swift-foundation/Sources/FoundationEssentials/Predicate/KeyPath+Inspection.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2023 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
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

extension UInt32 {
    private static var KEYPATH_HEADER_BUFFER_SIZE_MASK: UInt32 { 0x00FF_FFFF }
    
    private static var COMPONENT_HEADER_KIND_MASK: UInt32 { 0x7F00_0000 }
    private static var COMPONENT_HEADER_PAYLOAD_MASK: UInt32 { 0x00FF_FFFF }
    
    private static var COMPUTED_COMPONENT_PAYLOAD_ARGUMENTS_MASK: UInt32 { 0x0008_0000 }
    private static var COMPUTED_COMPONENT_PAYLOAD_SETTABLE_MASK: UInt32 { 0x0040_0000 }
    
    fileprivate var _keyPathHeader_bufferSize: Int {
        Int(self & Self.KEYPATH_HEADER_BUFFER_SIZE_MASK)
    }
    
    fileprivate var _keyPathComponentHeader_kind: Int {
        Int((self & Self.COMPONENT_HEADER_KIND_MASK) >> 24)
    }
    
    fileprivate var _keyPathComponentHeader_payload: UInt32 {
        self & Self.COMPONENT_HEADER_PAYLOAD_MASK
    }
    
    fileprivate var _keyPathComponentHeader_computedHasArguments: Bool {
        (_keyPathComponentHeader_payload & Self.COMPUTED_COMPONENT_PAYLOAD_ARGUMENTS_MASK) != 0
    }
    
    fileprivate var _keyPathComponentHeader_computedIsSettable: Bool {
        (_keyPathComponentHeader_payload & Self.COMPUTED_COMPONENT_PAYLOAD_SETTABLE_MASK) != 0
    }
}

private func _keyPathOffset<T>(_ root: T.Type, _ keyPath: AnyKeyPath) -> Int? {
    MemoryLayout<T>.offset(of: keyPath as! PartialKeyPath<T>)
}

extension AnyKeyPath {
    private static var WORD_SIZE: Int { MemoryLayout<Int>.size }
    
    func _validateForPredicateUsage(restrictArguments: Bool = false) {
        var ptr = unsafeBitCast(self, to: UnsafeRawPointer.self)
        ptr = ptr.advanced(by: Self.WORD_SIZE * 3) // skip isa, type metadata, and KVC string pointers
        let header = ptr.load(as: UInt32.self)
        ptr = ptr.advanced(by: Self.WORD_SIZE)
        let firstComponentHeader = ptr.load(as: UInt32.self)
        switch firstComponentHeader._keyPathComponentHeader_kind {
        case 1: // struct/tuple/self stored property
            fallthrough
        case 3: // class stored property
            // Key paths to stored properties are only single-component if MemoryLayout.offset(of:) returns an offset
            func project<T>(_: T.Type) -> Bool {
                _keyPathOffset(T.self, self) == nil
            }
            if _openExistential(Self.rootType, do: project) {
                fatalError("Predicate does not support keypaths with multiple components")
            }
        case 2: // computed
            var componentWords = 3
            if firstComponentHeader._keyPathComponentHeader_computedIsSettable {
                componentWords += 1
            }
            if firstComponentHeader._keyPathComponentHeader_computedHasArguments {
                if restrictArguments {
                    fatalError("Predicate does not support keypaths with arguments")
                }
                let capturesSize = ptr.advanced(by: Self.WORD_SIZE * componentWords).load(as: UInt.self)
                componentWords += 2 + (Int(capturesSize) / Self.WORD_SIZE)
            }
            if header._keyPathHeader_bufferSize > (Self.WORD_SIZE * componentWords) {
                fatalError("Predicate does not support keypaths with multiple components")
            }
        case 4: // optional chain
            fatalError("Predicate does not support keypaths with optional chaining/unwrapping")
        default: // unknown keypath component
            fatalError("Predicate does not support this type of keypath (\(firstComponentHeader._keyPathComponentHeader_kind))")
        }
    }
}