groupedKeys.swift

extension Dictionary where Value: Hashable {
	/// Groups keys with common values together. Example:
	/// ```
	/// let groceryList = ["Apple": 2, "Oranges": 3, "Pears": 2]
	/// groceryList.keysGroupedByValue
	/// // => [2: ["Apple", "Pears"], 3: ["Oranges"]]
	/// ```
	public var keysGroupedByValue: [Value: [Key]] {
		var commonalities: [Value: [Key]] = [:]
		for (key, value) in self {
			commonalities[value, default: []].append(key)
		}
		return commonalities
	}
}

class Dictionary_ExtensionsTests: XCTestCase {
	func testExtractKeysWithCommonValues() {
		let testingDict = ["Oranges": 2, "Pears": 5, "Strawberries": 4, "Apples": 2]
		let commonalities = testingDict.keysGroupedByValue
		XCTAssert(commonalities == [2: ["Oranges", "Apples"], 5: ["Pears"], 4: ["Strawberries"]])
	}
}