diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index b53f3a8..91029f0 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -18,6 +18,6 @@ jobs:
         python-version: ${{ matrix.python }}
     - name: Install dependencies
       run: |
-        pip install nose2 numpy scipy scikit-learn
+        pip install nose2 numpy scipy scikit-learn gmpy2
     - name: Run tests
       run: nose2 -v --pretty-assert
diff --git a/setup.py b/setup.py
index 4767be2..68c2aad 100644
--- a/setup.py
+++ b/setup.py
@@ -18,6 +18,7 @@
     platforms = 'any',
     install_requires = [
         'numpy',
+        'gmpy2',
     ],
     tests_require = [
         'nose2',
diff --git a/simhash/__init__.py b/simhash/__init__.py
index 4fd83be..789ee97 100644
--- a/simhash/__init__.py
+++ b/simhash/__init__.py
@@ -10,6 +10,7 @@
 from itertools import groupby
 
 import numpy as np
+from gmpy2 import popcount
 
 try:
     from collections.abc import Iterable
@@ -160,12 +161,7 @@ def _bitarray_from_bytes(b):
 
     def distance(self, another):
         assert self.f == another.f
-        x = (self.value ^ another.value) & ((1 << self.f) - 1)
-        ans = 0
-        while x:
-            ans += 1
-            x &= x - 1
-        return ans
+        return popcount(self.value ^ another.value)
 
 
 class SimhashIndex(object):