Getting started with fuzzing

// main.go
package main

import (
	"errors"
	"fmt"
	"unicode/utf8"
)

func Reverse(s string) (string, error) {

	if !utf8.ValidString(s) {
		return s, errors.New("input is not valid UTF-8")
	}

	b := []rune(s)

	for i, j := 0, len(b)-1; i < len(b)/2; i, j = i+1, j-1 {
		b[i], b[j] = b[j], b[i]
	}

	return string(b), nil
}

func main() {
	input := "The quick brown fox jumped over the lazy dog"
	rev, revErr := Reverse(input)
	doubleRev, doubleErr := Reverse(rev)
	fmt.Printf("original: %q \n", input)
	fmt.Printf("reverse: %q %v \n", rev, revErr)
	fmt.Printf("double: %q %v \n", doubleRev, doubleErr)
}

// reverse_test.go
package main

import (
	"testing"
	"unicode/utf8"
)

func FuzzReverse(f *testing.F) {
	testcases := []string{"Hello, world", " ", "!12345"}
	for _, tc := range testcases {
		f.Add(tc) // Use f.Add to provide a seed corpus
	}
	f.Fuzz(func(t *testing.T, orig string) {
		rev, err1 := Reverse(orig)
		if err1 != nil {
			return
		}
		doubleRev, err2 := Reverse(rev)
		if err2 != nil {
			return
		}
		if orig != doubleRev {
			t.Errorf("Before: %q, after: %q", orig, doubleRev)
		}
		if utf8.ValidString(orig) && !utf8.ValidString(rev) {
			t.Errorf("Reverse produced invalid UTF-8 string %q", rev)
		}
	})
}