class PathResolver {

    String relativePath(String path1, String path2) {
        [path1, path2].each{ checkPath(it) }
        def (from, to) = cutCommonBaseDir(path1, path2)
        (baseDir(from).replaceAll('[^/]+/', '../') ?: './') + to
    }

    private checkPath(path) {
        if (!path.startsWith('/')) {
            throw new IllegalArgumentException("It's not an absolute path: $path")
        }
        if (!path) {
            throw new IllegalArgumentException("Empty: $path")
        }
        if (path.find($/[\?*:|<>]/$)) {
            throw new IllegalArgumentException("Invalid chars: $path")
        }
    }

    private cutCommonBaseDir(path1, path2) {
        def commonBaseDir = commonBaseDir(path1, path2)
        [path1, path2].collect{ it.replaceFirst(commonBaseDir, '') }
    }

    private String commonBaseDir(path1, path2) {
        def result = []
        def splitPathToken = { baseDir(it).split('/') }
        def tokens1 = splitPathToken(path1)
        def tokens2 = splitPathToken(path2)
        for (int i : 0..<[tokens1, tokens2]*.size().min()) {
            if (tokens1[i] != tokens2[i]) break
            result << tokens1[i]
        }
        result << ''
        result.join('/')
    }

    private String baseDir(path) {
        path.replaceFirst('[^/]*$', '')
    }
}