modernc.org/strutil v1.2.1: Base64Decode

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 modernc.org/strutil.Base64Decode in pkg:golang/modernc.org/strutil@v1.2.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/modernc.org/strutil@v1.2.1
Demonstrate these symbols/APIs:
  - modernc.org/strutil.Base64Decode

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:f7fe53e26285ac21fe101579ee8241c4a40f797b3db4ccd1748e3f8afce887a5","contract":["strutil.Base64Decode decodes standard base64 encoded byte slices into the original binary data","strutil.Base64Decode returns an error and empty byte slice when given invalid base64 input","strutil.Base64Decode returns an error when reading from empty input due to immediate EOF","strutil.Base64Decode correctly reverses Base64Encode output across arbitrary binary payloads"],"goal":"verify modernc.org/strutil.Base64Decode in pkg:golang/modernc.org/strutil@v1.2.1","kind":"HOW","packages":["pkg:golang/modernc.org/strutil@v1.2.1"],"schemaVersion":1,"symbols":["modernc.org/strutil.Base64Decode"]},"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/modernc.org/strutil@v1.2.1"],"schemaVersion":1,"subject":"pkg:golang/modernc.org/strutil@v1.2.1","symbols":["modernc.org/strutil.Base64Decode"],"verifierAdapter":"golang@1"}

module sample

go 1.22

require modernc.org/strutil v1.2.1

github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
modernc.org/mathutil v1.7.1 h1:GCZVGXdaN8gTqB1Mf/usp1Y/hSqgI2vAGGP4jZMCxOU=
modernc.org/mathutil v1.7.1/go.mod h1:4p5IwJITfppl0G4sUEDtCr4DthTaT47/N3aT6MhfgJg=
modernc.org/strutil v1.2.1 h1:UneZBkQA+DX2Rp35KcM69cSsNES9ly8mQWD71HKlOA0=
modernc.org/strutil v1.2.1/go.mod h1:EHkiggD70koQxjVdSBM3JKM7k6L0FbGE5eymy9i3B9A=

package main

import (
	"fmt"

	"modernc.org/strutil"
)

func main() {
	encoded := []byte("SGVsbG8sIENvZGVTYW1wbGVYIQ==")
	decoded, err := strutil.Base64Decode(encoded)
	if err != nil {
		fmt.Printf("Base64Decode error: %v\n", err)
		return
	}
	fmt.Printf("Decoded string: %s\n", string(decoded))
}

{
  "schemaVersion": 1,
  "goal": "verify modernc.org/strutil.Base64Decode in pkg:golang/modernc.org/strutil@v1.2.1",
  "kind": "HOW",
  "packages": [
    "pkg:golang/modernc.org/strutil@v1.2.1"
  ],
  "symbols": [
    "modernc.org/strutil.Base64Decode"
  ]
}

package main

import (
	"bytes"
	"fmt"
	"os"

	"modernc.org/strutil"
)

func main() {
	// Assertion 1: strutil.Base64Decode decodes standard base64 encoded byte slices into the original binary data
	{
		input := []byte("SGVsbG8sIENvZGVTYW1wbGVYIQ==") // "Hello, CodeSampleX!"
		expected := []byte("Hello, CodeSampleX!")
		decoded, err := strutil.Base64Decode(input)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		if !bytes.Equal(decoded, expected) {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: expected %q, got %q\n", expected, decoded)
			os.Exit(1)
		}
	}

	// Assertion 2: strutil.Base64Decode returns an error and empty byte slice when given invalid base64 input
	{
		invalidInput := []byte("!@#$%^&*()_+-=")
		decoded, err := strutil.Base64Decode(invalidInput)
		if err == nil {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: expected error for invalid base64, got nil\n")
			os.Exit(1)
		}
		if len(decoded) != 0 {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: expected empty output on error, got %d bytes\n", len(decoded))
			os.Exit(1)
		}
	}

	// Assertion 3: strutil.Base64Decode returns an error when reading from empty input due to immediate EOF
	{
		decoded, err := strutil.Base64Decode([]byte(""))
		if err == nil {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: expected error on empty input, got nil\n")
			os.Exit(1)
		}
		if len(decoded) != 0 {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: expected 0 bytes on error, got %d bytes\n", len(decoded))
			os.Exit(1)
		}
	}

	// Assertion 4: strutil.Base64Decode correctly reverses Base64Encode output across arbitrary binary payloads
	{
		payload := []byte("arbitrary binary payload: \x00\x01\x02\xff\xfe\xfd with UTF-8 text \xe2\x9c\x93")
		encoded := strutil.Base64Encode(payload)
		decoded, err := strutil.Base64Decode(encoded)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 4 failed: unexpected error in roundtrip: %v\n", err)
			os.Exit(1)
		}
		if !bytes.Equal(decoded, payload) {
			fmt.Fprintf(os.Stderr, "Assertion 4 failed: roundtrip mismatch: got %v, expected %v\n", decoded, payload)
			os.Exit(1)
		}
	}

	fmt.Println("PASS: all Base64Decode contract assertions passed")
}