][(n/2%2<1?b<n/4?0:8:a==n/4&&b==n/4?4:a==n/4&&b==n/4+1?0:b<=n/4?0:b==n/4+1?4:8)+y%2*2+x%2]}})}
Q=->n{q[n].map{|s|p s}}

You can use it e.g. like

Q[3]

and the output will be

[8, 1, 6]
[3, 5, 7]
[4, 9, 2]

The solution is not yet fully golfed. Also huge squares are generated reasonably fast. It uses a recursive function q to generate the squares unless n is odd in which case the square is generated directly or n==4 which is hard-coded.

Answer 2

Q, 50

Works for odd numbers

{(+)a rotate'(+)(a:((!)x)-1)rotate'x cut 1+(!)x*x}

usage

q){(+)a rotate'(+)(a:((!)x)-1)rotate'x cut 1+(!)x*x} 5
24 1  8  15 17
5  7  14 16 23
6  13 20 22 4 
12 19 21 3  10
18 25 2  9  11