[CDG5] Random questions

[Max Poliakovski]

Compressed resources have a 18 bytes header at the beginning of the
resource data. That header tells some bits about the underlying
compression. It has the following format:

typedef struct ExtendedResource {
    uint32_t    signature; // must be 0xA89F6572
    uint16_t    hdrLength; // usually set to 0x12
    uint8_t      hdrVersion; // 8 - Donn, 9 - Gregg
    uint8_t      extAttributes; // bit 0 == 1 indicates compressed data
    union {
        struct {
            uint8_t      varTableRatio;
            uint8_t      overRun;
            uint16_t    decompressID;
            uint16_t    cTableID;
        } DonnSpecific;
        struct {
            uint16_t    defprocID; // decompressor resource ID
            uint16_t    decompressSlop;
            uint8_t      byteTabelSize;
            uint8_t      compressFlags;
        } GreggSpecific;
    } extHdr;
};

The DonnSpecific fields are only valid when hdrVersion == 8. Donn
decompression scheme seems to use two distinct algorithms: 0 for data, 1
for code.

Looking into the resources you sent to me, I found out that none of them
has been compressed using the Donn algo. ProcessManager's scod contains
defprocID = 0x0002 while glnd uses defprocID = 0x0003. The former seems to
indicate the GreggyBits algo (dcmp 2), while the latter looks like that
unknown algo added in OS9 (dcmp 3).

Next, I'll try to create a Python decompressor for the Greggy-compressed
resource...
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