1 files changed, 49 insertions, 0 deletions

diff --git a/dexcom_reader/packetwriter.py b/dexcom_reader/packetwriter.py
new file mode 100644
index 0000000..6e03342
--- /dev/null
+++ b/dexcom_reader/packetwriter.py
@@ -0,0 +1,49 @@
+import crc16
+import struct
+
+class PacketWriter(object):
+  MAX_PAYLOAD = 1584
+  MIN_LEN = 6
+  MAX_LEN = 1590
+  SOF = 0x01
+  OFFSET_SOF = 0
+  OFFSET_LENGTH = 1
+  OFFSET_CMD = 3
+  OFFSET_PAYLOAD = 4
+
+  def __init__(self):
+    self._packet = None
+
+  def Clear(self):
+    self._packet = None
+
+  def NewSOF(self, v):
+    self._packet[0] = chr(v)
+
+  def PacketString(self):
+    return ''.join(self._packet)
+ 
+  def AppendCrc(self):
+    self.SetLength()
+    ps = self.PacketString()
+    crc = crc16.crc16(ps, 0, len(ps))
+    for x in struct.pack('H', crc):
+      self._packet.append(x)
+
+  def SetLength(self):
+    self._packet[1] = chr(len(self._packet) + 2)
+
+  def _Add(self, x):
+    try:
+      len(x)
+      for y in x:
+        self._Add(y)
+    except:
+      self._packet.append(x)
+
+  def ComposePacket(self, command, payload=None):
+    assert self._packet is None
+    self._packet = ["\x01", None, "\x00", chr(command)]
+    if payload:
+      self._Add(payload)
+    self.AppendCrc()