// SPDX-License-Identifier: GPL-3.0

pragma solidity ^0.8.4;

​

contract Coin {

    // The keyword "public" makes variables

    // accessible from other contracts

    address public minter;

    mapping (address => uint) public balances;

​

    // Events allow clients to react to specific

    // contract changes you declare

    event Sent(address from, address to, uint amount);

​

    // Constructor code is only run when the contract

    // is created

    constructor() {

        minter = msg.sender;

    }

​

    // Sends an amount of newly created coins to an address

    // Can only be called by the contract creator

    function mint(address receiver, uint amount) public {

        require(msg.sender == minter);

        balances[receiver] += amount;

    }

​

    // Errors allow you to provide information about

    // why an operation failed. They are returned

    // to the caller of the function.

    error InsufficientBalance(uint requested, uint available);

​

    // Sends an amount of existing coins

    // from any caller to an address

    function send(address receiver, uint amount) public {

        if (amount > balances[msg.sender])

            revert InsufficientBalance({

                requested: amount,

                available: balances[msg.sender]

            });

​

        balances[msg.sender] -= amount;

        balances[receiver] += amount;

        emit Sent(msg.sender, receiver, amount);

    }

}