github.com/klauspost/compress v1.20.0: gzip.NewWriter

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify gzip.NewWriter in pkg:golang/github.com/klauspost/compress@v1.20.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/klauspost/compress@v1.20.0
Demonstrate these symbols/APIs:
  - gzip.NewWriter

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:7de77f4c95aceaf1eac2bb4877debb176b3c5ff447e302e8a7d248a560b9fbac","contract":["gzip.NewWriter creates a new Writer writing to the underlying io.Writer","Writer.Write compresses data into the gzip stream","Writer.Flush flushes any pending compressed data to the underlying writer","Writer.Close finalizes the gzip stream and writes the stream footer","Writer.Reset discards state and prepares the Writer to output to another stream","Compressed output from gzip.NewWriter decompresses back to the exact input data"],"goal":"verify gzip.NewWriter in pkg:golang/github.com/klauspost/compress@v1.20.0","kind":"HOW","packages":["pkg:golang/github.com/klauspost/compress@v1.20.0"],"schemaVersion":1,"symbols":["gzip.NewWriter"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/klauspost/compress@v1.20.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/klauspost/compress@v1.20.0","symbols":["gzip.NewWriter"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.25

require github.com/klauspost/compress v1.20.0

github.com/klauspost/compress v1.20.0 h1:a3C1ke2ohxFymNlb2HWAHjDeKCI90scRskErZkR0ezA=
github.com/klauspost/compress v1.20.0/go.mod h1:LUdAzn7YLVvxLpc7y3V1m40wESHTgc1422pwwBSKYuI=

package main

import (
	"bytes"
	"fmt"
	"io"
	"os"

	"github.com/klauspost/compress/gzip"
)

func main() {
	var buf bytes.Buffer
	w := gzip.NewWriter(&buf)

	data := "Demonstrating gzip.NewWriter compression with klauspost/compress"
	if _, err := w.Write([]byte(data)); err != nil {
		fmt.Fprintf(os.Stderr, "write error: %v\n", err)
		os.Exit(1)
	}

	if err := w.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "close error: %v\n", err)
		os.Exit(1)
	}

	compressedBytes := buf.Bytes()
	compressedSize := len(compressedBytes)

	r, err := gzip.NewReader(bytes.NewReader(compressedBytes))
	if err != nil {
		fmt.Fprintf(os.Stderr, "reader error: %v\n", err)
		os.Exit(1)
	}
	defer r.Close()

	decompressed, err := io.ReadAll(r)
	if err != nil {
		fmt.Fprintf(os.Stderr, "read error: %v\n", err)
		os.Exit(1)
	}

	fmt.Printf("Original size: %d, compressed size: %d\n", len(data), compressedSize)
	fmt.Printf("Decompressed string: %s\n", string(decompressed))
}

{
  "schemaVersion": 1,
  "goal": "verify gzip.NewWriter in pkg:golang/github.com/klauspost/compress@v1.20.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/klauspost/compress@v1.20.0"
  ],
  "symbols": [
    "gzip.NewWriter"
  ]
}

package main

import (
	"bytes"
	"fmt"
	"io"
	"os"

	"github.com/klauspost/compress/gzip"
)

func main() {
	// 1. gzip.NewWriter creates a new Writer writing to the underlying io.Writer
	var buf1 bytes.Buffer
	w1 := gzip.NewWriter(&buf1)
	if w1 == nil {
		fmt.Fprintf(os.Stderr, "expected non-nil gzip.Writer\n")
		os.Exit(1)
	}

	// 2. Writer.Write compresses data into the gzip stream
	part1 := []byte("Hello, ")
	part2 := []byte("world! This is a test of klauspost/compress gzip.NewWriter.")
	n1, err := w1.Write(part1)
	if err != nil || n1 != len(part1) {
		fmt.Fprintf(os.Stderr, "Write failed: err=%v, n=%d\n", err, n1)
		os.Exit(1)
	}
	n2, err := w1.Write(part2)
	if err != nil || n2 != len(part2) {
		fmt.Fprintf(os.Stderr, "Write failed: err=%v, n=%d\n", err, n2)
		os.Exit(1)
	}

	// 3. Writer.Flush flushes any pending compressed data to the underlying writer
	if err := w1.Flush(); err != nil {
		fmt.Fprintf(os.Stderr, "Flush failed: %v\n", err)
		os.Exit(1)
	}
	if buf1.Len() == 0 {
		fmt.Fprintf(os.Stderr, "expected flushed bytes in buffer, got 0\n")
		os.Exit(1)
	}

	// 4. Writer.Close finalizes the gzip stream and writes the stream footer
	lenBeforeClose := buf1.Len()
	if err := w1.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "Close failed: %v\n", err)
		os.Exit(1)
	}
	if buf1.Len() <= lenBeforeClose {
		fmt.Fprintf(os.Stderr, "expected footer written on Close\n")
		os.Exit(1)
	}

	// 5. Writer.Reset discards state and prepares the Writer to output to another stream
	var buf2 bytes.Buffer
	w1.Reset(&buf2)
	resetData := []byte("Second stream using reset writer")
	if _, err := w1.Write(resetData); err != nil {
		fmt.Fprintf(os.Stderr, "Write after Reset failed: %v\n", err)
		os.Exit(1)
	}
	if err := w1.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "Close after Reset failed: %v\n", err)
		os.Exit(1)
	}
	rReset, err := gzip.NewReader(&buf2)
	if err != nil {
		fmt.Fprintf(os.Stderr, "NewReader for reset stream failed: %v\n", err)
		os.Exit(1)
	}
	decompReset, err := io.ReadAll(rReset)
	_ = rReset.Close()
	if err != nil || !bytes.Equal(decompReset, resetData) {
		fmt.Fprintf(os.Stderr, "Reset verification failed: %s != %s\n", decompReset, resetData)
		os.Exit(1)
	}

	// 6. Compressed output from gzip.NewWriter decompresses back to the exact input data
	r1, err := gzip.NewReader(&buf1)
	if err != nil {
		fmt.Fprintf(os.Stderr, "NewReader failed: %v\n", err)
		os.Exit(1)
	}
	decomp1, err := io.ReadAll(r1)
	_ = r1.Close()
	if err != nil {
		fmt.Fprintf(os.Stderr, "ReadAll failed: %v\n", err)
		os.Exit(1)
	}
	expected1 := append(part1, part2...)
	if !bytes.Equal(decomp1, expected1) {
		fmt.Fprintf(os.Stderr, "decompressed data mismatch: got %q, expected %q\n", decomp1, expected1)
		os.Exit(1)
	}

	fmt.Println("All gzip.NewWriter contract assertions passed.")
}