solidity example contract

Endowment.sol

pragma solidity ^0.4.18;
contract Endowment {

    address public beneficiary;
    address public owner;
    uint public lastRedeem;
    uint constant public period = 1 minutes;
    uint public installmentAmount;
    
    function Endowment(address _beneficiary, uint _installmentAmount) payable  {
        require(_installmentAmount > 0);
        require(_beneficiary != 0);
        owner = msg.sender;
        beneficiary = _beneficiary;
        installmentAmount = _installmentAmount;
    }
    
    function () payable public {
    }
    
    event redeemed();
    function redeem() public {
        require(msg.sender == beneficiary);
        require (this.balance > 0);
        uint toSend = getRedeemable();
        require(toSend > 0);
        lastRedeem = now / period;
        beneficiary.transfer(toSend);
        redeemed();
    }
    
    function getRedeemable() public view returns (uint256 redeemableAmount) {
        uint installments;
        if( lastRedeem == 0) {
            installments = 1; // first redeem
        } else if(now/period <= lastRedeem) {
            installments = 0; 
        } else {
            installments = now /period - lastRedeem;
        }
        redeemableAmount = installments * installmentAmount;
        if(this.balance < redeemableAmount) {
            redeemableAmount = this.balance;
        }
        return redeemableAmount; 
    }

}