June 2018 Update

Task/Permutations-by-swapping/Lua/permutations-by-swapping-2.lua

@@ -0,0 +1,21 @@
+local wrap, yield = coroutine.wrap, coroutine.yield
+local function perm(n)
+    local r = {}
+    for i=1,n do r[i]=i end
+    local sign = 1
+  return wrap(function()
+    local function swap(m)
+      if m==0 then
+        sign = -sign, yield(sign,r)
+      else
+        for i=m,1,-1 do
+          r[i],r[m]=r[m],r[i]
+          swap(m-1)
+          r[i],r[m]=r[m],r[i]
+        end
+      end
+    end
+    swap(n)
+  end)
+end
+for sign,r in perm(3) do print(sign,table.unpack(r))end