Task 008: .7z File Format

Below is a detailed response addressing the requirements of TASK 008 for the .7Z file format. The .7Z file format is a compressed archive format developed by Igor Pavlov for the 7-Zip archiver. It is known for its high compression ratio, support for multiple compression methods, and advanced features like encryption. I'll first list the intrinsic properties of the .7Z file format based on its specifications, then provide implementations in Python, Java, JavaScript, and C to handle .7Z files and print these properties.

1. Properties of the .7Z File Format Intrinsic to Its File System

The .7Z file format is a binary archive format designed for compression, encryption, and aggregation of files. The following properties are intrinsic to its structure and functionality, as derived from official documentation and third-party specifications (e.g., 7-Zip source code, py7zr documentation, and other reliable sources):

File Signature (Magic Number):

• A 6-byte sequence at the start of the file: 0x37 0x7A 0xBC 0xAF 0x27 0x1C (ASCII: '7z' followed by 0xBC, 0xAF, 0x27, 0x1C).
• Identifies the file as a .7Z archive.

Version:

• Two bytes following the signature indicating the major and minor version of the format (e.g., 0x00 0x04 for version 0.4).

Start Header CRC:

• A 4-byte CRC32 checksum of the start header (signature and version).

Next Header Offset:

• A 64-bit integer specifying the offset to the next header (relative to the end of the start header).

Next Header Size:

• A 64-bit integer indicating the size of the next header.

Next Header CRC:

• A 4-byte CRC32 checksum of the next header.

Archive Properties:

• Optional metadata defining archive characteristics (e.g., compression methods, encryption settings).

Compression Methods:

• Support for multiple compression algorithms, including:
• LZMA (default, optimized LZ77).
• LZMA2 (improved LZMA).
• PPMD (Dmitry Shkarin’s PPMdH with modifications).
• BZip2 (Burrows-Wheeler Transform).
• Deflate (LZ77-based).
• Copy (no compression).

Filters:

• Pre-processing filters to enhance compression, such as:
• BCJ/BCJ2 (for x86 executables).
• ARM64, ARMT, ARM, PPC, SPARC, IA64 (architecture-specific converters).
• Delta (for WAV files).
• Swap2/Swap4 (byte order converters).

Encryption:

• Supports AES-256 encryption with a SHA-256-based key derivation function.
• Option to encrypt file names and archive headers.

File Size Support:

• Supports files up to 2^64 bytes (approximately 16 exabytes).

Unicode File Names:

• Supports Unicode (UTF-16) for file names, allowing international character sets.

Solid Compression:

• Groups similar files into a single compression stream to exploit redundancy, improving compression ratios.

Archive Header Compression:

• Headers can be compressed to reduce archive overhead.

Multi-Part Archives:

• Supports splitting archives into multiple parts (e.g., xxx.7z.001, xxx.7z.002).

Folder Structure:

• Organizes files into folders with associated coders (compression methods) and binding pairs for data streams.

Pack Info:

• Includes packed stream positions, number of pack streams, sizes, and CRCs.

Unpack Info:

• Contains folder information, unpack sizes, and CRCs for decompressed data.

SubStreams Info:

• Details the number of unpack streams per folder and their sizes.

Files Info:

• Metadata about files, including:
• Number of files.
• File names (Unicode).
• File attributes (e.g., is directory, modification time).
• Empty stream flags (for directories or empty files).

CRC Checks:

• Multiple CRC32 checksums for data integrity (e.g., for packed streams, folders, and headers).

Open Architecture:

• Modular design allowing custom compression, conversion, or encryption methods via plugins.

These properties are derived from the 7-Zip documentation, the py7zr specification, and the 7z format description in the 7-Zip source code (,,,,).

Note: The .7Z format does not store file system-specific metadata like owner/group permissions, which limits its use for backups on Linux/Unix systems ().

2. Python Class for .7Z File Handling

Below is a Python class using the py7zr library to open, read, write, and print the properties of a .7Z archive. The py7zr library is a robust implementation for handling .7Z files and provides access to most of the format's properties.

import py7zr
import binascii
import os
from datetime import datetime

class SevenZipHandler:
    def __init__(self, filepath):
        self.filepath = filepath
        self.archive = None

    def open_archive(self, mode='r'):
        """Open a .7Z archive for reading or writing."""
        try:
            self.archive = py7zr.SevenZipFile(self.filepath, mode=mode)
            return True
        except Exception as e:
            print(f"Error opening archive: {e}")
            return False

    def read_properties(self):
        """Read and print all intrinsic properties of the .7Z archive."""
        if not self.archive:
            print("Archive not opened.")
            return

        try:
            # Read header information
            with open(self.filepath, 'rb') as f:
                header = f.read(12)  # Signature (6) + Version (2) + CRC (4)
                signature = header[:6]
                version = header[6:8]
                start_header_crc = header[8:12]
                next_header_offset = int.from_bytes(f.read(8), 'little')
                next_header_size = int.from_bytes(f.read(8), 'little')
                next_header_crc = int.from_bytes(f.read(4), 'little')

            # Print basic properties
            print("=== .7Z Archive Properties ===")
            print(f"File Signature: {binascii.hexlify(signature).decode()}")
            print(f"Format Version: {version[0]}.{version[1]}")
            print(f"Start Header CRC: {binascii.hexlify(start_header_crc).decode()}")
            print(f"Next Header Offset: {next_header_offset}")
            print(f"Next Header Size: {next_header_size}")
            print(f"Next Header CRC: {next_header_crc}")

            # Archive properties from py7zr
            print("\nArchive Metadata:")
            print(f"Number of Files: {len(self.archive.files)}")
            print(f"Solid Compression: {self.archive.header.main_streams.solid}")
            print(f"Encrypted: {self.archive.header.main_streams.packinfo.encrypt}")

            # Compression methods and filters
            print("\nCompression Methods and Filters:")
            for folder in self.archive.header.main_streams.unpackinfo.folders:
                for coder in folder.coders:
                    method = coder.get('method', b'Unknown').decode('utf-8', errors='ignore')
                    print(f" - Method: {method}")
                    if 'properties' in coder:
                        print(f"   Properties: {binascii.hexlify(coder['properties']).decode()}")

            # File information
            print("\nFile Information:")
            for file_info in self.archive.files:
                print(f" - File: {file_info.filename}")
                print(f"   Size: {file_info.uncompressed}")
                print(f"   Is Directory: {file_info.is_directory}")
                print(f"   Modification Time: {file_info.lastwritetime}")
                print(f"   CRC: {file_info.crc}")

            # Multi-part archive check
            print(f"\nMulti-Part Archive: {self.archive.is_multipart}")

        except Exception as e:
            print(f"Error reading properties: {e}")

    def write_archive(self, files_to_add):
        """Write files to a new .7Z archive."""
        try:
            with py7zr.SevenZipFile(self.filepath, 'w') as archive:
                for file_path in files_to_add:
                    if os.path.exists(file_path):
                        archive.write(file_path)
                        print(f"Added {file_path} to archive")
                    else:
                        print(f"File {file_path} does not exist")
        except Exception as e:
            print(f"Error writing archive: {e}")

    def close_archive(self):
        """Close the archive."""
        if self.archive:
            self.archive.close()
            self.archive = None

# Example usage
if __name__ == "__main__":
    handler = SevenZipHandler("example.7z")
    if handler.open_archive():
        handler.read_properties()
        handler.close_archive()

    # Create a new archive
    handler = SevenZipHandler("new_archive.7z")
    handler.write_archive(["test.txt", "image.png"])

Dependencies: Install py7zr using pip install py7zr.

Explanation:

• The class uses py7zr to handle .7Z archives, which provides high-level access to archive properties.
• The read_properties method extracts and prints the file signature, version, CRCs, compression methods, file metadata, and other properties.
• The write_archive method creates a new .7Z archive with specified files.
• Error handling ensures robustness when opening or processing archives.

3. Java Class for .7Z File Handling

Java can use the Apache Commons Compress library with the SevenZFile class to handle .7Z archives. Below is a Java class that reads and writes .7Z files and prints their properties.

import org.apache.commons.compress.archivers.sevenz.SevenZFile;
import org.apache.commons.compress.archivers.sevenz.SevenZOutputFile;
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.Arrays;

public class SevenZipHandler {
    private String filepath;
    private SevenZFile archive;

    public SevenZipHandler(String filepath) {
        this.filepath = filepath;
        this.archive = null;
    }

    public boolean openArchive() {
        try {
            archive = new SevenZFile(new File(filepath));
            return true;
        } catch (IOException e) {
            System.err.println("Error opening archive: " + e.getMessage());
            return false;
        }
    }

    public void readProperties() {
        if (archive == null) {
            System.err.println("Archive not opened.");
            return;
        }

        try {
            // Read header manually
            File file = new File(filepath);
            try (java.io.RandomAccessFile raf = new java.io.RandomAccessFile(file, "r")) {
                byte[] header = new byte[32];
                raf.read(header);

                byte[] signature = Arrays.copyOfRange(header, 0, 6);
                byte[] version = Arrays.copyOfRange(header, 6, 8);
                byte[] startHeaderCrc = Arrays.copyOfRange(header, 8, 12);
                long nextHeaderOffset = ByteBuffer.wrap(Arrays.copyOfRange(header, 12, 20)).order(ByteOrder.LITTLE_ENDIAN).getLong();
                long nextHeaderSize = ByteBuffer.wrap(Arrays.copyOfRange(header, 20, 28)).order(ByteOrder.LITTLE_ENDIAN).getLong();
                int nextHeaderCrc = ByteBuffer.wrap(Arrays.copyOfRange(header, 28, 32)).order(ByteOrder.LITTLE_ENDIAN).getInt();

                System.out.println("=== .7Z Archive Properties ===");
                System.out.println("File Signature: " + bytesToHex(signature));
                System.out.println("Format Version: " + version[0] + "." + version[1]);
                System.out.println("Start Header CRC: " + bytesToHex(startHeaderCrc));
                System.out.println("Next Header Offset: " + nextHeaderOffset);
                System.out.println("Next Header Size: " + nextHeaderSize);
                System.out.println("Next Header CRC: " + nextHeaderCrc);
            }

            // File information
            System.out.println("\nFile Information:");
            SevenZFile.Entry entry;
            while ((entry = archive.getNextEntry()) != null) {
                System.out.println(" - File: " + entry.getName());
                System.out.println("   Size: " + entry.getSize());
                System.out.println("   Is Directory: " + entry.isDirectory());
                System.out.println("   Modification Time: " + entry.getLastModifiedDate());
                System.out.println("   CRC: " + Long.toHexString(entry.getCrcValue()));
                System.out.println("   Encrypted: " + entry.isEncrypted());
            }

        } catch (IOException e) {
            System.err.println("Error reading properties: " + e.getMessage());
        }
    }

    public void writeArchive(String[] filesToAdd) {
        try (SevenZOutputFile outArchive = new SevenZOutputFile(new File(filepath))) {
            for (String filePath : filesToAdd) {
                File file = new File(filePath);
                if (file.exists()) {
                    outArchive.putArchiveEntry(outArchive.createArchiveEntry(file, file.getName()));
                    java.nio.file.Files.copy(file.toPath(), outArchive);
                    outArchive.closeArchiveEntry();
                    System.out.println("Added " + filePath + " to archive");
                } else {
                    System.err.println("File " + filePath + " does not exist");
                }
            }
        } catch (IOException e) {
            System.err.println("Error writing archive: " + e.getMessage());
        }
    }

    public void closeArchive() {
        if (archive != null) {
            try {
                archive.close();
                archive = null;
            } catch (IOException e) {
                System.err.println("Error closing archive: " + e.getMessage());
            }
        }
    }

    private String bytesToHex(byte[] bytes) {
        StringBuilder sb = new StringBuilder();
        for (byte b : bytes) {
            sb.append(String.format("%02x", b));
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        SevenZipHandler handler = new SevenZipHandler("example.7z");
        if (handler.openArchive()) {
            handler.readProperties();
            handler.closeArchive();
        }

        // Create a new archive
        handler = new SevenZipHandler("new_archive.7z");
        handler.writeArchive(new String[]{"test.txt", "image.png"});
    }
}

Dependencies: Include commons-compress (e.g., org.apache.commons:commons-compress:1.26.1) in your project (e.g., via Maven).

Explanation:

• Uses SevenZFile for reading and SevenZOutputFile for writing .7Z archives.
• Reads the start header manually to extract signature, version, and CRCs.
• Prints file metadata (name, size, CRC, etc.) and checks for encryption.
• The writeArchive method creates a new archive with specified files.

4. JavaScript Class for .7Z File Handling

JavaScript lacks a robust native library for .7Z files, but the 7z-wasm library (a WebAssembly port of 7-Zip) can be used in Node.js. Below is a JavaScript class that uses 7z-wasm for basic .7Z handling. Note that WebAssembly-based libraries may have limitations in accessing low-level header details.

const SevenZip = require('7z-wasm');
const fs = require('fs').promises;

class SevenZipHandler {
    constructor(filepath) {
        this.filepath = filepath;
        this.sevenZip = new SevenZip();
    }

    async openArchive() {
        try {
            await this.sevenZip.ready; // Wait for WebAssembly module to load
            return true;
        } catch (e) {
            console.error(`Error initializing 7z-wasm: ${e}`);
            return false;
        }
    }

    async readProperties() {
        try {
            // Read header manually
            const fileBuffer = await fs.readFile(this.filepath);
            const signature = fileBuffer.slice(0, 6);
            const version = fileBuffer.slice(6, 8);
            const startHeaderCrc = fileBuffer.slice(8, 12);
            const nextHeaderOffset = fileBuffer.readBigInt64LE(12);
            const nextHeaderSize = fileBuffer.readBigInt64LE(20);
            const nextHeaderCrc = fileBuffer.readInt32LE(28);

            console.log('=== .7Z Archive Properties ===');
            console.log(`File Signature: ${signature.toString('hex')}`);
            console.log(`Format Version: ${version[0]}.${version[1]}`);
            console.log(`Start Header CRC: ${startHeaderCrc.toString('hex')}`);
            console.log(`Next Header Offset: ${nextHeaderOffset}`);
            console.log(`Next Header Size: ${nextHeaderSize}`);
            console.log(`Next Header CRC: ${nextHeaderCrc.toString(16)}`);

            // List files using 7z-wasm
            console.log('\nFile Information:');
            const output = await this.sevenZip.callMain(['l', this.filepath]);
            console.log(output); // Prints file listing (limited metadata)

        } catch (e) {
            console.error(`Error reading properties: ${e}`);
        }
    }

    async writeArchive(filesToAdd) {
        try {
            await this.sevenZip.ready;
            const args = ['a', this.filepath, ...filesToAdd];
            const output = await this.sevenZip.callMain(args);
            console.log(`Archive created: ${output}`);
        } catch (e) {
            console.error(`Error writing archive: ${e}`);
        }
    }

    async closeArchive() {
        // No explicit close needed for 7z-wasm
        console.log('Archive closed (no-op for 7z-wasm).');
    }
}

// Example usage
(async () => {
    const handler = new SevenZipHandler('example.7z');
    if (await handler.openArchive()) {
        await handler.readProperties();
        await handler.closeArchive();
    }

    // Create a new archive
    const newHandler = new SevenZipHandler('new_archive.7z');
    await newHandler.writeArchive(['test.txt', 'image.png']);
})();

Dependencies: Install 7z-wasm using npm install 7z-wasm.

Explanation:

• Uses 7z-wasm to interact with .7Z archives in Node.js.
• Reads the start header manually from the file buffer to extract signature, version, and CRCs.
• The readProperties method uses the 7z-wasm command-line interface to list files, but detailed metadata access is limited.
• The writeArchive method creates a new archive using the 7z-wasm command-line interface.

Limitations: The 7z-wasm library is less mature than Python or Java libraries, and detailed header parsing (e.g., compression methods, folder structure) may require additional WebAssembly bindings or manual parsing.

5. C Class for .7Z File Handling

C does not have a concept of classes, but we can use a struct and functions to achieve similar functionality. Below is a C implementation using the lzma-sdk (7-Zip's LZMA SDK) to handle .7Z files. This implementation focuses on basic header parsing and file listing due to the complexity of the .7Z format.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include "7z.h" // From LZMA SDK
#include "7zFile.h"
#include "7zCrc.h"

typedef struct {
    char* filepath;
    ISeqInStream inStream;
    ISeqOutStream outStream;
    CSzArEx archive;
    ISzAlloc allocImp;
    ISzAlloc allocTempImp;
} SevenZipHandler;

void SevenZipHandler_Init(SevenZipHandler* handler, const char* filepath) {
    handler->filepath = strdup(filepath);
    handler->allocImp.Alloc = SzAlloc;
    handler->allocImp.Free = SzFree;
    handler->allocTempImp.Alloc = SzAllocTemp;
    handler->allocTempImp.Free = SzFreeTemp;
}

int SevenZipHandler_OpenArchive(SevenZipHandler* handler) {
    CFileSeqInStream inStream;
    CLookToRead2 lookStream;
    FileSeqInStream_CreateVTable(&inStream);
    LookToRead2_CreateVTable(&lookStream);
    lookStream.realStream = &inStream.vt;

    if (InFile_Open(&inStream.file, handler->filepath) != 0) {
        printf("Error opening archive: %s\n", handler->filepath);
        return 0;
    }

    SzArEx_Init(&handler->archive);
    if (SzArEx_Open(&handler->archive, &lookStream.vt, &handler->allocImp, &handler->allocTempImp) != SZ_OK) {
        printf("Error opening archive structure\n");
        SzArEx_Free(&handler->archive, &handler->allocImp);
        File_Close(&inStream.file);
        return 0;
    }

    handler->inStream = lookStream.realStream;
    return 1;
}

void SevenZipHandler_ReadProperties(SevenZipHandler* handler) {
    FILE* file = fopen(handler->filepath, "rb");
    if (!file) {
        printf("Error opening file for reading\n");
        return;
    }

    // Read header
    uint8_t header[32];
    fread(header, 1, 32, file);
    fclose(file);

    uint8_t* signature = header;
    uint8_t* version = header + 6;
    uint8_t* startHeaderCrc = header + 8;
    uint64_t nextHeaderOffset = *(uint64_t*)(header + 12);
    uint64_t nextHeaderSize = *(uint64_t*)(header + 20);
    uint32_t nextHeaderCrc = *(uint32_t*)(header + 28);

    printf("=== .7Z Archive Properties ===\n");
    printf("File Signature: ");
    for (int i = 0; i < 6; i++) printf("%02x", signature[i]);
    printf("\n");
    printf("Format Version: %d.%d\n", version[0], version[1]);
    printf("Start Header CRC: ");
    for (int i = 0; i < 4; i++) printf("%02x", startHeaderCrc[i]);
    printf("\n");
    printf("Next Header Offset: %llu\n", nextHeaderOffset);
    printf("Next Header Size: %llu\n", nextHeaderSize);
    printf("Next Header CRC: %08x\n", nextHeaderCrc);

    // File information
    printf("\nFile Information:\n");
    for (UInt32 i = 0; i < handler->archive.NumFiles; i++) {
        char name[256] = {0};
        size_t nameLen;
        SzArEx_GetFileNameUtf16(&handler->archive, i, NULL, name, &nameLen);
        UInt64 size;
        SzArEx_GetFileSize(&handler->archive, i, &size);
        CFileItem* fileItem = handler->archive.Files + i;

        printf(" - File: %s\n", name);
        printf("   Size: %llu\n", size);
        printf("   Is Directory: %d\n", fileItem->IsDir);
        printf("   Modification Time: %u\n", fileItem->MTime.Low);
        printf("   CRC: %08x\n", fileItem->Crc);
    }
}

void SevenZipHandler_WriteArchive(SevenZipHandler* handler, const char** filesToAdd, int numFiles) {
    // Writing .7Z archives in C using LZMA SDK is complex and requires additional setup
    printf("Writing .7Z archives is not implemented in this example.\n");
    // Implement using SzArEx and compression APIs if needed
}

void SevenZipHandler_CloseArchive(SevenZipHandler* handler) {
    SzArEx_Free(&handler->archive, &handler->allocImp);
    free(handler->filepath);
}

int main() {
    SevenZipHandler handler;
    SevenZipHandler_Init(&handler, "example.7z");
    if (SevenZipHandler_OpenArchive(&handler)) {
        SevenZipHandler_ReadProperties(&handler);
        SevenZipHandler_CloseArchive(&handler);
    }
    return 0;
}

Dependencies: Requires the LZMA SDK (available from 7-Zip's official site). Compile with the SDK's C files (e.g., 7z, 7zFile, 7zCrc).

Explanation:

• Uses the LZMA SDK to parse the .7Z archive structure.
• Reads the start header manually and extracts file metadata using SzArEx.
• Writing archives is not fully implemented due to complexity; it requires setting up compression coders and streams.
• Prints key properties like signature, version, CRCs, and file metadata.

Limitations: The LZMA SDK is low-level, and writing .7Z archives requires significant additional code for compression and header construction.

Notes and Limitations

• Property Coverage: The implementations cover most intrinsic properties (signature, version, CRCs, file metadata, compression methods, etc.). However, some properties (e.g., detailed folder structures, custom codecs) are complex and may require deeper parsing of the header, which is library-dependent.
• Library Availability:
• Python's py7zr is the most comprehensive for .7Z handling.
• Java's Apache Commons Compress is robust but may not expose all header details.
• JavaScript's 7z-wasm is limited in metadata access due to its WebAssembly nature.
• C's LZMA SDK is powerful but requires low-level coding for full functionality.
• Writing Archives: Writing is implemented in Python and Java, but JavaScript and C implementations are limited due to library constraints.
• Error Handling: All implementations include basic error handling, but real-world applications should add more robust checks (e.g., for corrupted archives).
• Sources: The properties and implementations are based on 7-Zip documentation, py7zr, and the LZMA SDK (,,,,).

If you need further details or enhancements (e.g., specific compression method parsing, encryption handling), please let me know!

1. List of all the properties of the .7Z file format intrinsic to its file system

Based on the specifications of the .7Z file format (the archive format used by 7-Zip), the intrinsic properties refer to the core structural elements in the fixed signature header, which define the format's binary layout and are essential for identifying and accessing the archive's content. These are:

• Signature: A 6-byte magic sequence (hex: 37 7A BC AF 27 1C) that identifies the file as a .7Z archive.
• Major Version: A 1-byte value indicating the major version of the format (typically 0).
• Minor Version: A 1-byte value indicating the minor version of the format (typically 4).
• Start Header CRC: A 4-byte unsigned integer (little-endian) representing the CRC32 checksum of the start header fields (Next Header Offset, Next Header Size, and Next Header CRC).
• Next Header Offset: An 8-byte unsigned integer (little-endian) specifying the offset from the end of the signature header (position 32) to the start of the encoded or plain header database.
• Next Header Size: An 8-byte unsigned integer (little-endian) specifying the size of the header database.
• Next Header CRC: A 4-byte unsigned integer (little-endian) representing the CRC32 checksum of the header database.

These properties form the fixed 32-byte signature header at the beginning of every .7Z file.

2. Python class

import struct
import binascii
import os

class SevenZFile:
    def __init__(self, filename=None):
        self.signature = b'7z\xbc\xaf\x27\x1c'
        self.major = 0
        self.minor = 4
        self.start_crc = 0
        self.next_offset = 0
        self.next_size = 0
        self.next_crc = 0
        if filename:
            self.read_properties(filename)

    def read_properties(self, filename):
        with open(filename, 'rb') as f:
            data = f.read(32)
        if len(data) < 32:
            raise ValueError("File too small for .7Z header")
        self.signature = data[0:6]
        if self.signature != b'7z\xbc\xaf\x27\x1c':
            raise ValueError("Invalid .7Z signature")
        self.major = data[6]
        self.minor = data[7]
        self.start_crc = struct.unpack('<I', data[8:12])[0]
        self.next_offset = struct.unpack('<Q', data[12:20])[0]
        self.next_size = struct.unpack('<Q', data[20:28])[0]
        self.next_crc = struct.unpack('<I', data[28:32])[0]
        # Verify Start Header CRC
        start_header = data[12:32]
        calculated_crc = binascii.crc32(start_header) & 0xFFFFFFFF
        if self.start_crc != calculated_crc:
            raise ValueError("Invalid Start Header CRC")

    def get_properties(self):
        return {
            'signature': self.signature,
            'major': self.major,
            'minor': self.minor,
            'start_crc': self.start_crc,
            'next_offset': self.next_offset,
            'next_size': self.next_size,
            'next_crc': self.next_crc
        }

    def write_properties(self, filename):
        start_header = struct.pack('<Q', self.next_offset) + \
                      struct.pack('<Q', self.next_size) + \
                      struct.pack('<I', self.next_crc)
        self.start_crc = binascii.crc32(start_header) & 0xFFFFFFFF
        header_data = self.signature + \
                      struct.pack('<B', self.major) + \
                      struct.pack('<B', self.minor) + \
                      struct.pack('<I', self.start_crc) + \
                      start_header
        with open(filename, 'wb') as f:
            f.write(header_data)
        # Note: This writes only the signature header. Additional data/header must be appended manually for a complete file.

3. Java class

import java.io.*;
import java.nio.*;
import java.nio.channels.FileChannel;
import java.util.zip.CRC32;

public class SevenZFile {
    private byte[] signature = new byte[] {0x37, 0x7A, (byte)0xBC, (byte)0xAF, 0x27, 0x1C};
    private byte major = 0;
    private byte minor = 4;
    private int startCrc = 0;
    private long nextOffset = 0;
    private long nextSize = 0;
    private int nextCrc = 0;

    public SevenZFile(String filename) throws IOException {
        if (filename != null) {
            readProperties(filename);
        }
    }

    public void readProperties(String filename) throws IOException {
        try (RandomAccessFile raf = new RandomAccessFile(filename, "r")) {
            ByteBuffer buffer = ByteBuffer.allocate(32).order(ByteOrder.LITTLE_ENDIAN);
            raf.getChannel().read(buffer, 0);
            buffer.rewind();
            byte[] sig = new byte[6];
            buffer.get(sig);
            if (!java.util.Arrays.equals(sig, signature)) {
                throw new IOException("Invalid .7Z signature");
            }
            major = buffer.get();
            minor = buffer.get();
            startCrc = buffer.getInt();
            nextOffset = buffer.getLong();
            nextSize = buffer.getLong();
            nextCrc = buffer.getInt();
            // Verify Start Header CRC
            buffer.position(12);
            byte[] startHeader = new byte[20];
            buffer.get(startHeader);
            CRC32 crc = new CRC32();
            crc.update(startHeader);
            if (startCrc != (int) crc.getValue()) {
                throw new IOException("Invalid Start Header CRC");
            }
        }
    }

    public java.util.Map<String, Object> getProperties() {
        java.util.Map<String, Object> props = new java.util.HashMap<>();
        props.put("signature", signature);
        props.put("major", major);
        props.put("minor", minor);
        props.put("start_crc", startCrc);
        props.put("next_offset", nextOffset);
        props.put("next_size", nextSize);
        props.put("next_crc", nextCrc);
        return props;
    }

    public void writeProperties(String filename) throws IOException {
        ByteBuffer startHeader = ByteBuffer.allocate(20).order(ByteOrder.LITTLE_ENDIAN);
        startHeader.putLong(nextOffset);
        startHeader.putLong(nextSize);
        startHeader.putInt(nextCrc);
        CRC32 crc = new CRC32();
        crc.update(startHeader.array());
        startCrc = (int) crc.getValue();
        ByteBuffer headerData = ByteBuffer.allocate(32).order(ByteOrder.LITTLE_ENDIAN);
        headerData.put(signature);
        headerData.put(major);
        headerData.put(minor);
        headerData.putInt(startCrc);
        headerData.put(startHeader.array());
        try (FileOutputStream fos = new FileOutputStream(filename)) {
            fos.write(headerData.array());
        }
        // Note: This writes only the signature header. Additional data/header must be appended manually for a complete file.
    }
}

4. JavaScript class

const fs = require('fs');
const crypto = require('crypto'); // For CRC32 alternative, since Node has no built-in CRC32

class SevenZFile {
    constructor(filename = null) {
        this.signature = Buffer.from([0x37, 0x7A, 0xBC, 0xAF, 0x27, 0x1C]);
        this.major = 0;
        this.minor = 4;
        this.startCrc = 0;
        this.nextOffset = 0n;
        this.nextSize = 0n;
        this.nextCrc = 0;
        if (filename) {
            this.readProperties(filename);
        }
    }

    readProperties(filename) {
        const data = fs.readFileSync(filename).subarray(0, 32);
        if (data.length < 32) {
            throw new Error('File too small for .7Z header');
        }
        const sig = data.subarray(0, 6);
        if (!sig.equals(this.signature)) {
            throw new Error('Invalid .7Z signature');
        }
        this.major = data[6];
        this.minor = data[7];
        this.startCrc = data.readUInt32LE(8);
        this.nextOffset = data.readBigUInt64LE(12);
        this.nextSize = data.readBigUInt64LE(20);
        this.nextCrc = data.readUInt32LE(28);
        // Verify Start Header CRC
        const startHeader = data.subarray(12, 32);
        const calculatedCrc = this.crc32(startHeader);
        if (this.startCrc !== calculatedCrc) {
            throw new Error('Invalid Start Header CRC');
        }
    }

    getProperties() {
        return {
            signature: this.signature,
            major: this.major,
            minor: this.minor,
            start_crc: this.startCrc,
            next_offset: this.nextOffset,
            next_size: this.nextSize,
            next_crc: this.nextCrc
        };
    }

    writeProperties(filename) {
        const startHeader = Buffer.alloc(20);
        startHeader.writeBigUInt64LE(this.nextOffset, 0);
        startHeader.writeBigUInt64LE(this.nextSize, 8);
        startHeader.writeUInt32LE(this.nextCrc, 16);
        this.startCrc = this.crc32(startHeader);
        const headerData = Buffer.alloc(32);
        this.signature.copy(headerData, 0);
        headerData[6] = this.major;
        headerData[7] = this.minor;
        headerData.writeUInt32LE(this.startCrc, 8);
        startHeader.copy(headerData, 12);
        fs.writeFileSync(filename, headerData);
        // Note: This writes only the signature header. Additional data/header must be appended manually for a complete file.
    }

    // Simple CRC32 implementation
    crc32(buf) {
        const crcTable = new Uint32Array(256);
        for (let i = 0; i < 256; i++) {
            let c = i;
            for (let j = 0; j < 8; j++) {
                c = (c & 1) ? (0xEDB88320 ^ (c >>> 1)) : (c >>> 1);
            }
            crcTable[i] = c;
        }
        let crc = 0xFFFFFFFF;
        for (let i = 0; i < buf.length; i++) {
            crc = (crc >>> 8) ^ crcTable[(crc ^ buf[i]) & 0xFF];
        }
        return (crc ^ 0xFFFFFFFF) >>> 0;
    }
}

5. C class (implemented as C++ class for object-oriented support)

#include <fstream>
#include <stdexcept>
#include <cstdint>
#include <cstring>

class SevenZFile {
private:
    uint8_t signature[6] = {0x37, 0x7A, 0xBC, 0xAF, 0x27, 0x1C};
    uint8_t major = 0;
    uint8_t minor = 4;
    uint32_t start_crc = 0;
    uint64_t next_offset = 0;
    uint64_t next_size = 0;
    uint32_t next_crc = 0;

public:
    SevenZFile(const char* filename = nullptr) {
        if (filename) {
            read_properties(filename);
        }
    }

    void read_properties(const char* filename) {
        std::ifstream file(filename, std::ios::binary);
        if (!file) {
            throw std::runtime_error("Cannot open file");
        }
        uint8_t data[32];
        file.read(reinterpret_cast<char*>(data), 32);
        if (file.gcount() < 32) {
            throw std::runtime_error("File too small for .7Z header");
        }
        std::memcpy(signature, data, 6);
        if (std::memcmp(signature, "\x37\x7A\xBC\xAF\x27\x1C", 6) != 0) {
            throw std::runtime_error("Invalid .7Z signature");
        }
        major = data[6];
        minor = data[7];
        std::memcpy(&start_crc, data + 8, 4);
        std::memcpy(&next_offset, data + 12, 8);
        std::memcpy(&next_size, data + 20, 8);
        std::memcpy(&next_crc, data + 28, 4);
        // Verify Start Header CRC
        uint8_t start_header[20];
        std::memcpy(start_header, data + 12, 20);
        uint32_t calculated_crc = crc32(start_header, 20);
        if (start_crc != calculated_crc) {
            throw std::runtime_error("Invalid Start Header CRC");
        }
    }

    // Getter methods for properties (since no std::map in simple C++)
    const uint8_t* get_signature() const { return signature; }
    uint8_t get_major() const { return major; }
    uint8_t get_minor() const { return minor; }
    uint32_t get_start_crc() const { return start_crc; }
    uint64_t get_next_offset() const { return next_offset; }
    uint64_t get_next_size() const { return next_size; }
    uint32_t get_next_crc() const { return next_crc; }

    void write_properties(const char* filename) {
        uint8_t start_header[20];
        std::memcpy(start_header, &next_offset, 8);
        std::memcpy(start_header + 8, &next_size, 8);
        std::memcpy(start_header + 16, &next_crc, 4);
        start_crc = crc32(start_header, 20);
        uint8_t header_data[32];
        std::memcpy(header_data, signature, 6);
        header_data[6] = major;
        header_data[7] = minor;
        std::memcpy(header_data + 8, &start_crc, 4);
        std::memcpy(header_data + 12, start_header, 20);
        std::ofstream file(filename, std::ios::binary);
        if (!file) {
            throw std::runtime_error("Cannot write file");
        }
        file.write(reinterpret_cast<const char*>(header_data), 32);
        // Note: This writes only the signature header. Additional data/header must be appended manually for a complete file.
    }

private:
    uint32_t crc32(const uint8_t* buf, size_t len) {
        uint32_t crc_table[256];
        for (uint32_t i = 0; i < 256; i++) {
            uint32_t c = i;
            for (int j = 0; j < 8; j++) {
                c = (c & 1) ? (0xEDB88320 ^ (c >> 1)) : (c >> 1);
            }
            crc_table[i] = c;
        }
        uint32_t crc = 0xFFFFFFFF;
        for (size_t i = 0; i < len; i++) {
            crc = (crc >> 8) ^ crc_table[(crc ^ buf[i]) & 0xFF];
        }
        return crc ^ 0xFFFFFFFF;
    }
};