Allow mutating values in-place

Author: arennow

Sources/KeyedArray/KeyedArray.swift

@@ -61,6 +61,49 @@ public struct KeyedArray<Key: Hashable, Element: Equatable> {
 	}
 }
 
+extension KeyedArray {
+	/// Mutates an existing value without changing its position.
+	///
+	/// The mutation function will not be called if no value can be found for `key`.
+	///
+	/// - Complexity: O(n)
+	///
+	/// - Warning: The new element **must** produce the same key as the old value.
+	///
+	/// - Parameters:
+	///   - key: The key whose value to look up and mutate.
+	///   - function: A function that mutates the existing value.
+	public mutating func mutateValueInPlace(for key: Key, using function: (inout Element)->Void) {
+		guard var value = self[key], let index = self.array.firstIndex(of: value) else { return }
+		function(&value)
+		
+		precondition(self.keyExtractor(value) == key, "New value's key must match old value's key")
+		
+		self.array[index] = value
+		self.dictionary[key] = value
+	}
+	
+	/// Mutates an existing value without changing its position.
+	///
+	/// - Complexity: O(1)
+	/// 
+	/// - Warning: The new element **must** produce the same key as the old value.
+	///
+	/// - Parameters:
+	///   - index: The index whose value to mutate.
+	///   - function: A function that mutates the existing value.
+	public mutating func mutateValueInPlace(for index: Int, using function: (inout Element)->Void) {
+		var value = self[index]
+		let oldKey = self.keyExtractor(value)
+		function(&value)
+		
+		precondition(self.keyExtractor(value) == oldKey, "New value's key must match old value's key")
+		
+		self.array[index] = value
+		self.dictionary[oldKey] = value
+	}
+}
+
 extension KeyedArray where Element: Identifiable, Key == Element.ID {
 	public init() {
 		self.init(keyExtractor: \.id)

Tests/KeyedArrayTests/KeyedArrayTests.swift

@@ -2,9 +2,10 @@ import XCTest
 import KeyedArray
 
 final class KeyedArrayTests: XCTestCase {
-	struct DemoVal: Equatable, Identifiable, ExpressibleByStringLiteral {
+	struct DemoVal: Equatable, Identifiable, ExpressibleByStringLiteral, CustomStringConvertible {
 		let str: String
 		var id: Character { str.first! }
+		var description: String { str }
 
 		init(_ str: String) { self.str = str }
 		init(stringLiteral str: String) { self.str = str }
@@ -47,13 +48,31 @@ final class KeyedArrayTests: XCTestCase {
 		XCTAssertEqual(ar.array, ["alpha"])
 	}
 
-	func testReplaceByIndex() {
+	func testReplaceByAppend() {
 		ar.append("bravo")
 
 		XCTAssertEqual(ar.array, ["alpha", "bravo"])
 		XCTAssertEqual(ar.dictionary, ["a": "alpha", "b": "bravo"])
 	}
 
+	func testMutateValueInPlaceByKey() {
+		ar.mutateValueInPlace(for: "a") { existing in
+			existing = "alpine"
+		}
+		
+		XCTAssertEqual(ar.array, ["alpine", "beta"])
+		XCTAssertEqual(ar.dictionary, ["a": "alpine", "b": "beta"])
+	}
+	
+	func testMutateValueInPlaceByIndex() {
+		ar.mutateValueInPlace(for: 0) { existing in
+			existing = "alpine"
+		}
+		
+		XCTAssertEqual(ar.array, ["alpine", "beta"])
+		XCTAssertEqual(ar.dictionary, ["a": "alpine", "b": "beta"])
+	}
+	
 	func testArrayIsArray() {
 		XCTAssertEqual(Array(ar.array), ar.array)
 	}