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bytes_builder.dart
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bytes_builder.dart
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// Copyright (c) 2013, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
part of dart._internal;
/// Builds a list of bytes, allowing bytes and lists of bytes to be added at the
/// end.
///
/// Used to efficiently collect bytes and lists of bytes.
abstract interface class BytesBuilder {
/// Construct a new empty [BytesBuilder].
///
/// If [copy] is true (the default), the created builder is a *copying*
/// builder. A copying builder maintains its own internal buffer and copies
/// the bytes added to it eagerly.
///
/// If [copy] set to false, the created builder assumes that lists added
/// to it will not change.
/// Any [Uint8List] added using [add] is kept until
/// [toBytes] or [takeBytes] is called,
/// and only then are their contents copied.
/// A non-[Uint8List] may be copied eagerly.
/// If only a single [Uint8List] is added to the builder,
/// that list is returned by [toBytes] or [takeBytes] directly, without any copying.
/// A list added to a non-copying builder *should not* change its content
/// after being added, and it *must not* change its length after being added.
/// (Normal [Uint8List]s are fixed length lists, but growing lists implementing
/// [Uint8List] exist.)
factory BytesBuilder({bool copy = true}) =>
copy ? _CopyingBytesBuilder() : _BytesBuilder();
/// Appends [bytes] to the current contents of this builder.
///
/// Each value of [bytes] will be truncated
/// to an 8-bit value in the range 0 .. 255.
void add(List<int> bytes);
/// Appends [byte] to the current contents of this builder.
///
/// The [byte] will be truncated to an 8-bit value in the range 0 .. 255.
void addByte(int byte);
/// Returns the bytes currently contained in this builder and clears it.
///
/// The returned list may be a view of a larger buffer.
Uint8List takeBytes();
/// Returns a copy of the current byte contents of this builder.
///
/// Leaves the contents of this builder intact.
Uint8List toBytes();
/// The number of bytes in this builder.
int get length;
/// Whether the buffer is empty.
bool get isEmpty;
/// Whether the buffer is non-empty.
bool get isNotEmpty;
/// Clears the contents of this builder.
///
/// The current contents are discarded and this builder becomes empty.
void clear();
}
/// A [BytesBuilder] which appends bytes to a growing internal buffer.
class _CopyingBytesBuilder implements BytesBuilder {
/// Initial size of internal buffer.
static const int _initSize = 1024;
/// Reusable empty [Uint8List].
///
/// Safe for reuse because a fixed-length empty list is immutable.
static final _emptyList = Uint8List(0);
/// Current count of bytes written to buffer.
int _length = 0;
/// Internal buffer accumulating bytes.
///
/// Will grow as necessary
Uint8List _buffer;
_CopyingBytesBuilder() : _buffer = _emptyList;
void add(List<int> bytes) {
int byteCount = bytes.length;
if (byteCount == 0) return;
int required = _length + byteCount;
if (_buffer.length < required) {
_grow(required);
}
assert(_buffer.length >= required);
if (bytes is Uint8List) {
_buffer.setRange(_length, required, bytes);
} else {
for (int i = 0; i < byteCount; i++) {
_buffer[_length + i] = bytes[i];
}
}
_length = required;
}
void addByte(int byte) {
if (_buffer.length == _length) {
// The grow algorithm always at least doubles.
// If we added one to _length it would quadruple unnecessarily.
_grow(_length);
}
assert(_buffer.length > _length);
_buffer[_length] = byte;
_length++;
}
void _grow(int required) {
// We will create a list in the range of 2-4 times larger than
// required.
int newSize = required * 2;
if (newSize < _initSize) {
newSize = _initSize;
} else {
newSize = _pow2roundup(newSize);
}
var newBuffer = Uint8List(newSize);
newBuffer.setRange(0, _buffer.length, _buffer);
_buffer = newBuffer;
}
Uint8List takeBytes() {
if (_length == 0) return _emptyList;
var buffer = Uint8List.view(_buffer.buffer, _buffer.offsetInBytes, _length);
_clear();
return buffer;
}
Uint8List toBytes() {
if (_length == 0) return _emptyList;
return Uint8List.fromList(
Uint8List.view(_buffer.buffer, _buffer.offsetInBytes, _length));
}
int get length => _length;
bool get isEmpty => _length == 0;
bool get isNotEmpty => _length != 0;
void clear() {
_clear();
}
void _clear() {
_length = 0;
_buffer = _emptyList;
}
/// Rounds numbers <= 2^32 up to the nearest power of 2.
static int _pow2roundup(int x) {
assert(x > 0);
--x;
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
return x + 1;
}
}
/// A non-copying [BytesBuilder].
///
/// Accumulates lists of integers and lazily builds
/// a collected list with all the bytes when requested.
class _BytesBuilder implements BytesBuilder {
int _length = 0;
final List<Uint8List> _chunks = [];
void add(List<int> bytes) {
Uint8List typedBytes;
if (bytes is Uint8List) {
typedBytes = bytes;
} else {
typedBytes = Uint8List.fromList(bytes);
}
_chunks.add(typedBytes);
_length += typedBytes.length;
}
void addByte(int byte) {
// TODO(lrn): Optimize repeated `addByte` calls.
_chunks.add(Uint8List(1)..[0] = byte);
_length++;
}
Uint8List takeBytes() {
if (_length == 0) return _CopyingBytesBuilder._emptyList;
if (_chunks.length == 1) {
var buffer = _chunks[0];
_clear();
return buffer;
}
var buffer = Uint8List(_length);
int offset = 0;
for (var chunk in _chunks) {
buffer.setRange(offset, offset + chunk.length, chunk);
offset += chunk.length;
}
_clear();
return buffer;
}
Uint8List toBytes() {
if (_length == 0) return _CopyingBytesBuilder._emptyList;
var buffer = Uint8List(_length);
int offset = 0;
for (var chunk in _chunks) {
buffer.setRange(offset, offset + chunk.length, chunk);
offset += chunk.length;
}
return buffer;
}
int get length => _length;
bool get isEmpty => _length == 0;
bool get isNotEmpty => _length != 0;
void clear() {
_clear();
}
void _clear() {
_length = 0;
_chunks.clear();
}
}