ENH: Added popcount implementation for big python ints natively

[1/2] `popcount` object loop changes

numpy/core/_internal.py

@@ -78,6 +78,24 @@ def _usefields(adict, align):
                   "offsets": offsets,
                   "titles": titles}, align)
 
+_INT_SHIFT = 30
+_MASK = (2 ** _INT_SHIFT)
+
+# Get the internal ob_digit representation of python integer
+# Ref: https://rushter.com/blog/python-integer-implementation/
+def _get_ob_digit_array(num):
+    t = abs(num)
+
+    num_list = []
+    while t != 0:
+        # Get remainder from division
+        small_int = t % _MASK  # more efficient bitwise analogue: (t & (MASK-1))
+        num_list.append(small_int)
+
+        # Get integral part of the division (floor division)
+        t = t // _MASK  # more efficient bitwise analogue: t >>= SHIFT
+
+    return num_list
 
 # construct an array_protocol descriptor list
 #  from the fields attribute of a descriptor
@@ -793,6 +811,24 @@ def _gcd(a, b):
 def _lcm(a, b):
     return a // _gcd(a, b) * b
 
+def _popcount64(a):
+    """ Computes the number of 1-bits in a (64 bits) """
+
+    # Refer to npy_math_internal.h.src for more details.
+    a = abs(a)
+    a -= ((a >> 1) & 0x5555555555555555)
+    a = (a & 0x3333333333333333) + (a >> 2 & 0x3333333333333333)
+    return (((a + (a >> 4)) & 0xf0f0f0f0f0f0f0f) * 0x101010101010101 >> 56) & 0xff
+
+def _bit_count(a):
+    """ Computes the number of 1-bits in a (Python Integer) """
+    res = 0
+
+    for i in _get_ob_digit_array(a):
+        res += _popcount64(i)
+
+    return res
+
 def array_ufunc_errmsg_formatter(dummy, ufunc, method, *inputs, **kwargs):
     """ Format the error message for when __array_ufunc__ gives up. """
     args_string = ', '.join(['{!r}'.format(arg) for arg in inputs] +