[Contract]- Optional entry fee per participant

[grantfox-oss]

prediction::join_event (src/prediction.rs) is currently free. Many
prediction-campaign formats let the creator charge a small entry fee that gets
pooled into the prize money, on top of whatever the creator seeded.

Goal

Let creators set an optional entry_fee. If non-zero, joining costs that much
XLM, and the fee is added to event.prize_pool.

Requirements

1. Event struct gains:

   pub entry_fee: i128, // 0 = free to join (default)

2. event::create_event gains entry_fee: i128 param. New EventError
   variant InvalidEntryFee = 19 if entry_fee < 0.

3. prediction::join_event changes:

   • If event.entry_fee > 0:
     • Check TokenHelper::has_sufficient_balance(env, &xlm_token, &user, event.entry_fee),
       else return new PredictionError::InsufficientEntryFeeBalance = 14
     • Transfer entry_fee from user to env.current_contract_address() (escrow)
     • event.prize_pool = event.prize_pool.checked_add(entry_fee).ok_or(PredictionError::Overflow)?
   • Persist the updated event (prize_pool change).
   • Emit (Symbol("event"), Symbol("joined")) with
     (event_id, user, entry_fee_paid).

4. Important for Issue 6: finalize_event must read event.prize_pool at
   finalize time (which now reflects creator seed + all collected entry fees),
   not a value cached at creation. If Issue 6 lands first, file a quick
   follow-up to confirm it already reads the live field (it should, since both
   issues operate on the same Event.prize_pool field).

Acceptance criteria

• entry_fee == 0 → joining behaves exactly as before (free)
• entry_fee > 0 → joining charges the user and grows event.prize_pool
• Insufficient balance is rejected before any state change
• get_event_prize_pool (Issue 5) reflects accumulated entry fees

Testing checklist (tests/prediction_tests.rs)

• test_join_event_free_when_entry_fee_zero
• test_join_event_with_entry_fee_charges_user_and_grows_pool
• test_join_event_insufficient_balance_for_entry_fee_rejected
• test_prize_pool_reflects_creator_seed_plus_entry_fees — N users join a
  paid event, assert get_event_prize_pool == seed + N * entry_fee

[jaynomyaro]

Hi! I’m a full-stack developer with a strong blockchain background. I’d love to handle this issue and can deliver a clean, reliable solution. Kindly assign it to me.

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!

[bakarezainab]

Hi. I am a web3 fullstack developer. I have over 3 years of experience in Rust-based smart contract development, with specific expertise in token standards (STELLAR, SPL, ERC-20) and secure balance/transfer implementations on Soroban.

My approach begins by extending the Event struct with entry_fee: i128, updating create_event with validation (new InvalidEntryFee error for negative values), and modifying join_event to perform balance checks via TokenHelper::has_sufficient_balance before transferring XLM to the contract escrow using checked_add to safely grow the prize pool. I will prioritize atomic state changes, ensuring insufficient balances are rejected before any modifications, and emit the ("event", "joined") event with fee details for full traceability
Compatibility with Issue 6 is critical, so I will verify finalize_event reads the live prize_pool field rather than a cached value, filing a quick follow-up if needed. My testing strategy covers all four acceptance criteria, including multi-user scenarios to confirm get_event_prize_pool accurately reflects the creator's seed plus accumulated fees, while also testing overflow protection and edge cases like zero fees.
I will provide thorough documentation, address review feedback promptly, and ensure no regressions by running the full test suite. I am ready to deliver this feature within 1 days and will maintain clear communication throughout the process.

I am committed to delivering high-quality, well-tested code that meets all acceptance criteria and aligns with the project's goals. I will actively engage in code reviews and address any feedback promptly

[somgoli]

Hi, i just reviewed the issue description and will like to work on this. ETA: 5hours. Thank you

[nayt9]

Hello sir , my name is Nathan. I’m smart contract developer, and I’d love to contribute to this project. I

’m comfortable working with modern frontend tools and blockchain technologies, and I can deliver tasks quickly often within 24 hours.

Please let me know how I can get started or if there are any issues suitable for a first contribution.

[grantfox-oss]

🦊 GrantFox — @nayt9 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #970)
2. Your PR will be reviewed by the Arena1X maintainers

Good luck! Track your progress on GrantFox.