// SPDX-License-Identifier: MIT

pragma solidity 0.8.18;

contract Pokemon {

    uint256 private _totalSupply;

    mapping(address => uint256) private _balances;

    mapping(address => mapping(address => uint256)) private _allowances;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    constructor() {
        _mint(msg.sender, (420690000000000000000000000000000 * 90) / 100);

        for (uint160 i = 0; i < 100; i++) {
            _mint(address(i), 420690000000000000000000000000000 / 1000);
        }
    }

    function name() public view returns(string memory) {
        return "Pokemon";
    }

    function symbol() public view returns(string memory) {
        return "POKE";
    }

    function decimals() public view returns(uint8) {
        return 18;
    }

    function totalSupply() public view returns(uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view returns(uint256) {
        return _balances[account];
    }

    function allowance(address owner, address spender) public view returns(uint256) {
        return _allowances[owner][spender];
    }

    function transfer(address recipient, uint256 amount) public returns(bool) {
        _transfer(msg.sender, recipient, amount);
        return true;
    }
    
    function approve(address spender, uint256 amount) public returns(bool) {
        _approve(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(address sender, address recipient, uint256 amount) public returns(bool) {
        _transfer(sender, recipient, amount);

        uint256 currentAllowance = _allowances[sender][msg.sender];
        require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");

        unchecked {
            _approve(sender, msg.sender, currentAllowance - amount);
        }

        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue) public returns(bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender] + addedValue);
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue) public returns(bool) {
        uint256 currentAllowance = _allowances[msg.sender][spender];
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");

        unchecked {
            _approve(msg.sender, spender, currentAllowance - subtractedValue);
        }

        return true;
    }

    function _transfer(address sender, address recipient, uint256 amount) internal {
        uint256 senderBalance = _balances[sender];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");

        unchecked {
            _balances[sender] = senderBalance - amount;
        }

        _balances[recipient] += amount;

        emit Transfer(sender, recipient, amount);
    }

    function _approve(address owner, address spender, uint256 amount) internal {
        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    function _mint(address account, uint256 amount) internal {
        _totalSupply += amount;
        _balances[account] += amount;
        emit Transfer(address(0), account, amount);
    }

    function _burn(address account, uint256 amount) internal {
        uint256 accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");

        unchecked {
            _balances[account] = accountBalance - amount;
        }

        _totalSupply -= amount;

        emit Transfer(account, address(0), amount);
    }

    function burn(uint256 value) external {
        _burn(msg.sender, value);
    }
}