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

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.NewReaderDict 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.NewReaderDict

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:sha256:3f04228853406251d569367cd9dc06fd22c66cdd40d95f01076410e99a782e85","contract":["NewReaderDict successfully decompresses data compressed with a matching preset dictionary","NewReaderDict returns ErrDictionary when the compressed data requires a different preset dictionary","NewReaderDict decompresses standard zlib streams without a dictionary when nil dict is provided","NewReaderDict returns ErrHeader when provided invalid zlib header bytes","NewReaderDict returns an io.ReadCloser that implements the Resetter interface for reusing the decompressor"],"goal":"verify github.com/klauspost/compress/zlib.NewReaderDict 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.NewReaderDict"]},"contractCommand":["go","run","./test"],"environment":{"arch":"amd64","compiler":"go","compilerVersion":"1.26.6","distro":"ubuntu","ecosystem":"golang","language":"go","languageVersion":"1.26.6","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","packageManagerVersion":"1.26.6","runtime":"go","runtimeVersion":"1.26.6","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.NewReaderDict"],"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=

package main

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

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

func main() {
	dict := []byte("shared-zlib-preset-dictionary")
	rawMessage := []byte("Demonstrating zlib decompression with preset dictionary using klauspost/compress")

	var buf bytes.Buffer
	writer, err := zlib.NewWriterLevelDict(&buf, zlib.DefaultCompression, dict)
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to create zlib writer: %v\n", err)
		os.Exit(1)
	}
	if _, err := writer.Write(rawMessage); err != nil {
		fmt.Fprintf(os.Stderr, "failed to write data: %v\n", err)
		os.Exit(1)
	}
	if err := writer.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "failed to close writer: %v\n", err)
		os.Exit(1)
	}

	reader, err := zlib.NewReaderDict(&buf, dict)
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to create zlib reader with dict: %v\n", err)
		os.Exit(1)
	}
	defer reader.Close()

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

	fmt.Printf("Successfully decompressed %d bytes: %s\n", len(decompressed), string(decompressed))
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/klauspost/compress/zlib.NewReaderDict 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.NewReaderDict"
  ]
}

package main

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

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

func main() {
	// Assertion 1: NewReaderDict successfully decompresses data compressed with a matching preset dictionary
	{
		dict := []byte("contract-test-dictionary-preset-12345")
		original := []byte("The quick brown fox jumps over the lazy dog. Zlib compression with dictionary verification.")

		var buf bytes.Buffer
		w, err := zlib.NewWriterLevelDict(&buf, zlib.DefaultCompression, dict)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 setup failed: NewWriterLevelDict: %v\n", err)
			os.Exit(1)
		}
		if _, err := w.Write(original); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 setup failed: Write: %v\n", err)
			os.Exit(1)
		}
		if err := w.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 setup failed: Close: %v\n", err)
			os.Exit(1)
		}

		r, err := zlib.NewReaderDict(&buf, dict)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: NewReaderDict returned error: %v\n", err)
			os.Exit(1)
		}

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

		if !bytes.Equal(decompressed, original) {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: decompressed data does not match original\n")
			os.Exit(1)
		}
	}

	// Assertion 2: NewReaderDict returns ErrDictionary when the compressed data requires a different preset dictionary
	{
		dictA := []byte("dictionary-alpha-key-spec")
		dictB := []byte("dictionary-beta-different-key")
		payload := []byte("Payload compressed with dictionary alpha.")

		var buf bytes.Buffer
		w, err := zlib.NewWriterLevelDict(&buf, zlib.DefaultCompression, dictA)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 2 setup failed: NewWriterLevelDict: %v\n", err)
			os.Exit(1)
		}
		if _, err := w.Write(payload); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 2 setup failed: Write: %v\n", err)
			os.Exit(1)
		}
		if err := w.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 2 setup failed: Close: %v\n", err)
			os.Exit(1)
		}

		_, err = zlib.NewReaderDict(&buf, dictB)
		if !errors.Is(err, zlib.ErrDictionary) {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: expected ErrDictionary, got: %v\n", err)
			os.Exit(1)
		}
	}

	// Assertion 3: NewReaderDict decompresses standard zlib streams without a dictionary when nil dict is provided
	{
		payload := []byte("Standard zlib stream without preset dictionary.")

		var buf bytes.Buffer
		w := zlib.NewWriter(&buf)
		if _, err := w.Write(payload); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 setup failed: Write: %v\n", err)
			os.Exit(1)
		}
		if err := w.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 setup failed: Close: %v\n", err)
			os.Exit(1)
		}

		r, err := zlib.NewReaderDict(&buf, nil)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: NewReaderDict with nil dict returned error: %v\n", err)
			os.Exit(1)
		}

		decompressed, err := io.ReadAll(r)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: io.ReadAll returned error: %v\n", err)
			os.Exit(1)
		}
		if err := r.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: r.Close returned error: %v\n", err)
			os.Exit(1)
		}

		if !bytes.Equal(decompressed, payload) {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: decompressed data mismatch\n")
			os.Exit(1)
		}
	}

	// Assertion 4: NewReaderDict returns ErrHeader when provided invalid zlib header bytes
	{
		invalidHeader := []byte{0x00, 0x00, 0x00, 0x00}
		_, err := zlib.NewReaderDict(bytes.NewReader(invalidHeader), nil)
		if !errors.Is(err, zlib.ErrHeader) {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: expected ErrHeader, got: %v\n", err)
			os.Exit(1)
		}
	}

	// Assertion 5: NewReaderDict returns an io.ReadCloser that implements the Resetter interface for reusing the decompressor
	{
		dict1 := []byte("dictionary-first-run-777")
		dict2 := []byte("dictionary-second-run-888")
		msg1 := []byte("Message number one for reuse test.")
		msg2 := []byte("Message number two for reuse test with different dictionary.")

		var buf1 bytes.Buffer
		w1, _ := zlib.NewWriterLevelDict(&buf1, zlib.DefaultCompression, dict1)
		w1.Write(msg1)
		w1.Close()

		var buf2 bytes.Buffer
		w2, _ := zlib.NewWriterLevelDict(&buf2, zlib.DefaultCompression, dict2)
		w2.Write(msg2)
		w2.Close()

		r, err := zlib.NewReaderDict(&buf1, dict1)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: initial NewReaderDict: %v\n", err)
			os.Exit(1)
		}

		resetter, ok := r.(zlib.Resetter)
		if !ok {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: returned reader does not implement zlib.Resetter\n")
			os.Exit(1)
		}

		out1, err := io.ReadAll(r)
		if err != nil || !bytes.Equal(out1, msg1) {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: reading first stream: %v\n", err)
			os.Exit(1)
		}

		if err := resetter.Reset(&buf2, dict2); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: resetter.Reset: %v\n", err)
			os.Exit(1)
		}

		out2, err := io.ReadAll(r)
		if err != nil || !bytes.Equal(out2, msg2) {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: reading second stream after reset: %v\n", err)
			os.Exit(1)
		}

		if err := r.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: r.Close: %v\n", err)
			os.Exit(1)
		}
	}

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