Unescape Monero json encoded string
    /monero/external/rapidjson/reader.h

    :param st:
    :return:

def unescape_json_str(st):
    """
    Unescape Monero json encoded string
    /monero/external/rapidjson/reader.h

    :param st:
    :return:
    """
    c = 0
    ln = len(st)
    escape_chmap = {
        b'b': b'\b',
        b'f': b'\f',
        b'n': b'\n',
        b'r': b'\r',
        b't': b'\t',
        b'\\': b'\\',
        b'"': b'\"',
        b'/': b'\/'
    }

    ret = []

    def at(i):
        return st[i:i+1]

    while c < ln:
        if at(c) == b'\\':
            if at(c+1) == b'u':
                ret.append(bytes([int(st[c+2:c+6], 16)]))
                # ret.append(st[c:c+6].decode('unicode_escape').encode('utf8'))
                c += 6

            else:
                ret.append(escape_chmap[at(c+1)])
                c += 2

        else:
            ret.append(at(c))
            c += 1

    df = (b''.join(ret))
    return df