spiral(n) := block([a, i, j, k, p, di, dj, vi, vj, imin, imax, jmin, jmax],
a: zeromatrix(n, n),
vi: [1, 0, -1, 0],
vj: [0, 1, 0, -1],
imin: 0,
imax: n,
jmin: 1,
jmax: n + 1,
p: 1,
di: vi[p],
dj: vj[p],
i: 1,
j: 1,
for k from 1 thru n*n do (
   a[j, i]: k,
   i: i + di,
   j: j + dj,
   if i < imin or i > imax or j < jmin or j > jmax then (
      i: i - di,
      j: j - dj,
      p: mod(p, 4) + 1,
      di: vi[p],
      dj: vj[p],
      i: i + di,
      j: j + dj,
      if p = 1 then imax: imax - 1
      elseif p = 2 then jmax: jmax - 1
      elseif p = 3 then imin: imin + 1
      else jmin: jmin + 1
   )
),
a
)$

spiral(5);
/* matrix([ 1,  2,  3,  4,  5],
          [16, 17, 18, 19,  6],
          [15, 24, 25, 20,  7],
          [14, 23, 22, 21,  8],
          [13, 12, 11, 10,  9]) */