From 46ffb7923462deb2cc8b33cd0ab7e06d0e4cdb51 Mon Sep 17 00:00:00 2001
From: Junzi Sun <junzisun@gmail.com>
Date: Wed, 2 Dec 2020 22:29:42 +0100
Subject: [PATCH] add VDS check in ACAS decoding

---
 pyModeS/decoder/acas.py          | 35 +++++++++++++++++++++++++++++++-
 pyModeS/decoder/bds/bds61_st2.py |  4 ++--
 tests/test_acas.py               |  2 +-
 3 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/pyModeS/decoder/acas.py b/pyModeS/decoder/acas.py
index 9af8018..47a2dd8 100644
--- a/pyModeS/decoder/acas.py
+++ b/pyModeS/decoder/acas.py
@@ -3,6 +3,24 @@ Decoding Air-Air Surveillance (ACAS) DF=0/16
 """
 
 from pyModeS import common
+import warnings
+
+warnings.simplefilter("always", UserWarning)
+
+
+def isACAS(msg: str) -> bool:
+    """Check if the message is an ACAS coordination message.
+
+    :param msg: 28 hexdigits string
+    :return: if VDS is 3,1
+    """
+    mv = common.hex2bin(common.data(msg))
+
+    vds = mv[0:8]
+    if vds == "00110000":
+        return True
+    else:
+        return False
 
 
 def rac(msg: str) -> str:
@@ -11,10 +29,14 @@ def rac(msg: str) -> str:
     :param msg: 28 hexdigits string
     :return: RACs
     """
-    mv = common.hex2bin(common.data(msg))
+    if not isACAS(msg):
+        warnings.warn("Not an ACAS coordination message.")
+        return None
 
     RAC = []
 
+    mv = common.hex2bin(common.data(msg))
+
     if mv[22] == "1":
         RAC.append("do not pass below")
 
@@ -39,6 +61,10 @@ def rat(msg: str) -> bool:
     :param msg: 28 hexdigits string
     :return: if RA has been terminated
     """
+    if not isACAS(msg):
+        warnings.warn("Not an ACAS coordination message.")
+        return None
+
     mv = common.hex2bin(common.data(msg))
     mte = int(mv[26])
     return mte
@@ -50,6 +76,10 @@ def mte(msg: str) -> bool:
     :param msg: 28 hexdigits string
     :return: if there are multiple threats
     """
+    if not isACAS(msg):
+        warnings.warn("Not an ACAS coordination message.")
+        return None
+
     mv = common.hex2bin(common.data(msg))
     mte = int(mv[27])
     return mte
@@ -61,6 +91,9 @@ def ara(msg: str) -> str:
     :param msg: 28 bytes hexadecimal message string
     :return: RA charactristics
     """
+    if not isACAS(msg):
+        warnings.warn("Not an ACAS coordination message.")
+        return None
     mv = common.hex2bin(common.data(msg))
 
     mte = int(mv[27])
diff --git a/pyModeS/decoder/bds/bds61_st2.py b/pyModeS/decoder/bds/bds61_st2.py
index 846de0a..2726873 100644
--- a/pyModeS/decoder/bds/bds61_st2.py
+++ b/pyModeS/decoder/bds/bds61_st2.py
@@ -60,7 +60,7 @@ def threat_location(msg: str) -> Tuple:
 
 
 def has_multiple_threats(msg: str) -> bool:
-    """ Indicate if the ACAS is processing multiple threats simultaneously.
+    """Indicate if the ACAS is processing multiple threats simultaneously.
 
     :param msg: 28 hexdigits string
     :return: if there are multiple threats
@@ -76,7 +76,7 @@ def active_resolution_advisories(msg: str) -> str:
     :param msg: 28 bytes hexadecimal message string
     :return: RA charactristics
     """
-    return acars.ara(msg)
+    return acas.ara(msg)
 
 
 def is_ra_terminated(msg: str) -> bool:
diff --git a/tests/test_acas.py b/tests/test_acas.py
index 90cb12f..963e258 100644
--- a/tests/test_acas.py
+++ b/tests/test_acas.py
@@ -25,7 +25,7 @@ msgs = [
     "8526D57E4D963C92CDEE1B6C7C49",
     "802A613C93F65A7FF803A51B5ADB",
     "85B6C54279B67BE2A0001998FFDA",
-    "851944F15881648338D1AF4B7A27",
+    "851944F1 5881648338D1AF 4B7A27",
     "8321014858208000787905B0E800",
     "866DD078EDEBD330404FFFAE9BA5",
     "80E196905AB503260E835D849E35",