class RelativePathCalculator

final class RelativePathCalculator {
    
    /**
     * @psalm-var non-empty-string $baselineDirectory
     */
    private readonly string $baselineDirectory;
    
    /**
     * @psalm-param non-empty-string $baselineDirectory
     */
    public function __construct(string $baselineDirectory) {
        $this->baselineDirectory = $baselineDirectory;
    }
    
    /**
     * @psalm-param non-empty-string $filename
     *
     * @psalm-return non-empty-string
     */
    public function calculate(string $filename) : string {
        $result = implode('/', $this->parts($filename));
        assert($result !== '');
        return $result;
    }
    
    /**
     * @psalm-param non-empty-string $filename
     *
     * @psalm-return list<non-empty-string>
     */
    public function parts(string $filename) : array {
        $schemePosition = strpos($filename, '://');
        if ($schemePosition !== false) {
            $filename = substr($filename, $schemePosition + 3);
            assert($filename !== '');
        }
        $parentParts = explode('/', trim(str_replace('\\', '/', $this->baselineDirectory), '/'));
        $parentPartsCount = count($parentParts);
        $filenameParts = explode('/', trim(str_replace('\\', '/', $filename), '/'));
        $filenamePartsCount = count($filenameParts);
        $i = 0;
        for (; $i < $filenamePartsCount; $i++) {
            if ($parentPartsCount < $i + 1) {
                break;
            }
            $parentPath = implode('/', array_slice($parentParts, 0, $i + 1));
            $filenamePath = implode('/', array_slice($filenameParts, 0, $i + 1));
            if ($parentPath !== $filenamePath) {
                break;
            }
        }
        if ($i === 0) {
            return [
                $filename,
            ];
        }
        $dotsCount = $parentPartsCount - $i;
        assert($dotsCount >= 0);
        return array_merge(array_fill(0, $dotsCount, '..'), array_slice($filenameParts, $i));
    }

}