Greedy Sad Man is a Cairo challenge that presents a simple contract representing a person accepting donations.
As the description states, the objective is to set a sadness storage member to false.
Looking at the contract source code, we see that the implementation is short and simple, containing only three external
methods, of which only one seems to modify the state, the donate one. Since we have another file in the challenge where
a StorageArray type is implemented, and this type is used for the donations register, we can assume the clue should
be there. In this file, at line 59 in the read_at implementation we see something weird, which is a TStore::write
in a method that is supposed to read, we have our clue. The method is writing to storage the default value of the
generic T in the index position.
Back to our main contract, we see that the method using read_at is get_donation_by_index, and we can set the index.
We also see that donations is a StorageArray<felt252>, and since the default for felt252 is 0, if we pass
the position of the sadness storage member as index, the value of it will be set to 0, which is false in
bool representation. All we have left is to find what is the right index to pass, which as mentioned is the storage
address of sadness, being this the sn_keccak('sadness'), which can be computed in Cairo using the selector! macro.
The solution is then calling get_donation_by_index(selector!('sadness')).
Note that in Cairo, storage can be written to, even when the external function seems to be readonly (having self: @ContractState as first parameter).