File: _HeapNode.swift

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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Collections open source project
//
// Copyright (c) 2021 - 2024 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//

@usableFromInline @frozen
internal struct _HeapNode {
  @usableFromInline
  internal var offset: Int

  @usableFromInline
  internal var level: Int

  @inlinable
  internal init(offset: Int, level: Int) {
    assert(offset >= 0)
#if COLLECTIONS_INTERNAL_CHECKS
    assert(level == Self.level(forOffset: offset))
#endif
    self.offset = offset
    self.level = level
  }

  @inlinable
  internal init(offset: Int) {
    self.init(offset: offset, level: Self.level(forOffset: offset))
  }
}

extension _HeapNode: Comparable {
  @inlinable @inline(__always)
  internal static func ==(left: Self, right: Self) -> Bool {
    left.offset == right.offset
  }

  @inlinable @inline(__always)
  internal static func <(left: Self, right: Self) -> Bool {
    left.offset < right.offset
  }
}

extension _HeapNode: CustomStringConvertible {
  @usableFromInline
  internal var description: String {
    "(offset: \(offset), level: \(level))"
  }
}

extension _HeapNode {
  @inlinable @inline(__always)
  internal static func level(forOffset offset: Int) -> Int {
    (offset &+ 1)._binaryLogarithm()
  }

  @inlinable @inline(__always)
  internal static func firstNode(onLevel level: Int) -> _HeapNode {
    assert(level >= 0)
    return _HeapNode(offset: (1 &<< level) &- 1, level: level)
  }

  @inlinable @inline(__always)
  internal static func lastNode(onLevel level: Int) -> _HeapNode {
    assert(level >= 0)
    return _HeapNode(offset: (1 &<< (level &+ 1)) &- 2, level: level)
  }

  @inlinable @inline(__always)
  internal static func isMinLevel(_ level: Int) -> Bool {
    level & 0b1 == 0
  }
}

extension _HeapNode {
  /// The root node in the heap.
  @inlinable @inline(__always)
  internal static var root: Self {
    Self.init(offset: 0, level: 0)
  }

  /// The first max node in the heap. (I.e., the left child of the root.)
  @inlinable @inline(__always)
  internal static var leftMax: Self {
    Self.init(offset: 1, level: 1)
  }

  /// The second max node in the heap. (I.e., the right child of the root.)
  @inlinable @inline(__always)
  internal static var rightMax: Self {
    Self.init(offset: 2, level: 1)
  }

  @inlinable @inline(__always)
  internal var isMinLevel: Bool {
    Self.isMinLevel(level)
  }

  @inlinable @inline(__always)
  internal var isRoot: Bool {
    offset == 0
  }
}

extension _HeapNode {
  /// Returns the parent of this index, or `nil` if the index has no parent
  /// (i.e. when this is the root index).
  @inlinable @inline(__always)
  internal func parent() -> Self {
    assert(!isRoot)
    return Self(offset: (offset &- 1) / 2, level: level &- 1)
  }

  /// Returns the grandparent of this index, or `nil` if the index has
  /// no grandparent.
  @inlinable @inline(__always)
  internal func grandParent() -> Self? {
    guard offset > 2 else { return nil }
    return Self(offset: (offset &- 3) / 4, level: level &- 2)
  }

  /// Returns the left child of this node.
  @inlinable @inline(__always)
  internal func leftChild() -> Self {
    Self(offset: offset &* 2 &+ 1, level: level &+ 1)
  }

  /// Returns the right child of this node.
  @inlinable @inline(__always)
  internal func rightChild() -> Self {
    Self(offset: offset &* 2 &+ 2, level: level &+ 1)
  }

  @inlinable @inline(__always)
  internal func firstGrandchild() -> Self {
    Self(offset: offset &* 4 &+ 3, level: level &+ 2)
  }

  @inlinable @inline(__always)
  internal func lastGrandchild() -> Self {
    Self(offset: offset &* 4 &+ 6, level: level &+ 2)
  }

  @inlinable
  internal static func allNodes(
    onLevel level: Int,
    limit: Int
  ) -> ClosedRange<Self>? {
    let first = Self.firstNode(onLevel: level)
    guard first.offset < limit else { return nil }
    var last = self.lastNode(onLevel: level)
    if last.offset >= limit {
      last.offset = limit &- 1
    }
    return ClosedRange(uncheckedBounds: (first, last))
  }
}

extension ClosedRange where Bound == _HeapNode {
  @inlinable @inline(__always)
  internal func _forEach(_ body: (_HeapNode) -> Void) {
    assert(
      isEmpty || _HeapNode.level(forOffset: upperBound.offset) == lowerBound.level)
    var node = self.lowerBound
    while node.offset <= self.upperBound.offset {
      body(node)
      node.offset &+= 1
    }
  }
}