Golden File Test Helper

package main

import (
	"bytes"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

func WriteGolden(target string, data []byte) error {
	if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
		return fmt.Errorf("create golden dir: %w", err)
	}
	return os.WriteFile(target, data, 0o644)
}

func diffLines(want, got string) string {
	wantLines := strings.Split(strings.TrimSpace(want), "\n")
	gotLines := strings.Split(strings.TrimSpace(got), "\n")
	limit := len(wantLines)
	if len(gotLines) < limit {
		limit = len(gotLines)
	}
	var builder strings.Builder
	for i := 0; i < limit; i++ {
		if wantLines[i] != gotLines[i] {
			fmt.Fprintf(&builder, "- %s\n+ %s\n", wantLines[i], gotLines[i])
		}
	}
	if len(wantLines) != len(gotLines) {
		fmt.Fprintf(&builder, "line count: want %d got %d\n", len(wantLines), len(gotLines))
	}
	return builder.String()
}

func CompareGolden(target string, got []byte) error {
	want, err := os.ReadFile(target)
	if err != nil {
		return fmt.Errorf("read golden: %w", err)
	}
	if !bytes.Equal(bytes.TrimSpace(got), bytes.TrimSpace(want)) {
		return fmt.Errorf("golden mismatch for %s\n%s", filepath.Base(target), diffLines(string(want), string(got)))
	}
	return nil
}

func main() {
	dir, err := os.MkdirTemp("", "golden-*")
	if err != nil {
		panic(err)
	}
	defer os.RemoveAll(dir)

	target := filepath.Join(dir, "report.golden")
	report := []byte("users=3\norders=5\n")
	if err := WriteGolden(target, report); err != nil {
		panic(err)
	}
	if err := CompareGolden(target, report); err != nil {
		fmt.Println("unexpected mismatch:", err)
	} else {
		fmt.Println("match: ok")
	}
	if err := CompareGolden(target, []byte("users=4\norders=5\n")); err != nil {
		fmt.Println("mismatch detected:")
		fmt.Println(err)
	}
}