Add runningCount (#127)

README.md

@@ -124,6 +124,8 @@ All entry points always return an instance of the pipeline.
 | `flip()`    | Swaps keys and values. | `array_flip` |
 | `max()`     | Finds the highest value. | `max` |
 | `min()`     | Finds the lowest value. | `min` |
+| `count()`     | Counts values. Eagerly executed.| `array_count` |
+| `runningCount()` | Counts seen values using a reference argument. | |
 | `toArray()` | Returns an array with all values. Eagerly executed. | `dict`, `ToDictionary` |
 | `toArrayPreservingKeys()` | Returns an array with all values and keys. Eagerly executed. |  |
 | `runningVariance()` | Computes online statistics: sample mean, sample variance, standard deviation. | [Welford's method](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm) |

src/Standard.php

@@ -547,6 +547,29 @@ public function toArrayPreservingKeys(): array
         return $this->toArray(true);
     }
 
+    /**
+     * Counts seen values online.
+     *
+     * @param ?int &$count the current count; initialized unless provided
+     *
+     * @param-out int $count
+     *
+     * @return $this
+     */
+    public function runningCount(
+        ?int &$count
+    ): self {
+        $count ??= 0;
+
+        $this->cast(static function ($input) use (&$count) {
+            ++$count;
+
+            return $input;
+        });
+
+        return $this;
+    }
+
     /**
      * {@inheritdoc}
      *

tests/RunningCountTest.php

@@ -0,0 +1,92 @@
+<?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 PHPUnit\Framework\TestCase;
+use function Pipeline\map;
+use function Pipeline\take;
+use function range;
+
+/**
+ * @covers \Pipeline\Standard
+ *
+ * @internal
+ */
+final class RunningCountTest extends TestCase
+{
+    public function testRunningCount(): void
+    {
+        $countEven = 1;
+
+        $pipeline = map(fn () => yield from range(0, 100))
+            ->runningCount($countAll)
+            ->filter(fn (int $n) => 0 === $n % 2)
+            ->runningCount($countEven)
+            ->filter(fn (int $n) => $n % 3);
+
+        $this->assertSame(0, $countAll);
+        $this->assertSame(1, $countEven);
+
+        $this->assertSame(34, $pipeline->count());
+
+        $this->assertSame(101, $countAll);
+        $this->assertSame(51, $countEven - 1);
+    }
+
+    public function testRunningCountLazy(): void
+    {
+        $countEven = 1;
+
+        $pipeline = map(fn () => yield from range(0, 100))
+            ->runningCount($countAll)
+            ->filter(fn (int $n) => 0 === $n % 2)
+            ->runningCount($countEven)
+            ->filter(fn (int $n) => $n % 3);
+
+        $this->assertSame(0, $countAll);
+        $this->assertSame(1, $countEven);
+
+        foreach ($pipeline as $item) {
+            $this->assertSame(2, $item);
+
+            break;
+        }
+
+        // Because we need to inspect 3 numbers to get to 2: filtered out 0 and 1
+        $this->assertSame(3, $countAll);
+        $this->assertSame(3, $countEven);
+    }
+
+    public function testRunningCountArray(): void
+    {
+        $countEven = 1;
+
+        $pipeline = take(range(0, 100))
+            ->runningCount($countAll)
+            ->filter(fn (int $n) => 0 === $n % 2)
+            ->runningCount($countEven)
+            ->filter(fn (int $n) => $n % 3);
+
+        $this->assertSame(101, $countAll);
+        $this->assertSame(51, $countEven - 1);
+
+        $this->assertSame(34, $pipeline->count());
+    }
+}