Oblivion Mod:Mod File Format/Vs Morrowind

While the Oblivion file format is closely related to the Morrowind file format, the two formats also differ substantially both in structural elements and in details for most (all) record types. The introduction of groups, data compression and outsize subrecord data handling all vastly complicate reading and writing of the file, while the introduction of formids greatly complicates the logic of integrating and relating changes to files.

File Parsing Pseudocode

Oblivion

Basic pseudocode for parsing Oblivion's ESM/ESP files. Successfully tested on Oblivion.esm. Note that Header refers to the 20 byte record/group and the 6 byte subrecord headers.

Read Header   //Required TES4 record
Read Record

While Bytes Left in File
   Read Header
   Read Group
EndWhile

Read Group

   While Bytes Left in Group
      Read Header

      if Name = GRUP
         Read Group
      else
         Read Record
   EndWhile

End Group

Read Record

   if IsCompressed
      Read Compressed Record Data
   else
      Read Subrecords

End Record


Read Compressed Record Data
   Read Deflated Data Size (dword)
   Read Compressed Data (given by record size in header)
   Deflate Data (zLib)
   Read Subrecords (from the deflated data)
End Read Raw Data


Read Subrecords

   While Bytes Left in Record
      Read Subrecord Header
      
      if Name = XXXX
         Read Subrecord Data
         Subrecord data is the size of the next subrecord
      else if Previous Name = XXXX
         Ignore subrecord size in header, take size from previous XXXX subrecord
         Read Subrecord Data         
      else
         Read Subrecord Data
      
   EndWhile

End Read Subrecords

Morrowind

For comparison only, the pseudocode for parsing Morrowind's ESM/ESP/ESS files is trivial:

While Bytes Left in File
   Read Record Header

   While Bytes Left in Record
      Read Subrecord Header
      Read Subrecord Data
   EndWhile
EndWhile

Simple is good...