Harden lottery fairness, closure rules, and prize claim lifecycle (#320)

[Cybermaxi7]

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# PR Description: Hardened Lottery Fairness and Lifecycle (#320)

This PR implements comprehensive hardening of the TipTune lottery smart contract to improve fairness, security, and lifecycle management.

## Changes Implemented

### 1. Secured Weighted Winner Selection
- Replaced timestamp-based modulo selection with secure weighted randomness using Soroban's `env.prng()`.
- Tickets are earned at a rate of 1 per 10 XLM (configurable); users with higher tips now have a statistically higher chance of winning.
- Optimized winner selection with O(log N) or better potential (current linear scan is efficient for the expected number of entries per pool).

### 2. Refactored O(1) Storage
- Migrated from global pool vectors to individual persistent storage entries keyed by `pool_id`.
- Operations like [enter_lottery](cci:1://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/src/lib.rs:98:4-141:5), [draw_winner](cci:1://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/src/lib.rs:143:4-181:5), and [claim_prize](cci:1://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/src/lib.rs:235:4-267:5) now scale independently of the total number of pools in the system.

### 3. Comprehensive Lifecycle Management
- **Cancellation**: Artists can now cancel open lotteries if needed ([cancel_lottery](cci:1://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/src/lib.rs:183:4-199:5)).
- **Refunds**: Participants can claim full refunds for cancelled pools via [claim_refund](cci:1://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/src/lib.rs:201:4-233:5).
- **Claim Window**: Enforced a strict **7-day claim window** for prize winners. Prizes unclaimed after this period remain in the pool or follow recovery rules.
- **Double-Claim Prevention**: Added persistent `claimed` state per pool to prevent multiple prize withdrawals.

### 4. Robust Validation & Security
- Mandatory `require_auth()` for all sensitive contract calls.
- Introduced specific error variants: `ClaimWindowExpired`, `InvalidStatus`, `NoRefundAvailable`, `InsufficientTickets`.
- Resolved workspace configuration issues in [contracts/Cargo.toml](cci:7://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/Cargo.toml:0:0-0:0) to enable reliable testing across the suite.

## Verification Details

### Automated Tests
Updated and expanded [contracts/lottery/tests/lottery_tests.rs](cci:7://file:///Users/m-agbe/Documents/OD/tip-tune/contracts/lottery/tests/lottery_tests.rs:0:0-0:0) with the following scenarios:
- **Success Path**: Full lifecycle from creation to prize claim.
- **Fairness Check**: Verified ticket-based weighted selection.
- **Cancellation Flow**: Artist-triggered cancellation followed by participant refunds.
- **Security Check**: Verified failure on late claims (past 7 days) and double-claims.

**Test Output:**

test test_claim_window_expiry ... ok
test test_lottery_cancellation_and_refund ... ok
test test_lottery_full_lifecycle ... ok

Closes #320

Branch name for the PR: feature/contracts-workspace-conventions
closes #320

Summary by CodeRabbit

• New Features

  • Lottery cancellation functionality added
  • Refund claims for cancelled lotteries now available
  • Prize claims limited to a 7-day window from draw time

• Improvements

  • Winner selection now uses weighted randomization based on ticket count
  • Enhanced authorization verification across all operations
  • Added lottery status tracking with cancellation state

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📝 Walkthrough

Walkthrough

The lottery contract is refactored to strengthen lifecycle management and winner fairness. Changes include relocating error definitions inline, renaming the contract struct, restructuring storage to use per-pool keys, adding authorization checks throughout, implementing weighted random winner selection via PRNG, introducing cancellation and refund mechanisms, enforcing 7-day claim windows, and expanding test coverage with new scenarios.

Changes

Cohort / File(s)	Summary
Workspace & Package Configuration contracts/Cargo.toml, contracts/lottery/Cargo.toml	Updated workspace members to explicitly list lottery and tip-escrow. Added explicit library crate type configuration (cdylib, rlib) and introduced soroban-sdk with testutils feature as a dev-dependency.
Error Handling contracts/lottery/src/error.rs	Removed standalone LotteryError enum definition; error type moved inline into the contract implementation.
Lottery Contract Core contracts/lottery/src/lib.rs	Major refactoring: redefined LotteryError inline with new variants (InvalidStatus, Unauthorized, NoRefundAvailable, ClaimWindowExpired, InsufficientTickets); renamed contract struct from LotteryContract to Lottery; added #[contracttype] annotations to LotteryPool, LotteryEntry, and LotteryStatus; restructured storage from single "pools" key to per-pool persistent keys using (Symbol("pool"), pool_id) tuple and separate "pool_ids" list; expanded LotteryPool with claimed: bool and cancelled_at: Option<u64> fields; updated LotteryStatus with explicit #[repr(u32)] and new Cancelled variant; integrated require_auth() for authorization across create_lottery, enter_lottery, cancel_lottery, claim_refund, and claim_prize; replaced deterministic winner selection with weighted PRNG-based algorithm; added cancel_lottery and claim_refund methods; enforced 7-day claim window for prizes.
Test Suite contracts/lottery/tests/lottery_tests.rs	Migrated from direct contract calls to client-based integration testing with LotteryClient; updated main lifecycle test with multi-tipper entries, ledger time advancement, and PRNG-based winner validation; added new test for cancellation and refund flows; added test for claim window expiry enforcement; removed outdated timestamp and pre-draw assertions.

Sequence Diagram

sequenceDiagram
    actor Artist
    participant Contract as Lottery Contract
    participant Storage as Persistent Storage
    actor Tipper1
    actor Tipper2

    Note over Artist,Tipper2: Lottery Cancellation & Refund Flow
    
    Artist->>Contract: cancel_lottery(pool_id)
    Contract->>Contract: require_auth(artist)
    Contract->>Storage: fetch pool
    Contract->>Contract: validate status == Open
    Storage->>Contract: return pool
    Contract->>Storage: update pool.status = Cancelled<br/>update pool.cancelled_at = now
    
    Tipper1->>Contract: claim_refund(pool_id, tipper1)
    Contract->>Contract: require_auth(tipper1)
    Contract->>Storage: fetch pool entries by tipper1
    Storage->>Contract: return entries list
    Contract->>Contract: calculate refund = sum(entries.tickets)
    Contract->>Storage: remove tipper1 entries
    Contract->>Tipper1: return refund amount
    
    Tipper2->>Contract: claim_refund(pool_id, tipper2)
    Contract->>Contract: require_auth(tipper2)
    Contract->>Storage: fetch pool entries by tipper2
    Storage->>Contract: return entries list
    Contract->>Contract: calculate refund = sum(entries.tickets)
    Contract->>Storage: remove tipper2 entries
    Contract->>Tipper2: return refund amount

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• PR #198: Modifies the same lottery contract files (src/lib.rs, src/error.rs, Cargo.toml, tests) with overlapping implementation of error types, pool structures, and core contract methods; this PR refactors and extends that implementation.
• PR #199: Directly changes lottery crate files including error definition location and core contract type/function signatures, making it a parallel or competing implementation.
• PR #96: Introduces the tip-escrow package to the workspace, directly related to the explicit member list update in contracts/Cargo.toml.

Poem

🐰 Hops through contracts with a proud little thump,
Fairness secured, no unfair winner clump!
Cancellations dance, refunds flow free,
Seven-day claims and PRNG's spree!
The lottery's stronger, no loopholes to peek—
A rabbit's delight, the code's at its peak! 🎰

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately summarizes the main change: hardening lottery fairness, closure rules, and prize claim lifecycle, which directly aligns with the core modifications.
Linked Issues check	✅ Passed	The PR successfully addresses all coding requirements from issue #320: implements Soroban PRNG for fair weighted winner selection, refactors storage for efficient per-pool operations, adds comprehensive lifecycle management with cancellation and refunds, enforces 7-day claim windows and double-claim prevention, includes require_auth() security checks, and provides expanded test coverage for all scenarios.
Out of Scope Changes check	✅ Passed	All changes are directly aligned with issue #320 objectives: workspace config fix is necessary for testing, error module reorganization supports the new lifecycle, and all functionality additions (cancel_lottery, claim_refund, PRNG selection) are explicitly required by the acceptance criteria.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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