7z format specification

[SignatureHeader]
[PackedStreams]        // Compressed data
[PackedHeaderStreams]  // Compressed headers (optional)
[Header]               // Archive metadata

#define k7zStartHeaderSize 0x20  // 32 bytes
#define k7zSignatureSize 6

const Byte k7zSignature[k7zSignatureSize] = {'7', 'z', 0xBC, 0xAF, 0x27, 0x1C};

Offset  Size  Description
------  ----  -----------
0       6     Signature: 37 7A BC AF 27 1C
6       1     ArchiveVersion (Major)
7       1     ArchiveVersion (Minor)
8       4     StartHeaderCRC
12      8     NextHeaderOffset
20      8     NextHeaderSize
28      4     NextHeaderCRC

Header Types:
  kEnd                = 0x00
  kHeader             = 0x01
  kArchiveProperties  = 0x02
  kAdditionalStreamsInfo = 0x03
  kMainStreamsInfo    = 0x04
  kFilesInfo          = 0x05
  kPackInfo           = 0x06
  kUnpackInfo         = 0x07
  kSubStreamsInfo     = 0x08
  kSize               = 0x09
  kCRC                = 0x0A
  kFolder             = 0x0B
  kCodersUnpackSize   = 0x0C
  kNumUnpackStream    = 0x0D
  kEmptyStream        = 0x0E
  kEmptyFile          = 0x0F
  kAnti               = 0x10
  kName               = 0x11
  kCTime              = 0x12
  kATime              = 0x13
  kMTime              = 0x14
  kWinAttrib          = 0x15
  kComment            = 0x16
  kEncodedHeader      = 0x17
  kStartPos           = 0x18
  kDummy              = 0x19

typedef struct
{
  UInt32 NumCoders;     // Number of compression methods
  UInt32 NumBonds;      // Number of bonds between coders
  UInt32 NumPackStreams;
  UInt32 UnpackStream;
  UInt32 PackStreams[SZ_NUM_PACK_STREAMS_IN_FOLDER_MAX];
  CSzBond Bonds[SZ_NUM_BONDS_IN_FOLDER_MAX];
  CSzCoderInfo Coders[SZ_NUM_CODERS_IN_FOLDER_MAX];
} CSzFolder;

typedef struct
{
  size_t PropsOffset;   // Offset to properties in CodersData
  UInt32 MethodID;      // Compression method ID
  Byte NumStreams;      // Number of input/output streams
  Byte PropsSize;       // Size of properties
} CSzCoderInfo;

typedef struct
{
  CNtfsFileTime MTime;  // Modification time
  UInt64 Size;          // Uncompressed size
  UInt32 Crc;           // CRC32
  UInt32 Attrib;        // File attributes
  Byte HasStream;       // Has data stream
  Byte IsDir;           // Is directory
  Byte IsAnti;          // Anti-item (for updates)
  Byte CrcDefined;      // CRC is valid
  Byte MTimeDefined;    // MTime is valid
  Byte AttribDefined;   // Attrib is valid
} CSzFileItem;

#define FILE_ATTRIBUTE_READONLY  0x0001
#define FILE_ATTRIBUTE_HIDDEN    0x0002
#define FILE_ATTRIBUTE_SYSTEM    0x0004
#define FILE_ATTRIBUTE_DIRECTORY 0x0010
#define FILE_ATTRIBUTE_ARCHIVE   0x0020

UInt32 unixMode = (attrib >> 16) & 0xFFFF;

07 - [7z]
   01 - 7zAES (AES-256 + SHA-256)

Byte[1] - Version (0x00)
Byte[1] - Number of cycles (power of 2)
Byte[1] - Salt size (0-16)
Byte[*] - Salt data
Byte[*] - Password verification data
Byte[16] - IV

typedef struct
{
  UInt32 NumPackStreams;  // Number of packed streams
  UInt32 NumFolders;      // Number of folders

  UInt64 *PackPositions;  // Positions of packed streams
  CSzBitUi32s FolderCRCs; // CRCs for folders

  size_t *FoCodersOffsets;        // Offsets to coder data
  UInt32 *FoStartPackStreamIndex; // Start pack stream for folder
  UInt32 *FoToCoderUnpackSizes;   // Unpack sizes
  Byte *FoToMainUnpackSizeIndex;  // Main unpack size index
  UInt64 *CoderUnpackSizes;       // Unpack sizes for all coders

  Byte *CodersData;       // Coder properties data

  UInt64 RangeLimit;
} CSzAr;

// 1. Declare variables
CLookToRead2 lookStream;
CSzArEx db;
ISzAlloc allocImp;
ISzAlloc allocTempImp;

// 2. Initialize CRC table
CrcGenerateTable();

// 3. Initialize database
SzArEx_Init(&db);

// 4. Open archive
SRes res = SzArEx_Open(&db, &lookStream.vt, &allocImp, &allocTempImp);

if (res == SZ_OK) {
    // Archive opened successfully
    // db.NumFiles contains file count
}

// Extract file by index
UInt32 fileIndex = 0;
UInt32 blockIndex = 0xFFFFFFFF;
Byte *outBuffer = NULL;
size_t outBufferSize = 0;

for (UInt32 i = 0; i < db.NumFiles; i++) {
    size_t offset = 0;
    size_t outSizeProcessed = 0;
    
    SRes res = SzArEx_Extract(
        &db, &lookStream.vt, i,
        &blockIndex, &outBuffer, &outBufferSize,
        &offset, &outSizeProcessed,
        &allocImp, &allocTempImp
    );
    
    if (res == SZ_OK) {
        // File data is at: outBuffer + offset
        // Size: outSizeProcessed
    }
}

// Free output buffer
allocImp.Free(&allocImp, outBuffer);

class CHandler : public IInArchive, public IOutArchive
{
public:
    // IInArchive
    STDMETHOD(Open)(IInStream *stream, ...);
    STDMETHOD(Extract)(const UInt32 *indices, UInt32 numItems, ...);
    
    // IOutArchive
    STDMETHOD(UpdateItems)(ISequentialOutStream *outStream, ...);
};

// Update modes
enum EUpdateMode
{
    k_UpdateMode_Add,       // Add new file
    k_UpdateMode_Update,    // Update existing file
    k_UpdateMode_Delete,    // Delete file
    k_UpdateMode_Rename     // Rename file
};