[GR-13493] Add itertools.cycle

PullRequest: graalpython/374

Author: timfel

graalpython/com.oracle.graal.python.test/src/tests/test_iterator.py

@@ -129,3 +129,15 @@ def test_itertools_zip_longest():
     x = [1,2,3]
     y = [4,5,6,7]
     assert list(zip_longest(x,y)) == [(1, 4), (2, 5), (3, 6), (None, 7)], list(zip_longest(x,y))
+
+
+def test_itertools_cycle():
+    from itertools import cycle
+    x = [1,2,3]
+    r = []
+    for i in cycle(x):
+        r.append(i)
+        if len(r) > 10:
+            break
+    assert r == [1,2,3,1,2,3,1,2,3,1,2], r
+    assert [x for x in cycle([])] == []

graalpython/lib-graalpython/itertools.py

@@ -648,3 +648,52 @@ def __new__(subtype, iter1, *args, fillvalue=None):
         self.active = len(args) + 1
         self.iterators = [iter(iter1)] + [iter(arg) for arg in args]
         return self
+
+
+class cycle():
+    """
+    Make an iterator returning elements from the iterable and
+    saving a copy of each. When the iterable is exhausted, return
+    elements from the saved copy. Repeats indefinitely.
+
+    Equivalent to :
+
+    def cycle(iterable):
+        saved = []
+        for element in iterable:
+            yield element
+            saved.append(element)
+        while saved:
+            for element in saved:
+                yield element
+    """
+
+    def __init__(self, iterable):
+        self.saved = []
+        self.iterable = iter(iterable)
+        self.index = 0
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.index > 0:
+            if not self.saved:
+                raise StopIteration
+            if len(self.saved) > self.index:
+                obj = self.saved[self.index]
+                self.index += 1
+            else:
+                obj = self.saved[0]
+                self.index = 1
+        else:
+            try:
+                obj = next(self.iterable)
+            except StopIteration:
+                if not self.saved:
+                    raise
+                obj = self.saved[0]
+                self.index = 1
+            else:
+                self.saved.append(obj)
+        return obj