github.com/pmezard/go-difflib v1.0.0: difflib.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: verify github.com/pmezard/go-difflib/difflib.UnifiedDiff in pkg:golang/github.com/pmezard/go-difflib@v1.0.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/pmezard/go-difflib@v1.0.0
Demonstrate these symbols/APIs:
  - github.com/pmezard/go-difflib/difflib.UnifiedDiff
Constraints:
  - executionContext: node
Required runtime conditions:
  - ecosystem: npm
  - language: javascript
  - moduleSystem: cjs
  - packageManager: npm@10.9.8
  - runtime: node@22.23.2

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:sha256:1acc305944b545fd3072cf9084ac0b0695bf1650ad607f97e65c238434a771e0","contract":["UnifiedDiff struct configures line diff comparison with source and target line slices, file names, timestamps, and context line count","GetUnifiedDiffString generates unified diff formatted text including file headers, hunk range metadata, deletion prefixes (-), insertion prefixes (+), and surrounding context lines","GetUnifiedDiffString returns an empty string when the compared line sequences are identical","WriteUnifiedDiff formats and writes unified diff text with tab-separated timestamp headers directly to an io.Writer","UnifiedDiff with Context set to 0 outputs hunk modifications without surrounding context lines"],"goal":"verify github.com/pmezard/go-difflib/difflib.UnifiedDiff in pkg:golang/github.com/pmezard/go-difflib@v1.0.0","kind":"HOW","packages":["pkg:golang/github.com/pmezard/go-difflib@v1.0.0"],"schemaVersion":1,"symbols":["github.com/pmezard/go-difflib/difflib.UnifiedDiff"]},"contractCommand":["go","test","./..."],"environment":{"arch":"x64","compiler":"go","compilerVersion":"1.26.6","distro":"ubuntu","ecosystem":"golang","language":"go","languageVersion":"1.26.6","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","packageManagerVersion":"1.26.6","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":["github.com/pmezard/go-difflib/difflib.UnifiedDiff"],"verifierAdapter":"golang@1"}

module example.com/difflib-sample

go 1.22

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": "verify github.com/pmezard/go-difflib/difflib.UnifiedDiff in pkg:golang/github.com/pmezard/go-difflib@v1.0.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/pmezard/go-difflib@v1.0.0"
  ],
  "symbols": [
    "github.com/pmezard/go-difflib/difflib.UnifiedDiff"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "packageManager": "npm@10.9.8",
    "runtime": "node@22.23.2"
  }
}

package test

import (
	"bytes"
	"strings"
	"testing"

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

func TestUnifiedDiff_GetUnifiedDiffString(t *testing.T) {
	diff := difflib.UnifiedDiff{
		A:        difflib.SplitLines("foo\nbar\nbaz"),
		B:        difflib.SplitLines("foo\nqux\nbaz"),
		FromFile: "a.txt",
		ToFile:   "b.txt",
		Context:  3,
	}

	result, err := difflib.GetUnifiedDiffString(diff)
	if err != nil {
		t.Fatalf("GetUnifiedDiffString failed: %v", err)
	}

	expected := "--- a.txt\n+++ b.txt\n@@ -1,3 +1,3 @@\n foo\n-bar\n+qux\n baz\n"
	if result != expected {
		t.Fatalf("expected:\n%q\ngot:\n%q", expected, result)
	}
}

func TestUnifiedDiff_IdenticalSequences(t *testing.T) {
	diff := difflib.UnifiedDiff{
		A:        difflib.SplitLines("alpha\nbeta\ngamma"),
		B:        difflib.SplitLines("alpha\nbeta\ngamma"),
		FromFile: "orig.txt",
		ToFile:   "new.txt",
		Context:  3,
	}

	result, err := difflib.GetUnifiedDiffString(diff)
	if err != nil {
		t.Fatalf("GetUnifiedDiffString failed: %v", err)
	}

	if result != "" {
		t.Fatalf("expected empty diff for identical inputs, got: %q", result)
	}
}

func TestUnifiedDiff_WriteUnifiedDiffWithDates(t *testing.T) {
	var buf bytes.Buffer
	diff := difflib.UnifiedDiff{
		A:        []string{"line1\n", "line2\n"},
		B:        []string{"line1\n", "line2 modified\n"},
		FromFile: "file_a",
		FromDate: "2026-01-01 00:00:00",
		ToFile:   "file_b",
		ToDate:   "2026-01-02 00:00:00",
		Context:  1,
	}

	err := difflib.WriteUnifiedDiff(&buf, diff)
	if err != nil {
		t.Fatalf("WriteUnifiedDiff failed: %v", err)
	}

	output := buf.String()
	if !strings.Contains(output, "--- file_a\t2026-01-01 00:00:00\n") {
		t.Fatalf("expected from header with date, got:\n%s", output)
	}
	if !strings.Contains(output, "+++ file_b\t2026-01-02 00:00:00\n") {
		t.Fatalf("expected to header with date, got:\n%s", output)
	}
	if !strings.Contains(output, "-line2\n") || !strings.Contains(output, "+line2 modified\n") {
		t.Fatalf("expected hunk modifications, got:\n%s", output)
	}
}

func TestUnifiedDiff_ZeroContext(t *testing.T) {
	diff := difflib.UnifiedDiff{
		A:        difflib.SplitLines("header\ndel\nfooter"),
		B:        difflib.SplitLines("header\nins\nfooter"),
		FromFile: "old.txt",
		ToFile:   "new.txt",
		Context:  0,
	}

	result, err := difflib.GetUnifiedDiffString(diff)
	if err != nil {
		t.Fatalf("GetUnifiedDiffString failed: %v", err)
	}

	expectedHunkHeader := "@@ -2 +2 @@\n"
	if !strings.Contains(result, expectedHunkHeader) {
		t.Fatalf("expected hunk header %q, got:\n%s", expectedHunkHeader, result)
	}
	if !strings.Contains(result, "-del\n") || !strings.Contains(result, "+ins\n") {
		t.Fatalf("expected diff to show deletion and insertion, got:\n%s", result)
	}
	if strings.Contains(result, " header\n") || strings.Contains(result, " footer\n") {
		t.Fatalf("context 0 must not include unmodified context lines, got:\n%s", result)
	}
}