Make uniqued() lazy by default (#71)

* Make `uniqued()` lazy by default

Guides/Unique.md

@@ -5,7 +5,7 @@
 
 Methods to strip repeated elements from a sequence or collection.
 
-The `uniqued()` method returns an array, dropping duplicate elements
+The `uniqued()` method returns a sequence, dropping duplicate elements
 from a sequence. The `uniqued(on:)` method does the same, using 
 the result of the given closure to determine the "uniqueness" of each 
 element.
@@ -14,29 +14,36 @@ element.
 let numbers = [1, 2, 3, 3, 2, 3, 3, 2, 2, 2, 1]
 
 let unique = numbers.uniqued()
-// unique == [1, 2, 3]
+// Array(unique) == [1, 2, 3]
 ```
 
 ## Detailed Design
 
 Both methods are available for sequences, with the simplest limited to
 when the element type conforms to `Hashable`. Both methods preserve
-the relative order of the elements.
+the relative order of the elements. `uniqued(on:)` has a matching lazy
+version that is added to `LazySequenceProtocol`.
 
 ```swift
 extension Sequence where Element: Hashable {
-    func uniqued() -> [Element]
+    func uniqued() -> Uniqued<Self, Element>
 }
 
 extension Sequence {
-    func uniqued<T>(on: (Element) throws -> T) rethrows -> [Element]
-        where T: Hashable
+    func uniqued<Subject>(on projection: (Element) throws -> Subject) rethrows -> [Element]
+        where Subject: Hashable
+}
+
+extension LazySequenceProtocol {
+    func uniqued<Subject>(on projection: @escaping (Element) -> Subject) -> Uniqued<Self, Subject>
+        where Subject: Hashable
 }
 ```
 
 ### Complexity
 
-The `uniqued` methods are O(_n_) in both time and space complexity.
+The eager `uniqued(on:)` method is O(_n_) in both time and space complexity.
+The lazy versions are O(_1_).
 
 ### Comparison with other languages
 

Sources/Algorithms/Unique.swift

@@ -9,25 +9,82 @@
 //
 //===----------------------------------------------------------------------===//
 
+/// A sequence wrapper that leaves out duplicate elements of a base sequence.
+public struct Uniqued<Base: Sequence, Subject: Hashable> {
+  /// The base collection.
+  @usableFromInline
+  internal let base: Base
+
+  /// The projection function.
+  @usableFromInline
+  internal let projection: (Base.Element) -> Subject
+
+  @usableFromInline
+  internal init(base: Base, projection: @escaping (Base.Element) -> Subject) {
+    self.base = base
+    self.projection = projection
+  }
+}
+
+extension Uniqued: Sequence {
+  /// The iterator for a `Uniqued` sequence.
+  public struct Iterator: IteratorProtocol {
+    @usableFromInline
+    internal var base: Base.Iterator
+
+    @usableFromInline
+    internal let projection: (Base.Element) -> Subject
+
+    @usableFromInline
+    internal var seen: Set<Subject> = []
+
+    @usableFromInline
+    internal init(
+      base: Base.Iterator,
+      projection: @escaping (Base.Element) -> Subject
+    ) {
+      self.base = base
+      self.projection = projection
+    }
+
+    @inlinable
+    public mutating func next() -> Base.Element? {
+      while let element = base.next() {
+        if seen.insert(projection(element)).inserted {
+          return element
+        }
+      }
+      return nil
+    }
+  }
+
+  @inlinable
+  public func makeIterator() -> Iterator {
+    Iterator(base: base.makeIterator(), projection: projection)
+  }
+}
+
+extension Uniqued: LazySequenceProtocol where Base: LazySequenceProtocol {}
+
 //===----------------------------------------------------------------------===//
 // uniqued()
 //===----------------------------------------------------------------------===//
 
 extension Sequence where Element: Hashable {
-  /// Returns an array with only the unique elements of this sequence, in the
+  /// Returns a sequence with only the unique elements of this sequence, in the
   /// order of the first occurrence of each unique element.
   ///
   ///     let animals = ["dog", "pig", "cat", "ox", "dog", "cat"]
   ///     let uniqued = animals.uniqued()
-  ///     print(uniqued)
+  ///     print(Array(uniqued))
   ///     // Prints '["dog", "pig", "cat", "ox"]'
   ///
-  /// - Returns: An array with only the unique elements of this sequence.
+  /// - Returns: A sequence with only the unique elements of this sequence.
   ///  .
-  /// - Complexity: O(*n*), where *n* is the length of the sequence.
+  /// - Complexity: O(1).
   @inlinable
-  public func uniqued() -> [Element] {
-    uniqued(on: { $0 })
+  public func uniqued() -> Uniqued<Self, Element> {
+    Uniqued(base: self, projection: { $0 })
   }
 }
 
@@ -40,7 +97,7 @@ extension Sequence {
   /// first characters:
   ///
   ///     let animals = ["dog", "pig", "cat", "ox", "cow", "owl"]
-  ///     let uniqued = animals.uniqued(on: {$0.first})
+  ///     let uniqued = animals.uniqued(on: { $0.first })
   ///     print(uniqued)
   ///     // Prints '["dog", "pig", "cat", "ox"]'
   ///
@@ -67,3 +124,21 @@ extension Sequence {
     return result
   }
 }
+
+//===----------------------------------------------------------------------===//
+// lazy.uniqued()
+//===----------------------------------------------------------------------===//
+
+extension LazySequenceProtocol {
+  /// Returns a lazy sequence with the unique elements of this sequence (as
+  /// determined by the given projection), in the order of the first occurrence
+  /// of each unique element.
+  ///
+  /// - Complexity: O(1).
+  @inlinable
+  public func uniqued<Subject: Hashable>(
+    on projection: @escaping (Element) -> Subject
+  ) -> Uniqued<Self, Subject> {
+    Uniqued(base: self, projection: projection)
+  }
+}

Tests/SwiftAlgorithmsTests/UniqueTests.swift

@@ -17,10 +17,13 @@ final class UniqueTests: XCTestCase {
     let a = repeatElement(1...10, count: 15).joined().shuffled()
     let b = a.uniqued()
     XCTAssertEqual(b.sorted(), Set(a).sorted())
-    XCTAssertEqual(10, b.count)
+    XCTAssertEqual(10, Array(b).count)
 
     let c: [Int] = []
-    XCTAssertEqual(c.uniqued(), [])
+    XCTAssertEqualSequences(c.uniqued(), [])
+
+    let d = Array(repeating: 1, count: 10)
+    XCTAssertEqualSequences(d.uniqued(), [1])
   }
 
   func testUniqueOn() {
@@ -30,5 +33,21 @@ final class UniqueTests: XCTestCase {
 
     let c: [Int] = []
     XCTAssertEqual(c.uniqued(on: { $0.bitWidth }), [])
+
+    let d = Array(repeating: "Andromeda", count: 10)
+    XCTAssertEqualSequences(d.uniqued(on: { $0.first }), ["Andromeda"])
+  }
+
+  func testLazyUniqueOn() {
+    let a = ["Albemarle", "Abeforth", "Astrology", "Brandywine", "Beatrice", "Axiom"]
+    let b = a.lazy.uniqued(on: { $0.first })
+    XCTAssertEqualSequences(b, ["Albemarle", "Brandywine"])
+    XCTAssertLazySequence(b)
+
+    let c: [Int] = []
+    XCTAssertEqualSequences(c.lazy.uniqued(on: { $0.bitWidth }), [])
+
+    let d = Array(repeating: "Andromeda", count: 10)
+    XCTAssertEqualSequences(d.lazy.uniqued(on: { $0.first }), ["Andromeda"])
   }
 }