Allow each Int Set to specialize its creation of combinations

Sources/SwiftDocC/Infrastructure/Link Resolution/PathHierarchy+TypeSignatureDisambiguation.swift

@@ -73,10 +73,10 @@ extension PathHierarchy.DisambiguationContainer {
 
         // Check if any columns are common for all overloads so that type name combinations with those columns can be skipped.
         let allOverloads = IntSet(0 ..< listOfOverloadTypeNames.count)
-        let typeNameIndicesToCheck = (0 ..< numberOfTypes).filter {
+        let typeNameIndicesToCheck = IntSet((0 ..< numberOfTypes).filter {
             // It's sufficient to check the first row because this column has to be the same for all rows
             table[0][$0] != allOverloads
-        }
+        })
 
         guard !typeNameIndicesToCheck.isEmpty else {
             // Every type name is common across the overloads. This information can't be used to disambiguate the overloads.
@@ -88,15 +88,16 @@ extension PathHierarchy.DisambiguationContainer {
             var shortestDisambiguationSoFar: (indicesToInclude: IntSet, length: Int)?
 
             // For each overload we iterate over the possible parameter combinations with increasing number of elements in each combination.
-            for typeNamesToInclude in typeNameIndicesToCheck.combinations(ofCount: 1...) {
+            for typeNamesToInclude in typeNameIndicesToCheck.combinationsToCheck() {
                 // Stop if we've already found a match with fewer parameters than this
                 guard typeNamesToInclude.count <= (shortestDisambiguationSoFar?.indicesToInclude.count ?? .max) else {
                     break
                 }
 
-                let firstTypeNameToInclude = typeNamesToInclude.first! // The generated `typeNamesToInclude` is never empty.
+                var iterator = typeNamesToInclude.makeIterator()
+                let firstTypeNameToInclude = iterator.next()! // The generated `typeNamesToInclude` is never empty.
                 // Compute which other overloads this combinations of type names also could refer to.
-                let overlap = typeNamesToInclude.dropFirst().reduce(into: table[row][firstTypeNameToInclude]) { partialResult, index in
+                let overlap = IteratorSequence(iterator).reduce(into: table[row][firstTypeNameToInclude]) { partialResult, index in
                     partialResult.formIntersection(table[row][index])
                 }
 
@@ -133,12 +134,36 @@ extension PathHierarchy.DisambiguationContainer {
 // MARK: Int Set
 
 /// A private protocol that abstracts sets of integers.
-private protocol _IntSet: SetAlgebra<Int> {
+private protocol _IntSet: SetAlgebra<Int>, Sequence<Int> {
     // In addition to the general SetAlgebra, the code in this file checks the number of elements in the set.
     var count: Int { get }
+
+    // Let each type specialize the creation of possible combinations to check.
+    associatedtype CombinationSequence: Sequence<Self>
+    func combinationsToCheck() -> CombinationSequence
+}
+
+
+extension Set<Int>: _IntSet {
+    func combinationsToCheck() -> some Sequence<Self> {
+        // For `Set<Int>`, use the Swift Algorithms implementation to generate the possible combinations.
+        self.combinations(ofCount: 1...).lazy.map { Set($0) }
+    }
+}
+extension _TinySmallValueIntSet: _IntSet {
+    func combinationsToCheck() -> [Self] {
+        // For `_TinySmallValueIntSet`, leverage the fact that bits of an Int represent the possible combinations.
+        let smallest = storage.trailingZeroBitCount
+        return (1 ... storage >> smallest)
+            .compactMap {
+                let combination = Self(storage: UInt64($0 << smallest))
+                // Filter out any combinations that include columns that are the same for all overloads
+                return self.isSuperset(of: combination) ? combination : nil
+            }
+            // The bits of larger and larger Int values won't be in order of number of bits set, so we sort them.
+            .sorted(by: { $0.count < $1.count })
+    }
 }
-extension Set<Int>: _IntSet {}
-extension _TinySmallValueIntSet: _IntSet {}
 
 /// A specialized set-algebra type that only stores the possible values `0 ..< 64`.
 ///
@@ -233,3 +258,38 @@ struct _TinySmallValueIntSet: SetAlgebra {
         storage ^= other.storage
     }
 }
+
+extension _TinySmallValueIntSet: Sequence {
+    func makeIterator() -> Iterator {
+        Iterator(set: self)
+    }
+
+    struct Iterator: IteratorProtocol {
+        typealias Element = Int
+
+        private let set: _TinySmallValueIntSet
+        private var current: Int
+        private let end: Int
+
+        @inlinable
+        init(set: _TinySmallValueIntSet) {
+            self.set = set
+            self.current = set.storage.trailingZeroBitCount
+            self.end = 64 - set.storage.leadingZeroBitCount
+        }
+
+        @inlinable
+        mutating func next() -> Int? {
+            defer { current += 1 }
+
+            while !set.contains(current) {
+                current += 1
+                if end <= current {
+                    return nil
+                }
+            }
+
+            return current
+        }
+    }
+}

Tests/SwiftDocCTests/Infrastructure/TinySmallValueIntSetTests.swift

@@ -9,6 +9,7 @@
 */
 
 import XCTest
+import Algorithms
 @testable import SwiftDocC
 
 class TinySmallValueIntSetTests: XCTestCase {
@@ -67,4 +68,49 @@ class TinySmallValueIntSetTests: XCTestCase {
         XCTAssertEqual(tiny.contains(9), real.contains(9))
         XCTAssertEqual(tiny.count, real.count)
     }
+
+    func testCombinations() {
+        do {
+            let tiny: _TinySmallValueIntSet = [0,1,2]
+            XCTAssertEqual(tiny.combinationsToCheck().map { $0.sorted() }, [
+                [0], [1], [2],
+                [0,1], [0,2], [1,2],
+                [0,1,2]
+            ])
+        }
+
+        do {
+            let tiny: _TinySmallValueIntSet = [2,5,9]
+            XCTAssertEqual(tiny.combinationsToCheck().map { $0.sorted() }, [
+                [2], [5], [9],
+                [2,5], [2,9], [5,9],
+                [2,5,9]
+            ])
+        }
+
+        do {
+            let tiny: _TinySmallValueIntSet = [3,4,7,11,15,16]
+
+            let expected = Array(tiny).combinations(ofCount: 1...)
+            let actual   = tiny.combinationsToCheck().map { Array($0) }
+
+            XCTAssertEqual(expected.count, actual.count)
+
+            // The two implementations doesn't need to provide combinations in the same order within a size
+            let expectedBySize: [[[Int]]] = expected.grouped(by: \.count).sorted(by: \.key).map(\.value)
+            let actualBySize:   [[[Int]]] = actual  .grouped(by: \.count).sorted(by: \.key).map(\.value)
+
+            for (expectedForSize, actualForSize) in zip(expectedBySize, actualBySize) {
+                XCTAssertEqual(expectedForSize.count, actualForSize.count)
+
+                // Comparing [Int] descriptions to allow each same-size combination list to have different orders.
+                // For example, these two lists of combinations (with the last 2 elements swapped) are considered equivalent:
+                // [1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]
+                // [1, 2, 3], [1, 2, 4], [2, 3, 4], [1, 3, 4]
+                XCTAssertEqual(expectedForSize.map(\.description).sorted(),
+                               actualForSize  .map(\.description).sorted())
+
+            }
+        }
+    }
 }