github.com/pmezard/go-difflib v1.0.0: generate unified diff string with UnifiedDiff

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: generate unified diff string with UnifiedDiff
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/pmezard/go-difflib@v1.0.0
Demonstrate these symbols/APIs:
  - difflib.GetUnifiedDiffString
  - difflib.UnifiedDiff

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:fc06c93b2e8e31ab55ec797b95e082e7941932708b6e2e9874afae67fc53fb4a","contract":["GenerateUnifiedDiff returns empty string when inputs are identical","GenerateUnifiedDiff formats unified diff with file headers and modified lines","difflib.GetUnifiedDiffString generates unified diff string from UnifiedDiff struct"],"goal":"generate unified diff string with UnifiedDiff","kind":"HOW","packages":["pkg:golang/github.com/pmezard/go-difflib@v1.0.0"],"schemaVersion":1,"symbols":["difflib.GetUnifiedDiffString","difflib.UnifiedDiff"]},"contractCommand":["go","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/github.com/pmezard/go-difflib@v1.0.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/pmezard/go-difflib@v1.0.0","symbols":["difflib.GetUnifiedDiffString","difflib.UnifiedDiff"],"verifierAdapter":"golang@1"}

package sample

import (
	"github.com/pmezard/go-difflib/difflib"
)

// GenerateUnifiedDiff generates a unified diff between two text strings.
func GenerateUnifiedDiff(fromFile, toFile, fromText, toText string, context int) (string, error) {
	diff := difflib.UnifiedDiff{
		A:        difflib.SplitLines(fromText),
		B:        difflib.SplitLines(toText),
		FromFile: fromFile,
		ToFile:   toFile,
		Context:  context,
	}
	return difflib.GetUnifiedDiffString(diff)
}

module sample

go 1.20

require github.com/pmezard/go-difflib v1.0.0

github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=

{
  "schemaVersion": 1,
  "goal": "generate unified diff string with UnifiedDiff",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/pmezard/go-difflib@v1.0.0"
  ],
  "symbols": [
    "difflib.GetUnifiedDiffString",
    "difflib.UnifiedDiff"
  ]
}

package test

import (
	"strings"
	"testing"

	"github.com/pmezard/go-difflib/difflib"
	"sample"
)

func TestContract_IdenticalInputsProduceEmptyDiff(t *testing.T) {
	text := "line1\nline2\nline3\n"
	diff, err := sample.GenerateUnifiedDiff("a.txt", "b.txt", text, text, 3)
	if err != nil {
		t.Fatalf("unexpected error: %v", err)
	}
	if diff != "" {
		t.Fatalf("expected empty diff for identical inputs, got: %q", diff)
	}
}

func TestContract_UnifiedDiffFormattingAndHeaders(t *testing.T) {
	orig := "alpha\nbeta\ngamma\n"
	updated := "alpha\nbeta-modified\ngamma\n"
	diff, err := sample.GenerateUnifiedDiff("orig.txt", "updated.txt", orig, updated, 1)
	if err != nil {
		t.Fatalf("unexpected error: %v", err)
	}

	if !strings.Contains(diff, "--- orig.txt") {
		t.Errorf("missing fromFile header in diff: %s", diff)
	}
	if !strings.Contains(diff, "+++ updated.txt") {
		t.Errorf("missing toFile header in diff: %s", diff)
	}
	if !strings.Contains(diff, "-beta\n") {
		t.Errorf("missing deleted line in diff: %s", diff)
	}
	if !strings.Contains(diff, "+beta-modified\n") {
		t.Errorf("missing added line in diff: %s", diff)
	}
}

func TestContract_DirectUnifiedDiffStructAndGetUnifiedDiffString(t *testing.T) {
	seqA := difflib.SplitLines("first line\nsecond line\n")
	seqB := difflib.SplitLines("first line\nchanged line\n")

	ud := difflib.UnifiedDiff{
		A:        seqA,
		B:        seqB,
		FromFile: "fileA.txt",
		ToFile:   "fileB.txt",
		Context:  0,
	}

	result, err := difflib.GetUnifiedDiffString(ud)
	if err != nil {
		t.Fatalf("unexpected error from GetUnifiedDiffString: %v", err)
	}

	if !strings.Contains(result, "--- fileA.txt") || !strings.Contains(result, "+++ fileB.txt") {
		t.Fatalf("expected headers with file names, got: %s", result)
	}
	if !strings.Contains(result, "-second line\n") || !strings.Contains(result, "+changed line\n") {
		t.Fatalf("expected diff to contain changed line, got: %s", result)
	}
}