pragma solidity ^0.4.8;

contract Token {
  /// @return total amount of tokens
  function totalSupply() constant returns (uint256 supply) {}
  /// @param _owner The address from which the balance will be retrieved
  /// @return The balance
  function balanceOf(address _owner) constant returns (uint256 balance) {}
  /// @notice send `_value` token to `_to` from `msg.sender`
  /// @param _to The address of the recipient
  /// @param _value The amount of token to be transferred
  /// @return Whether the transfer was successful or not
  function transfer(address _to, uint256 _value) returns (bool success) {}
  /// @notice send `_value` token to `_to` from `_from` on the condition it is approved by `_from`
  /// @param _from The address of the sender
  /// @param _to The address of the recipient
  /// @param _value The amount of token to be transferred
  /// @return Whether the transfer was successful or not
  function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {}
  /// @notice `msg.sender` approves `_addr` to spend `_value` tokens
  /// @param _spender The address of the account able to transfer the tokens
  /// @param _value The amount of wei to be approved for transfer
  /// @return Whether the approval was successful or not
  function approve(address _spender, uint256 _value) returns (bool success) {}
  /// @param _owner The address of the account owning tokens
  /// @param _spender The address of the account able to transfer the tokens
  /// @return Amount of remaining tokens allowed to spent
  function allowance(address _owner, address _spender) constant returns (uint256 remaining) {}
  event Transfer(address indexed _from, address indexed _to, uint256 _value);
  event Approval(address indexed _owner, address indexed _spender, uint256 _value);
  uint public decimals;
  string public name;
}

contract TokenExchange {
    mapping (address => mapping (address => uint256)) amounts;
    mapping (address => mapping (address => uint256)) prices;

    // Allows user to deposit some number of tokens that you are willing to sell for some price.
    // Should also check that the user has has given the exchange an allowance of this token
    // (and transfer the tokens into the contracts control).
    function depositToken(address _token, uint _amount) {
        Token t = Token(_token);
        if (_amount < 0) throw;
        // Note: a bit confused about the tradeoff between security and accuracy
        // amount should only change if there is a 'successful' transaction
        // (same with withdrawToken)
        amounts[_token][msg.sender] += _amount;
        if (!t.transferFrom(msg.sender, this, _amount)) throw;
    }

    // Allows users who have previously deposited tokens to reclaim possession of them.
    // Also allows users who have bought tokens to claim possession of them.
    // The decentralized exchange should use the tokens transfer functon
    function withdrawToken(address _token, uint _amount) {
        Token t = Token(_token);
        if (amounts[_token][msg.sender] < _amount) throw;
        if (_amount < 0) throw;
        amounts[_token][msg.sender] -= _amount;
        if (!t.transfer(msg.sender, _amount)) throw;
    }

    // Returns the number of tokens that a user has deposited of that specific _token.
    function getTokenBalance(address _owner, address _token) constant returns (uint256 balance) {
        Token t = Token(_token);
        return amounts[_token][_owner];
    }

    // Set the price you are willing to sell your tokens for (in Ether)
    function setPrice(address _token, uint _price) {
        Token t = Token(_token);
        if (_price < 0) throw;
        prices[_token][msg.sender] = _price;
    }

    // Send this function with at least _amount * _price of token for that
    // specific seller, and have the tokens transferred into your ownership
    // (within this contract only, not in the token contract yet - that's
    // what the withdrawTokens function is for).
    function buyToken(address _token, address _seller, uint _amount) payable {
        Token t = Token(_token);
        if (_amount < amounts[_token][_seller]) throw;
        if (!this.send(_amount * prices[_token][_seller])) throw;
        amounts[_token][_seller] -= _amount;
        amounts[_token][this] += _amount;
    }

    // withdraw _amount of Ether that you have in the exchange (from selling tokens).
    function withdrawEther(uint _amount) {
        // sender.send for transfering ether
        if (_amount < 0) throw;
        if (!msg.sender.send(_amount)) throw;
    }
}