// before
elapsed := time.Now().Sub(start)
// after
elapsed := time.Since(start)

// before
remaining := deadline.Sub(time.Now())
// after
remaining := time.Until(deadline)

// before
s := ""
for _, item := range items {
    s += item
}
// after
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}
s := b.String()

// before
if err == io.EOF {
    return
}
// after
if errors.Is(err, io.EOF) {
    return
}

// before
//go:build linux && amd64
// +build linux,amd64

package foo
// after
//go:build linux && amd64

package foo

// before — pointer arithmetic via uintptr
p2 := unsafe.Pointer(uintptr(ptr) + uintptr(offset))
// after
p2 := unsafe.Add(ptr, offset)

// before — building a slice from a base pointer
s := (*[1 << 30]byte)(unsafe.Pointer(p))[:n:n]
// after
s := unsafe.Slice(p, n)

// before
func decode(v interface{}) error { ... }
// after
func decode(v any) error { ... }

// before
if i := strings.Index(s, "="); i >= 0 {
    key, val := s[:i], s[i+1:]
}
// after
if key, val, found := strings.Cut(s, "="); found {
    ...
}

// before
if i := bytes.Index(b, sep); i >= 0 {
    before, after := b[:i], b[i+len(sep):]
}
// after
before, after, found := bytes.Cut(b, sep)

// before
buf = append(buf, fmt.Sprintf("x=%d", x)...)
// after
buf = fmt.Appendf(buf, "x=%d", x)

// before
var ready int32
atomic.StoreInt32(&ready, 1)
if atomic.LoadInt32(&ready) == 1 { ... }

// after
var ready atomic.Int32
ready.Store(1)
if ready.Load() == 1 { ... }

// before
var cache atomic.Value
cache.Store(&Config{})
cfg := cache.Load().(*Config)

// after
var cache atomic.Pointer[Config]
cache.Store(&Config{})
cfg := cache.Load()

// before
s2 := string([]byte(s)) // force copy
// after
s2 := strings.Clone(s)

// before
dst := make([]byte, len(src))
copy(dst, src)
// after
dst := bytes.Clone(src)

// before
if strings.HasPrefix(s, "pre_") {
    s = s[len("pre_"):]
}
// after
if rest, ok := strings.CutPrefix(s, "pre_"); ok {
    s = rest
}

// before
if strings.HasSuffix(s, ".txt") {
    s = s[:len(s)-len(".txt")]
}
// after
if rest, ok := strings.CutSuffix(s, ".txt"); ok {
    s = rest
}

// before
return fmt.Errorf("load config: %w: %w", err1, err2)
// after
return errors.Join(fmt.Errorf("load config"), err1, err2)

// before
ctx, cancel := context.WithCancel(parent)
// ... somewhere ...
cancel()

// after
ctx, cancel := context.WithCancelCause(parent)
cancel(ErrShutdown)
// caller: context.Cause(ctx) → ErrShutdown

// before
lo := a
if b < lo {
    lo = b
}
// after
lo := min(a, b)

// before
if x < 0 {
    x = 0
}
if x > 100 {
    x = 100
}
// after
x = min(max(x, 0), 100)

// before
for k := range m {
    delete(m, k)
}
// after
clear(m)

// before
for i := range s {
    s[i] = 0
}
// after
clear(s)

// before → after: slices.Contains(items, target)
found := false
for _, x := range items {
    if x == target {
        found = true
        break
    }
}

// before → after: slices.Index(items, target)
for i, x := range items {
    if x == target {
        return i
    }
}
return -1

// before → after: slices.SortFunc(items, cmp.Compare)
sort.Slice(items, func(i, j int) bool { return items[i] < items[j] })

// before → after: slices.Max(items) / slices.Min(items)
max := items[0]
for _, v := range items[1:] {
    if v > max {
        max = v
    }
}

// before → after: slices.Reverse(s)
for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
    s[i], s[j] = s[j], s[i]
}

// before → after: slices.Compact(s)
i := 0
for j := 1; j < len(s); j++ {
    if s[j] != s[i] {
        i++
        s[i] = s[j]
    }
}
s = s[:i+1]

// before → after: slices.Clip(s)
s = s[:len(s):len(s)]

// before → after: slices.Clone(src)
dst := make([]T, len(src))
copy(dst, src)

// before — element removal (classic aliasing/leak footgun)
s = append(s[:i], s[i+1:]...)
// after
s = slices.Delete(s, i, i+1)

// before — insert at index i
s = append(s[:i], append([]T{v}, s[i:]...)...)
// after
s = slices.Insert(s, i, v)

// before
if reflect.DeepEqual(got, want) { ... }   // got, want are []string
// after
if slices.Equal(got, want) { ... }

// before → after: maps.Clone(m)
dst := make(map[K]V)
for k, v := range src {
    dst[k] = v
}

// before → after: maps.Copy(dst, src)
for k, v := range src {
    dst[k] = v
}

// before → after: maps.DeleteFunc(m, func(k K, v V) bool { return v == 0 })
for k, v := range m {
    if v == 0 {
        delete(m, k)
    }
}

// before
all := append(append([]int(nil), a...), b...)
// after
all := slices.Concat(a, b)

// before — three-way concat
merged := append(append(append([]string(nil), x...), y...), z...)
// after
merged := slices.Concat(x, y, z)

// before
var once sync.Once
func init() { once.Do(func() { setup() }) }

// after
var initOnce = sync.OnceFunc(func() { setup() })

// before
var once sync.Once
var cfg *Config
func getConfig() *Config {
    once.Do(func() { cfg = loadConfig() })
    return cfg
}
// after
var getConfig = sync.OnceValue(func() *Config { return loadConfig() })

// before
go func() {
    <-ctx.Done()
    conn.Close()
}()
// after
stop := context.AfterFunc(ctx, func() { conn.Close() })

// before
ctx, cancel := context.WithTimeout(parent, 5*time.Second)
// after
ctx, cancel := context.WithTimeoutCause(parent, 5*time.Second, ErrTimeout)

// before
for i := 0; i < len(items); i++ {
    process(i, items[i])
}
// after
for i := range len(items) {
    process(i, items[i])
}

// before
for i := 0; i < n; i++ {
    doWork()
}
// after
for range n {
    doWork()
}

// before
for _, x := range items {
    x := x          // capture for goroutine
    go func() { use(x) }()
}
// after
for _, x := range items {
    go func() { use(x) }()
}

// before
name := os.Getenv("NAME")
if name == "" {
    name = os.Getenv("USER")
}
if name == "" {
    name = "anonymous"
}
// after
name := cmp.Or(os.Getenv("NAME"), os.Getenv("USER"), "anonymous")

// before
t := reflect.TypeOf((*MyType)(nil)).Elem()
// after
t := reflect.TypeFor[MyType]()

// before
mux.HandleFunc("/api/", func(w http.ResponseWriter, r *http.Request) {
    id := strings.TrimPrefix(r.URL.Path, "/api/")
    ...
})
// after
mux.HandleFunc("GET /api/{id}", func(w http.ResponseWriter, r *http.Request) {
    id := r.PathValue("id")
    ...
})

// before
import "math/rand"
n := rand.Intn(100)
// after
import "math/rand/v2"
n := rand.IntN(100)

// before — type-specific bound
d := time.Duration(rand.Int63n(int64(max)))
// after — generic N works for any integer type
d := rand.N(max)

// before — callback-based iteration
func (t *Tree) Each(fn func(v int)) {
    for _, v := range t.values {
        fn(v)
    }
}
// caller: t.Each(func(v int) { use(v) })

// after — standard iterator, usable with range
func (t *Tree) All() iter.Seq[int] {
    return func(yield func(int) bool) {
        for _, v := range t.values {
            if !yield(v) { return }
        }
    }
}
// caller: for v := range t.All() { use(v) }

// before
var keys []string
for k := range m {
    keys = append(keys, k)
}
// after
keys := slices.Collect(maps.Keys(m))

// before
var vals []int
for _, v := range m {
    vals = append(vals, v)
}
// after
vals := slices.Collect(maps.Values(m))

// before
for _, part := range strings.Split(line, ",") {
    process(part)
}
// after
for part := range strings.SplitSeq(line, ",") {
    process(part)
}

// before
for _, part := range bytes.Split(data, sep) {
    process(part)
}
// after
for part := range bytes.SplitSeq(data, sep) {
    process(part)
}

// before
for i := len(items) - 1; i >= 0; i-- {
    process(items[i])
}
// after
for _, v := range slices.Backward(items) {
    process(v)
}

// before — index still needed
for i := len(items) - 1; i >= 0; i-- {
    fmt.Println(i, items[i])
}
// after
for i, v := range slices.Backward(items) {
    fmt.Println(i, v)
}

// before
func TestFetch(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    result := fetch(ctx)
}
// after
func TestFetch(t *testing.T) {
    result := fetch(t.Context())
}

// before
sc := bufio.NewScanner(strings.NewReader(s))
for sc.Scan() {
    process(sc.Text())
}
// after
for line := range strings.Lines(s) {
    process(strings.TrimSuffix(line, "\n"))
}

// before — hand-rolled path-traversal guard
p := filepath.Join(base, name)
if !strings.HasPrefix(filepath.Clean(p), filepath.Clean(base)+string(os.PathSeparator)) {
    return errUnsafePath
}
f, err := os.Open(p)
// after — the OS enforces the boundary
root, err := os.OpenRoot(base)
if err != nil { return err }
defer root.Close()
f, err := root.Open(name) // symlinks/".." escaping base are rejected

// before
type Config struct {
    Timeout time.Duration `json:"timeout,omitempty"`
    Labels  []string      `json:"labels,omitempty"`
}
// after
type Config struct {
    Timeout time.Duration `json:"timeout,omitzero"`
    Labels  []string      `json:"labels,omitzero"`
}

// before
func BenchmarkHash(b *testing.B) {
    for i := 0; i < b.N; i++ {
        hash(input)
    }
}
// after
func BenchmarkHash(b *testing.B) {
    for b.Loop() {
        hash(input)
    }
}

// before
var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func(item Item) {
        defer wg.Done()
        process(item)
    }(item)
}
wg.Wait()
// after
var wg sync.WaitGroup
for _, item := range items {
    wg.Go(func() { process(item) })
}
wg.Wait()

// before
timeout := 30
debug := true
cfg := Config{
    Timeout: &timeout,
    Debug:   &debug,
}
// after
cfg := Config{
    Timeout: new(30),
    Debug:   new(true),
}

// before
func ptr[T any](v T) *T { return &v }
cfg := Config{Count: ptr(10)}

// after
cfg := Config{Count: new(10)}

// before
var pathErr *os.PathError
if errors.As(err, &pathErr) {
    log.Println(pathErr.Path)
}
// after
if pathErr, ok := errors.AsType[*os.PathError](err); ok {
    log.Println(pathErr.Path)
}

type Base struct {
    ID   int
    Name string
}
type User struct {
    Base
    Age int
}

// before
u := User{
    Base: Base{ID: 1, Name: "alice"},
    Age:  30,
}
// after
u := User{
    ID:   1,
    Name: "alice",
    Age:  30,
}

## Modernization Summary

| File | Transformations |
|---|---|
| main.go | any, strings.Cut, min/max (2 occurrences) |
| pkg/handler.go | range over int (3), slices.Contains, t.Context() |
| pkg/util.go | new(expr) (1), errors.AsType → errors.Is |

**3 files modified, 10 transformations applied**

Skipped (requires higher Go version):
- new(expr): requires go 1.26 (project is go 1.24)
- WaitGroup.Go: requires go 1.25 (project is go 1.24)

go fix ./...