my first lua

nextperm.lua

--[[ Psuedo-code given in chapter:
NextPerm(p1, p2, ..., pn)

Let k be the largest index such that P(k) < P(k+1)
  If no such k exists then (p1, p2, ..., pn) is the last permutation

Let j be the largest index such that P(j) > P(k)
Swap P(j) and P(k)
Reverse the order of P(k+1), ..., P(n)
--]]

function nextperm(inset)
  local j, k = nil, nil
  local set = { table.unpack(inset) } -- copy set instead of modifying

  for i = #set - 1, 1, -1 do
    if set[i] < set[i+1] then
      k = i  -- loop index is out-of-scope after break
      break
    end
  end

  -- already last perm
  if k == nil then return {} end

  for i = #set, k, -1 do
    if set[i] > set[k] then
      j = i
      break
    end
  end

  -- i don't think this can fail
  if j == nil then return {} end

  set[k], set[j] = set[j], set[k]

  -- reverse order after k
  for i = 1, (#set - k) / 2, 1 do
    set[i+k], set[#set+1-i] = set[#set+1-i], set[i+k]
  end

  return set
end