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package stringbuffer
import (
"sync"
"unicode/utf16"
)
// TODO: worth exporting and using in mkwinsyscall?
// Uint16BufferSize is the buffer size in the pool, chosen somewhat arbitrarily to accommodate
// large path strings:
// MAX_PATH (260) + size of volume GUID prefix (49) + null terminator = 310.
const MinWStringCap = 310
// use *[]uint16 since []uint16 creates an extra allocation where the slice header
// is copied to heap and then referenced via pointer in the interface header that sync.Pool
// stores.
var pathPool = sync.Pool{ // if go1.18+ adds Pool[T], use that to store []uint16 directly
New: func() interface{} {
b := make([]uint16, MinWStringCap)
return &b
},
}
func newBuffer() []uint16 { return *(pathPool.Get().(*[]uint16)) }
// freeBuffer copies the slice header data, and puts a pointer to that in the pool.
// This avoids taking a pointer to the slice header in WString, which can be set to nil.
func freeBuffer(b []uint16) { pathPool.Put(&b) }
// WString is a wide string buffer ([]uint16) meant for storing UTF-16 encoded strings
// for interacting with Win32 APIs.
// Sizes are specified as uint32 and not int.
//
// It is not thread safe.
type WString struct {
// type-def allows casting to []uint16 directly, use struct to prevent that and allow adding fields in the future.
// raw buffer
b []uint16
}
// NewWString returns a [WString] allocated from a shared pool with an
// initial capacity of at least [MinWStringCap].
// Since the buffer may have been previously used, its contents are not guaranteed to be empty.
//
// The buffer should be freed via [WString.Free]
func NewWString() *WString {
return &WString{
b: newBuffer(),
}
}
func (b *WString) Free() {
if b.empty() {
return
}
freeBuffer(b.b)
b.b = nil
}
// ResizeTo grows the buffer to at least c and returns the new capacity, freeing the
// previous buffer back into pool.
func (b *WString) ResizeTo(c uint32) uint32 {
// already sufficient (or n is 0)
if c <= b.Cap() {
return b.Cap()
}
if c <= MinWStringCap {
c = MinWStringCap
}
// allocate at-least double buffer size, as is done in [bytes.Buffer] and other places
if c <= 2*b.Cap() {
c = 2 * b.Cap()
}
b2 := make([]uint16, c)
if !b.empty() {
copy(b2, b.b)
freeBuffer(b.b)
}
b.b = b2
return c
}
// Buffer returns the underlying []uint16 buffer.
func (b *WString) Buffer() []uint16 {
if b.empty() {
return nil
}
return b.b
}
// Pointer returns a pointer to the first uint16 in the buffer.
// If the [WString.Free] has already been called, the pointer will be nil.
func (b *WString) Pointer() *uint16 {
if b.empty() {
return nil
}
return &b.b[0]
}
// String returns the returns the UTF-8 encoding of the UTF-16 string in the buffer.
//
// It assumes that the data is null-terminated.
func (b *WString) String() string {
// Using [windows.UTF16ToString] would require importing "golang.org/x/sys/windows"
// and would make this code Windows-only, which makes no sense.
// So copy UTF16ToString code into here.
// If other windows-specific code is added, switch to [windows.UTF16ToString]
s := b.b
for i, v := range s {
if v == 0 {
s = s[:i]
break
}
}
return string(utf16.Decode(s))
}
// Cap returns the underlying buffer capacity.
func (b *WString) Cap() uint32 {
if b.empty() {
return 0
}
return b.cap()
}
func (b *WString) cap() uint32 { return uint32(cap(b.b)) }
func (b *WString) empty() bool { return b == nil || b.cap() == 0 }
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