pragma cashscript >= 0.7.0;

// v20220522

// This is an experimental perpetuity contract 
// from: bitcoin-cash-forever npm package.
contract Perpetuity(

    // interval for payouts, in blocks
    int period,

    // PKH of the beneficiary, the address receiving payments
    bytes20 recipientPkh,

    // extra allowance for administration of contract
    // fees are paid from executors' allowance. 
    int executorAllowance,

    // divisor for the payout, each payout must be greater than the total
    // amount held on the contract divided by this number
    int decay
) {
    function execute() {

        // Check that the first output sends to the recipient
        bytes25 recipientLockingBytecode = new LockingBytecodeP2PKH(recipientPkh);
        require(tx.outputs[0].lockingBytecode == recipientLockingBytecode);

        // Check that time has passed and that time locks are enabled
        require(tx.age >= period);
            
        // require the second output to match the active bytecode
        require(tx.outputs[1].lockingBytecode == new LockingBytecodeP2SH(hash160(this.activeBytecode)));


        // Get the total value on the contract
        int currentValue = tx.inputs[this.activeInputIndex].value;

        // The payout is the current value divided by the decay
        int installment = currentValue/decay;

        // Calculate value returned to the contract
        int returnedValue = currentValue - installment - executorAllowance;

        // Check that the outputs send the correct amounts
        require(tx.outputs[0].value >= installment);
        require(tx.outputs[1].value >= returnedValue);
            
    }

}