Swift 里字符串（十）修改字符串
2024-09-02 17:56:28
以append操作为例
public mutating func append(_ other: String) {
if self.isEmpty && !_guts.hasNativeStorage {
self = other
return
}
self._guts.append(other._guts)
}
_StringGuts 做了实际的工作
下面是实际进行append的地方
internal mutating func append(_ slicedOther: _StringGutsSlice) {
defer { self._invariantCheck() }
if self.isSmall && slicedOther._guts.isSmall {
// TODO: In-register slicing
let smolSelf = self.asSmall
if let smol = slicedOther.withFastUTF8({ otherUTF8 in
return _SmallString(smolSelf, appending: _SmallString(otherUTF8)!)
}) {
self = _StringGuts(smol)
return
}
}
// 进行 copy-on-write 操作
prepareForAppendInPlace(otherUTF8Count: slicedOther.utf8Count)
if slicedOther.isFastUTF8 {
let otherIsASCII = slicedOther.isASCII
slicedOther.withFastUTF8 { otherUTF8 in
self.appendInPlace(otherUTF8, isASCII: otherIsASCII)
}
return
}
_foreignAppendInPlace(slicedOther)
}
首先判断是否可以安装小字符串处理，然后是重头戏。
我们以实际存储的是native string为例分析。
分配内存操作
private mutating func prepareForAppendInPlace(
otherUTF8Count otherCount: Int
) {
defer {
_internalInvariant(self.uniqueNativeUnusedCapacity != nil,
"growth should produce uniqueness")
_internalInvariant(self.uniqueNativeUnusedCapacity! >= otherCount,
"growth should produce enough capacity")
}
// See if we can accomodate without growing or copying. If we have
// sufficient capacity, we do not need to grow, and we can skip the copy if
// unique. Otherwise, growth is required.
let sufficientCapacity: Bool
if let unused = self.nativeUnusedCapacity, unused >= otherCount {
sufficientCapacity = true
} else {
sufficientCapacity = false
}
//naivestorage的字符串，一定不是UniqueNative的
if self.isUniqueNative && sufficientCapacity {
return
}
let totalCount = self.utf8Count + otherCount
// Non-unique storage: just make a copy of the appropriate size, otherwise
// grow like an array.
let growthTarget: Int
if sufficientCapacity {
growthTarget = totalCount
} else {
growthTarget = Swift.max(
totalCount, _growArrayCapacity(nativeCapacity ?? 0))
}
//最后会走到这里来
self.grow(growthTarget)
}
一定会调用grow函数。
internal mutating func grow(_ n: Int) {
defer { self._invariantCheck() }
_internalInvariant(
self.uniqueNativeCapacity == nil || self.uniqueNativeCapacity! < n)
let growthTarget = Swift.max(n, (self.uniqueNativeCapacity ?? 0) * 2)
if _fastPath(isFastUTF8) {
let isASCII = self.isASCII
let storage = self.withFastUTF8 {
// 分配了内存
__StringStorage.create(
initializingFrom: $0, capacity: growthTarget, isASCII: isASCII)
}
self = _StringGuts(storage)
return
}
_foreignGrow(growthTarget)
}
在grow函数里，分配了内存
在分配好的内存里进行内存copy
internal mutating func appendInPlace(
_ other: UnsafeBufferPointer<UInt8>, isASCII: Bool
) {
self._object.nativeStorage.appendInPlace(other, isASCII: isASCII)
// We re-initialize from the modified storage to pick up new count, flags,
// etc.
self = _StringGuts(self._object.nativeStorage)
}
@_effects(releasenone)
internal func appendInPlace(
_ other: UnsafeBufferPointer<UInt8>, isASCII: Bool
) {
_internalInvariant(self.capacity >= other.count)
let srcAddr = other.baseAddress._unsafelyUnwrappedUnchecked
let srcCount = other.count
self.mutableEnd.initialize(from: srcAddr, count: srcCount)
_postAppendAdjust(appendedCount: srcCount, appendedIsASCII: isASCII)
}
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