parse FIX msg to key/value pairS #repeating group

http://bigblog.tanbin.com/2012/03/design-considerations-my-first-homemade.html is the context.

    /**
     * Parsing to array is faster than parsing to a HashMap
     */
    public static ArrayList quickParse(String fromServer) {
        ArrayList ret = new ArrayList(Collections.nCopies(
            MAX_FIX_TAG, (String) null));
        for (String pair : fromServer.split(SOH)) {
            String[] tmp = pair.split("=");
            if (tmp.length != 2)  continue;
            int tag = Integer.parseInt(tmp[0]);
            ret.set(tag, tmp[1]); //treat ArrayList as array -- efficient
        }
        return ret;
    }

Repeating group is not supported in this simplistic scheme, but could be accommodated. I can think one simple and one robust design:

Design 1: array<list<string>>, so a regular non-repeating tag will hit a singular list.

Design 2: we ought to know which tags are repeating. The message specification must tell us which tag (NoRoutingIds in the example) signals the arrival of a repeating group. Therefore, at compile time (not runtime) system knows what tags are special i.e. potentially repeating. We will build a separate data structure for them, such as a list<groupMember>. So each FIX message will have one regular array + one list

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