pragma solidity ^0.4.24;

library SafeMath {
  function mul(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    if (_a == 0) { return 0; }
    c = _a * _b;
    assert(c / _a == _b);
    return c;
  }

  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
    return _a / _b;
  }

  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
    assert(_b <= _a);
    return _a - _b;
  }

  function add(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    c = _a + _b;
    assert(c >= _a);
    return c;
  }
}

contract ERC20 {
  function totalSupply() public view returns (uint256);
  function balanceOf(address _who) public view returns (uint256);
  function allowance(address _owner, address _spender) public view returns (uint256);
  function transfer(address _to, uint256 _value) public returns (bool);
  function approve(address _spender, uint256 _value) public returns (bool);
  function transferFrom(address _from, address _to, uint256 _value) public returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract Ownable {
  address public owner;

  event OwnershipRenounced(address indexed previousOwner);
  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
  modifier onlyOwner() { require(msg.sender == owner); _; }

  constructor() public { owner = msg.sender; }

  function renounceOwnership() public onlyOwner() {
    emit OwnershipRenounced(owner);
    owner = address(0);
  }

  function transferOwnership(address _newOwner) public onlyOwner() {
    _transferOwnership(_newOwner);
  }

  function _transferOwnership(address _newOwner) internal {
    require(_newOwner != address(0));
    emit OwnershipTransferred(owner, _newOwner);
    owner = _newOwner;
  }
}

contract AdjustableToken is ERC20, Ownable {
  using SafeMath for uint256;

  uint256 totalSupply_;
  bool public adjusted;
  mapping(address => uint256) balances;
  mapping (address => mapping (address => uint256)) internal allowed;

  event Adjusted(address indexed who, uint256 value);
  modifier onlyOnce() { require(!adjusted); _; }

  function totalSupply() public view returns (uint256) {
    return totalSupply_;
  }

  function balanceOf(address _owner) public view returns (uint256) {
    return balances[_owner];
  }

  function allowance(address _owner, address _spender) public view returns (uint256) {
    return allowed[_owner][_spender];
  }

  function transfer(address _to, uint256 _value) public returns (bool) {
    require(_value <= balances[msg.sender]);
    require(_to != address(0));

    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    emit Transfer(msg.sender, _to, _value);
    return true;
  }

  function approve(address _spender, uint256 _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
  }

  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);
    require(_to != address(0));

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    emit Transfer(_from, _to, _value);
    return true;
  }

  function increaseApproval(address _spender, uint256 _addedValue) public returns (bool) {
    allowed[msg.sender][_spender] = (allowed[msg.sender][_spender].add(_addedValue));
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

  function decreaseApproval(address _spender, uint256 _subtractedValue) public returns (bool) {
    uint256 oldValue = allowed[msg.sender][_spender];
    if (_subtractedValue >= oldValue) {
      allowed[msg.sender][_spender] = 0;
    } else {
      allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
    }
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

  function adjustSupply(uint256 _value) external onlyOwner() onlyOnce() {
    adjusted = true;
    require(_value <= balances[msg.sender]);
    balances[msg.sender] = balances[msg.sender].sub(_value);
    totalSupply_ = totalSupply_.sub(_value);
    emit Adjusted(msg.sender, _value);
    emit Transfer(msg.sender, address(0), _value);
  }
}

contract BitSongToken is AdjustableToken {
  string public name;
  string public symbol;
  uint8 public decimals;

  constructor(string _name, string _symbol, uint8 _decimals, uint256 _initialSupply) public {
    name = _name;
    symbol = _symbol;
    decimals = _decimals;
    totalSupply_ = _initialSupply * 10**uint256(decimals);
    balances[msg.sender] = totalSupply_;
  }
}