Personal Carbon Footprint Tracker

What it does:
Tracks personal carbon emissions from activities (travel, energy use, consumption) and allows users to monitor, reduce, and offset their footprint on-chain.

Why it matters:
Encourages eco-conscious behavior, provides verifiable carbon data, enables tokenized incentives or rewards for reduction, and integrates sustainability into personal lifestyle management.

How it works:

Users log activities contributing to carbon emissions
Smart contract calculates total footprint based on predefined emission factors
Users can purchase carbon offsets or participate in sustainability rewards
Integrates with Carbon Credit Trading, Green Energy Sharing, or Recycling Reward Smart Contract
Dashboards show emissions history, reductions, and offset achievements
Reputation or token incentives can be awarded for footprint reduction

      // SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/access/Ownable.sol";

/**
 * @title PersonalCarbonTracker
 * @author Nam
 * @notice Tracks personal carbon footprint and offsets on-chain
 */
contract PersonalCarbonTracker is Ownable {

    struct Activity {
        string description;
        uint256 carbonAmount; // in grams CO2 equivalent
        uint256 timestamp;
        bool offset;
    }

    mapping(address =&gt; Activity[]) private userActivities;
    mapping(address =&gt; uint256) public totalCarbon; // grams CO2

    // -------------------- EVENTS --------------------

    event ActivityLogged(address indexed user, string description, uint256 carbonAmount);
    event CarbonOffsetPurchased(address indexed user, uint256 carbonAmount, uint256 payment);

    // -------------------- ACTIVITY LOGGING --------------------

    function logActivity(string calldata _description, uint256 _carbonAmount) external {
        require(_carbonAmount &gt; 0, "Carbon must be &gt;0");

        userActivities[msg.sender].push(Activity({
            description: _description,
            carbonAmount: _carbonAmount,
            timestamp: block.timestamp,
            offset: false
        }));

        totalCarbon[msg.sender] += _carbonAmount;

        emit ActivityLogged(msg.sender, _description, _carbonAmount);
    }

    // -------------------- CARBON OFFSET --------------------

    function purchaseOffset(uint256 _activityIndex) external payable {
        Activity storage a = userActivities[msg.sender][_activityIndex];
        require(!a.offset, "Already offset");
        require(msg.value &gt;= a.carbonAmount * 1e12, "Insufficient payment"); // e.g., 1 wei per gram, adjustable

        a.offset = true;
        totalCarbon[msg.sender] -= a.carbonAmount;

        emit CarbonOffsetPurchased(msg.sender, a.carbonAmount, msg.value);

        // Payment can be sent to a sustainability fund or DAO
        // payable(owner()).transfer(msg.value);
    }

    // -------------------- VIEW FUNCTIONS --------------------

    function getActivities(address _user) external view returns (Activity[] memory) {
        return userActivities[_user];
    }

    function getTotalCarbon(address _user) external view returns (uint256) {
        return totalCarbon[_user];
    }
}