merge-zip-files-without-recompression

main.go

package main

import (
	"archive/zip"
	"fmt"
	"io"
	"os"
)

func addZipToWriter(zw *zip.Writer, srcPath string) error {
	r, err := zip.OpenReader(srcPath)
	if err != nil {
		return err
	}
	defer r.Close()

	for _, file := range r.File {
		// Open the file to get the raw compressed data
		rawReader, err := file.OpenRaw()
		if err != nil {
			return fmt.Errorf("failed to open raw reader for %s: %w", file.Name, err)
		}

		// Create a new file in the target zip with the same header
		rawWriter, err := zw.CreateRaw(&file.FileHeader)
		if err != nil {
			return fmt.Errorf("failed to create raw writer for %s: %w", file.Name, err)
		}

		// Copy the raw compressed data directly from the source to the target
		_, err = io.Copy(rawWriter, rawReader)
		if err != nil {
			return fmt.Errorf("failed to copy raw data for %s: %w", file.Name, err)
		}
	}

	return nil
}

func MergeZips(targetPath string, sources []string) error {
	// Create the target zip file
	targetFile, err := os.Create(targetPath)
	if err != nil {
		return fmt.Errorf("failed to create target file: %w", err)
	}
	defer targetFile.Close()

	// Create a new zip writer for the target file
	zipWriter := zip.NewWriter(targetFile)
	defer zipWriter.Close()

	for _, src := range sources {
		// Add the contents of each source zip to the target zip
		err = addZipToWriter(zipWriter, src)
		if err != nil {
			return fmt.Errorf("failed processing %s: %w", src, err)
		}
	}

	return nil
}

func main() {
	// Usage example: merge "part1.zip" and "part2.zip" into "output.zip"
	sources := []string{"part1.zip", "part2.zip"}
	output := "output.zip"

	fmt.Printf("Merging %v into %s...\n", sources, output)
	err := MergeZips(output, sources)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("Merge completed successfully.")
}