golang.org/x/text v0.42.0: encoding.Replacement

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 golang.org/x/text/encoding.Replacement in pkg:golang/golang.org/x/text@v0.42.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/text@v0.42.0
Demonstrate these symbols/APIs:
  - golang.org/x/text/encoding.Replacement

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:sha256:3ac6445160475838b4a737c4782dde9a00b6482be9d684ad2b2bb0ff6c7d70f9","contract":["encoding.Replacement decoder converts any input into a single Unicode replacement character","encoding.Replacement decoder returns transform.ErrShortDst when destination buffer is smaller than 3 bytes","encoding.Replacement encoder preserves valid UTF-8 sequences unchanged","encoding.Replacement encoder replaces invalid UTF-8 byte sequences with Unicode replacement characters"],"goal":"verify golang.org/x/text/encoding.Replacement in pkg:golang/golang.org/x/text@v0.42.0","kind":"HOW","packages":["pkg:golang/golang.org/x/text@v0.42.0"],"schemaVersion":1,"symbols":["golang.org/x/text/encoding.Replacement"]},"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/golang.org/x/text@v0.42.0"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/text@v0.42.0","symbols":["golang.org/x/text/encoding.Replacement"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require golang.org/x/text v0.42.0

golang.org/x/text v0.42.0 h1:JbOZXgfeCPU9gacVtYliJqOhD+zhrEqK4LfdpmlUZqI=
golang.org/x/text v0.42.0/go.mod h1:ojzP1Z+2QtioaF8DTtO8K5q7JWVVYwZKenzujK0Zd0E=

package main

import (
	"fmt"
	"os"

	"golang.org/x/text/encoding"
)

func main() {
	decoder := encoding.Replacement.NewDecoder()
	decoded, err := decoder.Bytes([]byte("Arbitrary input text"))
	if err != nil {
		fmt.Fprintf(os.Stderr, "decoder failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Printf("Decoded: %s (runes: %d)\n", string(decoded), len([]rune(string(decoded))))

	encoder := encoding.Replacement.NewEncoder()
	input := []byte("Valid UTF-8 and invalid \xff\xfe bytes")
	encoded, err := encoder.Bytes(input)
	if err != nil {
		fmt.Fprintf(os.Stderr, "encoder failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Printf("Encoded: %s\n", string(encoded))
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/text/encoding.Replacement in pkg:golang/golang.org/x/text@v0.42.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/text@v0.42.0"
  ],
  "symbols": [
    "golang.org/x/text/encoding.Replacement"
  ]
}

package main

import (
	"bytes"
	"errors"
	"fmt"
	"os"
	"unicode/utf8"

	"golang.org/x/text/encoding"
	"golang.org/x/text/transform"
)

func main() {
	if err := runTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract assertions passed.")
}

func runTests() error {
	// Assertion 1: encoding.Replacement decoder converts any input into a single Unicode replacement character
	decoder := encoding.Replacement.NewDecoder()
	testInputs := [][]byte{
		[]byte("hello world"),
		[]byte(""),
		[]byte("\xff\xfe\xfd"),
		[]byte("1234567890"),
	}
	for _, in := range testInputs {
		out, err := decoder.Bytes(in)
		if err != nil {
			return fmt.Errorf("decoder.Bytes(%q) returned error: %w", in, err)
		}
		if !bytes.Equal(out, []byte(string(utf8.RuneError))) {
			return fmt.Errorf("decoder.Bytes(%q) expected %q, got %q", in, string(utf8.RuneError), string(out))
		}
	}

	// Assertion 2: encoding.Replacement decoder returns transform.ErrShortDst when destination buffer is smaller than 3 bytes
	decoder.Reset()
	shortDst := make([]byte, 2)
	nDst, nSrc, err := decoder.Transform(shortDst, []byte("data"), true)
	if !errors.Is(err, transform.ErrShortDst) {
		return fmt.Errorf("expected transform.ErrShortDst with short buffer, got err=%v, nDst=%d, nSrc=%d", err, nDst, nSrc)
	}

	// Assertion 3: encoding.Replacement encoder preserves valid UTF-8 sequences unchanged
	encoder := encoding.Replacement.NewEncoder()
	validTexts := []string{
		"Hello, World!",
		"日本語テキスト",
		"한글 테스트",
		"ASCII and Unicode: äöü, αβγ, 🌟",
	}
	for _, text := range validTexts {
		out, err := encoder.Bytes([]byte(text))
		if err != nil {
			return fmt.Errorf("encoder.Bytes(%q) returned error: %w", text, err)
		}
		if string(out) != text {
			return fmt.Errorf("encoder.Bytes(%q) expected %q, got %q", text, text, string(out))
		}
	}

	// Assertion 4: encoding.Replacement encoder replaces invalid UTF-8 byte sequences with Unicode replacement characters
	invalidCases := []struct {
		input    []byte
		expected string
	}{
		{
			input:    []byte("bad \xff byte"),
			expected: "bad " + string(utf8.RuneError) + " byte",
		},
		{
			input:    []byte("bad \xff\xfe sequence"),
			expected: "bad " + string(utf8.RuneError) + string(utf8.RuneError) + " sequence",
		},
		{
			input:    []byte("\x80\x81"),
			expected: string(utf8.RuneError) + string(utf8.RuneError),
		},
	}
	for _, tc := range invalidCases {
		out, err := encoder.Bytes(tc.input)
		if err != nil {
			return fmt.Errorf("encoder.Bytes(%q) returned error: %w", tc.input, err)
		}
		if string(out) != tc.expected {
			return fmt.Errorf("encoder.Bytes(%q) expected %q, got %q", tc.input, tc.expected, string(out))
		}
	}

	return nil
}