Lua, permutation with coroutine

permu_with_coroutine.lua

local function printResult(a)
  for i = 1, #a do
    io.write(a[i], " ")
  end
  io.write("\n")
end

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

local function permutations(a)
  local co = coroutine.create(function () permgen(a) end)
  return function ()
          local code, res = coroutine.resume(co)
          return res
         end
end

local function test()
  for p in permutations({"a", "b", "c"}) do
    printResult(p)
  end
end

local function main()
  test()
end

main()