Make is_range

Author: mroeschke

pandas/_libs/lib.pyi

@@ -231,3 +231,7 @@ def is_range_indexer(
     left: np.ndarray,
     n: int,  # np.ndarray[np.int64, ndim=1]
 ) -> bool: ...
+def is_range(
+    left: np.ndarray,
+    diff: int,  # np.ndarray[np.int64, ndim=1]
+) -> bool: ...

pandas/_libs/lib.pyx

@@ -659,20 +659,40 @@ ctypedef fused int6432_t:
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def is_range_indexer(ndarray[int6432_t, ndim=1] left, Py_ssize_t n, int diff=1) -> bool:
+def is_range_indexer(ndarray[int6432_t, ndim=1] left, Py_ssize_t n) -> bool:
     """
     Perform an element by element comparison on 1-d integer arrays, meant for indexer
     comparisons
     """
     cdef:
         Py_ssize_t i
 
-    if left.size != n or diff == 0:
+    if left.size != n:
         return False
 
     for i in range(n):
 
-        if left[i] != left[0] + i * diff:
+        if left[i] != i:
+            return False
+
+    return True
+
+
+@cython.wraparound(False)
+@cython.boundscheck(False)
+def is_range(ndarray[int6432_t, ndim=1] sequence, int64_t diff) -> bool:
+    """
+    Check if sequence is equivalent to a range with the specified diff.
+    """
+    cdef:
+        Py_ssize_t i, n = len(sequence)
+
+    if diff == 0:
+        return False
+
+    for i in range(n):
+
+        if sequence[i] != sequence[0] + i * diff:
             return False
 
     return True

pandas/core/indexes/range.py

@@ -476,9 +476,7 @@ def _shallow_copy(self, values, name: Hashable = no_default):
             # GH 46675 & 43885: If values is equally spaced, return a
             # more memory-compact RangeIndex instead of Index with 64-bit dtype
             diff = values[1] - values[0]
-            if not missing.isna(diff) and lib.is_range_indexer(
-                values, len(values), diff
-            ):
+            if not missing.isna(diff) and lib.is_range(values, diff):
                 new_range = range(values[0], values[-1] + diff, diff)
                 return type(self)._simple_new(new_range, name=name)
         return self._constructor._simple_new(values, name=name)