a case study from a blockchain company interview

[astr0-g]

I am applying for solidity developer jobs and one of the company gave me a small case study. I will put the case study requirements below:

Bingo Game Exercise

Design and implement the smart contract for a decentralized, provably fair Bingo game. Bingo is a luck-based game in which players match a randomized board of numbers with random numbers drawn by a host. The first player to achieve a line of numbers on their board and claim Bingo wins. Please adhere to the following rules and requirements:

**Bingo rules:**

- 5x5 board, middle spot is free
- Sequence of 5 marked numbers in a row, column, or diagonal to win

**Project requirements:**

- Support multiple players in a game
- Support multiple concurrent games
- Each player pays an ERC20 entry fee, transferred on join
- Winner wins the pot of entry fees, transferred on win
- Games have a minimum join duration before start
- Games have a minimum turn duration between draws
- Admin can update the entry fee, join duration, and turn duration

**For the sake of this exercise, feel free to make the following assumptions:**

- Players are online to claim a winning board
- Each game number may be between 0 and 255, inclusive (a byte)
- Boards may have duplicate numbers that can be marked by one drawn number
- Duplicate numbers may be drawn, but have no effect on the game
- Random numbers may be generated with `blockhash(block.number - 1)`

In addition to the game contract, provide an interface that exposes a public API, including the necessary getters and events to allow a frontend to render the current and historical game state. Include a summary of the worst-case gas costs for game operations.

I don't know if this is ok to post this discussion in this space, but here is the only space I know, feel free to ask me to delete this.

Ok, the thing is I got rejected by the company because their staff think I was a bit lacking in gas optimization (should be <1m) and understanding in random numbers. I am really appeciate if anyone has any ideas or suggestions for the smart contract, so I can learn and study from you guys.

Here is my repository , please only focus on the smart contracts.

[pacelliv]

For now I can see that you declared all variables as public. You already know many values so you can declare many as constant. Also, you have a couple of variables that will have a permanent value after construction, these can be declared as immutable. This should reduce some cost by embedding these variables in the bytcode.

I've not run the test, to get the gas-report but something tells me that drawWinner is a expensive function. I know you told us to focus on the contracts, but I will like to run the tests to get the gas-report; in the tests I saw many for-loop can I reduce the number of iterations ?

[astr0-g]

For constant and public variables, I did want to make them constant or immutable, but it says ‘Admin can update the entry fee, join duration, and turn duration’ in the requirements, and for the maximum number players and Boolean returnbet, I made them all to be changeable too because of it really needs to be changeable in the reality cases.

and for the loop in the test, feel free to change them on your needs. But for the contract ifself, do you think do while functions for drawwinner or generate numbers function could be replace?

[mattjaf]

Oh, that a pretty cool exercise, how long did they give you?

VRF for the win!

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couldn't you do it with an array of 25 numbers generated from chainlink VRF using modulo,

hmm, if elements 0-4, 5-9, 10-14, 15-19, 20-24 (across) || elements [0,5,10,15,20], [1,6,11,16,21], [2,7,12,17,22], [3,8,13,18,23], [4,9,14,19,24] (vertical) || [0,6,12,18,24] or [4,8,12,16,20] (diagonal)

middle spot is free

[mattjaf]

You gotta at least get it under 20 mill or it won't even work on polygon.

Also know that view functions cost gas when u call them within a mutating function,

Try to avoid using loops and reduce the big O notation in terms of complexity

Even though they said u could use that boo boo RNG logic, I feel like it's a trap.

I haven't looked too deep into your RNG logic, chances are the way u did it is not secure what so ever.

RNG can become a huge security vulnerability, if it's not done right and doing it wrong even though the instructions tell you can, is straight up negligence and a poor reflection on work ethics.

It's like if your employee tell you to do some stuff that doesn't make sense, logical explain explain why and provide an alternative solution

[astr0-g]

Try to avoid using loops

sould I just list them out?

reduce the big O notation in terms of complexity

got it

I feel like it's a trap.

I feel the same now

[mattjaf]

What do u mean by "list them out"

[astr0-g]

like change
gameRounds[_gameRound].players[_player].gameBoardMatchs[ i * 5 + 0 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ i * 5 + 1 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ i * 5 + 2 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ i * 5 + 3 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ i * 5 + 4 ] == 1
to
gameRounds[_gameRound].players[_player].gameBoardMatchs[ 0 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ 1 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ 2 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ 3 ] == 1 && gameRounds[_gameRound].players[_player].gameBoardMatchs[ 4 ] == 1 ..........

just rewrite the loop, I know this is not your points tho, I just don't other ways to make it better

[mattjaf]

If they won they won, pretty sure you don't have to loop thru every element to prove that