PERF: Avoid np.divmod in RangeIndex._shallow_copy

Author: mroeschke

pandas/_libs/lib.pyx

@@ -659,20 +659,20 @@ ctypedef fused int6432_t:
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def is_range_indexer(ndarray[int6432_t, ndim=1] left, Py_ssize_t n) -> bool:
+def is_range_indexer(ndarray[int6432_t, ndim=1] left, Py_ssize_t n, int diff=1) -> 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:
+    if left.size != n or diff == 0:
         return False
 
     for i in range(n):
 
-        if left[i] != i:
+        if left[i] != left[0] + i * diff:
             return False
 
     return True

pandas/core/indexes/range.py

@@ -476,14 +476,11 @@ 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 diff != 0:
-                maybe_range_indexer, remainder = np.divmod(values - values[0], diff)
-                if (
-                    lib.is_range_indexer(maybe_range_indexer, len(maybe_range_indexer))
-                    and not remainder.any()
-                ):
-                    new_range = range(values[0], values[-1] + diff, diff)
-                    return type(self)._simple_new(new_range, name=name)
+            if not missing.isna(diff) and lib.is_range_indexer(
+                values, len(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)
 
     def _view(self) -> Self: