mvc-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 output results in a JSON file. It’s a representation used to build locking and unlocking scripts. We call this file a contract description file.

There are three 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
      asm: true // set this flag to be `true` to get the asm file output
      optimize: false //set this flag to be `true` to get optimized asm opcode
      sourceMap: true //set this flag to be `true` to get source map
    }
  );

1. Run npx command in CLI:

  # install compiler binary
  npx scryptlib download
  # compiling contract
  npx scryptlib your_directory/your_scrypt.scrypt

Types

1. Basic Types

All basic types of the sCrypt language have their corresponding javascript classes in scryptlib. In this way, the type of parameters could be checked and potential bugs can be detected before running.

Types (scrypt)	scryptlib (javascript/typescript)
int	new Int(1) or number or bigint
bool	new Bool(true) or boolean
bytes	new Bytes('0001') or new String("hello world 😊")
PubKey	new PubKey('0001')
PrivKey	new PrivKey(1)
Sig	new Sig('0001')
Ripemd160	new Ripemd160('0001')
Sha1	new Sha1('0001')
Sha256	new Sha256('0001')
SigHashType	new SigHashType('01')
SigHashPreimage	new SigHashPreimage('010001')
OpCodeType	new OpCodeType('76')

2. Array Types

scryptlib uses javascript array to represent the array types of the sCrypt language.

[[1, 3, 1]] // represent `int[1][3]` in **sCrypt** language

[new Bytes("00"), new Bytes("00"), new Bytes("00")] // represent `bytes[3]` in **sCrypt** language

3. Structure and Type Aliases

Composite types, including structs and type aliases, are dynamically generated by buildTypeClasses. When creating a structure, all members must specify values. Use dot to access structure members.

Structure and type aliases defined in sCrypt:

struct Person {
    bytes addr;
    bool isMale;
    int age;
}

struct Block {
    bytes hash;
    bytes header;
    int time;
}

type Male = Person;
type Female = Person;

contract Main {
    Person person;
    int x;

    ...

}

Access Structure and type aliases by SDK :

const PersonContract = buildContractClass(loadDescription('person_desc.json'));

/*Person is structure and Male, Female are type aliases */
const { Person, Male, Female } = buildTypeClasses(PersonContract);

let man = new Person({
    isMale: true,
    age: 14,
    addr: new Bytes("68656c6c6f20776f726c6421")
  });

man.age = 20;

let woman = new Female({
    isMale: false,
    age: 18,
    addr: new Bytes("68656c6c6f20776f726c6421")
  });

woman.addr = new Bytes("")

4. Library

Library is another composite types. When the constructor parameter of the contract contains library, we need to create library through sdk.

Library defined in sCrypt:

library L {
  private int x;

  constructor(int a, int b) {
    this.x = a + b;
  }
  function f() : int {
    return this.x;
  }
}

contract Test {
  public int x;
  L l;

  public function unlock(int x) {
    require(this.l.f() == x + this.x);
  }
}

Access Library by SDK :

const Test = buildContractClass(loadDescription('test_desc.json'));

const { L } = buildTypeClasses(Test);

let test = new Test(1, new L(1, 2));

As you can see, creating a library instance is similar to creating a contract instance. Sometimes the constructor parameters of the library may be generic types. At this time, the sdk will deduce the generic type based on the constructor arguments you pass.

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();

const {Person, Male, Female} = buildTypeClasses(JSON.parse(descFileContent));

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. Declare any property that is part of the state with a decorator @state in a contract, for example:

contract Counter {
    @state
    int counter;

    constructor(int counter) {
        this.counter = counter;
    }
}

Use the initial state to instantiate the contract and read the state by accessing the properties of the contract instance.

const instance = new Counter(0);

let state = instance.counter;
// update state
instance.counter++;

Then use instance.getNewStateScript() to get a locking script that includes the new state. It accepts an object as a parameter. Each key of the object is the name of a state property, and each value is the value of the state property. You should provide all state properties in the object.

const tx = newTx(inputSatoshis);
let newLockingScript = instance.getNewStateScript({
    counter: 1
});

tx.addOutput(new bsv.Transaction.Output({
  script: newLockingScript,
  satoshis: outputAmount
}))

preimage = getPreimage(tx, instance.lockingScript, inputSatoshis)

You can also access the state of the contract by accessing the properties of the instance.

instance.counter++;
instance.person.name = new Bytes('0001');

You can also maintain state manually to, for example, optimize your contract or use customized state de/serialization rawstate.

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.

Construct contracts from raw transactions

In addition to using a constructor to create a contract, you can also use a raw transaction to construct it.

const axios = require('axios');

const Counter = buildContractClass(loadDesc("counter_debug_desc.json"));
let response = await axios.get("https://api.whatsonchain.com/v1/bsv/test/tx/7b9bc5c67c91a3caa4b3212d3a631a4b61e5c660f0369615e6e3a969f6bef4de/hex")
// constructor from raw Transaction.
let counter = Counter.fromTransaction(response.data, 0/** output index**/);

// constructor from Utxo lockingScript
let counterClone = Counter.fromHex(counter.lockingScript.toHex());

Chained APIs

Chained APIs makes building transactions easy.

Support browsers that are not compatible with BigInt

Some contracts use Bigint to construct or unlock. but some browsers do not support Bigint, such as IE11. In this case, we use strings to build Bigint.

// polyfill

import 'react-app-polyfill/ie11';  
import 'core-js/features/number';
import 'core-js/features/string';
import 'core-js/features/array';



let demo = new Demo("11111111111111111111111111111111111", 1);

let result = demo.add(new Int("11111111111111111111111111111111112")).verify();

console.assert(result.success, result.error)