Add grouped(by:) and keyed(by:)

Guides/Grouped.md

@@ -0,0 +1,68 @@
+# Grouped
+
+[[Source](https://github.com/apple/swift-algorithms/blob/main/Sources/Algorithms/Grouped.swift) | 
+ [Tests](https://github.com/apple/swift-algorithms/blob/main/Tests/SwiftAlgorithmsTests/GroupedTests.swift)]
+
+Groups up elements of a sequence into a new Dictionary, whose values are Arrays of grouped elements, each keyed by the result of the given closure.
+
+```swift
+let fruits = ["Apricot", "Banana", "Apple", "Cherry", "Avocado", "Coconut"]
+let fruitsByLetter = fruits.grouped(by: { $0.first! })
+// Results in:
+// [
+//     "B": ["Banana"],
+//     "A": ["Apricot", "Apple", "Avocado"],
+//     "C": ["Cherry", "Coconut"],
+// ]
+```
+
+If you wish to achieve a similar effect but for single values (instead of Arrays of grouped values), see [`keyed(by:)`](Keyed.md).
+
+## Detailed Design
+
+The `grouped(by:)` method is declared as a `Sequence` extension returning
+`[GroupKey: [Element]]`.
+
+```swift
+extension Sequence {
+    public func grouped<GroupKey>(
+        by keyForValue: (Element) throws -> GroupKey
+    ) rethrows -> [GroupKey: [Element]]
+}
+```
+
+### Complexity
+
+Calling `grouped(by:)` is an O(_n_) operation.
+
+### Comparison with other languages
+
+| Language      | Grouping API |
+|---------------|--------------|
+| Java          | [`groupingBy`](https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/util/stream/Collectors.html#groupingBy(java.util.function.Function)) |
+| Kotlin        | [`groupBy`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/group-by.html) |
+| C#            | [`GroupBy`](https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.groupby?view=net-7.0#system-linq-enumerable-groupby) |
+| Rust          | [`group_by`](https://doc.rust-lang.org/std/primitive.slice.html#method.group_by) |
+| Ruby          | [`group_by`](https://ruby-doc.org/3.2.2/Enumerable.html#method-i-group_by) |
+| Python        | [`groupby`](https://docs.python.org/3/library/itertools.html#itertools.groupby) |
+| PHP (Laravel) | [`groupBy`](https://laravel.com/docs/10.x/collections#method-groupby) |
+
+#### Naming
+
+All the surveyed languages name this operation with a variant of "grouped" or "grouping". The past tense `grouped(by:)` best fits [Swift's API Design Guidelines](https://www.swift.org/documentation/api-design-guidelines/).
+
+#### Customization points
+
+Java and C# are interesting in that they provide multiple overloads with several points of customization:
+
+1. Changing the type of the groups.
+    1. E.g. the groups can be Sets instead of Arrays.
+    1. Akin to calling `.transformValues { group in Set(group) }` on the resultant dictionary, but avoiding the intermediate allocation of Arrays of each group.
+2. Picking which elements end up in the groupings.
+    1. The default is the elements of the input sequence, but can be changed.
+    2. Akin to calling `.transformValues { group in group.map(someTransform) }` on the resultant dictionary, but avoiding the intermediate allocation of Arrays of each group.
+3. Changing the type of the outermost collection.
+    1. E.g using an `OrderedDictionary`, `SortedDictionary` or `TreeDictionary` instead of the default (hashed, unordered) `Dictionary`.
+    2. There's no great way to achieve this with the  `grouped(by:)`. One could wrap the resultant dictionary in an initializer to one of the other dictionary types, but that isn't sufficient: Once the `Dictionary` loses the ordering, there's no way to get it back when constructing one of the ordered dictionary variants.
+
+It is not clear which of these points of customization are worth supporting, or what the best way to express them might be.

Guides/Keyed.md

@@ -0,0 +1,78 @@
+# Keyed
+
+[[Source](https://github.com/apple/swift-algorithms/blob/main/Sources/Algorithms/Keyed.swift) | 
+ [Tests](https://github.com/apple/swift-algorithms/blob/main/Tests/SwiftAlgorithmsTests/KeyedTests.swift)]
+
+Stores the elements of a sequence as the values of a Dictionary, keyed by the result of the given closure.
+
+```swift
+let fruits = ["Apricot", "Banana", "Apple", "Cherry", "Blackberry", "Avocado", "Coconut"]
+let fruitByLetter = fruits.keyed(by: { $0.first! })
+// Results in:
+// [
+//     "A": "Avocado",
+//     "B": "Blackberry",
+//     "C": "Coconut",
+// ]
+```
+
+On a key-collision, the latest element is kept by default. Alternatively, you can provide a closure which specifies which value to keep:
+
+```swift
+let fruits = ["Apricot", "Banana", "Apple", "Cherry", "Blackberry", "Avocado", "Coconut"]
+let fruitsByLetter = fruits.keyed(
+    by: { $0.first! },
+    resolvingConflictsWith: { key, old, new in old } // Always pick the first fruit
+)
+// Results in:
+// [
+//     "A": "Apricot",
+//     "B": "Banana",
+//     "C": "Cherry",
+// ]
+```
+
+## Detailed Design
+
+The `keyed(by:)` and `keyed(by:resolvingConflictsWith:)` methods are declared in an `Sequence` extension, both returning `[Key: Element]`.
+
+```swift
+extension Sequence {
+    public func keyed<Key>(
+        by keyForValue: (Element) throws -> Key
+    ) rethrows -> [Key: Element]
+
+    public func keyed<Key>(
+        by keyForValue: (Element) throws -> Key,
+        resolvingConflictsWith resolve: ((Key, Element, Element) throws -> Element)? = nil
+    ) rethrows -> [Key: Element]
+}
+```
+
+### Complexity
+
+Calling `keyed(by:)` is an O(_n_) operation.
+
+### Comparison with other languages
+
+| Language      | "Keying" API |
+|---------------|-------------|
+| Java          | [`toMap`](https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/util/stream/Collectors.html#toMap(java.util.function.Function,java.util.function.Function)) |
+| Kotlin        | [`associatedBy`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/associate-by.html) |
+| C#            | [`ToDictionary`](https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.todictionary?view=net-7.0#system-linq-enumerable-todictionary) |
+| Ruby (ActiveSupport) | [`index_by`](https://rubydoc.info/gems/activesupport/7.0.5/Enumerable#index_by-instance_method) |
+| PHP (Laravel) | [`keyBy`](https://laravel.com/docs/10.x/collections#method-keyby) |
+
+#### Rejected alternative names
+
+1. Java's `toMap` is referring to `Map`/`HashMap`, their naming for Dictionaries and other associative collections. It's easy to confuse with the transformation function, `Sequence.map(_:)`.
+2. C#'s `toXXX()` naming doesn't suite Swift well, which tends to prefer `Foo.init` over `toFoo()` methods.
+3. Ruby's `index_by` naming doesn't fit Swift well, where "index" is a specific term (e.g. the `associatedtype Index` on `Collection`). There is also a [`index(by:)`](Index.md) method in swift-algorithms, is specifically to do with matching elements up with their indices, and not any arbitrary derived value.
+
+#### Alternative names
+
+Kotlin's `associatedBy` naming is a good alterative, and matches the past tense of [Swift's API Design Guidelines](https://www.swift.org/documentation/api-design-guidelines/), though perhaps we'd spell it `associated(by:)`.
+
+#### Customization points
+
+Java and C# are interesting in that they provide overloads that let you customize the type of the outermost collection. E.g. using an `OrderedDictionary` instead of the default (hashed, unordered) `Dictionary`.

README.md

@@ -45,8 +45,10 @@ Read more about the package, and the intent behind it, in the [announcement on s
 - [`adjacentPairs()`](https://github.com/apple/swift-algorithms/blob/main/Guides/AdjacentPairs.md): Lazily iterates over tuples of adjacent elements.
 - [`chunked(by:)`, `chunked(on:)`, `chunks(ofCount:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Chunked.md): Eager and lazy operations that break a collection into chunks based on either a binary predicate or when the result of a projection changes or chunks of a given count.
 - [`firstNonNil(_:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/FirstNonNil.md): Returns the first non-`nil` result from transforming a sequence's elements.
+- [`grouped(by:)](https://github.com/apple/swift-algorithms/blob/main/Guides/Grouped.md): Group up elements using the given closure, returning a Dictionary of those groups, keyed by the results of the closure.
 - [`indexed()`](https://github.com/apple/swift-algorithms/blob/main/Guides/Indexed.md): Iterate over tuples of a collection's indices and elements. 
 - [`interspersed(with:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Intersperse.md): Place a value between every two elements of a sequence.
+- [`keyed(by:)`, `keyed(by:resolvingConflictsBy:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Keyed.md): Returns a Dictionary that associates elements of a sequence with the keys returned by the given closure.
 - [`partitioningIndex(where:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Partition.md): Returns the starting index of the partition of a collection that matches a predicate.
 - [`reductions(_:)`, `reductions(_:_:)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Reductions.md): Returns all the intermediate states of reducing the elements of a sequence or collection.
 - [`split(maxSplits:omittingEmptySubsequences:whereSeparator)`, `split(separator:maxSplits:omittingEmptySubsequences)`](https://github.com/apple/swift-algorithms/blob/main/Guides/Split.md): Lazy versions of the Standard Library's eager operations that split sequences and collections into subsequences separated by the specified separator element.

Sources/Algorithms/Grouped.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+extension Sequence {
+  /// Groups up elements of `self` into a new Dictionary,
+  /// whose values are Arrays of grouped elements,
+  /// each keyed by the group key returned by the given closure.
+  /// - Parameters:
+  ///   - keyForValue: A closure that returns a key for each element in
+  ///     `self`.
+  /// - Returns: A dictionary containing grouped elements of self, keyed by
+  ///     the keys derived by the `keyForValue` closure.
+  @inlinable
+  public func grouped<GroupKey>(by keyForValue: (Element) throws -> GroupKey) rethrows -> [GroupKey: [Element]] {
+    try Dictionary(grouping: self, by: keyForValue)
+  }
+}

Sources/Algorithms/Keyed.swift

@@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+extension Sequence {
+  /// Creates a new Dictionary from the elements of `self`, keyed by the
+  /// results returned by the given `keyForValue` closure.
+  ///
+  /// If the key derived for a new element collides with an existing key from a previous element,
+  /// the latest value will be kept.
+  ///
+  /// - Parameters:
+  ///   - keyForValue: A closure that returns a key for each element in `self`.
+  @inlinable
+  public func keyed<Key>(
+    by keyForValue: (Element) throws -> Key
+  ) rethrows -> [Key: Element] {
+    return try self.keyed(by: keyForValue, resolvingConflictsWith: { _, old, new in new })
+  }
+
+  /// Creates a new Dictionary from the elements of `self`, keyed by the
+  /// results returned by the given `keyForValue` closure. As the dictionary is
+  /// built, the initializer calls the `resolve` closure with the current and
+  /// new values for any duplicate keys. Pass a closure as `resolve` that
+  /// returns the value to use in the resulting dictionary: The closure can
+  /// choose between the two values, combine them to produce a new value, or
+  /// even throw an error.
+  ///
+  /// - Parameters:
+  ///   - keyForValue: A closure that returns a key for each element in `self`.
+  ///   - resolve: A closure that is called with the values for any duplicate
+  ///     keys that are encountered. The closure returns the desired value for
+  ///     the final dictionary.
+  @inlinable
+  public func keyed<Key>(
+    by keyForValue: (Element) throws -> Key,
+    resolvingConflictsWith resolve: (Key, Element, Element) throws -> Element
+  ) rethrows -> [Key: Element] {
+    var result = [Key: Element]()
+
+    for element in self {
+      let key = try keyForValue(element)
+
+      if let oldValue = result.updateValue(element, forKey: key) {
+        let valueToKeep = try resolve(key, oldValue, element)
+
+        // This causes a second look-up for the same key. The standard library can avoid that
+        // by calling `mutatingFind` to get access to the bucket where the value will end up,
+        // and updating in place.
+        // Swift Algorithms doesn't have access to that API, so we make do.
+        // When this gets merged into the standard library, we should optimize this.
+        result[key] = valueToKeep
+      }
+    }
+
+    return result
+  }
+}

Tests/SwiftAlgorithmsTests/GroupedTests.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import Algorithms
+
+final class GroupedTests: XCTestCase {
+  private class SampleError: Error {}
+
+  // Based on https://github.com/apple/swift/blob/4d1d8a9de5ebc132a17aee9fc267461facf89bf8/validation-test/stdlib/Dictionary.swift#L1974-L1988
+
+  func testGroupedBy() {
+    let r = 0..<10
+
+    let d1 = r.grouped(by: { $0 % 3 })
+    XCTAssertEqual(3, d1.count)
+    XCTAssertEqual(d1[0]!, [0, 3, 6, 9])
+    XCTAssertEqual(d1[1]!, [1, 4, 7])
+    XCTAssertEqual(d1[2]!, [2, 5, 8])
+
+    let d2 = r.grouped(by: { $0 })
+    XCTAssertEqual(10, d2.count)
+
+    let d3 = (0..<0).grouped(by: { $0 })
+    XCTAssertEqual(0, d3.count)
+  }
+
+  func testThrowingFromKeyFunction() {
+    let input = ["Apple", "Banana", "Cherry"]
+    let error = SampleError()
+
+    XCTAssertThrowsError(
+      try input.grouped(by: { (_: String) -> Character in throw error })
+    ) { thrownError in
+      XCTAssertIdentical(error, thrownError as? SampleError)
+    }
+  }
+}

Tests/SwiftAlgorithmsTests/KeyedTests.swift

@@ -0,0 +1,88 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import Algorithms
+
+final class KeyedTests: XCTestCase {
+  private class SampleError: Error {}
+
+  func testUniqueKeys() {
+    let d = ["Apple", "Banana", "Cherry"].keyed(by: { $0.first! })
+    XCTAssertEqual(d.count, 3)
+    XCTAssertEqual(d["A"]!, "Apple")
+    XCTAssertEqual(d["B"]!, "Banana")
+    XCTAssertEqual(d["C"]!, "Cherry")
+    XCTAssertNil(d["D"])
+  }
+
+  func testEmpty() {
+    let d = EmptyCollection<String>().keyed(by: { $0.first! })
+    XCTAssertEqual(d.count, 0)
+  }
+
+  func testNonUniqueKeys() throws {
+    let d = ["Apple", "Avocado", "Banana", "Cherry"].keyed(by: { $0.first! })
+    XCTAssertEqual(d.count, 3)
+    XCTAssertEqual(d["A"]!, "Avocado", "On a key-collision, keyed(by:) should take the latest value.")
+    XCTAssertEqual(d["B"]!, "Banana")
+    XCTAssertEqual(d["C"]!, "Cherry")
+  }
+
+  func testNonUniqueKeysWithMergeFunction() {
+    var resolveCallHistory = [(key: Character, current: String, new: String)]()
+    let expectedCallHistory = [
+      (key: "A", current: "Apple", new: "Avocado"),
+      (key: "C", current: "Cherry", new: "Coconut"),
+    ]
+
+    let d = ["Apple", "Avocado", "Banana", "Cherry", "Coconut"].keyed(
+      by: { $0.first! },
+      resolvingConflictsWith: { key, older, newer in
+        resolveCallHistory.append((key, older, newer))
+        return "\(older)-\(newer)"
+      }
+    )
+
+    XCTAssertEqual(d.count, 3)
+    XCTAssertEqual(d["A"]!, "Apple-Avocado")
+    XCTAssertEqual(d["B"]!, "Banana")
+    XCTAssertEqual(d["C"]!, "Cherry-Coconut")
+    XCTAssertNil(d["D"])
+
+    XCTAssertEqual(
+      resolveCallHistory.map(String.init(describing:)), // quick/dirty workaround: tuples aren't Equatable
+      expectedCallHistory.map(String.init(describing:))
+    )
+  }
+
+  func testThrowingFromKeyFunction() {
+    let input = ["Apple", "Banana", "Cherry"]
+    let error = SampleError()
+
+    XCTAssertThrowsError(
+      try input.keyed(by: { (_: String) -> Character in throw error })
+    ) { thrownError in
+      XCTAssertIdentical(error, thrownError as? SampleError)
+    }
+  }
+
+  func testThrowingFromCombineFunction() {
+    let input = ["Apple", "Avocado", "Banana", "Cherry"]
+    let error = SampleError()
+
+    XCTAssertThrowsError(
+      try input.keyed(by: { $0.first! }, resolvingConflictsWith: { _, _, _ in throw error })
+    ) { thrownError in
+      XCTAssertIdentical(error, thrownError as? SampleError)
+    }
+  }
+}