[GR-12668] Add groupby into itertools module.

Author: ppisl

graalpython/lib-graalpython/itertools.py

@@ -445,3 +445,45 @@ def __next__(self):
             self._iter = iter([])
             raise StopIteration()
         return value
+
+class groupby(object):
+    """Make an iterator that returns consecutive keys and groups from the
+    iterable. The key is a function computing a key value for each
+    element. If not specified or is None, key defaults to an identity
+    function and returns the element unchanged. Generally, the
+    iterable needs to already be sorted on the same key function.
+
+    The returned group is itself an iterator that shares the
+    underlying iterable with groupby(). Because the source is shared,
+    when the groupby object is advanced, the previous group is no
+    longer visible. So, if that data is needed later, it should be
+    stored as a list:
+
+       groups = []
+       uniquekeys = []
+       for k, g in groupby(data, keyfunc):
+           groups.append(list(g))      # Store group iterator as a list
+           uniquekeys.append(k)
+    """    
+    def __init__(self, iterable, key=None):
+        if key is None:
+            key = lambda x: x
+        self._keyfunc = key
+        self._iter = iter(iterable)
+        self._tgtkey = self._currkey = self._currvalue = xrange(0)
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        while self._currkey == self._tgtkey:
+            self._currvalue = next(self._iter) # Exit on StopIteration
+            self._currkey = self._keyfunc(self._currvalue)
+        self._tgtkey = self._currkey
+        return (self._currkey, self._grouper(self._tgtkey))
+
+    def _grouper(self, tgtkey):
+        while self._currkey == tgtkey:
+            yield self._currvalue
+            self._currvalue = next(self._iter) # Exit on StopIteration
+            self._currkey = self._keyfunc(self._currvalue)