scan() generates a sequence of accumulated values

[sanmai]

$numbers = Pipeline\fromValues(1, 2, 3, 4, 5);

$runningTotal = $numbers->scan(function ($accumulator, $value) {
    return $accumulator + $value;
}, 0);

foreach ($runningTotal as $total) {
    echo $total . ', ';
}
// Output: 0, 1, 3, 6, 10, 15,

Possible implementation:

/**
 * Performs a scan operation on the pipeline.
 *
 * @param callable $func Function that takes the accumulator and an element and returns the new accumulator.
 * @param mixed $initial Initial value for the accumulator.
 * @return self
 */
public function scan(callable $func, $initial = null): self
{
    if (null === $initial) {
        $this->pipeline = self::makeNonRewindable($this->pipeline);
        $initial = $this->pipeline->current();
        $this->pipeline->next();
    }

    $this->pipeline = self::performScan($this->pipeline, $func, $initial);

    return $this;
}

private static function performScan(Generator $input, callable $func, $accumulator): Generator
{
    yield $accumulator;
    foreach ($input as $value) {
        $accumulator = $func($accumulator, $value);
        yield $accumulator;
    }
}