Add strain (#589)

* Add `strain`

* Reapply formatting

* Swap stub and example

config.json

@@ -301,6 +301,14 @@
         "prerequisites": [],
         "difficulty": 2
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "3c03a3f1-972e-44d0-a558-43480d862021",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "sum-of-multiples",
         "name": "Sum of Multiples",

exercises/practice/strain/.docs/instructions.md

@@ -0,0 +1,29 @@
+# Instructions
+
+Implement the `keep` and `discard` operation on collections.
+Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.
+
+For example, given the collection of numbers:
+
+- 1, 2, 3, 4, 5
+
+And the predicate:
+
+- is the number even?
+
+Then your keep operation should produce:
+
+- 2, 4
+
+While your discard operation should produce:
+
+- 1, 3, 5
+
+Note that the union of keep and discard is all the elements.
+
+The functions may be called `keep` and `discard`, or they may need different names in order to not clash with existing functions or concepts in your language.
+
+## Restrictions
+
+Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
+Solve this one yourself using other basic tools instead.

exercises/practice/strain/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "lib/strain.dart"
+    ],
+    "test": [
+      "test/strain_test.dart"
+    ],
+    "example": [
+      ".meta/lib/example.dart"
+    ]
+  },
+  "blurb": "Implement the `keep` and `discard` operation on collections.",
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "http://graysoftinc.com/"
+}

exercises/practice/strain/.meta/lib/example.dart

@@ -0,0 +1,17 @@
+class Strain {
+  List<T> keep<T>(List<T> list, bool Function(T) predicate) {
+    final result = <T>[];
+
+    for (final item in list) {
+      if (predicate(item)) {
+        result.add(item);
+      }
+    }
+
+    return result;
+  }
+
+  List<T> discard<T>(List<T> list, bool Function(T) predicate) {
+    return keep(list, (item) => !predicate(item));
+  }
+}

exercises/practice/strain/.meta/tests.toml

@@ -0,0 +1,52 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[26af8c32-ba6a-4eb3-aa0a-ebd8f136e003]
+description = "keep on empty list returns empty list"
+
+[f535cb4d-e99b-472a-bd52-9fa0ffccf454]
+description = "keeps everything"
+
+[950b8e8e-f628-42a8-85e2-9b30f09cde38]
+description = "keeps nothing"
+
+[92694259-6e76-470c-af87-156bdf75018a]
+description = "keeps first and last"
+
+[938f7867-bfc7-449e-a21b-7b00cbb56994]
+description = "keeps neither first nor last"
+
+[8908e351-4437-4d2b-a0f7-770811e48816]
+description = "keeps strings"
+
+[2728036b-102a-4f1e-a3ef-eac6160d876a]
+description = "keeps lists"
+
+[ef16beb9-8d84-451a-996a-14e80607fce6]
+description = "discard on empty list returns empty list"
+
+[2f42f9bc-8e06-4afe-a222-051b5d8cd12a]
+description = "discards everything"
+
+[ca990fdd-08c2-4f95-aa50-e0f5e1d6802b]
+description = "discards nothing"
+
+[71595dae-d283-48ca-a52b-45fa96819d2f]
+description = "discards first and last"
+
+[ae141f79-f86d-4567-b407-919eaca0f3dd]
+description = "discards neither first nor last"
+
+[daf25b36-a59f-4f29-bcfe-302eb4e43609]
+description = "discards strings"
+
+[a38d03f9-95ad-4459-80d1-48e937e4acaf]
+description = "discards lists"

exercises/practice/strain/lib/strain.dart

@@ -0,0 +1,3 @@
+class Strain {
+  // your code here...
+}

exercises/practice/strain/pubspec.yaml

@@ -0,0 +1,5 @@
+name: 'strain'
+environment:
+  sdk: '>=3.2.0 <4.0.0'
+dev_dependencies:
+  test: '<2.0.0'

exercises/practice/strain/test/strain_test.dart

@@ -0,0 +1,149 @@
+import 'package:strain/strain.dart';
+import 'package:test/test.dart';
+
+void main() {
+  final strain = Strain();
+
+  group('Strain', () {
+    group('Keep', () {
+      test('keep on empty list returns empty list', () {
+        final values = [];
+        final fn = (x) => true;
+        final result = strain.keep(values, fn);
+        final expected = [];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps everything', () {
+        final values = [1, 3, 5];
+        final fn = (x) => true;
+        final result = strain.keep(values, fn);
+        final expected = [1, 3, 5];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps nothing', () {
+        final values = [1, 3, 5];
+        final fn = (x) => false;
+        final result = strain.keep(values, fn);
+        final expected = [];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps first and last', () {
+        final values = [1, 2, 3];
+        final fn = (x) => x % 2 == 1;
+        final result = strain.keep(values, fn);
+        final expected = [1, 3];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps neither first nor last', () {
+        final values = [1, 2, 3];
+        final fn = (x) => x % 2 == 0;
+        final result = strain.keep(values, fn);
+        final expected = [2];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps strings', () {
+        final values = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"];
+        final fn = (String x) => x.startsWith('z');
+        final result = strain.keep(values, fn);
+        final expected = ["zebra", "zombies", "zealot"];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('keeps lists', () {
+        final values = [
+          [1, 2, 3],
+          [5, 5, 5],
+          [5, 1, 2],
+          [2, 1, 2],
+          [1, 5, 2],
+          [2, 2, 1],
+          [1, 2, 5]
+        ];
+        final fn = (List<int> x) => x.contains(5);
+        final result = strain.keep(values, fn);
+        final expected = [
+          [5, 5, 5],
+          [5, 1, 2],
+          [1, 5, 2],
+          [1, 2, 5]
+        ];
+        expect(result, equals(expected));
+      }, skip: false);
+    });
+
+    group('Discard', () {
+      test('discard on empty list returns empty list', () {
+        final values = [];
+        final fn = (x) => true;
+        final result = strain.discard(values, fn);
+        final expected = [];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards everything', () {
+        final values = [1, 3, 5];
+        final fn = (x) => true;
+        final result = strain.discard(values, fn);
+        final expected = [];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards nothing', () {
+        final values = [1, 3, 5];
+        final fn = (x) => false;
+        final result = strain.discard(values, fn);
+        final expected = [1, 3, 5];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards first and last', () {
+        final values = [1, 2, 3];
+        final fn = (x) => x % 2 == 1;
+        final result = strain.discard(values, fn);
+        final expected = [2];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards neither first nor last', () {
+        final values = [1, 2, 3];
+        final fn = (x) => x % 2 == 0;
+        final result = strain.discard(values, fn);
+        final expected = [1, 3];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards strings', () {
+        final values = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"];
+        final fn = (String x) => x.startsWith('z');
+        final result = strain.discard(values, fn);
+        final expected = ["apple", "banana", "cherimoya"];
+        expect(result, equals(expected));
+      }, skip: false);
+
+      test('discards lists', () {
+        final values = [
+          [1, 2, 3],
+          [5, 5, 5],
+          [5, 1, 2],
+          [2, 1, 2],
+          [1, 5, 2],
+          [2, 2, 1],
+          [1, 2, 5]
+        ];
+        final fn = (List<int> x) => x.contains(5);
+        final result = strain.discard(values, fn);
+        final expected = [
+          [1, 2, 3],
+          [2, 1, 2],
+          [2, 2, 1]
+        ];
+        expect(result, equals(expected));
+      }, skip: false);
+    });
+  });
+}