Initial working rank with no tiebreaker

Author: WillAyd

pandas/_libs/groupby_helper.pxi.in

@@ -444,6 +444,39 @@ def group_nth_{{name}}(ndarray[{{dest_type2}}, ndim=2] out,
                 else:
                     out[i, j] = resx[i, j]
 
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+def group_rank_{{name}}(ndarray[{{dest_type2}}, ndim=2] out,
+                        ndarray[{{c_type}}, ndim=2] values,
+                        ndarray[int64_t] labels,
+                        bint is_datetimelike):
+    """
+    Only transforms on axis=0
+    """
+    cdef:
+        Py_ssize_t i, j, N, K
+        int64_t lab, idx, counter=1
+        ndarray[int64_t] _as
+
+    N, K = (<object> values).shape
+
+    _as = np.lexsort((values[:, 0], labels))
+
+    with nogil:
+        for i in range(N):
+            idx = _as[i]
+            lab = labels[idx]
+            if i > 0 and lab == labels[_as[i-1]]:
+                counter += 1
+            else:
+                counter = 1
+            if lab < 0:
+                continue
+
+            for j in range(K):
+                out[idx, j] = counter
+
 {{endfor}}
 
 #----------------------------------------------------------------------

pandas/core/groupby.py

@@ -1756,6 +1756,12 @@ def cumcount(self, ascending=True):
         cumcounts = self._cumcount_array(ascending=ascending)
         return Series(cumcounts, index)
 
+    @Substitution(name='groupby')
+    @Appender(_doc_template)
+    def rank(self, axis=0, *args, **kwargs):
+        """Rank within each group"""
+        return self._cython_transform('rank', **kwargs)
+
     @Substitution(name='groupby')
     @Appender(_doc_template)
     def cumprod(self, axis=0, *args, **kwargs):
@@ -2171,6 +2177,7 @@ def get_group_levels(self):
             'cumsum': 'group_cumsum',
             'cummin': 'group_cummin',
             'cummax': 'group_cummax',
+            'rank': 'group_rank',
         }
     }