#!/usr/bin/lua

function permgen (a, n)
 n = n or #a
 if n <= 1 then
 coroutine.yield(a)
 else
 for i = 1, n do
 -- put i-th element as the last one
 a[n], a[i] = a[i], a[n]

 -- generate all permutations of the other elements
 permgen(a, n-1)

 -- restore i-th element
 a[n], a[i] = a[i], a[n]

 end
 end
end

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

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

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