github.com/klauspost/compress v1.19.1: zlib.NewWriterLevelDict

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 github.com/klauspost/compress/zlib.NewWriterLevelDict in pkg:golang/github.com/klauspost/compress@v1.19.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/klauspost/compress@v1.19.1
Demonstrate these symbols/APIs:
  - github.com/klauspost/compress/zlib.NewWriterLevelDict

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:sha256:4d36a079a9db467a39c8c96251bfdae4f783b6f9a0a208deaf1bbaac8d9b7718","contract":["zlib.NewWriterLevelDict initializes a Writer with specified compression level and dictionary","zlib.NewWriterLevelDict produces compressed data successfully decompressed using the matching dictionary","zlib.NewWriterLevelDict requires the identical dictionary during decompression and fails when omitted or mismatched","zlib.NewWriterLevelDict accepts a nil dictionary behaving equivalent to standard compression","zlib.NewWriterLevelDict returns an error when given an invalid compression level"],"goal":"verify github.com/klauspost/compress/zlib.NewWriterLevelDict in pkg:golang/github.com/klauspost/compress@v1.19.1","kind":"HOW","packages":["pkg:golang/github.com/klauspost/compress@v1.19.1"],"schemaVersion":1,"symbols":["github.com/klauspost/compress/zlib.NewWriterLevelDict"]},"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@v1.19.1"],"schemaVersion":1,"subject":"pkg:golang/github.com/klauspost/compress@v1.19.1","symbols":["github.com/klauspost/compress/zlib.NewWriterLevelDict"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

require github.com/klauspost/compress v1.19.1

github.com/klauspost/compress v1.19.1 h1:VsB4HPswih7mmZ8WleSFQ75c/Ui1M4trX5oAsJnhSlk=
github.com/klauspost/compress v1.19.1/go.mod h1:cwPg85FWrGar70rWktvGQj8/hthj3wpl0PGDogxkrSQ=

{
  "schemaVersion": 1,
  "goal": "verify github.com/klauspost/compress/zlib.NewWriterLevelDict in pkg:golang/github.com/klauspost/compress@v1.19.1",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/klauspost/compress@v1.19.1"
  ],
  "symbols": [
    "github.com/klauspost/compress/zlib.NewWriterLevelDict"
  ]
}

package main

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

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

func main() {
	dict := []byte("frequently-occurring-common-dictionary-string-pattern")

	// Assertion 1: zlib.NewWriterLevelDict initializes a Writer with specified compression level and dictionary
	var buf1 bytes.Buffer
	w1, err := sample.NewDictWriter(&buf1, zlib.BestSpeed, dict)
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 1: NewWriterLevelDict returned error: %v\n", err)
		os.Exit(1)
	}
	if w1 == nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 1: NewWriterLevelDict returned nil writer\n")
		os.Exit(1)
	}

	// Assertion 2: zlib.NewWriterLevelDict produces compressed data successfully decompressed using the matching dictionary
	payload := []byte("frequently-occurring-common-dictionary-string-pattern and some unique payload data for verification")
	compressed, err := sample.CompressWithDict(payload, zlib.BestCompression, dict)
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 2: CompressWithDict failed: %v\n", err)
		os.Exit(1)
	}
	decompressed, err := sample.DecompressWithDict(compressed, dict)
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 2: DecompressWithDict failed: %v\n", err)
		os.Exit(1)
	}
	if !bytes.Equal(decompressed, payload) {
		fmt.Fprintf(os.Stderr, "FAIL assertion 2: payload mismatch: got %q, expected %q\n", decompressed, payload)
		os.Exit(1)
	}

	// Assertion 3: zlib.NewWriterLevelDict requires the identical dictionary during decompression and fails when omitted or mismatched
	rNoDict, err := zlib.NewReader(bytes.NewReader(compressed))
	if err == nil {
		_, readErr := io.ReadAll(rNoDict)
		rNoDict.Close()
		if readErr == nil {
			fmt.Fprintf(os.Stderr, "FAIL assertion 3: expected decompression error without dictionary, got success\n")
			os.Exit(1)
		}
	} else if !errors.Is(err, zlib.ErrDictionary) {
		// NewReader itself may return ErrDictionary directly
	}

	wrongDict := []byte("different-dictionary-string-pattern-mismatch")
	rWrongDict, err := zlib.NewReaderDict(bytes.NewReader(compressed), wrongDict)
	if err == nil {
		_, readErr := io.ReadAll(rWrongDict)
		rWrongDict.Close()
		if readErr == nil {
			fmt.Fprintf(os.Stderr, "FAIL assertion 3: expected decompression error with mismatched dictionary, got success\n")
			os.Exit(1)
		}
	}

	// Assertion 4: zlib.NewWriterLevelDict accepts a nil dictionary behaving equivalent to standard compression
	nilDictCompressed, err := sample.CompressWithDict(payload, zlib.DefaultCompression, nil)
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 4: CompressWithDict with nil dict failed: %v\n", err)
		os.Exit(1)
	}
	rStandard, err := zlib.NewReader(bytes.NewReader(nilDictCompressed))
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 4: NewReader failed on nil dict compressed data: %v\n", err)
		os.Exit(1)
	}
	nilDictDecompressed, err := io.ReadAll(rStandard)
	rStandard.Close()
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 4: ReadAll failed on nil dict compressed data: %v\n", err)
		os.Exit(1)
	}
	if !bytes.Equal(nilDictDecompressed, payload) {
		fmt.Fprintf(os.Stderr, "FAIL assertion 4: nil dict payload mismatch: got %q, expected %q\n", nilDictDecompressed, payload)
		os.Exit(1)
	}

	// Assertion 5: zlib.NewWriterLevelDict returns an error when given an invalid compression level
	var bufBad bytes.Buffer
	_, errBadLevel := sample.NewDictWriter(&bufBad, 999, dict)
	if errBadLevel == nil {
		fmt.Fprintf(os.Stderr, "FAIL assertion 5: expected error for invalid compression level 999, got nil\n")
		os.Exit(1)
	}

	fmt.Println("PASS: github.com/klauspost/compress/zlib.NewWriterLevelDict contract verified")
}

package sample

import (
	"bytes"
	"io"

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

// NewDictWriter wraps zlib.NewWriterLevelDict to create a zlib writer with a custom compression level and preset dictionary.
func NewDictWriter(w io.Writer, level int, dict []byte) (*zlib.Writer, error) {
	return zlib.NewWriterLevelDict(w, level, dict)
}

// CompressWithDict compresses input data using zlib.NewWriterLevelDict with the specified compression level and dictionary.
func CompressWithDict(data []byte, level int, dict []byte) ([]byte, error) {
	var buf bytes.Buffer
	w, err := zlib.NewWriterLevelDict(&buf, level, dict)
	if err != nil {
		return nil, err
	}
	if _, err := w.Write(data); err != nil {
		return nil, err
	}
	if err := w.Close(); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// DecompressWithDict decompresses data that was compressed with a preset dictionary using zlib.NewReaderDict.
func DecompressWithDict(compressed []byte, dict []byte) ([]byte, error) {
	r, err := zlib.NewReaderDict(bytes.NewReader(compressed), dict)
	if err != nil {
		return nil, err
	}
	defer r.Close()
	return io.ReadAll(r)
}