fix DF4,20 altitude decoding

tests/test_ehs.py

@@ -1,5 +1,5 @@
 from pyModeS import ehs
-
+from pyModeS import modes_common
 
 def test_ehs_icao():
     assert ehs.icao("A0001839CA3800315800007448D9") == '400940'
@@ -49,3 +49,20 @@ def test_ehs_BDS60_functions():
     assert ehs.mach60("A000029CFFBAA11E2004727281F1") == 0.48
     assert ehs.vr60baro("A000029CFFBAA11E2004727281F1") == 0
     assert ehs.vr60ins("A000029CFFBAA11E2004727281F1") == 3648
+
+def test_greycode_to_altitude():
+    assert modes_common.grey2alt('00000000010') == -1000
+    assert modes_common.grey2alt('00000001010') == -500
+    assert modes_common.grey2alt('00000011011') == -100
+    assert modes_common.grey2alt('00000011010') == 0
+    assert modes_common.grey2alt('00000011110') == 100
+    assert modes_common.grey2alt('00000010011') == 600
+    assert modes_common.grey2alt('00000110010') == 1000
+    assert modes_common.grey2alt('00001001001') == 5800
+    assert modes_common.grey2alt('00011100100') == 10300
+    assert modes_common.grey2alt('01100011010') == 32000
+    assert modes_common.grey2alt('01110000100') == 46300
+    assert modes_common.grey2alt('01010101100') == 50200
+    assert modes_common.grey2alt('11011110100') == 73200
+    assert modes_common.grey2alt('10000000011') == 126600
+    assert modes_common.grey2alt('10000000001') == 126700