ARK-OS/arkos/arkrt/swift-system/System/FilePath/FilePathComponentView.swift

/*
 This source file is part of the Swift System open source project

 Copyright (c) 2020 Apple Inc. and the Swift System project authors
 Licensed under Apache License v2.0 with Runtime Library Exception

 See https://swift.org/LICENSE.txt for license information
*/

// MARK: - API

@available(System 0.0.2, *)
extension FilePath {
  /// A bidirectional, range replaceable collection of the non-root components
  /// that make up a file path.
  ///
  /// ComponentView provides access to standard `BidirectionalCollection`
  /// algorithms for accessing components from the front or back, as well as
  /// standard `RangeReplaceableCollection` algorithms for modifying the
  /// file path using component or range of components granularity.
  ///
  /// Example:
  ///
  ///     var path: FilePath = "/./home/./username/scripts/./tree"
  ///     let scriptIdx = path.components.lastIndex(of: "scripts")!
  ///     path.components.insert("bin", at: scriptIdx)
  ///     // path is "/./home/./username/bin/scripts/./tree"
  ///
  ///     path.components.removeAll { $0.kind == .currentDirectory }
  ///     // path is "/home/username/bin/scripts/tree"
  @available(System 0.0.2, *)
  public struct ComponentView: Sendable {
    internal var _path: FilePath
    internal var _start: SystemString.Index

    internal init(_ path: FilePath) {
      self._path = path
      self._start = path._relativeStart
      _invariantCheck()
    }
  }

  /// View the non-root components that make up this path.
  public var components: ComponentView {
    __consuming get { ComponentView(self) }
    _modify {
      // RRC's empty init means that we can't guarantee that the yielded
      // view will restore our root. So copy it out first.
      //
      // TODO(perf): Small-form root (especially on Unix). Have Root
      // always copy out (not worth ref counting). Make sure that we're
      // not needlessly sliding values around or triggering a COW
      let rootStr = self.root?._systemString ?? SystemString()
      var comp = ComponentView(self)
      self = FilePath()
      defer {
        self = comp._path
        if root?._slice.elementsEqual(rootStr) != true {
          self.root = Root(rootStr)
        }
      }
      yield &comp
    }
  }
}

@available(System 0.0.2, *)
extension FilePath.ComponentView: BidirectionalCollection {
  public typealias Element = FilePath.Component

  @available(System 0.0.2, *)
  public struct Index: Sendable, Comparable, Hashable {
    internal typealias Storage = SystemString.Index

    internal var _storage: Storage

    public static func < (lhs: Self, rhs: Self) -> Bool {
      lhs._storage < rhs._storage
    }

    fileprivate init(_ idx: Storage) {
      self._storage = idx
    }
  }

  public var startIndex: Index { Index(_start) }
  public var endIndex: Index { Index(_path._storage.endIndex) }

  public func index(after i: Index) -> Index {
    return Index(_path._parseComponent(startingAt: i._storage).nextStart)
  }

  public func index(before i: Index) -> Index {
    Index(_path._parseComponent(priorTo: i._storage).lowerBound)
  }

  public subscript(position: Index) -> FilePath.Component {
    let end = _path._parseComponent(startingAt: position._storage).componentEnd
    return FilePath.Component(_path, position._storage ..< end)
  }
}

@available(System 0.0.2, *)
extension FilePath.ComponentView: RangeReplaceableCollection {
  public init() {
    self.init(FilePath())
  }

  // TODO(perf): We probably want to have concrete overrides or generic
  // specializations taking FP.ComponentView and
  // FP.ComponentView.SubSequence because we
  // can just memcpy in those cases. We
  // probably want to do that for all RRC operations.

  public mutating func replaceSubrange<C>(
    _ subrange: Range<Index>, with newElements: C
  ) where C : Collection, Self.Element == C.Element {
    defer {
      _path._invariantCheck()
      _invariantCheck()
    }
    if isEmpty {
      _path = FilePath(root: _path.root, newElements)
      return
    }
    let range = subrange.lowerBound._storage ..< subrange.upperBound._storage
    if newElements.isEmpty {
      let fromEnd = subrange.upperBound == endIndex
      _path._storage.removeSubrange(range)
      if fromEnd {
        _path._removeTrailingSeparator()
      }
      return
    }

    // TODO(perf): Avoid extra allocation by sliding elements down and
    // filling in the bytes ourselves.

    // If we're inserting at the end, we need a leading separator.
    var str = SystemString()
    let atEnd = subrange.lowerBound == endIndex
    if atEnd {
      str.append(platformSeparator)
    }
    str.appendComponents(components: newElements)
    if !atEnd {
      str.append(platformSeparator)
    }
    _path._storage.replaceSubrange(range, with: str)
  }
}

@available(System 0.0.2, *)
extension FilePath {
  /// Create a file path from a root and a collection of components.
  public init<C: Collection>(
    root: Root?, _ components: C
  ) where C.Element == Component {
    var str = root?._systemString ?? SystemString()
    str.appendComponents(components: components)
    self.init(str)
  }

  /// Create a file path from a root and any number of components.
  public init(root: Root?, components: Component...) {
    self.init(root: root, components)
  }

  /// Create a file path from an optional root and a slice of another path's
  /// components.
  public init(root: Root?, _ components: ComponentView.SubSequence) {
    var str = root?._systemString ?? SystemString()
    let (start, end) =
      (components.startIndex._storage, components.endIndex._storage)
    str.append(contentsOf: components.base._slice[start..<end])
    self.init(str)
  }
}

// MARK: - Internals

@available(System 0.0.2, *)
extension FilePath.ComponentView: _PathSlice {
  internal var _range: Range<SystemString.Index> {
    _start ..< _path._storage.endIndex
  }

  internal init(_ str: SystemString) {
    fatalError("TODO: consider dropping proto req")
  }
}

// MARK: - Invariants

@available(System 0.0.2, *)
extension FilePath.ComponentView {
  internal func _invariantCheck() {
    #if DEBUG
    if isEmpty {
      precondition(_path.isEmpty == (_path.root == nil))
      return
    }

    // If path has a root,
    if _path.root != nil {
      precondition(first!._slice.startIndex > _path._storage.startIndex)
      precondition(first!._slice.startIndex == _path._relativeStart)
    }

    self.forEach { $0._invariantCheck() }

    if let base = last {
      precondition(base._slice.endIndex == _path._storage.endIndex)
    }

    precondition(FilePath(root: _path.root, self) == _path)
    #endif // DEBUG
  }
}