First shot at a Gcd class.

grmn/gcd.py

@@ -0,0 +1,163 @@
+# -*- coding: utf-8 -*-
+# Thanks to TurboCCC and kunix for all your work!
+
+from . import chksum
+from . import devices
+from binascii import hexlify
+from struct import pack, unpack
+import sys
+
+GCD_SIG = b"G\x41RM\x49Nd\00"
+DEFAULT_COPYRIGHT = b"Copyright 1996-2017 by G\x61rm\x69n Ltd. or its subsidiaries."
+DEFAULT_FIRST_PADDING = 21
+DEFAULT_ALIGN = 0x1000
+
+TLV_TYPES = {
+    0x0001: "Checksum rectifier",
+    0x0002: "Padding",
+    0x0003: "Part number?",
+    0x0005: "Copyright notice",
+    0x0006: "Block Type 7 format definition",
+    0x0007: "Binary descriptor",
+    0x0008: "Binary Region 0C (boot.bin)",
+    0x0401: "Binary Component Firmware (SensorHub, ANT_BLE_BT, GPS, WiFi)",
+    0x0505: "Binary Region 05",
+    0x0555: "Binary Region 55",
+    0x0557: "Binary Region 57",
+    0x02bd: "Binary Region 0E (fw_all.bin)",
+    0xffff: "EOF marker",
+}
+
+class TLV:
+    def __init__(self, type_id: int, expected_length: int, value=None):
+        self.type_id = type_id
+        self.comment = TLV_TYPES.get(type_id, "Type {:04x} / {:d}".format(type_id, type_id))
+        self.length = expected_length
+        self.value = None
+        if value is not None:
+            self.value = bytes(value)
+
+    @staticmethod
+    def factory(header: bytes):
+        (type_id, length) = unpack("<HH", payload)
+        if type_id == 0x0006:
+            return TLV6(type_id, length)
+        elif type_id == 0x0007:
+            return TLV7(type_id, length)
+        return TLV(type_id, length)
+
+    def set_value(self, new_value: bytes):
+        self.value = new_value
+
+    def get_length(self):
+        return self.length
+
+    def get_actual_length(self):
+        if self.value is None:
+            return 0
+        return len(self.value)
+
+    def get_record_length(self):
+        # Length including record definition
+        return self.get_actual_length() + 4
+    
+    def get_value(self):
+        return self.value
+
+    def get(self):
+        header = pack("HH", self.type_id, self.get_actual_length())
+        if self.value is None:
+            return header
+        return header + self.value
+
+class TLV6(TLV):
+    """
+    Describes following TLV7:
+    http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=137838&whichpage=12
+    """
+
+    # Field ids in Type 6 payloads (describe Type 7 data format)
+    # First nibble might be data type: 0 = B, 1 = H, 2 = L
+    FIELD_TYPES = {
+        0x000a: ["B", "XOR flag/value"],
+        0x000b: ["B", "Reset/Downgrade flag"],
+        0x1009: ["H", "Device hw_id"],
+        0x100a: ["H", "Block type"],
+        0x100d: ["H", "Firmware version"],
+        0x1014: ["H", "Field 1014"],
+        0x1015: ["H", "Field 1015"],
+        0x1016: ["H", "Field 1016 (WiFi fw)"],
+        0x2015: ["L", "Block size"],
+        0x5003: ["", "End of definition marker"],
+    }
+
+    def __init__(self, type_id: int, expected_length: int, value=None):
+        super.__init__(type_id, expected_length, value)
+        self.is_parsed = False
+
+    def parse(self):
+        if len(self.value) % 2 != 0:
+            raise Exception("Invalid TLV6 payload length!")
+
+        self.fids = []
+        self.format = ""
+        self.fields = []
+
+        for i in range(0, len(self.value), 2):
+            fid = unpack("H", self.value[i:i+2])[0]
+            fdef = self.FIELD_TYPES[fid]
+            self.fids.append(fid)
+            self.format += fdef[0]
+            self.fields.append(fdef[1])
+
+        self.is_parsed = True
+
+class TLV7(TLV):
+    def set_tlv6(self, tlv6: TLV6):
+        self.tlv6 = tlv6
+
+    def parse(self):
+        if not self.tlv6.is_parsed:
+            # Make sure we have the structure analysed
+            self.tlv6.parse()
+        values = unpack("<" + self.tlv6.format, self.value)
+        for i, v in enumerate(values):
+            fid = self.tlv6.fids[i]
+            fdesc = self.tlv6.fields[i]
+            if fid == 0x1009:
+                print("  - {:>20}: 0x{:04x} / {:d} ({})".format(fdesc, v, v, devices.DEVICES.get(v, "Unknown device")))
+            elif fid == 0x2015:
+                print("  - {:>20}: {} Bytes".format(fdesc, v))
+            else:
+                print("  - {:>20}: 0x{:04x} / {:d}".format(fdesc, v, v))
+
+class Gcd:
+    def __init__(self, filename: str=None):
+        self.filename = filename
+        self.struct = []
+        if filename is not None:
+            self.load()
+
+    def load(self):
+        if self.filename is None:
+            return False
+        last_tlv6 = None
+        with open(self.filename, "rb") as f:
+            sig = f.read(8)
+            if sig != GCD_SIG:
+                raise Exception("Signature mismatch ({}, should be {})!".format(repr(sig), repr(GCD_SIG)))
+            while True:
+                header = f.read(4)
+                tlv = TLV.factory(header)
+                self.struct.append(tlv)
+                if tlv.type_id == 0xFFFF:
+                    # End of file reached
+                    break
+                tlength = tlv.get_length()
+                payload = f.read(tlength)
+                tlv.set_value(payload)
+                if tlv.type_id == 0x0006:
+                    last_tlv6 = tlv
+                elif tlv.type_id == 0x0007:
+                    tlv.set_tlv6(last_tlv6)
+            f.close()