How to let evm execute one function from a smart contract and receive outputs in a handy way?

Question

I am a newbie to this field so I apologize in advance if this seems trivial.

Say I have written some code in Solidity, for instance something like the quick sort code (https://gist.github.com/subhodi/b3b86cc13ad2636420963e692a4d896f).

Now I want to locally run it with my evm virtual machine (due to certain reason, we prefer to run it locally with virtual machine, instead of using some web services...), so I would use commands like:

solc --bin --optimize -o . qs.sol
./evm --debug --code $(cat QuickSort.bin) run

So after executing the above commands, I do see some logs printed out; however, I am still unclear about:

1. How exactly can I specify a function to execute? In the above quick sort case I only have one "public" function, but what if I have many?

2. How to pass in parameters into the evm when executing certain functions?

3. Is there any convenient way I can expect to read some output? For instance can evm return a sorted array and somehow print it out, or write into some log files?

Answer 1

Some theory basics are confusing you.

Everything "runs" on the EVM which is a distributed state machine. You can set up a local private blockchain or ganache-cli or even Remix to "run" locally in an isolated context.

Your constant function sort() is read-only and therefore will always run locally in all contexts. constant is deprecated. You could use pure.

To use the function, you would sign and call sort(arguments[]) using Web3. It should return a sorted array if everything goes as planned.

I would caution that this is not an ideal project for getting acquainted with Ethereum and also possibly a horrible idea for a production system owing to the gas cost and inherent limitations of performing this sort process on-chain. Sort should be avoided if at all possible. It usually is possible. https://blog.b9lab.com/the-joy-of-minimalism-in-smart-contract-design-2303010c8b09

Hope it helps.

Answer 2

Regarding your first two points: Parameters are passed via so-called "Call Data". Call data is retrieved via certain EVM opcodes (CALLDATACOPY, CALLDATASIZE, CALLDATALOAD). In your testing environment, using the evm tool, you may provide call data with the --input parameter, eg: --input 11223344 will provide 4 bytes of data, 0x11223344.

The public ABI of Ethereum contracts specify that the first 4 bytes of call data are your public function selector (calculated as the top 4-bytes of the Keccak256 hash of a public function normalized prototype). That hash is provided by solc when you compile your source.

That should answer points 1 and 2. You will find more information on public ABI here: https://solidity.readthedocs.io/en/latest/abi-spec.html