py: Implement builtin reversed() function.

reversed function now implemented, and works for tuple, list, str, bytes
and user objects with __len__ and __getitem__.

Renamed mp_builtin_len to mp_obj_len to make it publically available (eg
for reversed).

py/objreversed.c

@@ -0,0 +1,74 @@
+/*
+ * This file is part of the Micro Python project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <stdlib.h>
+#include <assert.h>
+
+#include "mpconfig.h"
+#include "nlr.h"
+#include "misc.h"
+#include "qstr.h"
+#include "obj.h"
+#include "runtime.h"
+
+typedef struct _mp_obj_reversed_t {
+    mp_obj_base_t base;
+    mp_obj_t seq;           // sequence object that we are reversing
+    mp_uint_t cur_index;    // current index, plus 1; 0=no more, 1=last one (index 0)
+} mp_obj_reversed_t;
+
+STATIC mp_obj_t reversed_make_new(mp_obj_t type_in, uint n_args, uint n_kw, const mp_obj_t *args) {
+    mp_arg_check_num(n_args, n_kw, 1, 1, false);
+
+    mp_obj_reversed_t *o = m_new_obj(mp_obj_reversed_t);
+    o->base.type = &mp_type_reversed;
+    o->seq = args[0];
+    o->cur_index = mp_obj_get_int(mp_obj_len(args[0])); // start at the end of the sequence
+
+    return o;
+}
+
+STATIC mp_obj_t reversed_iternext(mp_obj_t self_in) {
+    assert(MP_OBJ_IS_TYPE(self_in, &mp_type_reversed));
+    mp_obj_reversed_t *self = self_in;
+
+    // "raise" stop iteration if we are at the end (the start) of the sequence
+    if (self->cur_index == 0) {
+        return MP_OBJ_STOP_ITERATION;
+    }
+
+    // pre-decrement and index sequence
+    self->cur_index -= 1;
+    return mp_obj_subscr(self->seq, MP_OBJ_NEW_SMALL_INT(self->cur_index), MP_OBJ_SENTINEL);
+}
+
+const mp_obj_type_t mp_type_reversed = {
+    { &mp_type_type },
+    .name = MP_QSTR_reversed,
+    .make_new = reversed_make_new,
+    .getiter = mp_identity,
+    .iternext = reversed_iternext,
+};