From d1f6deb47d90a57acd1774f3f51315ef941a946c Mon Sep 17 00:00:00 2001
From: Alexey Kopytko <alexey@kopytko.com>
Date: Tue, 13 Jun 2023 11:51:40 +0900
Subject: [PATCH] Add skipWhile() (#130)

Fixes #129
---
 README.md                            |  1 +
 src/Standard.php                     | 29 +++++++++++
 tests/Helper/RunningVarianceTest.php |  8 +--
 tests/SkipWhileTest.php              | 73 ++++++++++++++++++++++++++++
 4 files changed, 105 insertions(+), 6 deletions(-)
 create mode 100644 tests/SkipWhileTest.php

diff --git a/README.md b/README.md
index 36131e6..61fd732 100644
--- a/README.md
+++ b/README.md
@@ -118,6 +118,7 @@ All entry points always return an instance of the pipeline.
 | `unpack()`  | Unpacks arrays into arguments for a callback. Flattens inputs if no callback provided. |             |
 | `chunk()` | Chunks the pipeline into arrays of specified length. | `array_chunk` |
 | `filter()`  | Removes elements unless a callback returns true. Removes falsey values if no callback provided.  |  `array_filter`, `Where`                |
+| `skipWhile()` | Skips elements while the predicate returns true, and keeps everything after the predicate return false just once. |  | 
 | `slice()`  | Extracts a slice from the inputs. Keys are not discarded intentionally. Suppors negative values for both arguments. |  `array_slice`                |
 | `fold()`  | Reduces input values to a single value. Defaults to summation. Requires an initial value. | `array_reduce`, `Aggregate`, `Sum` |
 | `reduce()`  | Alias to `fold()` with a reversed order of arguments. | `array_reduce` |
diff --git a/src/Standard.php b/src/Standard.php
index 486cad1..c9d3a6a 100644
--- a/src/Standard.php
+++ b/src/Standard.php
@@ -446,6 +446,35 @@ private static function resolvePredicate(?callable $func): callable
         return $func;
     }
 
+    /**
+     * Skips elements while the predicate returns true, and keeps everything after the predicate return false just once.
+     *
+     * @param callable $predicate a callback returning boolean value
+     */
+    public function skipWhile(callable $predicate): self
+    {
+        // No-op: an empty array or null.
+        if ($this->empty()) {
+            return $this;
+        }
+
+        $predicate = self::resolvePredicate($predicate);
+
+        $this->filter(static function ($value) use ($predicate): bool {
+            static $done = false;
+
+            if ($predicate($value) && !$done) {
+                return false;
+            }
+
+            $done = true;
+
+            return true;
+        });
+
+        return $this;
+    }
+
     /**
      * Reduces input values to a single value. Defaults to summation. This is a terminal operation.
      *
diff --git a/tests/Helper/RunningVarianceTest.php b/tests/Helper/RunningVarianceTest.php
index 42fa946..6aff20a 100644
--- a/tests/Helper/RunningVarianceTest.php
+++ b/tests/Helper/RunningVarianceTest.php
@@ -242,11 +242,7 @@ public function testMullerTransform(int $count, float $mean, float $sigma): void
                 $numbers
             ), $sigma / 10);
         } catch (Throwable $e) {
-            if ($mean > 1E10) {
-                $this->markTestSkipped($e->getMessage());
-            }
-
-            throw $e;
+            $this->assertGreaterThan(1E10, $mean, "Naive standard deviation calculation failed where it should not: {$e->getMessage()}");
         }
     }
 
@@ -261,7 +257,7 @@ public function testMullerTransform(int $count, float $mean, float $sigma): void
     private static function getRandomNumbers(float $mean, float $sigma): iterable
     {
         $two_pi = 2 * M_PI;
-        $epsilon = 1E-6; // Arbitrary number
+        $epsilon = PHP_FLOAT_EPSILON;
 
         while (true) {
             do {
diff --git a/tests/SkipWhileTest.php b/tests/SkipWhileTest.php
new file mode 100644
index 0000000..6f6a500
--- /dev/null
+++ b/tests/SkipWhileTest.php
@@ -0,0 +1,73 @@
+<?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 Pipeline\Standard;
+use function Pipeline\map;
+use function Pipeline\take;
+
+/**
+ * @covers \Pipeline\Standard
+ *
+ * @internal
+ */
+final class SkipWhileTest extends TestCase
+{
+    public function testSkipEmpty(): void
+    {
+        $pipeline = new Standard();
+
+        $result = $pipeline
+            ->skipWhile(fn ($number) => 1 === $number)
+            ->toArray();
+
+        $this->assertSame([], $result);
+    }
+
+    public function testSkipNever(): void
+    {
+        $result = take([2])
+            ->skipWhile(fn ($number) => 1 === $number)
+            ->toArray();
+
+        $this->assertSame([2], $result);
+    }
+
+    public function testSkipWhileOnce(): void
+    {
+        $result = take([1, 1, 1, 2, 3, 4, 1, 2, 3])
+            ->skipWhile(fn ($number) => 1 === $number)
+            ->toArray();
+
+        $this->assertSame([2, 3, 4, 1, 2, 3], $result);
+    }
+
+    public function testSkipWhileTwice()
+    {
+        $result = map(fn () => yield from [1, 1, 1, 2, 2, 3, 4, 5, 1, 2])
+            ->skipWhile(fn ($number) => 1 === $number)
+            ->skipWhile(fn ($number) => 2 === $number)
+            ->filter(fn ($number) => 1 === $number % 2)
+            ->toArray();
+
+        $this->assertSame([3, 5, 1], $result);
+    }
+}