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When the Bitcoin client looks at the input of a transaction, it know the hash of the previous transaction from which the funds are coming, however then, how does it know that this output has not been spent already? Does it need to scan the whole blocklist for another transaction spending that same output? Isn't that very inefficient?
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Bitcoin-Qt marks outputs as spent in the database once they are spent. To check whether an input's prev-out is spent, Bitcoin just needs to look up the transaction and check whether the output is marked spent.
Other ways of doing it are to have a set of all unspent outputs and check whether the prev-out is in this set (future versions of Bitcoin will do something like this), or to index inputs by their prev-outs.
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It is similar to the question here
Suppose A, a peer, receives 'B' BTCs from various transactions: B1,B2,B3, etc. Also, A had spent 'S' BTCs in various transactions S1,S2,S3, etc. Now, whenever a new transaction is initiated from A to C (eg: A wants to transfer X bitcoins to C), the transaction has to be verified as follows.
Please refer to this link as well- Good article on how individual transactions are processed.
Thanks for clarifying. So the sum of all unspent outputs is the balance for an address? – Philip Frank – 2013-01-25T00:16:32.733
@bananer Yes, that's right. – theymos – 2013-01-25T00:19:02.063
Where does this appear in the code? – Kinnard Hockenhull – 2013-01-25T02:41:44.297
@KinnardHockenhull For 0.7.x and below, search for vSpent. – theymos – 2013-01-25T04:52:16.580
So I imagine there is some sort of hashtable indexed on the transaction hash, with some flag indicating if it was spent. – Flavien – 2013-01-25T09:59:08.707