github.com/klauspost/compress v1.20.0: zlib.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 zlib.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:
  - zlib.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:sha256:28a86aceb62cf23d4023d6fc9536786994e2e75bb4f423880a654de90af31477","contract":["zlib.NewWriter compresses data stream and zlib.NewReader decompresses to restore exact original payload","zlib.NewWriterLevel compresses with specified compression level including BestSpeed and BestCompression","zlib.NewWriter and zlib.NewReader correctly roundtrip empty input payload","zlib.NewReader returns error when reading invalid or corrupted zlib stream","zlib.NewWriter and zlib.NewReader support stream reuse via Reset","zlib.NewWriterLevelDict and zlib.NewReaderDict compress and decompress with preset dictionary"],"goal":"verify zlib.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":["zlib.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":["zlib.NewWriter"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

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"
	"log"

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

func main() {
	original := []byte("Hello, CodeSampleX with klauspost/compress zlib!")

	var buf bytes.Buffer
	writer := zlib.NewWriter(&buf)
	if _, err := writer.Write(original); err != nil {
		log.Fatalf("failed to write compressed data: %v", err)
	}
	if err := writer.Close(); err != nil {
		log.Fatalf("failed to close zlib writer: %v", err)
	}

	reader, err := zlib.NewReader(&buf)
	if err != nil {
		log.Fatalf("failed to create zlib reader: %v", err)
	}
	defer reader.Close()

	decompressed, err := io.ReadAll(reader)
	if err != nil {
		log.Fatalf("failed to read decompressed data: %v", err)
	}

	fmt.Printf("Original:     %s\n", string(original))
	fmt.Printf("Decompressed: %s\n", string(decompressed))
}

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

package main

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

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

func main() {
	// 1. zlib.NewWriter compresses data stream and zlib.NewReader decompresses to restore exact original payload
	{
		original := []byte("CodeSampleX verification payload for github.com/klauspost/compress/zlib stream compression.")
		var buf bytes.Buffer
		zw := zlib.NewWriter(&buf)
		if _, err := zw.Write(original); err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewWriter Write failed: %v\n", err)
			os.Exit(1)
		}
		if err := zw.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewWriter Close failed: %v\n", err)
			os.Exit(1)
		}

		zr, err := zlib.NewReader(&buf)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewReader failed: %v\n", err)
			os.Exit(1)
		}
		decompressed, err := io.ReadAll(zr)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewReader ReadAll failed: %v\n", err)
			os.Exit(1)
		}
		if err := zr.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewReader Close failed: %v\n", err)
			os.Exit(1)
		}
		if !bytes.Equal(original, decompressed) {
			fmt.Fprintf(os.Stderr, "decompressed mismatch: got %q, want %q\n", string(decompressed), string(original))
			os.Exit(1)
		}
	}

	// 2. zlib.NewWriterLevel compresses with specified compression level including BestSpeed and BestCompression
	{
		sampleData := bytes.Repeat([]byte("High repetition pattern for testing klauspost zlib compression levels. "), 40)
		for _, level := range []int{zlib.BestSpeed, zlib.BestCompression, zlib.HuffmanOnly} {
			var b bytes.Buffer
			zw, err := zlib.NewWriterLevel(&b, level)
			if err != nil {
				fmt.Fprintf(os.Stderr, "zlib.NewWriterLevel(%d) failed: %v\n", level, err)
				os.Exit(1)
			}
			if _, err := zw.Write(sampleData); err != nil {
				fmt.Fprintf(os.Stderr, "zlib write with level %d failed: %v\n", level, err)
				os.Exit(1)
			}
			if err := zw.Close(); err != nil {
				fmt.Fprintf(os.Stderr, "zlib close with level %d failed: %v\n", level, err)
				os.Exit(1)
			}

			zr, err := zlib.NewReader(&b)
			if err != nil {
				fmt.Fprintf(os.Stderr, "zlib.NewReader for level %d failed: %v\n", level, err)
				os.Exit(1)
			}
			decomp, err := io.ReadAll(zr)
			if err != nil {
				fmt.Fprintf(os.Stderr, "zlib read for level %d failed: %v\n", level, err)
				os.Exit(1)
			}
			if err := zr.Close(); err != nil {
				fmt.Fprintf(os.Stderr, "zlib reader close for level %d failed: %v\n", level, err)
				os.Exit(1)
			}
			if !bytes.Equal(sampleData, decomp) {
				fmt.Fprintf(os.Stderr, "level %d roundtrip mismatch\n", level)
				os.Exit(1)
			}
		}
	}

	// 3. zlib.NewWriter and zlib.NewReader correctly roundtrip empty input payload
	{
		var emptyBuf bytes.Buffer
		zwEmpty := zlib.NewWriter(&emptyBuf)
		if err := zwEmpty.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zlib empty write close failed: %v\n", err)
			os.Exit(1)
		}
		zrEmpty, err := zlib.NewReader(&emptyBuf)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib empty reader failed: %v\n", err)
			os.Exit(1)
		}
		emptyDecomp, err := io.ReadAll(zrEmpty)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib empty read failed: %v\n", err)
			os.Exit(1)
		}
		if err := zrEmpty.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zlib empty reader close failed: %v\n", err)
			os.Exit(1)
		}
		if len(emptyDecomp) != 0 {
			fmt.Fprintf(os.Stderr, "expected empty decompressed payload, got %d bytes\n", len(emptyDecomp))
			os.Exit(1)
		}
	}

	// 4. zlib.NewReader returns error when reading invalid or corrupted zlib stream
	{
		corrupted := []byte{0x78, 0x9c, 0x00, 0x00, 0x99, 0x88, 0x77}
		zrBad, err := zlib.NewReader(bytes.NewReader(corrupted))
		if err == nil {
			_, readErr := io.ReadAll(zrBad)
			_ = zrBad.Close()
			if readErr == nil {
				fmt.Fprintf(os.Stderr, "expected error reading corrupted zlib stream, got nil\n")
				os.Exit(1)
			}
		}
	}

	// 5. zlib.NewWriter and zlib.NewReader support stream reuse via Reset
	{
		var buf1 bytes.Buffer
		zwReuse := zlib.NewWriter(&buf1)
		p1 := []byte("first stream payload for reusable zlib writer")
		if _, err := zwReuse.Write(p1); err != nil {
			fmt.Fprintf(os.Stderr, "p1 write failed: %v\n", err)
			os.Exit(1)
		}
		if err := zwReuse.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "p1 close failed: %v\n", err)
			os.Exit(1)
		}

		var buf2 bytes.Buffer
		zwReuse.Reset(&buf2)
		p2 := []byte("second stream payload after zlib writer reset")
		if _, err := zwReuse.Write(p2); err != nil {
			fmt.Fprintf(os.Stderr, "p2 write failed: %v\n", err)
			os.Exit(1)
		}
		if err := zwReuse.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "p2 close failed: %v\n", err)
			os.Exit(1)
		}

		zrReuse, err := zlib.NewReader(&buf1)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zrReuse buf1 failed: %v\n", err)
			os.Exit(1)
		}
		res1, err := io.ReadAll(zrReuse)
		if err != nil || !bytes.Equal(res1, p1) {
			fmt.Fprintf(os.Stderr, "p1 mismatch: %v\n", err)
			os.Exit(1)
		}

		resetter, ok := zrReuse.(zlib.Resetter)
		if !ok {
			fmt.Fprintf(os.Stderr, "zrReuse does not implement zlib.Resetter\n")
			os.Exit(1)
		}
		if err := resetter.Reset(&buf2, nil); err != nil {
			fmt.Fprintf(os.Stderr, "resetter.Reset failed: %v\n", err)
			os.Exit(1)
		}
		res2, err := io.ReadAll(zrReuse)
		if err != nil || !bytes.Equal(res2, p2) {
			fmt.Fprintf(os.Stderr, "p2 mismatch: %v\n", err)
			os.Exit(1)
		}
		_ = zrReuse.Close()
	}

	// 6. zlib.NewWriterLevelDict and zlib.NewReaderDict compress and decompress with preset dictionary
	{
		dict := []byte("frequently-used-dictionary-tokens-for-compression-efficiency")
		var buf bytes.Buffer
		zwDict, err := zlib.NewWriterLevelDict(&buf, zlib.DefaultCompression, dict)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewWriterLevelDict failed: %v\n", err)
			os.Exit(1)
		}
		payload := []byte("frequently-used-dictionary-tokens-for-compression-efficiency and some extra body text")
		if _, err := zwDict.Write(payload); err != nil {
			fmt.Fprintf(os.Stderr, "zwDict write failed: %v\n", err)
			os.Exit(1)
		}
		if err := zwDict.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zwDict close failed: %v\n", err)
			os.Exit(1)
		}

		zrDict, err := zlib.NewReaderDict(&buf, dict)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zlib.NewReaderDict failed: %v\n", err)
			os.Exit(1)
		}
		decomp, err := io.ReadAll(zrDict)
		if err != nil {
			fmt.Fprintf(os.Stderr, "zrDict read failed: %v\n", err)
			os.Exit(1)
		}
		if err := zrDict.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "zrDict close failed: %v\n", err)
			os.Exit(1)
		}
		if !bytes.Equal(payload, decomp) {
			fmt.Fprintf(os.Stderr, "dict roundtrip mismatch\n")
			os.Exit(1)
		}
	}

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