golang.org/x/text v0.31.0: 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@v0.31.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/text@v0.31.0
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:f23321dd0f9bb024a6c2b20479cff5d3bad90ad239c267a6fdf621f90f687249","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@v0.31.0","kind":"HOW","packages":["pkg:golang/golang.org/x/text@v0.31.0"],"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@v0.31.0"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/text@v0.31.0","symbols":["golang.org/x/text/transform.ErrShortDst"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require golang.org/x/text v0.31.0

golang.org/x/text v0.31.0 h1:aC8ghyu4JhP8VojJ2lEHBnochRno1sgL6nEi9WGFGMM=
golang.org/x/text v0.31.0/go.mod h1:tKRAlv61yKIjGGHX/4tP1LTbc13YSec1pxVEWXzfoeM=

package main

import (
	"errors"
	"fmt"
	"log"

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

// duplicateTransformer repeats every input byte twice into the output.
type duplicateTransformer struct {
	transform.NopResetter
}

func (duplicateTransformer) 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("hello")

	// 1. Direct transform with short destination buffer returns ErrShortDst
	smallDst := make([]byte, 2)
	nDst, nSrc, err := transform.Nop.Transform(smallDst, src, true)
	if errors.Is(err, transform.ErrShortDst) {
		fmt.Printf("Buffer too small: read %d, wrote %d, error: %v\n", nSrc, nDst, err)
	} else {
		log.Fatalf("expected ErrShortDst, got: %v", err)
	}

	// 2. Direct transform with adequate buffer succeeds
	fullDst := make([]byte, len(src))
	nDst, nSrc, err = transform.Nop.Transform(fullDst, src, true)
	if err != nil {
		log.Fatalf("unexpected error: %v", err)
	}
	fmt.Printf("Copied %d bytes: %s\n", nDst, string(fullDst[:nDst]))

	// 3. transform.Bytes automatically grows destination buffer when ErrShortDst is returned
	t := duplicateTransformer{}
	doubled, n, err := transform.Bytes(t, []byte("abc"))
	if err != nil {
		log.Fatalf("transform.Bytes failed: %v", err)
	}
	fmt.Printf("Transformed %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@v0.31.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/text@v0.31.0"
  ],
  "symbols": [
    "golang.org/x/text/transform.ErrShortDst"
  ]
}

package main

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

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

type repeatTransformer struct {
	transform.NopResetter
}

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

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

	// Assertion 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, "Assertion 3 failed: expected nil error, got %v\n", err)
		os.Exit(1)
	}
	if nDst != len(src) || nSrc != len(src) {
		fmt.Fprintf(os.Stderr, "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, "Assertion 3 failed: destination content mismatch\n")
		os.Exit(1)
	}

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

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

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