scryptlib

scryptlib

Javascript/TypeScript SDK for integration of Bitcoin SV Smart Contracts written in the sCrypt language.

You can install scryptlib in your project as below:

$ npm install scryptlib

A smart contract is compiled to a locking script template. A contract function call is transformed to an unlocking script. Developers are responsible for setting the locking and unlocking scripts of a transaction properly before sending it to the Bitcoin network. This may include some actions described below:

• Instantiate locking script: replace the constructor formal parameters, represented by placeholders in the locking script template, with actual parameters/arguments to form the complete locking script.

• Assemble unlocking script: convert the arguments of a contract function call to script format and concatenate them to form the unlocking script.

By using scryptlib, both scripts can be obtained with ease.

Contract Description File

The compiler outputs results in a JSON file. It’s a representation used to build locking and unlocking scripts. We call this file a contract description file. Here's its structure:

{
  "compilerVersion": "0.1.0+commit.312f643",    // version of compiler used to produce this file
  "contract": "DemoP2PKH",    // name of the contract
  "md5": "01234...",    // md5 of the contract source code file
  "abi": [    // ABI of the contract: interfaces of its public functions and constructor.
    {
        "type": "constructor",
        "name": "constructor",
        "params": [
            {
                "name": "pubKeyHash",
                "type": "Ripemd160"
            }
        ]
    },
    {
        "type": "function",
        "name": "unlock",
        "index": 0,
        "params": [
            {
                "name": "sig",
                "type": "Sig"
            },
            {
                "name": "pubKey",
                "type": "PubKey"
            }
        ]
    },
    ...
  ],
  "asm": "$pubKeyHash OP_OVER OP_HASH160 ..."    // locking script of the contract in ASM format, including placeholders for constructor parameters
}

There are two ways to generate this file (named as xxx_desc.json):

1. Use sCrypt VS Code extension to compile manually;
2. Use the function compile programmatically:

  import { compile } from 'scryptlib';
  
  ...
  
  compile( 
    { 
      path: contractFilePath  //  the file path of the contract
    }, 
    {
      desc: true  // set this flag to be `true` to get the description file output
    }
  );

Deploy A Contract and Call Its Function

Both deploying a contract and calling a contract functionare achieved by sending a transaction. Generally speaking,

• Deploying a contract needs the locking script in the output of this transaction to be set properly;
• Calling a contract function needs the unlocking script in the input of this transaction to be set properly;

You can use the description file to build a reflected contract class in Javascript/TypeScript like this:

const MyContract = buildContractClass(JSON.parse(descFileContent));

To create an instance of the contract class, for example:

const instance = new MyContract(1234, true, ...parameters);

To get the locking script, use:

const lockingScript = instance.lockingScript;
// To convert it to ASM/hex format
const lockingScriptASM = lockingScript.toASM();
const lockingScriptHex = lockingScript.toHex();

To get the unlocking script, just call the function and turn the result to bsv.Script object, for example:

const funcCall = instance.someFunc(new Sig('0123456'), new Bytes('aa11ff'), ...parameters);
const unlockingScript = funcCall.toScript();
// To convert it to ASM/hex format
const unlockingScriptASM = unlockingScript.toASM();
const unlockingScriptHex = unlockingScript.toHex();

Note that parameters in both constructor and function call are mapped to sCrypt types as follows:

• boolean: mapped to sCrypt bool
• number: mapped to sCrypt int
• new Bytes(x) / new Sig(x) / new PubKey(x) / new Ripemd160(x) / … : mapped to sCrypt bytes / Sig / PubKey / Ripemd160 / … , where x is hex string

In this way, the type of parameters could be checked and potential bugs can be detected before running.

Local Unit Tests

A useful method verify(txContext) is provided for each contract function call. It would execute the function call with the given context locally. The txContext argument provides some context information of the current transaction, needed only if signature is checked inside the contract.

{
  tx?: any;                 // current transaction represented in bsv.Transaction object
  inputIndex?: number;      // input index, default value: 0
  inputSatoshis?: number;   // input amount in satoshis
}

It returns an object:

{
  success: boolean;       // script evaluates to true or false
  error: string;          // error message, empty if success
}

It usually appears in unit tests, like:

const context = { tx, inputIndex, inputSatoshis };

// 1) set context per verify()
const funcCall = instance.someFunc(new Sig('0123456'), new Bytes('aa11ff'), ...parameters);
const result = funcCall.verify(context);
// 2) alternatively, context can be set at instance level and all following verify() will use it
instance.txContext = context;
const result = funcCall.verify();

expect(result.success, result.error).to.be.true;
assert.isFalse(result.success, result.error);

Contracts with State

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

const dataPart = instance.dataPart;
const dataPartASM = instance.dataPart.toASM();
const dataPartHex = instance.dataPart.toHex();
// to set it using ASM
instance.setDataPart(dataInASM);
// to set it using state object (no nesting)
let state = {'counter': 11, 'bytes': '1234', 'flag': true}
instance.setDataPart(state)

After that, the instance.lockingScript would include the data part automatically.

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();

Instantiate Inline Assembly Variables

Assembly variables can be replaced with literal Script in ASM format using replace(). Each variable is prefixed by its unique scope, namely, the contract and the function it is under.

const asmVars = {
  'contract1.function1.variable1': 'ff41',
  'contract2.function2.variable2': 'OP_4'
};
instance.replaceAsmVars(asmVars);

You could find more examples using scryptlib in the boilerplate repository.