Quote from: Remember remember the 5th of November on December 03, 2013, 01:01:07 PM

I am writing my own blockchain parser, I wanted to export the UTXOs, I asked around in the bitcoin-dev IRC channel, but my questions got ignored(sort of).

What must I do to verify the blockchain myself? I want to for instance verify that some transaction can actually spend some funds, or that the block in which this transaction is, is not an orphan block and is part of the longest difficulty-wise chain etc.
I was suggested to use "bitcoind", but I am writing my own parser, I want to be able to do these things independently.

Note: Recreating the reference client's blockchain validation is the most nebulous part about implementing bitcoin. And the reasons why it's hard range from the lack of hand-holding documentation, the bugs that you must recreate, and - most particularly - the implications of getting it all wrong.

First of all, I'd start at the hardest part first: https://en.bitcoin.it/wiki/Script, which will have you implement https://en.bitcoin.it/wiki/OP_CHECKSIG. It will also have you implement DER encode/decode and the menagerie of utility functions you'll need along the way.

Once you're at a place where you can (valid-tx-input? x) and it will (eval-script (concat input-script output-script)), return a boolean, and handle OP_CHECKSIG - even if it's a naive implementation - then you're making good progress.

My naive approach for handling the most-valid-branch of the blockchain in the simplest way is to always append incoming blocks to my blockchain db if they validate (inputs validate, difficulty is correct, prev-outputs are unspent on this branch, block-hash is correct, etc). A fork is represented when multiple blocks point back to the same prev-block. For example, there may be multiple blocks at depth 42.

I calculate (and cache) the most-valid-branch (branch with hardest summed difficulty) with a function that I apply to the genesis block. It returns a set of those blocks in the most-valid-branch. So if Tx ABC exists in two blocks at depth 42, I need a way to get the most valid one, and it's just a matter of intersecting its parent-block with the most-valid block set.

Just know that in a couple months you might find yourself going "What the fuck am I doing?", but that's just weakness leaving the body.