Subject: Re: [Bitcoin-development] Ultimate Blockchain Compression w/ trust-free lite node

From: Alan Reiner <[email-masked]>
Date: Tue, 19 Jun 2012 14:12:19 -0400

On 06/19/2012 01:59 PM, Gregory Maxwell wrote:
> On Tue, Jun 19, 2012 at 1:33 PM, Alan Reiner<[email-masked]>  wrote:
>>   One app developer updates their
>> RB tree code which updated the RB-tree optimizations/rebalancing, and
>> now a significant portion of the network can't agree on the correct
>> root.  Not only would that be disruptive, it would be a disaster to
>> track down.
> This is why good comprehensive tests and a well specified algorithim
> are important. The tree update algorithm would be normative in that
> scheme. Worrying that implementers might get it wrong would be like
> worrying that they'd get SHA256 wrong.

The point is not that they get it *wrong*, it's that the implement it 
*differently*.  Given a set of 100 TxOuts, there's a seemingly-infinite 
number of ways to construct a binary tree.  Put them in in a different 
order, and you get a different tree. *They're all correct and legal* in 
terms of satisfying expectations of insert, delete and query runtime -- 
but they will produce different root hashes.   And the differences in 
underlying structure are completely transparent to the calling code.

I'm extremely uncomfortable with the idea the you can have all the nodes 
in the tree, but have to replay X years of blockchain history just to 
get the same tree configuration as someone else.  However, a trie 
configuration is history-independent -- given an unspent-TxOut list, 
there's only one way to construct that tree.  That's an important 
property to me.

I can't tell if you're joking about Judy structures: I've never heard of 
them.  But I'll look into it anyway...