-- Heap’s algorithm for all permutations

function printResult (a)
   for _,v in ipairs(a) do
      io.write(v, " ")
   end
   io.write("\n")
end

function heap (a, n)
   n = n or #a
   if n <= 1 then
      coroutine.yield(a)
   else
      for i=1,n-1 do
         heap(a, n-1)
         if n%2 == 0 then
            a[n], a[i] = a[i], a[n]
         else
            a[n], a[1] = a[1], a[n]
         end
      end
      heap(a, n-1)
   end
end

function permutations (a)
   return coroutine.wrap(function () heap(a) end)
end

-- test
for p in permutations{1,2,3,4} do
   printResult(p)
end