Introduce a new lastBy() function (#223)
Similar to groupBy(), except that it only keeps the latest value
corresponding to a given key.
Prior art: [Kotlin](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/associate-by.html)
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 806a8b0..f36e987 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
## 1.16.1-dev
+* Add a top-level `lastBy()` function that converts an `Iterable` to a `Map` by
+ grouping its elements using a function, keeping the last element for each
+ computed key. Also available as an extension method on `Iterable`.
+
## 1.16.0
* Add an `Iterable.slices` extension method.
diff --git a/lib/src/functions.dart b/lib/src/functions.dart
index 64a38ee..8f60b26 100644
--- a/lib/src/functions.dart
+++ b/lib/src/functions.dart
@@ -41,6 +41,13 @@
return result;
}
+/// Associates the elements in [values] by the value returned by [key].
+///
+/// Returns a map from keys computed by [key] to the last value for which [key]
+/// returns that key.
+Map<T, S> lastBy<S, T>(Iterable<S> values, T Function(S) key) =>
+ {for (var element in values) key(element): element};
+
/// Groups the elements in [values] by the value returned by [key].
///
/// Returns a map from keys computed by [key] to a list of all values for which
diff --git a/lib/src/iterable_extensions.dart b/lib/src/iterable_extensions.dart
index ac82112..88f320d 100644
--- a/lib/src/iterable_extensions.dart
+++ b/lib/src/iterable_extensions.dart
@@ -7,6 +7,7 @@
import 'package:collection/src/utils.dart';
import 'algorithms.dart';
+import 'functions.dart' as functions;
/// Extensions that apply to all iterables.
///
@@ -353,6 +354,12 @@
return null;
}
+ /// Associates the elements in [this] by the value returned by [key].
+ ///
+ /// Returns a map from keys computed by [key] to the last value for which [key]
+ /// returns that key.
+ Map<K, T> lastBy<K>(K Function(T) key) => functions.lastBy(this, key);
+
/// Groups elements by [keyOf] then folds the elements in each group.
///
/// A key is found for each element using [keyOf].
diff --git a/test/extensions_test.dart b/test/extensions_test.dart
index 1ccebde..ce4b2e9 100644
--- a/test/extensions_test.dart
+++ b/test/extensions_test.dart
@@ -520,6 +520,25 @@
expect(iterable([1, 3, 5]).singleOrNull, null);
});
});
+ group('.lastBy', () {
+ test('empty', () {
+ expect(iterable([]).lastBy((dynamic _) {}), {});
+ });
+ test('single', () {
+ expect(iterable([1]).lastBy(toString), {
+ '1': 1,
+ });
+ });
+ test('multiple', () {
+ expect(
+ iterable([1, 2, 3, 4, 5]).lastBy((x) => x.isEven),
+ {
+ false: 5,
+ true: 4,
+ },
+ );
+ });
+ });
group('.groupFoldBy', () {
test('empty', () {
expect(iterable([]).groupFoldBy(unreachable, unreachable), {});
diff --git a/test/functions_test.dart b/test/functions_test.dart
index 97bb4ab..9d41625 100644
--- a/test/functions_test.dart
+++ b/test/functions_test.dart
@@ -79,6 +79,26 @@
});
});
+ group('lastBy()', () {
+ test('returns an empty map for an empty iterable', () {
+ expect(
+ lastBy([], (_) => fail("Must not be called for empty input")),
+ isEmpty,
+ );
+ });
+
+ test("keeps the latest element for the function's return value", () {
+ expect(
+ lastBy(['foo', 'bar', 'baz', 'bop', 'qux'],
+ (String string) => string[1]),
+ equals({
+ 'o': 'bop',
+ 'a': 'baz',
+ 'u': 'qux',
+ }));
+ });
+ });
+
group('groupBy()', () {
test('returns an empty map for an empty iterable', () {
expect(groupBy([], expectAsync1((dynamic _) {}, count: 0)), isEmpty);
