How to create a compressed reader or writer with a custom compression level in Go?

To create a compressed reader or writer with a custom compression level in Go, you can use the compress/gzip or compress/zlib packages, which provide functionality for gzip and zlib compression respectively. Here's an example of how you can achieve this:

Compressed Reader:

package main

import (
	"compress/gzip"
	"fmt"
	"io"
	"os"
)

func main() {
	// Open the input file
	inputFile, err := os.Open("input.txt")
	if err != nil {
		fmt.Println("Failed to open input file:", err)
		return
	}
	defer inputFile.Close()

	// Create a gzip reader
	gzipReader, err := gzip.NewReader(inputFile)
	if err != nil {
		fmt.Println("Failed to create gzip reader:", err)
		return
	}
	defer gzipReader.Close()

	// Read and print the contents of the compressed file
	buf := make([]byte, 1024)
	for {
		n, err := gzipReader.Read(buf)
		if err != nil && err != io.EOF {
			fmt.Println("Failed to read from gzip reader:", err)
			return
		}
		if n == 0 {
			break
		}
		fmt.Print(string(buf[:n]))
	}
}

Compressed Writer:

package main

import (
	"compress/gzip"
	"fmt"
	"os"
)

func main() {
	// Open the output file
	outputFile, err := os.Create("output.txt.gz")
	if err != nil {
		fmt.Println("Failed to create output file:", err)
		return
	}
	defer outputFile.Close()

	// Create a gzip writer with custom compression level
	gzipWriter, err := gzip.NewWriterLevel(outputFile, gzip.BestCompression)
	if err != nil {
		fmt.Println("Failed to create gzip writer:", err)
		return
	}
	defer gzipWriter.Close()

	// Write content to the compressed file
	content := "Hello, World!"
	_, err = gzipWriter.Write([]byte(content))
	if err != nil {
		fmt.Println("Failed to write to gzip writer:", err)
		return
	}

	fmt.Println("Content compressed and written to output file")
}

In both examples, the compress/gzip package is used. If you want to use zlib compression instead, you can replace the import statement with compress/zlib and use the zlib.NewReader or zlib.NewWriterLevel functions accordingly.