Add filter method for ordered and unordered map

stdlib/public/Cxx/CxxDictionary.swift

@@ -25,9 +25,10 @@ public protocol CxxDictionary<Key, Value> {
   associatedtype Size: BinaryInteger
   associatedtype InsertionResult
 
+  init()
+
   /// Do not implement this function manually in Swift.
   func __findUnsafe(_ key: Key) -> RawIterator
-
   /// Do not implement this function manually in Swift.
   mutating func __findMutatingUnsafe(_ key: Key) -> RawMutableIterator
 
@@ -39,6 +40,9 @@ public protocol CxxDictionary<Key, Value> {
   @discardableResult
   mutating func erase(_ key: Key) -> Size
 
+  /// Do not implement this function manually in Swift.
+  func __beginUnsafe() -> RawIterator
+
   /// Do not implement this function manually in Swift.
   func __endUnsafe() -> RawIterator
 
@@ -72,4 +76,22 @@ extension CxxDictionary {
       }
     }
   }
+
+  public func filter(_ isIncluded: (_ key: Key, _ value: Value) throws -> Bool) rethrows -> Self {
+    var filteredDictionary = Self.init()
+    var iterator = __beginUnsafe()
+    let endIterator = __endUnsafe()
+
+    while iterator != endIterator {
+      let pair = iterator.pointee
+
+      if try isIncluded(pair.first, pair.second) {
+        filteredDictionary.__insertUnsafe(pair)
+      }
+
+      iterator = iterator.successor()
+    }
+
+    return filteredDictionary
+  }
 }

test/Interop/Cxx/stdlib/Inputs/std-map.h

@@ -2,8 +2,8 @@
 #define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_MAP_H
 
 #include <map>
-#include <unordered_map>
 #include <string>
+#include <unordered_map>
 
 using Map = std::map<int, int>;
 using MapStrings = std::map<std::string, std::string>;
@@ -12,5 +12,7 @@ using UnorderedMap = std::unordered_map<int, int>;
 
 inline Map initMap() { return {{1, 3}, {2, 2}, {3, 3}}; }
 inline UnorderedMap initUnorderedMap() { return {{1, 3}, {3, 3}, {2, 2}}; }
+inline Map initEmptyMap() { return {}; }
+inline UnorderedMap initEmptyUnorderedMap() { return {}; }
 
 #endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_MAP_H

test/Interop/Cxx/stdlib/use-std-map.swift

@@ -88,6 +88,38 @@ StdMapTestSuite.test("UnorderedMap.subscript") {
   expectNil(m[-1])
 }
 
+StdMapTestSuite.test("Map.filter") {
+  var m = initMap()
+  var n = initEmptyMap()
+
+  expectNotNil(m[1])
+  expectEqual(n.size(), 0)
+
+  m = m.filter { k, v in k != 1 }
+  n = n.filter { k, v in false }
+
+  expectNil(m[1])
+  expectEqual(m[2], 2)
+  expectEqual(m[3], 3)
+  expectTrue(n.empty())
+}
+
+StdMapTestSuite.test("UnorderedMap.filter") {
+  var m = initUnorderedMap()
+  var n = initEmptyUnorderedMap()
+
+  expectNotNil(m[1])
+  expectEqual(n.size(), 0)
+
+  m = m.filter { k, v in k != 1 }
+  n = n.filter { k, v in false }
+
+  expectNil(m[1])
+  expectEqual(m[2], 2)
+  expectEqual(m[3], 3)
+  expectTrue(n.empty())
+}
+
 StdMapTestSuite.test("Map.erase") {
   var m = initMap()
   expectNotNil(m[1])