class DiffOpOutputBuilder

final class DiffOpOutputBuilder implements DiffOutputBuilderInterface {
    
    /**
     * A constant to manage removal+addition as a single operation.
     */
    private const CHANGED = 999;
    
    /**
     * {@inheritdoc}
     */
    public function getDiff(array $diff) : string {
        return serialize($this->toOpsArray($diff));
    }
    
    /**
     * Converts the output of Differ to an array of DiffOp* value objects.
     *
     * @param array $diff
     *   The array output of Differ::diffToArray().
     *
     * @return \Drupal\Component\Diff\Engine\DiffOp[]
     *   An array of DiffOp* value objects.
     */
    public function toOpsArray(array $diff) : array {
        $ops = [];
        $hunkMode = NULL;
        $hunkSource = [];
        $hunkTarget = [];
        for ($i = 0; $i < count($diff); $i++) {
            // Handle a sequence of removals + additions as a sequence of changes, and
            // manages the tail if required.
            if ($diff[$i][1] === Differ::REMOVED) {
                if ($hunkMode !== NULL) {
                    $ops[] = $this->hunkOp($hunkMode, $hunkSource, $hunkTarget);
                    $hunkSource = [];
                    $hunkTarget = [];
                }
                for ($n = $i; $n < count($diff) && $diff[$n][1] === Differ::REMOVED; $n++) {
                    $hunkSource[] = $diff[$n][0];
                }
                for (; $n < count($diff) && $diff[$n][1] === Differ::ADDED; $n++) {
                    $hunkTarget[] = $diff[$n][0];
                }
                if (count($hunkTarget) === 0) {
                    $ops[] = $this->hunkOp(Differ::REMOVED, $hunkSource, $hunkTarget);
                }
                else {
                    $ops[] = $this->hunkOp(self::CHANGED, $hunkSource, $hunkTarget);
                }
                $hunkMode = NULL;
                $hunkSource = [];
                $hunkTarget = [];
                $i = $n - 1;
                continue;
            }
            // When here, we are adding or copying the item. Removing or changing is
            // managed above.
            if ($hunkMode === NULL) {
                $hunkMode = $diff[$i][1];
            }
            elseif ($hunkMode !== $diff[$i][1]) {
                $ops[] = $this->hunkOp($hunkMode, $hunkSource, $hunkTarget);
                $hunkMode = $diff[$i][1];
                $hunkSource = [];
                $hunkTarget = [];
            }
            $hunkSource[] = $diff[$i][0];
        }
        if ($hunkMode !== NULL) {
            $ops[] = $this->hunkOp($hunkMode, $hunkSource, $hunkTarget);
        }
        return $ops;
    }
    
    /**
     * Returns the proper DiffOp object based on the hunk mode.
     *
     * @param int $mode
     *   A Differ constant or self::CHANGED.
     * @param string[] $source
     *   An array of strings to be changed/added/removed/copied.
     * @param string[] $source
     *   The array of strings to be changed to when self::CHANGED is specified.
     *
     * @return \Drupal\Component\Diff\Engine\DiffOp
     *   A DiffOp* value object.
     *
     * @throw \InvalidArgumentException
     *   When $mode is not valid.
     */
    private function hunkOp(int $mode, array $source, array $target) : DiffOp {
        switch ($mode) {
            case Differ::OLD:
                return new DiffOpCopy($source);
            case self::CHANGED:
                return new DiffOpChange($source, $target);
            case Differ::ADDED:
                return new DiffOpAdd($source);
            case Differ::REMOVED:
                return new DiffOpDelete($source);
        }
        throw new \InvalidArgumentException("Invalid \$mode {$mode} specified");
    }

}