MAINT: Unify polynomial power functions

These power functions are all the same - the algorithm used does not care about the basis.

`polypow` and `chebpow` have some optimizations in their versions, which this maintains

1 files changed, 33 insertions, 0 deletions

diff --git a/numpy/polynomial/polyutils.py b/numpy/polynomial/polyutils.py
index 1a73f47d8..5128e35e0 100644
--- a/numpy/polynomial/polyutils.py
+++ b/numpy/polynomial/polyutils.py
@@ -690,6 +690,39 @@ def _fit(vander_f, x, y, deg, rcond=None, full=False, w=None):
         return c
 
 
+def _pow(mul_f, c, pow, maxpower):
+    """
+    Helper function used to implement the ``<type>pow`` functions.
+
+    Parameters
+    ----------
+    vander_f : function(array_like, int) -> ndarray
+        The 1d vander function, such as ``polyvander``
+    pow, maxpower :
+        See the ``<type>pow`` functions for more detail
+    mul_f : function(array_like, array_like) -> ndarray
+        The ``<type>mul`` function, such as ``polymul``
+    """
+    # c is a trimmed copy
+    [c] = as_series([c])
+    power = int(pow)
+    if power != pow or power < 0:
+        raise ValueError("Power must be a non-negative integer.")
+    elif maxpower is not None and power > maxpower:
+        raise ValueError("Power is too large")
+    elif power == 0:
+        return np.array([1], dtype=c.dtype)
+    elif power == 1:
+        return c
+    else:
+        # This can be made more efficient by using powers of two
+        # in the usual way.
+        prd = c
+        for i in range(2, power + 1):
+            prd = mul_f(prd, c)
+        return prd
+
+
 def _deprecate_as_int(x, desc):
     """
     Like `operator.index`, but emits a deprecation warning when passed a float