Add flip() (#100)

src/Standard.php

@@ -20,6 +20,7 @@
 namespace Pipeline;
 
 use function array_filter;
+use function array_flip;
 use function array_map;
 use function array_reduce;
 use function array_shift;
@@ -773,4 +774,37 @@ public function max()
 
         return $max;
     }
+
+    /**
+     * @return $this
+     */
+    public function flip()
+    {
+        if (null === $this->pipeline) {
+            // No-op: null.
+            return $this;
+        }
+
+        if ([] === $this->pipeline) {
+            // No-op: an empty array.
+            return $this;
+        }
+
+        if (is_array($this->pipeline)) {
+            $this->pipeline = array_flip($this->pipeline);
+
+            return $this;
+        }
+
+        $this->pipeline = self::flipKeysAndValues($this->pipeline);
+
+        return $this;
+    }
+
+    private static function flipKeysAndValues(iterable $previous): iterable
+    {
+        foreach ($previous as $key => $value) {
+            yield $value => $key;
+        }
+    }
 }

tests/FlipTest.php

@@ -0,0 +1,149 @@
+<?php
+/**
+ * Copyright 2017, 2018 Alexey Kopytko <alexey@kopytko.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+declare(strict_types=1);
+
+namespace Tests\Pipeline;
+
+use function array_flip;
+use function array_reverse;
+use ArrayIterator;
+use function call_user_func;
+use function chr;
+use function count;
+use IteratorIterator;
+use PHPUnit\Framework\TestCase;
+use function Pipeline\map;
+use Pipeline\Standard;
+use function Pipeline\take;
+use function shuffle;
+
+/**
+ * @covers \Pipeline\Standard
+ *
+ * @internal
+ */
+final class FlipTest extends TestCase
+{
+    public function testFlipDiscardKeys(): void
+    {
+        $keys = map(static function () {
+            yield 'a' => 1;
+            yield 'b' => 2;
+            yield 'c' => 2;
+            yield 'd' => 3;
+        })->flip()->toArray();
+
+        $this->assertSame(['a', 'b', 'c', 'd'], $keys);
+    }
+
+    public function testFlipRespectKeys(): void
+    {
+        $keys = map(static function () {
+            yield 'a' => 1;
+            yield 'b' => 2;
+            yield 'c' => 2;
+            yield 'd' => 3;
+        })->flip()->toArray(true);
+
+        $this->assertSame([1 => 'a', 'c', 'd'], $keys);
+    }
+
+    private function provideInputs(): iterable
+    {
+        yield [];
+
+        yield ['one' => 1, 'two' => 2, 3 => 1, 'index' => 1];
+        yield ['key' => 1, 'two' => 'TWO', 'three' => 3];
+        yield [
+            'one',
+            1 => 'int_key',
+            -2 => 'negative_key',
+            8.9 => 'float_key',
+            012 => 'octal_key',
+            0x34 => 'hex_key',
+            'key' => 'string_key1',
+            'two' => 'string_key2',
+            '' => 'string_key3',
+            ' ' => 'string_key4',
+            'a'.chr(0).'b' => 'binary_key1',
+        ];
+
+        yield [1 => 'value', 2 => 'VALUE', 3 => 4];
+
+        yield ['key' => 1, 'two' => 'TWO', 'three' => 3, 'key1' => 'FOUR'];
+
+        yield ['key1' => 'value1', 'key2' => '2', 'key3' => 'value1'];
+    }
+
+    private function provideRandomizedInputs(): iterable
+    {
+        foreach ($this->provideInputs() as $input) {
+            yield $input;
+
+            if ([] === $input) {
+                continue;
+            }
+
+            yield array_reverse($input);
+
+            if (count($input) <= 2) {
+                continue;
+            }
+
+            shuffle($input);
+
+            yield $input;
+        }
+    }
+
+    public function provideFlipInputs(): iterable
+    {
+        foreach ($this->provideRandomizedInputs() as $input) {
+            $expected = array_flip($input);
+
+            yield [$expected, $input];
+
+            yield [$expected, new ArrayIterator($input)];
+
+            yield [$expected, new IteratorIterator(new ArrayIterator($input))];
+
+            yield [$expected, call_user_func(function () use ($input) {
+                yield from $input;
+            })];
+
+            yield [$expected, call_user_func(function () use ($input) {
+                foreach ($input as $key => $value) {
+                    yield $key => $value;
+                }
+            })];
+        }
+    }
+
+    /**
+     * @dataProvider provideFlipInputs
+     */
+    public function testFlip(array $expected, iterable $input): void
+    {
+        $this->assertSame($expected, take($input)->flip()->toArray(true));
+    }
+
+    public function testNonPrimedFlip(): void
+    {
+        $this->assertSame([], (new Standard())->flip()->toArray());
+    }
+}