diff --git a/.gitignore b/.gitignore
index 453dfd0..c6d576d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,9 @@ __pycache__/
 *.py[cod]
 .pytest_cache/
 
+#cython
+.c
+
 # C extensions
 *.so
 
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..70cc54d
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,19 @@
+install:
+	pip install . --upgrade
+
+uninstall:
+	pip uninstall pyModeS -y
+
+ext:
+	python setup.py build_ext --inplace
+
+test:
+	python -m pytest
+
+clean:
+	find pyModeS/decoder -type f -name '*.c' -delete
+	find pyModeS/decoder -type f -name '*.so' -delete
+	find . | grep -E "(__pycache__|\.pyc|\.pyo$$)" | xargs rm -rf
+	rm -rf *.egg-info
+	rm -rf .pytest_cache
+	rm -rf build/*
diff --git a/README.rst b/README.rst
index b6fc7c0..fa0cb09 100644
--- a/README.rst
+++ b/README.rst
@@ -321,8 +321,19 @@ Here is an example:
 
 Unit test
 ---------
-To perform unit tests. First, install ``tox`` through pip. Then, run the following commands:
+To perform unit tests, ``pytest`` must be install first.
 
-.. code:: bash
+Build Cython extensions
+::
 
-  $ tox
+  $ make ext
+
+Run unit tests
+::
+
+  $ make test
+
+Clean build files
+::
+
+  $ make clean
diff --git a/pyModeS/__init__.py b/pyModeS/__init__.py
index bc491d9..a2014df 100644
--- a/pyModeS/__init__.py
+++ b/pyModeS/__init__.py
@@ -3,11 +3,16 @@ from __future__ import absolute_import, print_function, division
 import os
 import warnings
 
-from .decoder.common import *
+try:
+    from .decoder import c_common as common
+    from .decoder.c_common import *
+except:
+    from .decoder import common
+    from .decoder.common import *
+
 from .decoder import tell
 from .decoder import adsb
 from .decoder import commb
-from .decoder import common
 from .decoder import bds
 from .extra import aero
 from .extra import tcpclient
diff --git a/pyModeS/decoder/acas.py b/pyModeS/decoder/acas.py
index c275030..e22759b 100644
--- a/pyModeS/decoder/acas.py
+++ b/pyModeS/decoder/acas.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 """
 Decoding Air-Air Surveillance (ACAS) DF=0/16
 
diff --git a/pyModeS/decoder/adsb.py b/pyModeS/decoder/adsb.py
index a84dc1b..d28d77a 100644
--- a/pyModeS/decoder/adsb.py
+++ b/pyModeS/decoder/adsb.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2015 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 """ADS-B Wrapper.
 
 The ADS-B wrapper also imports functions from the following modules:
@@ -38,7 +23,7 @@ from pyModeS.decoder import uncertainty
 from pyModeS.decoder.bds.bds05 import (
     airborne_position,
     airborne_position_with_ref,
-    altitude,
+    altitude as altitude05,
 )
 from pyModeS.decoder.bds.bds06 import (
     surface_position,
@@ -143,18 +128,13 @@ def altitude(msg):
     if tc < 5 or tc == 19 or tc > 22:
         raise RuntimeError("%s: Not a position message" % msg)
 
-    if tc >= 5 and tc <= 8:
+    elif tc >= 5 and tc <= 8:
         # surface position, altitude 0
         return 0
 
-    msgbin = common.hex2bin(msg)
-    q = msgbin[47]
-    if q:
-        n = common.bin2int(msgbin[40:47] + msgbin[48:52])
-        alt = n * 25 - 1000
-        return alt
     else:
-        return None
+        # airborn position
+        return altitude05(msg)
 
 
 def velocity(msg, rtn_sources=False):
@@ -177,7 +157,7 @@ def velocity(msg, rtn_sources=False):
             as reference, 'mag_north' for magnetic north as reference),
             rate of climb/descent source ('Baro' for barometer, 'GNSS'
             for GNSS constellation).
-            
+
             In the case of surface messages, None will be put in place
             for vertical rate and its respective sources.
     """
diff --git a/pyModeS/decoder/allcall.py b/pyModeS/decoder/allcall.py
index 6978e42..1901698 100644
--- a/pyModeS/decoder/allcall.py
+++ b/pyModeS/decoder/allcall.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 """
 Decoding all call replies DF=11
 
diff --git a/pyModeS/decoder/bds/bds05.py b/pyModeS/decoder/bds/bds05.py
index 15a64fe..dce5742 100644
--- a/pyModeS/decoder/bds/bds05.py
+++ b/pyModeS/decoder/bds/bds05.py
@@ -1,19 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-
 # ------------------------------------------
 #   BDS 0,5
 #   ADS-B TC=9-18
diff --git a/pyModeS/decoder/bds/bds06.py b/pyModeS/decoder/bds/bds06.py
index 08dccdd..d882569 100644
--- a/pyModeS/decoder/bds/bds06.py
+++ b/pyModeS/decoder/bds/bds06.py
@@ -1,19 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-
 # ------------------------------------------
 #   BDS 0,6
 #   ADS-B TC=5-8
diff --git a/pyModeS/decoder/bds/bds08.py b/pyModeS/decoder/bds/bds08.py
index cd8068c..bdd7106 100644
--- a/pyModeS/decoder/bds/bds08.py
+++ b/pyModeS/decoder/bds/bds08.py
@@ -1,19 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-
 # ------------------------------------------
 #   BDS 0,8
 #   ADS-B TC=1-4
diff --git a/pyModeS/decoder/bds/bds09.py b/pyModeS/decoder/bds/bds09.py
index 1b36d64..1a26ca6 100644
--- a/pyModeS/decoder/bds/bds09.py
+++ b/pyModeS/decoder/bds/bds09.py
@@ -1,19 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-
 # ------------------------------------------
 #   BDS 0,9
 #   ADS-B TC=19
diff --git a/pyModeS/decoder/bds/bds10.py b/pyModeS/decoder/bds/bds10.py
index 1bea8cb..02cb658 100644
--- a/pyModeS/decoder/bds/bds10.py
+++ b/pyModeS/decoder/bds/bds10.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 1,0
 # Data link capability report
diff --git a/pyModeS/decoder/bds/bds17.py b/pyModeS/decoder/bds/bds17.py
index 07a58af..5614fce 100644
--- a/pyModeS/decoder/bds/bds17.py
+++ b/pyModeS/decoder/bds/bds17.py
@@ -1,19 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-
 # ------------------------------------------
 #   BDS 1,7
 #   Common usage GICB capability report
diff --git a/pyModeS/decoder/bds/bds20.py b/pyModeS/decoder/bds/bds20.py
index 63f28d6..32ced8c 100644
--- a/pyModeS/decoder/bds/bds20.py
+++ b/pyModeS/decoder/bds/bds20.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 2,0
 # Aircraft identification
diff --git a/pyModeS/decoder/bds/bds30.py b/pyModeS/decoder/bds/bds30.py
index cb4d94c..59a7a9a 100644
--- a/pyModeS/decoder/bds/bds30.py
+++ b/pyModeS/decoder/bds/bds30.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 3,0
 # ACAS active resolution advisory
diff --git a/pyModeS/decoder/bds/bds40.py b/pyModeS/decoder/bds/bds40.py
index 181149f..a9891ad 100644
--- a/pyModeS/decoder/bds/bds40.py
+++ b/pyModeS/decoder/bds/bds40.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 4,0
 # Selected vertical intention
diff --git a/pyModeS/decoder/bds/bds44.py b/pyModeS/decoder/bds/bds44.py
index 50fc0cc..7bd59eb 100644
--- a/pyModeS/decoder/bds/bds44.py
+++ b/pyModeS/decoder/bds/bds44.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 4,4
 # Meteorological routine air report
diff --git a/pyModeS/decoder/bds/bds45.py b/pyModeS/decoder/bds/bds45.py
index 3dd5c0a..adf3197 100644
--- a/pyModeS/decoder/bds/bds45.py
+++ b/pyModeS/decoder/bds/bds45.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 4,5
 # Meteorological hazard report
diff --git a/pyModeS/decoder/bds/bds50.py b/pyModeS/decoder/bds/bds50.py
index ae65786..91ec924 100644
--- a/pyModeS/decoder/bds/bds50.py
+++ b/pyModeS/decoder/bds/bds50.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 5,0
 # Track and turn report
diff --git a/pyModeS/decoder/bds/bds53.py b/pyModeS/decoder/bds/bds53.py
index 962e260..6673489 100644
--- a/pyModeS/decoder/bds/bds53.py
+++ b/pyModeS/decoder/bds/bds53.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 5,3
 # Air-referenced state vector
diff --git a/pyModeS/decoder/bds/bds60.py b/pyModeS/decoder/bds/bds60.py
index 8998644..75f2927 100644
--- a/pyModeS/decoder/bds/bds60.py
+++ b/pyModeS/decoder/bds/bds60.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 # ------------------------------------------
 # BDS 6,0
 # Heading and speed report
diff --git a/pyModeS/decoder/c_common.pxd b/pyModeS/decoder/c_common.pxd
new file mode 100644
index 0000000..e311ddf
--- /dev/null
+++ b/pyModeS/decoder/c_common.pxd
@@ -0,0 +1,23 @@
+# cython: language_level=3
+
+cdef int char_to_int(unsigned char binstr)
+cdef unsigned char int_to_char(unsigned char i)
+
+cpdef str hex2bin(str hexstr)
+cpdef long bin2int(str binstr)
+cpdef long hex2int(str binstr)
+
+cpdef unsigned char df(str msg)
+cpdef long crc(str msg, bint encode=*)
+
+cpdef long floor(double x)
+cpdef str icao(str msg)
+cpdef bint is_icao_assigned(str icao)
+
+cpdef int typecode(str msg)
+cpdef int cprNL(double lat)
+cpdef str idcode(str msg)
+cpdef int altcode(str msg)
+
+cdef str data(str msg)
+cpdef bint allzeros(str msg)
diff --git a/pyModeS/decoder/c_common.pyx b/pyModeS/decoder/c_common.pyx
new file mode 100644
index 0000000..2c76cf7
--- /dev/null
+++ b/pyModeS/decoder/c_common.pyx
@@ -0,0 +1,431 @@
+# cython: language_level=3
+
+cimport cython
+from cpython cimport array
+from cpython.bytes cimport PyBytes_GET_SIZE
+from cpython.bytearray cimport PyByteArray_GET_SIZE
+
+from libc.math cimport cos, acos, fabs, M_PI as pi, floor as c_floor
+
+
+cdef int char_to_int(unsigned char binstr):
+    if 48 <= binstr <= 57:  # 0 to 9
+        return binstr - 48
+    if 97 <= binstr <= 102: # a to f
+        return binstr - 97 + 10
+    if 65 <= binstr <= 70: # A to F
+        return binstr - 65 + 10
+    return 0
+
+cdef unsigned char int_to_char(unsigned char i):
+    if i < 10:
+        return 48 + i  # "0" + i
+    return 97 - 10 + i  # "a" - 10 + i
+
+@cython.boundscheck(False)
+@cython.overflowcheck(False)
+cpdef str hex2bin(str hexstr):
+    """Convert a hexdecimal string to binary string, with zero fillings."""
+    # num_of_bits = len(hexstr) * 4
+    cdef hexbytes = bytes(hexstr.encode())
+    cdef Py_ssize_t len_hexstr = PyBytes_GET_SIZE(hexbytes)
+    # binstr = bin(int(hexbytes, 16))[2:].zfill(int(num_of_bits))
+    cdef bytearray _binstr = bytearray(4 * len_hexstr)
+    cdef unsigned char[:] binstr = _binstr
+    cdef unsigned char int_
+    cdef Py_ssize_t i
+    for i in range(len_hexstr):
+        int_ = char_to_int(hexbytes[i])
+        binstr[4*i] = int_to_char((int_  >> 3) & 1)
+        binstr[4*i+1] = int_to_char((int_ >> 2) & 1)
+        binstr[4*i+2] = int_to_char((int_ >> 1) & 1)
+        binstr[4*i+3] = int_to_char((int_) & 1)
+    return _binstr.decode()
+
+@cython.boundscheck(False)
+cpdef long bin2int(str binstr):
+    """Convert a binary string to integer."""
+    # return int(binstr, 2)
+    cdef bytearray binbytes = bytearray(binstr.encode())
+    cdef Py_ssize_t len_ = PyByteArray_GET_SIZE(binbytes)
+    cdef long cumul = 0
+    cdef unsigned char[:] v_binstr = binbytes
+    for i in range(len_):
+        cumul = 2*cumul + char_to_int(v_binstr[i])
+    return cumul
+
+@cython.boundscheck(False)
+cpdef long hex2int(str hexstr):
+    """Convert a binary string to integer."""
+    # return int(hexstr, 2)
+    cdef bytearray binbytes = bytearray(hexstr.encode())
+    cdef Py_ssize_t len_ = PyByteArray_GET_SIZE(binbytes)
+    cdef long cumul = 0
+    cdef unsigned char[:] v_hexstr = binbytes
+    for i in range(len_):
+        cumul = 16*cumul + char_to_int(v_hexstr[i])
+    return cumul
+
+@cython.boundscheck(False)
+cpdef unsigned char df(str msg):
+    """Decode Downlink Format vaule, bits 1 to 5."""
+    cdef str dfbin = hex2bin(msg[:2])
+    # return min(bin2int(dfbin[0:5]), 24)
+    cdef long df = bin2int(dfbin[0:5])
+    if df > 24:
+        return 24
+    return df
+
+# the CRC generator
+# G = [int("11111111", 2), int("11111010", 2), int("00000100", 2), int("10000000", 2)]
+cdef array.array _G = array.array('l', [0b11111111, 0b11111010, 0b00000100, 0b10000000])
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef long crc(str msg, bint encode=False):
+    """Mode-S Cyclic Redundancy Check.
+
+    Detect if bit error occurs in the Mode-S message. When encode option is on,
+    the checksum is generated.
+
+    Args:
+        msg (string): 28 bytes hexadecimal message string
+        encode (bool): True to encode the date only and return the checksum
+    Returns:
+        int: message checksum, or partity bits (encoder)
+
+    """
+    # the CRC generator
+    # G = [int("11111111", 2), int("11111010", 2), int("00000100", 2), int("10000000", 2)]
+    # cdef array.array _G = array.array('l', [0b11111111, 0b11111010, 0b00000100, 0b10000000])
+    cdef long[4] G = _G
+
+    # msgbin_split = wrap(msgbin, 8)
+    # mbytes = list(map(bin2int, msgbin_split))
+    cdef bytearray _msgbin = bytearray(hex2bin(msg).encode())
+    cdef unsigned char[:] msgbin = _msgbin
+
+    cdef Py_ssize_t len_msgbin = PyByteArray_GET_SIZE(_msgbin)
+    cdef Py_ssize_t len_mbytes = len_msgbin // 8
+    cdef Py_ssize_t i
+
+    if encode:
+        for i in range(len_msgbin - 24, len_msgbin):
+            msgbin[i] = 0
+
+    cdef array.array _mbytes = array.array(
+        'l', [bin2int(_msgbin[8*i:8*i+8].decode()) for i in range(len_mbytes)]
+    )
+
+    cdef long[:] mbytes = _mbytes
+
+    cdef long bits, mask
+    cdef Py_ssize_t ibyte, ibit
+
+    for ibyte in range(len_mbytes - 3):
+        for ibit in range(8):
+            mask = 0x80 >> ibit
+            bits = mbytes[ibyte] & mask
+
+            if bits > 0:
+                mbytes[ibyte] = mbytes[ibyte] ^ (G[0] >> ibit)
+                mbytes[ibyte + 1] = mbytes[ibyte + 1] ^ (
+                    0xFF & ((G[0] << 8 - ibit) | (G[1] >> ibit))
+                )
+                mbytes[ibyte + 2] = mbytes[ibyte + 2] ^ (
+                    0xFF & ((G[1] << 8 - ibit) | (G[2] >> ibit))
+                )
+                mbytes[ibyte + 3] = mbytes[ibyte + 3] ^ (
+                    0xFF & ((G[2] << 8 - ibit) | (G[3] >> ibit))
+                )
+
+    cdef long result = (mbytes[len_mbytes-3] << 16) | (mbytes[len_mbytes-2] << 8) | mbytes[len_mbytes-1]
+
+    return result
+
+
+
+cpdef long floor(double x):
+    """Mode-S floor function.
+
+    Defined as the greatest integer value k, such that k <= x
+    For example: floor(3.6) = 3 and floor(-3.6) = -4
+
+    """
+    return <long> c_floor(x)
+
+cpdef str icao(str msg):
+    """Calculate the ICAO address from an Mode-S message.
+
+    Applicable only with DF4, DF5, DF20, DF21 messages.
+
+    Args:
+        msg (String): 28 bytes hexadecimal message string
+
+    Returns:
+        String: ICAO address in 6 bytes hexadecimal string
+
+    """
+    cdef unsigned char DF = df(msg)
+    cdef long c0, c1
+
+    if DF in (11, 17, 18):
+        addr = msg[2:8]
+    elif DF in (0, 4, 5, 16, 20, 21):
+        c0 = crc(msg, encode=True)
+        c1 = hex2int(msg[-6:])
+        addr = "%06X" % (c0 ^ c1)
+    else:
+        addr = None
+
+    return addr
+
+
+cpdef bint is_icao_assigned(str icao):
+    """Check whether the ICAO address is assigned (Annex 10, Vol 3)."""
+    if (icao is None) or (not isinstance(icao, str)) or (len(icao) != 6):
+        return False
+
+    cdef long icaoint = hex2int(icao)
+
+    if 0x200000 < icaoint < 0x27FFFF:
+        return False  # AFI
+    if 0x280000 < icaoint < 0x28FFFF:
+        return False  # SAM
+    if 0x500000 < icaoint < 0x5FFFFF:
+        return False  # EUR, NAT
+    if 0x600000 < icaoint < 0x67FFFF:
+        return False  # MID
+    if 0x680000 < icaoint < 0x6F0000:
+        return False  # ASIA
+    if 0x900000 < icaoint < 0x9FFFFF:
+        return False  # NAM, PAC
+    if 0xB00000 < icaoint < 0xBFFFFF:
+        return False  # CAR
+    if 0xD00000 < icaoint < 0xDFFFFF:
+        return False  # future
+    if 0xF00000 < icaoint < 0xFFFFFF:
+        return False  # future
+
+    return True
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef int typecode(str msg):
+    """Type code of ADS-B message
+
+    Args:
+        msg (string): 28 bytes hexadecimal message string
+
+    Returns:
+        int: type code number
+    """
+    if df(msg) not in (17, 18):
+        return -1
+        # return None
+
+    cdef str tcbin = hex2bin(msg[8:10])
+    return bin2int(tcbin[0:5])
+
+@cython.cdivision(True)
+cpdef int cprNL(double lat):
+    """NL() function in CPR decoding."""
+
+    if lat == 0:
+        return 59
+
+    if lat == 87 or lat == -87:
+        return 2
+
+    if lat > 87 or lat < -87:
+        return 1
+
+    cdef int nz = 15
+    cdef double a = 1 - cos(pi / (2 * nz))
+    cdef double b = cos(pi / 180.0 * fabs(lat)) ** 2
+    cdef double nl = 2 * pi / (acos(1 - a / b))
+    NL = floor(nl)
+    return NL
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef str idcode(str msg):
+    """Compute identity (squawk code).
+
+    Applicable only for DF5 or DF21 messages, bit 20-32.
+    credit: @fbyrkjeland
+
+    Args:
+        msg (String): 28 bytes hexadecimal message string
+
+    Returns:
+        string: squawk code
+
+    """
+    if df(msg) not in [5, 21]:
+        raise RuntimeError("Message must be Downlink Format 5 or 21.")
+
+    cdef bytearray _mbin = bytearray(hex2bin(msg).encode())
+    cdef unsigned char[:] mbin = _mbin
+
+    cdef bytearray _idcode = bytearray(4)
+    cdef unsigned char[:] idcode = _idcode
+
+    cdef unsigned char C1 = mbin[19]
+    cdef unsigned char A1 = mbin[20]
+    cdef unsigned char C2 = mbin[21]
+    cdef unsigned char A2 = mbin[22]
+    cdef unsigned char C4 = mbin[23]
+    cdef unsigned char A4 = mbin[24]
+    # _ = mbin[25]
+    cdef unsigned char B1 = mbin[26]
+    cdef unsigned char D1 = mbin[27]
+    cdef unsigned char B2 = mbin[28]
+    cdef unsigned char D2 = mbin[29]
+    cdef unsigned char B4 = mbin[30]
+    cdef unsigned char D4 = mbin[31]
+
+    # byte1 = int(A4 + A2 + A1, 2)
+    # byte2 = int(B4 + B2 + B1, 2)
+    # byte3 = int(C4 + C2 + C1, 2)
+    # byte4 = int(D4 + D2 + D1, 2)
+
+    idcode[0] = int_to_char((char_to_int(A4)*2 + char_to_int(A2))*2 + char_to_int(A1))
+    idcode[1] = int_to_char((char_to_int(B4)*2 + char_to_int(B2))*2 + char_to_int(B1))
+    idcode[2] = int_to_char((char_to_int(C4)*2 + char_to_int(C2))*2 + char_to_int(C1))
+    idcode[3] = int_to_char((char_to_int(D4)*2 + char_to_int(D2))*2 + char_to_int(D1))
+
+    return _idcode.decode()
+
+    #return str(byte1) + str(byte2) + str(byte3) + str(byte4)
+
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef int altcode(str msg):
+    """Compute the altitude.
+
+    Applicable only for DF4 or DF20 message, bit 20-32.
+    credit: @fbyrkjeland
+
+    Args:
+        msg (String): 28 bytes hexadecimal message string
+
+    Returns:
+        int: altitude in ft
+
+    """
+    if df(msg) not in [0, 4, 16, 20]:
+        raise RuntimeError("Message must be Downlink Format 0, 4, 16, or 20.")
+
+    # Altitude code, bit 20-32
+    cdef bytearray _mbin = bytearray(hex2bin(msg).encode())
+    cdef unsigned char[:] mbin = _mbin
+
+    cdef char mbit = mbin[25]  # M bit: 26
+    cdef char qbit = mbin[27]  # Q bit: 28
+    cdef int alt = 0
+    cdef bytearray vbin
+    cdef bytearray _graybytes = bytearray(11)
+    cdef unsigned char[:] graybytes = _graybytes
+
+    if mbit == 48:  # unit in ft, "0" -> 48
+        if qbit == 49:  # 25ft interval, "1" -> 49
+            vbin = _mbin[19:25] + _mbin[26:27] + _mbin[28:32]
+            alt = bin2int(vbin.decode()) * 25 - 1000
+        if qbit == 48:  # 100ft interval, above 50175ft, "0" -> 48
+            graybytes[8] = mbin[19]
+            graybytes[2] = mbin[20]
+            graybytes[9] = mbin[21]
+            graybytes[3] = mbin[22]
+            graybytes[10] = mbin[23]
+            graybytes[4] = mbin[24]
+            # _ = mbin[25]
+            graybytes[5] = mbin[26]
+            # cdef char D1 = mbin[27]  # always zero
+            graybytes[6] = mbin[28]
+            graybytes[0] = mbin[29]
+            graybytes[7] = mbin[30]
+            graybytes[1] = mbin[31]
+            # graybytes = D2 + D4 + A1 + A2 + A4 + B1 + B2 + B4 + C1 + C2 + C4
+
+            alt = gray2alt(_graybytes.decode())
+
+    if mbit == 49:  # unit in meter, "1" -> 49
+        vbin = _mbin[19:25] + _mbin[26:31]
+        alt = int(bin2int(vbin.decode()) * 3.28084)  # convert to ft
+
+    return alt
+
+
+
+cpdef int gray2alt(str codestr):
+    cdef str gc500 = codestr[:8]
+    cdef int n500 = gray2int(gc500)
+
+    # in 100-ft step must be converted first
+    cdef str gc100 = codestr[8:]
+    cdef int n100 = gray2int(gc100)
+
+    if n100 in [0, 5, 6]:
+        return -1
+        #return None
+
+    if n100 == 7:
+        n100 = 5
+
+    if n500 % 2:
+        n100 = 6 - n100
+
+    alt = (n500 * 500 + n100 * 100) - 1300
+    return alt
+
+
+cdef int gray2int(str graystr):
+    """Convert greycode to binary."""
+    cdef int num = bin2int(graystr)
+    num ^= num >> 8
+    num ^= num >> 4
+    num ^= num >> 2
+    num ^= num >> 1
+    return num
+
+
+cdef str data(str msg):
+    """Return the data frame in the message, bytes 9 to 22."""
+    return msg[8:-6]
+
+
+cpdef bint allzeros(str msg):
+    """Check if the data bits are all zeros.
+
+    Args:
+        msg (String): 28 bytes hexadecimal message string
+
+    Returns:
+        bool: True or False
+
+    """
+    d = hex2bin(data(msg))
+
+    if bin2int(d) > 0:
+        return False
+    else:
+        return True
+
+
+def wrongstatus(data, sb, msb, lsb):
+    """Check if the status bit and field bits are consistency.
+
+    This Function is used for checking BDS code versions.
+
+    """
+    # status bit, most significant bit, least significant bit
+    status = int(data[sb - 1])
+    value = bin2int(data[msb - 1 : lsb])
+
+    if not status:
+        if value != 0:
+            return True
+
+    return False
diff --git a/pyModeS/decoder/common.py b/pyModeS/decoder/common.py
index 17f55c6..b1edf35 100644
--- a/pyModeS/decoder/common.py
+++ b/pyModeS/decoder/common.py
@@ -15,32 +15,11 @@ def hex2int(hexstr):
     return int(hexstr, 16)
 
 
-def int2hex(n):
-    """Convert a integer to hexadecimal string."""
-    # strip 'L' for python 2
-    return hex(n)[2:].rjust(6, "0").upper().rstrip("L")
-
-
 def bin2int(binstr):
     """Convert a binary string to integer."""
     return int(binstr, 2)
 
 
-def bin2hex(hexstr):
-    """Convert a hexdecimal string to integer."""
-    return int2hex(bin2int(hexstr))
-
-
-def bin2np(binstr):
-    """Convert a binary string to numpy array."""
-    return np.array([int(i) for i in binstr])
-
-
-def np2bin(npbin):
-    """Convert a binary numpy array to string."""
-    return np.array2string(npbin, separator="")[1:-1]
-
-
 def df(msg):
     """Decode Downlink Format value, bits 1 to 5."""
     dfbin = hex2bin(msg[:2])
@@ -102,7 +81,7 @@ def crc_legacy(msg, encode=False):
     )
     ng = len(generator)
 
-    msgnpbin = bin2np(hex2bin(msg))
+    msgnpbin = np.array([int(i) for i in hex2bin(msg)])
 
     if encode:
         msgnpbin[-24:] = [0] * 24
@@ -116,7 +95,9 @@ def crc_legacy(msg, encode=False):
         msgnpbin[i : i + ng] = np.bitwise_xor(msgnpbin[i : i + ng], generator)
 
     # last 24 bits
-    reminder = bin2int(np2bin(msgnpbin[-24:]))
+    msgbin = np.array2string(msgnpbin[-24:], separator="")[1:-1]
+    reminder = bin2int(msgbin)
+
     return reminder
 
 
@@ -148,7 +129,7 @@ def icao(msg):
         addr = msg[2:8]
     elif DF in (0, 4, 5, 16, 20, 21):
         c0 = crc(msg, encode=True)
-        c1 = hex2int(msg[-6:])
+        c1 = int(msg[-6:], 16)
         addr = "%06X" % (c0 ^ c1)
     else:
         addr = None
@@ -161,7 +142,7 @@ def is_icao_assigned(icao):
     if (icao is None) or (not isinstance(icao, str)) or (len(icao) != 6):
         return False
 
-    icaoint = hex2int(icao)
+    icaoint = int(icao, 16)
 
     if 0x200000 < icaoint < 0x27FFFF:
         return False  # AFI
diff --git a/pyModeS/decoder/surv.py b/pyModeS/decoder/surv.py
index 45f219d..91b8c38 100644
--- a/pyModeS/decoder/surv.py
+++ b/pyModeS/decoder/surv.py
@@ -1,18 +1,3 @@
-# Copyright (C) 2018 Junzi Sun (TU Delft)
-
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 """
 Warpper for short roll call surveillance replies DF=4/5
 
diff --git a/setup.py b/setup.py
index e24ce41..9f62029 100644
--- a/setup.py
+++ b/setup.py
@@ -15,6 +15,13 @@ Steps for deploying a new version:
 # Always prefer setuptools over distutils
 from setuptools import setup, find_packages
 
+# Compile some parts
+from setuptools.extension import Extension
+from Cython.Build import cythonize
+
+extensions = [Extension("pyModeS.decoder.c_common", ["pyModeS/decoder/c_common.pyx"])]
+
+
 # To use a consistent encoding
 from codecs import open
 from os import path
@@ -57,6 +64,7 @@ setup(
         "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 3",
     ],
+    ext_modules=cythonize(extensions),
     # What does your project relate to?
     keywords="Mode-S ADS-B EHS ELS Comm-B",
     # You can just specify the packages manually here if your project is
diff --git a/tests/benchmark.py b/tests/benchmark.py
new file mode 100644
index 0000000..c7e5276
--- /dev/null
+++ b/tests/benchmark.py
@@ -0,0 +1,130 @@
+import sys
+import time
+import pandas as pd
+from tqdm import tqdm
+from pyModeS.decoder import adsb
+
+fin = sys.argv[1]
+
+df = pd.read_csv(fin, names=["ts", "df", "icao", "msg"])
+df_adsb = df[df["df"] == 17].copy()
+
+total = df_adsb.shape[0]
+
+
+def native():
+
+    from pyModeS.decoder import common
+
+    # airborne position
+    m_air_0 = None
+    m_air_1 = None
+
+    # surface position
+    m_surf_0 = None
+    m_surf_1 = None
+
+    for i, r in tqdm(df_adsb.iterrows(), total=total):
+        ts = r.ts
+        m = r.msg
+
+        downlink_format = common.df(m)
+        crc = common.crc(m)
+        icao = adsb.icao(m)
+        tc = adsb.typecode(m)
+
+        if 1 <= tc <= 4:
+            category = adsb.category(m)
+            callsign = adsb.callsign(m)
+        if tc == 19:
+            velocity = adsb.velocity(m)
+
+        if 5 <= tc <= 8:
+            if adsb.oe_flag(m):
+                m_surf_1 = m
+                t1 = ts
+            else:
+                m_surf_0 = m
+                t0 = ts
+
+            if m_surf_0 and m_surf_1:
+                position = adsb.surface_position(
+                    m_surf_0, m_surf_1, t0, t1, 50.01, 4.35
+                )
+                altitude = adsb.altitude(m)
+
+        if 9 <= tc <= 18:
+            if adsb.oe_flag(m):
+                m_air_1 = m
+                t1 = ts
+            else:
+                m_air_0 = m
+                t0 = ts
+
+            if m_air_0 and m_air_1:
+                position = adsb.position(m_air_0, m_air_1, t0, t1)
+                altitude = adsb.altitude(m)
+
+
+def cython():
+
+    from pyModeS.decoder import c_common as common
+
+    # airborne position
+    m_air_0 = None
+    m_air_1 = None
+
+    # surface position
+    m_surf_0 = None
+    m_surf_1 = None
+
+    for i, r in tqdm(df_adsb.iterrows(), total=total):
+        ts = r.ts
+        m = r.msg
+
+        downlink_format = common.df(m)
+        crc = common.crc(m)
+        icao = adsb.icao(m)
+        tc = adsb.typecode(m)
+
+        if 1 <= tc <= 4:
+            category = adsb.category(m)
+            callsign = adsb.callsign(m)
+        if tc == 19:
+            velocity = adsb.velocity(m)
+
+        if 5 <= tc <= 8:
+            if adsb.oe_flag(m):
+                m_surf_1 = m
+                t1 = ts
+            else:
+                m_surf_0 = m
+                t0 = ts
+
+            if m_surf_0 and m_surf_1:
+                position = adsb.surface_position(
+                    m_surf_0, m_surf_1, t0, t1, 50.01, 4.35
+                )
+                altitude = adsb.altitude(m)
+
+        if 9 <= tc <= 18:
+            if adsb.oe_flag(m):
+                m_air_1 = m
+                t1 = ts
+            else:
+                m_air_0 = m
+                t0 = ts
+
+            if m_air_0 and m_air_1:
+                position = adsb.position(m_air_0, m_air_1, t0, t1)
+                altitude = adsb.altitude(m)
+
+
+if __name__ == "__main__":
+    t1 = time.time()
+    native()
+    dt1 = time.time() - t1
+
+    t2 = time.time()
+    cython()
+    dt2 = time.time() - t2
diff --git a/tests/sample_run_adsb.py b/tests/sample_run_adsb.py
index c6c7c16..f0d5134 100644
--- a/tests/sample_run_adsb.py
+++ b/tests/sample_run_adsb.py
@@ -1,44 +1,46 @@
-from __future__ import print_function
-from pyModeS import adsb, ehs
+import sys
+import time
+import csv
+
+if len(sys.argv) > 1 and sys.argv[1] == "cython":
+    from pyModeS.c_decoder import adsb
+else:
+    from pyModeS.decoder import adsb
+
+print("===== Decode ADS-B sample data=====")
+
+f = open("tests/data/sample_data_adsb.csv", "rt")
+
+msg0 = None
+msg1 = None
+
+tstart = time.time()
+for i, r in enumerate(csv.reader(f)):
+
+    ts = int(r[0])
+    m = r[1].encode()
+
+    icao = adsb.icao(m)
+    tc = adsb.typecode(m)
+
+    if 1 <= tc <= 4:
+        print(ts, m, icao, tc, adsb.category(m), adsb.callsign(m))
+    if tc == 19:
+        print(ts, m, icao, tc, adsb.velocity(m))
+    if 5 <= tc <= 18:
+        if adsb.oe_flag(m):
+            msg1 = m
+            t1 = ts
+        else:
+            msg0 = m
+            t0 = ts
+
+        if msg0 and msg1:
+            pos = adsb.position(msg0, msg1, t0, t1)
+            alt = adsb.altitude(m)
+            print(ts, m, icao, tc, pos, alt)
 
 
-# === Decode sample data file ===
+dt = time.time() - tstart
 
-
-def adsb_decode_all(n=None):
-    print("===== Decode ADS-B sample data=====")
-    import csv
-
-    f = open("tests/data/sample_data_adsb.csv", "rt")
-
-    msg0 = None
-    msg1 = None
-
-    for i, r in enumerate(csv.reader(f)):
-        if n and i > n:
-            break
-
-        ts = r[0]
-        m = r[1]
-        icao = adsb.icao(m)
-        tc = adsb.typecode(m)
-        if 1 <= tc <= 4:
-            print(ts, m, icao, tc, adsb.category(m), adsb.callsign(m))
-        if tc == 19:
-            print(ts, m, icao, tc, adsb.velocity(m))
-        if 5 <= tc <= 18:
-            if adsb.oe_flag(m):
-                msg1 = m
-                t1 = ts
-            else:
-                msg0 = m
-                t0 = ts
-
-            if msg0 and msg1:
-                pos = adsb.position(msg0, msg1, t0, t1)
-                alt = adsb.altitude(m)
-                print(ts, m, icao, tc, pos, alt)
-
-
-if __name__ == "__main__":
-    adsb_decode_all(n=100)
+print("Execution time: {} seconds".format(dt))
diff --git a/tests/test_c_common.py b/tests/test_c_common.py
new file mode 100644
index 0000000..71599b9
--- /dev/null
+++ b/tests/test_c_common.py
@@ -0,0 +1,60 @@
+from pyModeS.decoder import c_common as common
+
+
+def test_conversions():
+    assert common.hex2bin("6E406B") == "011011100100000001101011"
+
+
+def test_crc_decode():
+
+    assert common.crc("8D406B902015A678D4D220AA4BDA") == 0
+    assert common.crc("8d8960ed58bf053cf11bc5932b7d") == 0
+    assert common.crc("8d45cab390c39509496ca9a32912") == 0
+    assert common.crc("8d74802958c904e6ef4ba0184d5c") == 0
+    assert common.crc("8d4400cd9b0000b4f87000e71a10") == 0
+    assert common.crc("8d4065de58a1054a7ef0218e226a") == 0
+
+    assert common.crc("c80b2dca34aa21dd821a04cb64d4") == 10719924
+    assert common.crc("a800089d8094e33a6004e4b8a522") == 4805588
+    assert common.crc("a8000614a50b6d32bed000bbe0ed") == 5659991
+    assert common.crc("a0000410bc900010a40000f5f477") == 11727682
+    assert common.crc("8d4ca251204994b1c36e60a5343d") == 16
+    assert common.crc("b0001718c65632b0a82040715b65") == 353333
+
+
+def test_crc_encode():
+    parity = common.crc("8D406B902015A678D4D220AA4BDA", encode=True)
+    assert parity == 11160538
+
+
+def test_icao():
+    assert common.icao("8D406B902015A678D4D220AA4BDA") == "406B90"
+    assert common.icao("A0001839CA3800315800007448D9") == "400940"
+    assert common.icao("A000139381951536E024D4CCF6B5") == "3C4DD2"
+    assert common.icao("A000029CFFBAA11E2004727281F1") == "4243D0"
+
+
+def test_modes_altcode():
+    assert common.altcode("A02014B400000000000000F9D514") == 32300
+
+
+def test_modes_idcode():
+    assert common.idcode("A800292DFFBBA9383FFCEB903D01") == "1346"
+
+
+def test_graycode_to_altitude():
+    assert common.gray2alt("00000000010") == -1000
+    assert common.gray2alt("00000001010") == -500
+    assert common.gray2alt("00000011011") == -100
+    assert common.gray2alt("00000011010") == 0
+    assert common.gray2alt("00000011110") == 100
+    assert common.gray2alt("00000010011") == 600
+    assert common.gray2alt("00000110010") == 1000
+    assert common.gray2alt("00001001001") == 5800
+    assert common.gray2alt("00011100100") == 10300
+    assert common.gray2alt("01100011010") == 32000
+    assert common.gray2alt("01110000100") == 46300
+    assert common.gray2alt("01010101100") == 50200
+    assert common.gray2alt("11011110100") == 73200
+    assert common.gray2alt("10000000011") == 126600
+    assert common.gray2alt("10000000001") == 126700
diff --git a/tests/test_common.py b/tests/test_common.py
index df2ac36..571b0cf 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -1,10 +1,8 @@
-from pyModeS import common
+from pyModeS.decoder import common
 
 
 def test_conversions():
     assert common.hex2bin("6E406B") == "011011100100000001101011"
-    assert common.bin2hex("011011100100000001101011") == "6E406B"
-    assert common.int2hex(11160538) == "AA4BDA"
 
 
 def test_crc_decode():
@@ -28,7 +26,7 @@ def test_crc_decode():
 
 def test_crc_encode():
     parity = common.crc("8D406B902015A678D4D220AA4BDA", encode=True)
-    assert common.int2hex(parity) == "AA4BDA"
+    assert parity == 11160538
 
 
 def test_icao():
diff --git a/tox.ini b/tox.ini
deleted file mode 100644
index 9a0ca74..0000000
--- a/tox.ini
+++ /dev/null
@@ -1,11 +0,0 @@
-[tox]
-toxworkdir = /tmp/tox
-envlist = py2,py3
-
-[testenv]
-deps =
-  pytest
-  numpy
-  pyzmq
-  pyrtlsdr
-commands = py.test