golang.org/x/text v0.42.0: transform.ErrShortDst

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/transform.ErrShortDst 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/transform.ErrShortDst

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:sha256:5f54466ba1a804464d57aef28e13eb49b5a36dc4df4cf911420788848d3be2e9","contract":["ErrShortDst error string equals transform: short destination buffer","transform.Nop.Transform returns ErrShortDst when destination buffer is smaller than source buffer","transform.Nop.Transform returns nil error when destination buffer is sufficiently sized","Custom Transformer signals ErrShortDst when destination buffer cannot hold the transformed chunk","transform.Bytes expands destination buffer upon receiving ErrShortDst until transformation completes","transform.String expands destination buffer upon receiving ErrShortDst until transformation completes"],"goal":"verify golang.org/x/text/transform.ErrShortDst 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/transform.ErrShortDst"]},"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/transform.ErrShortDst"],"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 (
	"errors"
	"fmt"
	"log"

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

// DoublingTransformer duplicates each input byte into two output bytes.
type DoublingTransformer struct {
	transform.NopResetter
}

// Transform writes duplicated bytes to dst or returns ErrShortDst if dst is too short.
func (DoublingTransformer) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
	for i, b := range src {
		if nDst+2 > len(dst) {
			return nDst, i, transform.ErrShortDst
		}
		dst[nDst] = b
		dst[nDst+1] = b
		nDst += 2
	}
	return nDst, len(src), nil
}

func main() {
	src := []byte("CodeSampleX")

	// 1. Direct Transform with a buffer smaller than required
	smallDst := make([]byte, 4)
	nDst, nSrc, err := transform.Nop.Transform(smallDst, src, true)
	if errors.Is(err, transform.ErrShortDst) {
		fmt.Printf("Destination buffer too short: read %d src bytes, wrote %d dst bytes (%s)\n", nSrc, nDst, err)
	} else {
		log.Fatalf("expected transform.ErrShortDst, got: %v", err)
	}

	// 2. Direct Transform with sufficient buffer capacity
	fullDst := make([]byte, len(src))
	nDst, nSrc, err = transform.Nop.Transform(fullDst, src, true)
	if err != nil {
		log.Fatalf("transform failed: %v", err)
	}
	fmt.Printf("Full transform succeeded: read %d bytes, output: %s\n", nSrc, string(fullDst[:nDst]))

	// 3. Custom transformer with transform.Bytes handling ErrShortDst automatically
	t := DoublingTransformer{}
	doubled, n, err := transform.Bytes(t, []byte("CSX"))
	if err != nil {
		log.Fatalf("transform.Bytes failed: %v", err)
	}
	fmt.Printf("Doubled %d bytes into: %s\n", n, string(doubled))
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/text/transform.ErrShortDst 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/transform.ErrShortDst"
  ]
}

package main

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

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

type doublingTransformer struct {
	transform.NopResetter
}

func (doublingTransformer) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
	for i, b := range src {
		if nDst+2 > len(dst) {
			return nDst, i, transform.ErrShortDst
		}
		dst[nDst] = b
		dst[nDst+1] = b
		nDst += 2
	}
	return nDst, len(src), nil
}

func main() {
	// 1. ErrShortDst error string equals "transform: short destination buffer"
	if transform.ErrShortDst.Error() != "transform: short destination buffer" {
		fmt.Fprintf(os.Stderr, "Contract assertion 1 failed: %s\n", transform.ErrShortDst.Error())
		os.Exit(1)
	}

	// 2. transform.Nop.Transform returns ErrShortDst when destination buffer is smaller than source buffer
	src := []byte("sample input")
	smallDst := make([]byte, 5)
	nDst, nSrc, err := transform.Nop.Transform(smallDst, src, true)
	if !errors.Is(err, transform.ErrShortDst) {
		fmt.Fprintf(os.Stderr, "Contract assertion 2 failed: expected ErrShortDst, got %v\n", err)
		os.Exit(1)
	}
	if nDst != 5 || nSrc != 5 {
		fmt.Fprintf(os.Stderr, "Contract assertion 2 failed: expected nDst=5, nSrc=5; got nDst=%d, nSrc=%d\n", nDst, nSrc)
		os.Exit(1)
	}
	if !bytes.Equal(smallDst, []byte("sampl")) {
		fmt.Fprintf(os.Stderr, "Contract assertion 2 failed: unexpected dst bytes: %s\n", string(smallDst))
		os.Exit(1)
	}

	// 3. transform.Nop.Transform returns nil error when destination buffer is sufficiently sized
	fullDst := make([]byte, len(src))
	nDst, nSrc, err = transform.Nop.Transform(fullDst, src, true)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Contract assertion 3 failed: expected nil error, got %v\n", err)
		os.Exit(1)
	}
	if nDst != len(src) || nSrc != len(src) {
		fmt.Fprintf(os.Stderr, "Contract assertion 3 failed: expected nDst=%d, nSrc=%d; got nDst=%d, nSrc=%d\n", len(src), len(src), nDst, nSrc)
		os.Exit(1)
	}
	if !bytes.Equal(fullDst, src) {
		fmt.Fprintf(os.Stderr, "Contract assertion 3 failed: dst mismatch\n")
		os.Exit(1)
	}

	// 4. Custom Transformer signals ErrShortDst when destination buffer cannot hold the transformed chunk
	t := doublingTransformer{}
	doublingDst := make([]byte, 3)
	nDst, nSrc, err = t.Transform(doublingDst, []byte("abc"), true)
	if !errors.Is(err, transform.ErrShortDst) {
		fmt.Fprintf(os.Stderr, "Contract assertion 4 failed: expected ErrShortDst, got %v\n", err)
		os.Exit(1)
	}
	if nDst != 2 || nSrc != 1 {
		fmt.Fprintf(os.Stderr, "Contract assertion 4 failed: expected nDst=2, nSrc=1; got nDst=%d, nSrc=%d\n", nDst, nSrc)
		os.Exit(1)
	}

	// 5. transform.Bytes expands destination buffer upon receiving ErrShortDst until transformation completes
	resBytes, n, err := transform.Bytes(t, []byte("xyz"))
	if err != nil {
		fmt.Fprintf(os.Stderr, "Contract assertion 5 failed: transform.Bytes error: %v\n", err)
		os.Exit(1)
	}
	if n != 3 || string(resBytes) != "xxyyzz" {
		fmt.Fprintf(os.Stderr, "Contract assertion 5 failed: expected n=3 and res=xxyyzz; got n=%d, res=%s\n", n, string(resBytes))
		os.Exit(1)
	}

	// 6. transform.String expands destination buffer upon receiving ErrShortDst until transformation completes
	resStr, n, err := transform.String(t, "abc")
	if err != nil {
		fmt.Fprintf(os.Stderr, "Contract assertion 6 failed: transform.String error: %v\n", err)
		os.Exit(1)
	}
	if n != 3 || resStr != "aabbcc" {
		fmt.Fprintf(os.Stderr, "Contract assertion 6 failed: expected n=3 and res=aabbcc; got n=%d, res=%s\n", n, resStr)
		os.Exit(1)
	}
}