Gas used for the same deterministic operation is different

Question

standard ICO function which exchanges Ether to ERC20 token balance.

Call 1, used 49431: https://ropsten.etherscan.io/tx/0x6671c8fd39d195b09868d914d80f0e884e42259c7346b69afb483f2e18fae263

Call 2, used 34431: https://ropsten.etherscan.io/tx/0x990f9eebf09968cdfc29a4d2b4f0090b96e4cdff196fb2a0a10ea1a1d7f02164

Same code, same wallets.

Does it mean that gas consumption of working with mapping structure isn't deterministic? it's because it's hashmap?

Answer 1

This is expected behaviour. The difference is 15.000. It's because the first call is changing a slot in storage from 0x0 to some value (cost = 20.000). The second call is changing the same, already initialized slot, which costs 5.000. 15.000 less. If you make a third call, it will cost the same like the second.