sanitize path

//example:
/a/b/..       -> /a/
/a/b/./       -> /a/b/
/a/b//        -> /a/b/
/a/b/../../   -> /
/////         -> /
/a/b/../../.. -> throw

string sanitize-path(path):
    if path == null || (path = path.trim()) == "":
        throw
    // assume split removes empty entries
    tokens = path.split("/")
    target = 0
    for i = 0, i < tokens.length, i++:
        if tokens[i] == ".":
            // do nothing
        else if tokens[i] == "..":
            target--
            if target < 0:
                throw "invalid input"
        else:
            tokens[target] == tokens[i]
            target++
    buffer = new
    buffer.append("/")
    for i = 0, i < target, i++:
        buffer.append(tokens[i])
        buffer.append("/")
    return buffer.tostring()

[Hat tip to LM]