Manually Maintain Mutable State

One way to implement state in contract is dividing the contract in the locking script into two parts: data and code. Data part is the state. Code part contains the business logic of a contract that encodes rules for state transition.

sCrypt offers stateful contracts. Let us look at a simple example of a stateful contract: a counter contract tracking how many times its function increment() has been called. Its code is shown below with comments inline.

contract Counter {
    public function increment(SigHashPreimage txPreimage, int amount) {
        require(Tx.checkPreimage(txPreimage));

        // deserialize state (i.e., counter value)
        bytes scriptCode = Util.scriptCode(txPreimage);
        int scriptLen = len(scriptCode);
        // counter is at the end
        int counter = unpack(scriptCode[scriptLen - Util.DataLen :]);

        // increment counter
        counter++;

        // serialize state
        bytes outputScript = scriptCode[: scriptLen - Util.DataLen] + num2bin(counter, Util.DataLen);

        bytes output = Util.buildOutput(outputScript, amount);
        // ensure output is expected: amount is same with specified
        // also output script is the same with scriptCode except counter incremented
        require(hash256(output) == Util.hashOutputs(txPreimage));
    }
}

OP_RETURN data of the contract locking script can be accessed by using an accessor named dataPart, for example:

const Counter = buildContractClass(loadDescription('counter_desc.json'));
counter = new Counter();
const dataPart = counter.dataPart;
const dataPartASM = counter.dataPart.toASM();
const dataPartHex = counter.dataPart.toHex();
// to set it using ASM
counter.setDataPart('01');

After that, the counter.lockingScript would include the data part automatically. You can use it to calculate preimage.

const preimage = getPreimage(tx, counter.lockingScript, inputSatoshis)

If you want to access the code part of the contract's locking script including the trailing OP_RETURN, use:

const codePart = instance.codePart;
const codePartASM = instance.codePart.toASM();
const codePartHex = instance.codePart.toHex();