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Encoding & Decoding

SCALE codec

The SCALE (Simple Concatenated Aggregate Little-Endian) Codec is a lightweight, efficient, binary serialization and deserialization codec.

It is designed for high-performance, copy-free encoding and decoding of data in resource-constrained execution contexts, like the Substrate runtime. It is not self-describing in any way and assumes the decoding context has all type knowledge about the encoded data.

Reading values

byte[] msg = readSomeData();
ScaleCodecReader rdr = new ScaleCodecReader(msg);

// there are few shorthand methods for common types

// to read a single byte, and convert it to int
int a = rdr.readUByte();
// to read a number encoded as Compact Int
int b = rdr.readCompactInt();

// otherwise you should use a ScaleReader<T> readers

// UInt32Reader reads longs that are encoded as 32 bit values
long c = rdr.read(new UInt32Reader());
// CompactBigIntReader reads a BigInteger encoded as CompactInt, i.e. values up to 2^536-1
BigInteger d = rdr.read(new CompactBigIntReader());

// or, if the value is optional:
Optional<Long> optionalC = rdr.readOptional(new UInt32Reader());

// to read a list of, say, booleans you should use ListReader<T> with reader for items
List<Boolean> e = rdr.read(new ListReader<>(new BoolReader()));

// read an enumerated union, depending on tag in the encoded message, it will use different readers
UnionValue<Number> f = rdr.read(new UnionReader<>(
        // value with tag 0 is read as unsigned long
        new UInt32Reader(),
        // value with tag 1 is read as unsigned long
        new UInt32Reader(),
        // value with tag 2 is read as compact integer
        new CompactUIntReader()
));
System.out.println("Union read #" + f.getIndex() + " = " + f.getValue());
The full list of predefined readers is:

  • BoolOptionalReader → Optional<Boolean>

  • BoolReader → Boolean

  • CompactBigIntReader → unsigned BigInteger encoded as Compact Integer

  • CompactUintReader → unsigned Integer encoded as Compact Integer

  • EnumReader<T extends Enum<?>> → Java enum, whose ordinal id is encoded as a single byte

  • Int32Reader → signed Integer encoded as 32 bits

  • ListReader<T> → List<T>, where you should also specify reader for actual items of the list

  • StringReader → UTF-8 encoded String

  • UByteReader → unsigned Integer encoded as a single byte (i.e., 0..255)

  • UInt16Reader → unsigned Integer encoded as 16 bits

  • UInt32Reader → unsigned Long encoded as 32 bits

  • UInt128Reader → unsigned BigInteger encoded as 128 bits

  • UnionReader → a enumeration, where individual readers are tagged

Using custom reader

But what if we have a class that we want to read (or write) as a whole, without manual reading each time. For example class Status below (getter and setter are omitted for simplicity)

class Status {
    private long version;
    private long minVersion;
    private byte roles;
    private long height;
    private Hash256 bestHash;
    private Hash256 genesis;
}

Now you can implement a reader, which implements ScaleReader<Status> interface. Inside the methods read you specify all the readings, and build a resulting object.

class StatusReader implements ScaleReader<Status> {

    @Override
    public Status read(ScaleCodecReader rdr) {
        Status status = new Status();
        status.version = rdr.readUint32();
        status.minVersion = rdr.readUint32();
        status.roles = rdr.readByte();
        rdr.skip(1);
        status.height = rdr.readUint32();
        status.bestHash = new Hash256(rdr.readUint256());
        status.genesis = new Hash256(rdr.readUint256());
        return status;
    }
}

Now you can package it even as a separate library, and other people can read Status without bothering about internal structure. To read Status instance they just pass the reader StatusReader. Of course, it can be used together with ListReader, UnionReader, read as Optional<Status> by `` and other structures

// Read Status message encoded with SCALE codec
byte[] msg = readMessage();
// Initialize SCALE Reader
ScaleCodecReader rdr = new ScaleCodecReader(msg);
// Call it providing a custom reader for the expected class
Status status = rdr.read(new StatusReader());

// All read
System.out.println("Decoded Status: height=" + status.height + ", hash=" + status.bestHash.toString());

Which would print something like this:

Status: height=381, hash=bb931fd17f85fb26e8209eb7af5747258163df29a7dd8f87fa7617963fcfa1aa

Writing values

Writing is pretty similar to reading, you have to create ScaleCodecWriter with an OutputStream, and either use shorthand methods, or ScaleWriter writers.

ByteArrayOutputStream buf = new ByteArrayOutputStream();
// first, open writer as try-with-resources
try(ScaleCodecWriter wrt = new ScaleCodecWriter(buf)) {
    // same as for reading, there are few shorthand methods for common types

    // write a single byte
    wrt.writeByte(1);

    // write a compact integer
    wrt.writeCompact(2);

    // and same as for reader, use ScaleWriter<T> for writing more complex types

    // write unsigned int as 32 bits
    wrt.write(new UInt32Writer(), 3);
    // write big integer as compact integer
    wrt.write(new CompactBigIntWriter(), new BigInteger("112233445566778899", 16));

    // to write an enumerated union you have to define it's structure first
    UnionWriter<Number> union = new UnionWriter<>(
            // value with tag 0 is read as unsigned long
            new UInt32Writer(),
            // value with tag 1 is read as unsigned long
            new UInt32Writer(),
            // value with tag 2 is read as compact integer
            new CompactUIntWriter()
    );
    // then write pass it, with actual value
    // at this case we write under tag 2, which will write actual value 101 as Compact Integer
    wrt.write(union, new UnionValue<>(2, 101));
}
System.out.println("Encoded: " + Hex.encodeHexString(buf.toByteArray()));

Using custom writer

In the same way, you can implement a writer for your Status class

class StatusWriter implements ScaleWriter<Status> {

    @Override
    public void write(ScaleCodecWriter wrt, Status value) throws IOException {
        wrt.writeUint32(value.version);
        wrt.writeUint32(value.minVersion);
        wrt.writeByte(value.roles);
        wrt.writeByte(0);
        wrt.writeUint32(value.height);
        wrt.writeUint256(value.bestHash.getBytes());
        wrt.writeUint256(value.genesis.getBytes());
    }
}

And then use it to write a value

// Write status as bytes
ByteArrayOutputStream buf = new ByteArrayOutputStream();
ScaleCodecWriter writer = new ScaleCodecWriter(buf);
writer.write(new StatusWriter(), status);
// don't forget to close writer
writer.close();

System.out.println("Encoded Status: " + Hex.encodeHexString(buf.toByteArray()));

SS58

Encode pubkey as Address

byte[] pubkey = Hex.decodeHex(
        // a pubkey is 32 byte value, for this example it's hardcoded as hex
        "9053cc32597892cc2cd43ea6e3c0db7a3b4c52e5fe6052762080dbc3e3222c0b"
);
String address = SS58Codec.getInstance().encode(
        // using Kusama here. but for Polkadot mainnet use SS58Type.Network.LIVE
        SS58Type.Network.CANARY,
        // pubkey as bytes
        pubkey
);
System.out.println("Address: " + address);

Which would print:

Address: FqZJib4Kz759A1VFd2cXX4paQB42w7Uamsyhi4z3kGgCkQy

Decode pubkey

SS58 address = SS58Codec.getInstance().decode("FqZJib4Kz759A1VFd2cXX4paQB42w7Uamsyhi4z3kGgCkQy");

if (address.getType() != SS58Type.Network.CANARY) {
    throw new IllegalStateException("Not Kusama address");
}

System.out.println(
        "Pub key: " + Hex.encodeHexString(address.getValue())
);

Which would print:

Pub key: 9053cc32597892cc2cd43ea6e3c0db7a3b4c52e5fe6052762080dbc3e3222c0b