cwr_next(): move invariants out of loops.

This simplifies and clarifies the code, and gives a small speedup.

Modules/itertoolsmodule.c

@@ -2713,20 +2713,20 @@ cwr_next(cwrobject *co)
     PyObject *result = co->result;
     Py_ssize_t n = PyTuple_GET_SIZE(pool);
     Py_ssize_t r = co->r;
-    Py_ssize_t i, j, index;
+    Py_ssize_t i, index;
 
     if (co->stopped)
         return NULL;
 
     if (result == NULL) {
-        /* On the first pass, initialize result tuple using the indices */
+        /* On the first pass, initialize result tuple with pool[0] */
         result = PyTuple_New(r);
         if (result == NULL)
             goto empty;
         co->result = result;
+        elem = PyTuple_GET_ITEM(pool, 0);
         for (i=0; i<r ; i++) {
-            index = indices[i];
-            elem = PyTuple_GET_ITEM(pool, index);
+            assert(indices[i] == 0);
             Py_INCREF(elem);
             PyTuple_SET_ITEM(result, i, elem);
         }
@@ -2761,15 +2761,11 @@ cwr_next(cwrobject *co)
 
         /* Increment the current index which we know is not at its
            maximum.  Then set all to the right to the same value. */
-        indices[i]++;
-        for (j=i+1 ; j<r ; j++)
-            indices[j] = indices[j-1];
-
-        /* Update the result tuple for the new indices
-           starting with i, the leftmost index that changed */
-        for ( ; i<r ; i++) {
-            index = indices[i];
+        index = indices[i] + 1;
+        assert(index < n);
         elem = PyTuple_GET_ITEM(pool, index);
+        for ( ; i<r ; i++) {
+            indices[i] = index;
             Py_INCREF(elem);
             oldelem = PyTuple_GET_ITEM(result, i);
             PyTuple_SET_ITEM(result, i, elem);