+OrderedSet add filter #158

Sources/OrderedCollections/OrderedSet/OrderedSet.swift

@@ -494,3 +494,27 @@ extension OrderedSet {
     return _elements.remove(at: index)
   }
 }
+
+extension OrderedSet {
+  /// Returns a new ordered set containing the values pairs of the ordered set
+  /// that satisfy the given predicate.
+  ///
+  /// - Parameter isIncluded: A closure that takes a value as its
+  ///   argument and returns a Boolean value indicating whether the value
+  ///   should be included in the returned dictionary.
+  ///
+  /// - Returns: An ordered set of the values that `isIncluded` allows.
+  ///
+  /// - Complexity: O(`count`)
+  @inlinable
+  public func filter(
+          _ isIncluded: (Element) throws -> Bool
+  ) rethrows -> Self {
+    var result: OrderedSet = Self(minimumCapacity: _minimumCapacity)
+    for element in self where try isIncluded(element) {
+      result._appendNew(element)
+    }
+    result._checkInvariants()
+    return result
+  }
+}

Sources/_CollectionsTestSupport/AssertionContexts/Assertions.swift

@@ -402,3 +402,7 @@ public func expectThrows<T>(
     errorHandler(error)
   }
 }
+
+public func expectType<T>(_ actual: T, _ expectedType: Any.Type) {
+  expectTrue(type(of: actual) == expectedType)
+}

Tests/OrderedCollectionsTests/OrderedSet/OrderedSetTests.swift

@@ -1360,4 +1360,24 @@ class OrderedSetTests: CollectionTestCase {
       }
     }
   }
+
+  func test_filter() {
+    withOrderedSetLayouts(scales: [0, 5, 6]) { layout in
+      withEvery("factor", in: [1, 2, 3, 5, 10]) { factor in
+        let count = layout.count
+        let input = OrderedSet(layout: layout, contents: 0 ..< count)
+        let expected = (0 ..< count).filter { $0 % factor == 0 }
+        let actual = input.filter { $0 % factor == 0 }
+
+        expectEqualElements(actual, expected)
+      }
+    }
+  }
+  func test_filter_type() {
+    let s = Set([1, 2, 3, 4]).filter { $0.isMultiple(of: 2) }
+    expectType(s, Set<Int>.self)
+
+    let os = OrderedSet([1, 2, 3, 4]).filter { $0.isMultiple(of: 2) }
+    expectType(os, OrderedSet<Int>.self)
+  }
 }