AkiLottoDrawer

A Starknet Cairo contract for a lottery with an owner-driven draw and user-driven double-or-nothing spins. Integrates with Pragma VRF for secure randomness and ERC20 for fee handling.

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Overview

• Owner-controlled draw: Picks a winner using VRF randomness and a ticket-weighted mechanism.
• User double-or-nothing: Each connected user may gamble their tickets once per draw window, doubling or losing them based on the parity of a VRF-provided random word.
• Concurrency-safe: Separate randomness storage for draws vs. per-user spins to prevent overlap.
• Wallet integration: Tracks ticket balance (u256), connection state, and spin state per user.
• Fees handled via ERC20: Charges VRF provider fees via a token, using OpenZeppelin’s ERC20 dispatcher.

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Dependencies

• pragma_lib – Integration with a zero-knowledge VRF oracle (Pragma VRF). Used for requesting/receiving randomness securely.
• openzeppelin_token – ERC20 dispatcher interface for fee payments.

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Design & Security Considerations

Randomness Architecture

• Per-user randomness (user_spin_random: Map<ContractAddress, felt252>): Prevents concurrency issues by storing each user’s randomness separately.
• Single draw randomness (draw_random_word: felt252): Held centrally and used only once during the owner’s draw process.

Access Control

• Methods like set_owner, add_tickets, draw, double_spin are restricted by appropriate asserts.
• Each user may only spin once per draw window — tracked by has_spinned.
• Draws cannot be repeated once marked by has_drawed.

Ticket Management

• Ticket counts use u256 to prevent overflow during doubling.
• Total tickets maintained consistently both per-user and globally.

VRF & Fee Workflow

1. Owner calls request_randomness_from_pragma (includes fee approval).
2. On callback, randomness is stored in either draw_random_word (owner draw) or user_spin_random (user spin).
3. Nonces are cleared after consumption to prevent reuse.

Workflow

• User Spin:

  1. User requests owner for randomness.
  2. User calls double_spin.
  3. Then checks parity of the random word:
  • If even, doubles the tickets.
  • If odd, burns the tickets.
  5. Updates user state and emits event.

• Owner Draw:

  1. Owner requests randomness.
  2. Upon callback, calculates winner based on ticket weight.
  3. Owner can then call draw to finalize the draw.
  4. Emits Drawn event with winner details.

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Usage

• Sepolia:
  • Deployed @ 0x0033907d0cf21be5f0eabfb9e1c16369f60723bff403168a79cb96c684c2e4d1
• Mainnet:
  • Deployed @ 0x00800ba849776dec2b96ff533312b097fcd4aa023285eaf534f4deeb595aa8a7