Auction Smart Contract

This repository contains a Solidity smart contract for an auction system, developed as the final project for Module 2 of the Ethereum Developer Pack course. I built this contract to learn about smart contract development. The contract is deployed on the Sepolia testnet and verified with source code.

Contract Details

• Deployed Address: 0x46Eafa2B0392805964D41e29bE0A1D4979Fe72ad
• Solidity Version: >=0.8.26
• License: GPL-3.0
• File: Auction.sol

Overview

The Auction smart contract allows users to bid on an item. Key features include:

• Bids must be at least 5% higher than the current highest bid.
• The auction extends by 10 minutes if a valid bid is placed in the last 10 minutes.
• Non-winners are refunded their last bid minus a 2% commission after the auction ends.
• Bidders can request a partial refund during the auction (see Notes for limitations).
• Events notify participants of new bids and auction completion.

Auction Logic Example

Time	User	Offer	Action
T0	User 1	1 ETH	Places bid, becomes highest bidder
T1	User 2	2 ETH	Places bid, becomes highest bidder
T2	User 1	3 ETH	Places bid, can request refund (see Notes)

Variables

• owner (address, private): The contract deployer's address, used for admin tasks like refunds.
• startTime (uint256, private): Timestamp when the auction starts.
• limitDate (uint256, private): Timestamp when the auction ends (7 days after start).
• winner (Bider, private): Struct with the highest bidder's address and bid amount.
• biders (mapping(address => Bider), private): Maps addresses to bidder data.
• biderAddresses (address[], private): Tracks bidder addresses for refund processing.
• Bider (struct):
  • value (uint256): Current bid amount for the bidder.
  • lastOffer (uint256): Previous bid amount (used for refunds).
  • bider (address): Bidder's address.

Events

• NewOffer(address indexed bider, uint256 amount): Emitted when a new bid is placed, with the bidder's address and amount.
• AuctionEnded(): Emitted when the auction ends and refunds are processed.

Functions

• constructor(): Sets up the auction with a 7-day duration and assigns the deployer as the owner.
• bid(): Allows bidding (payable). Requires the bid to be 5% higher than the current highest and the auction to be active. Updates bidder data, extends the auction if needed, and emits NewOffer.
• showWinner(): Returns the current highest bidder's data (Bider struct).
• showOffers(): Returns an array of all bids. Note: Gas-intensive, use carefully.
• refund(): Refunds non-winners' last bids minus 2% commission, callable by the owner after the auction ends. Clears bidder data and emits AuctionEnded.
• partialRefund(): Allows bidders to withdraw their previous bid during the auction. Note: Currently refunds lastOffer (see Notes).
• extendLimitDate() (private): Extends the auction by 10 minutes if a bid is placed in the last 10 minutes.
• receive(): Reverts accidental ETH transfers, ensuring funds go through bid().

Modifiers

• isActive(): Checks that the auction is ongoing (block.timestamp < limitDate).
• isOwner(): Restricts access to the contract owner.
• hasEnded(): Ensures the auction has ended (block.timestamp >= limitDate).

Deployment and Usage

1. Deployment:

   • Deployed on Sepolia at 0x46Eafa2B0392805964D41e29bE0A1D4979Fe72ad.
   • Source code verified on Etherscan.
   • Deploy using Remix with Solidity compiler >=0.8.26.

2. Interaction:

   • Bidding: Call bid() with ETH, ensuring the amount is 5% higher than the current highest bid.
   • Partial Refund: Call partialRefund() during the auction to withdraw previous bids.
   • View Data: Use showWinner() and showOffers() to check bids.
   • End Auction: After limitDate, the owner calls refund() to process refunds.

3. Testing:

   • Use Sepolia test ETH for bids and refunds.
   • Check events on Etherscan to verify state changes.
   • Ensure sufficient ETH for bid() calls.

Notes

• Partial Refund Limitation: The partialRefund function currently refunds the previous bid (lastOffer) instead of the excess over the last valid bid (e.g., 1 ETH from T0 in the example). This is a known issue due to the contract's logic, where lastOffer is set to the current value before refunding.
• Gas Usage: showOffers and refund iterate over biderAddresses, which can be costly for many bidders. Suitable for this project but not for large-scale auctions.
• Security: The contract uses call for safe ETH transfers, checks inputs with require, and clears data to prevent reentrancy.