[Low level] Store SourceLanguage properties outside of the main structure use a new bit-set type for "sets" of languages

Sources/DocCCommon/CMakeLists.txt

@@ -8,4 +8,6 @@ See https://swift.org/LICENSE.txt for license information
 #]]
 
 add_library(DocCCommon STATIC
+  FixedSizeBitSet.swift
+  Mutex.swift
   SourceLanguage.swift)

Sources/DocCCommon/FixedSizeBitSet.swift

@@ -0,0 +1,294 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2024-2025 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 Swift project authors
+*/
+
+/// A fixed size bit set, used for storing very small amounts of small integer values.
+///
+/// This type can only store values that are `0 ..< Storage.bitWidth` which makes it _unsuitable_ as a general purpose set-algebra type.
+/// However, in specialized cases where the caller can guarantee that all values are in bounds, this type can offer a memory and performance improvement.
+package struct _FixedSizeBitSet<Storage: FixedWidthInteger & Sendable>: Sendable {
+    package typealias Element = Int
+
+    package init() {}
+
+    @usableFromInline
+    private(set) var storage: Storage = 0
+
+    @inlinable
+    init(storage: Storage) {
+        self.storage = storage
+    }
+}
+
+// MARK: Set Algebra
+
+extension _FixedSizeBitSet: SetAlgebra {
+    private static func mask(_ number: Int) -> Storage {
+        precondition(number < Storage.bitWidth, "Number \(number) is out of bounds (0..<\(Storage.bitWidth))")
+        return 1 &<< number
+    }
+
+    @inlinable
+    @discardableResult
+    mutating package func insert(_ member: Int) -> (inserted: Bool, memberAfterInsert: Int) {
+        let newStorage = storage | _FixedSizeBitSet.mask(member)
+        defer {
+            storage = newStorage
+        }
+        return (newStorage != storage, member)
+    }
+
+    @inlinable
+    @discardableResult
+    mutating package func remove(_ member: Int) -> Int? {
+        let newStorage = storage & ~_FixedSizeBitSet.mask(member)
+        defer {
+            storage = newStorage
+        }
+        return newStorage != storage ? member : nil
+    }
+
+    @inlinable
+    @discardableResult
+    mutating package func update(with member: Int) -> Int? {
+        let (inserted, _) = insert(member)
+        return inserted ? nil : member
+    }
+
+    @inlinable
+    package func contains(_ member: Int) -> Bool {
+        storage & _FixedSizeBitSet.mask(member) != 0
+    }
+
+    @inlinable
+    package func isSuperset(of other: Self) -> Bool {
+        (storage & other.storage) == other.storage
+    }
+
+    @inlinable
+    package func union(_ other: Self) -> Self {
+        .init(storage: storage | other.storage)
+    }
+
+    @inlinable
+    package func intersection(_ other: Self) -> Self {
+        .init(storage: storage & other.storage)
+    }
+
+    @inlinable
+    package func symmetricDifference(_ other: Self) -> Self {
+        .init(storage: storage ^ other.storage)
+    }
+
+    @inlinable
+    mutating package func formUnion(_ other: Self) {
+        storage |= other.storage
+    }
+
+    @inlinable
+    mutating package func formIntersection(_ other: Self) {
+        storage &= other.storage
+    }
+
+    @inlinable
+    mutating package func formSymmetricDifference(_ other: Self) {
+        storage ^= other.storage
+    }
+
+    @inlinable
+    package var isEmpty: Bool {
+        storage == 0
+    }
+}
+
+// MARK: Sequence
+
+extension _FixedSizeBitSet: Sequence {
+    @inlinable
+    package func makeIterator() -> some IteratorProtocol<Int> {
+        _Iterator(set: self)
+    }
+
+    private struct _Iterator: IteratorProtocol {
+        typealias Element = Int
+
+        private var storage: Storage
+        private var current: Int = -1
+
+        @inlinable
+        init(set: _FixedSizeBitSet) {
+            self.storage = set.storage
+        }
+
+        @inlinable
+        mutating func next() -> Int? {
+            guard storage != 0 else {
+                return nil
+            }
+            // If the set is somewhat sparse, we can find the next element faster by shifting to the next value.
+            // This saves needing to do `contains()` checks for all the numbers since the previous element.
+            let amountToShift = storage.trailingZeroBitCount + 1
+            storage &>>= amountToShift
+
+            current &+= amountToShift
+            return current
+        }
+    }
+}
+
+// MARK: Collection
+
+extension _FixedSizeBitSet: Collection {
+    // Collection conformance requires an `Index` type, that the collection can advance, and `startIndex` and `endIndex` accessors that follow certain requirements.
+    //
+    // For this design, as a hidden implementation detail, the `Index` holds the bit offset to the element.
+
+    @inlinable
+    package subscript(position: Index) -> Int {
+        precondition(position.bit < Storage.bitWidth, "Index \(position.bit) out of bounds")
+        // Because the index stores the bit offset, which is also the value, we can simply return the value without accessing the storage.
+        return Int(position.bit)
+    }
+
+    package struct Index: Comparable {
+        // The bit offset into the storage to the value
+        fileprivate var bit: UInt8
+
+        package static func < (lhs: Self, rhs: Self) -> Bool {
+            lhs.bit < rhs.bit
+        }
+    }
+
+    @inlinable
+    package var startIndex: Index {
+        // This is the index (bit offset) to the smallest value in the bit set.
+        Index(bit: UInt8(storage.trailingZeroBitCount))
+    }
+
+    @inlinable
+    package var endIndex: Index {
+        // For a valid collection, the end index is required to be _exactly_ one past the last in-bounds index, meaning; `index(after: LAST_IN-BOUNDS_INDEX)`
+        // If the collection implementation doesn't satisfy this requirement, it will have an infinitely long `indices` collection.
+        // This either results in infinite implementations or hits internal preconditions in other Swift types that that collection has more elements than its `count`.
+
+        // See `index(after:)` below for explanation of how the index after is calculated.
+        let lastInBoundsBit = UInt8(Storage.bitWidth &- storage.leadingZeroBitCount)
+        return Index(bit: lastInBoundsBit &+ UInt8((storage &>> lastInBoundsBit).trailingZeroBitCount))
+    }
+
+    @inlinable
+    package func index(after currentIndex: Index) -> Index {
+        // To advance the index we have to find the next 1 bit _after_ the current bit.
+        // For example, consider the following 16 bits, where values are represented from right to left:
+        //   0110 0010 0110 0010
+        //
+        // To go from the first index to the second index, we need to count the number of 0 bits between it and the next 1 bit.
+        // We get this value by shifting the bits by one past the current index:
+        //   0110 0010 0110 0010
+        //                    ╰╴current index
+        //   0001 1000 1001 1000
+        //                   ~~~ 3 trailing zero bits
+        //
+        // The second index's absolute value is the one past the first index's value plus the number of trailing zero bits in the shifted value.
+        //
+        // For the third index we repeat the same process, starting by shifting the bits by one past second index:
+        //   0110 0010 0110 0010
+        //               ╰╴current index
+        //   0000 0001 1000 1001
+        //                      0 trailing zero bits
+        //
+        // This time there are no trailing zero bits in the shifted value, so the third index's absolute value is just one past the second index.
+        let shift = currentIndex.bit &+ 1
+        return Index(bit: shift &+ UInt8((storage &>> shift).trailingZeroBitCount))
+    }
+
+    @inlinable
+    package func formIndex(after index: inout Index)  {
+        // See `index(after:)` above for explanation.
+        index.bit &+= 1
+        index.bit &+= UInt8((storage &>> index.bit).trailingZeroBitCount)
+    }
+
+    @inlinable
+    package func distance(from start: Index, to end: Index) -> Int {
+        // To compute the distance between two indices we have to find the number 1 bit from the start index to (but excluding) the end index.
+        // For example, consider the following 16 bits, where values are represented from right to left:
+        //   0110 0010 0110 0010
+        //      end╶╯    ╰╴start
+        //
+        // To find the distance between the second index and the fourth index, we need to count the number of 0 bits between it and the next 1 bit.
+        // We limit the calculation to this range in two steps.
+        //
+        // First, we mask out all the bits above the end index:
+        //      end╶╮    ╭╴start
+        //   0110 0010 0110 0010
+        //   0000 0011 1111 1111  mask
+        //
+        // Because collections can have end indices that extend out-of-bounds we need to clamp the mask from a larger integer type to avoid it wrapping around to 0.
+        let mask = Storage(clamping: (1 &<< UInt(end.bit)) &- 1)
+        var distance = storage & mask
+
+        // Then, we shift away all the bits below the start index:
+        //      end╶╮    ╭╴start
+        //   0000 0010 0110 0010
+        //   0000 0000 0000 1001
+        distance &>>= start.bit
+
+        // The distance from start to end is the number of 1 bits in this number.
+        return distance.nonzeroBitCount
+    }
+
+    @inlinable
+    package var first: Element? {
+        isEmpty ? nil : storage.trailingZeroBitCount
+    }
+
+    @inlinable
+    package func min() -> Element? {
+        first // The elements are already sorted
+    }
+
+    @inlinable
+    package func sorted() -> [Element] {
+        Array(self) // The elements are already sorted
+    }
+
+    @inlinable
+    package var count: Int {
+        storage.nonzeroBitCount
+    }
+}
+
+// MARK: Hashable
+
+extension _FixedSizeBitSet: Hashable {}
+
+// MARK: Combinations
+
+extension _FixedSizeBitSet {
+    /// Returns a list of all possible combinations of the elements in the set, in order of increasing number of elements.
+    package func allCombinationsOfValues() -> [Self] {
+        // Leverage the fact that bits of an Int represent the possible combinations.
+        let smallest = storage.trailingZeroBitCount
+
+        var combinations: [Self] = []
+        combinations.reserveCapacity((1 &<< count /*known to be less than Storage.bitWidth */) - 1)
+
+        for raw in 1 ... storage &>> smallest {
+            let combination = Self(storage: Storage(raw &<< smallest))
+
+            // Filter out any combinations that include columns that are the same for all overloads
+            guard self.isSuperset(of: combination) else { continue }
+
+            combinations.append(combination)
+        }
+        // The bits of larger and larger Int values won't be in order of number of bits set, so we sort them.
+        return combinations.sorted(by: { $0.count < $1.count })
+    }
+}

Sources/DocCCommon/Mutex.swift

@@ -0,0 +1,48 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2025 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 Swift project authors
+*/
+
+#if os(macOS) || os(iOS)
+import Darwin
+
+// This type is designs to have the same API surface as 'Synchronization.Mutex'.
+// It's different from 'SwiftDocC.Synchronized' which requires that the wrapped values is `Copyable`.
+//
+// When we can require macOS 15.0 we can remove this custom type and use 'Synchronization.Mutex' directly on all platforms.
+struct Mutex<Value: ~Copyable>: ~Copyable, @unchecked Sendable {
+    private var value: UnsafeMutablePointer<Value>
+    private var lock: UnsafeMutablePointer<os_unfair_lock>
+
+    init(_ initialValue: consuming sending Value) {
+        value = UnsafeMutablePointer<Value>.allocate(capacity: 1)
+        value.initialize(to: initialValue)
+
+        lock = UnsafeMutablePointer<os_unfair_lock>.allocate(capacity: 1)
+        lock.initialize(to: os_unfair_lock())
+    }
+
+    deinit {
+        value.deallocate()
+        lock.deallocate()
+    }
+
+    borrowing func withLock<Result: ~Copyable, E: Error>(_ body: (inout sending Value) throws(E) -> sending Result) throws(E) -> sending Result {
+        os_unfair_lock_lock(lock)
+        defer { os_unfair_lock_unlock(lock) }
+
+        return try body(&value.pointee)
+    }
+}
+
+//extension Mutex:  where Value: ~Copyable {}
+#else
+import Synchronization
+
+typealias Mutex = Synchronization.Mutex
+#endif