How to calculate the outputId?

Question

How can I calculate the output id with transaction id and output index?

An OutputId is the the concatenation of transactionId+outputIndex where outputIndex needs to be converted to little endian first. Hex-encoded with 0x prefix. (The transactionId is the BLAKE2b-256 hash of the transaction payload bytes).

Answer 1

An example OutputId is 0x0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a790000:

Hex-prefix	TransactionId	OutputIndex	OutputIndex Decimal
0x	0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a79	0000	0
0x	0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a79	0100	1
0x	0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a79	0200	2
0x	0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a79	2a00	42
0x	0c78e998f5177834ecb3bae1596d5056af76e487386eecb19727465b4be86a79	8000	128

Code examples to hex encoded the OutputIndex:

Rust:

fn main() {
    fn output_index_to_hex(index: u16) -> String {
        let bytes = u16::to_le_bytes(index);

        bytes
            .iter()
            .map(|b| format!("{b:02x}"))
            .collect::<Vec<String>>()
            .join("")
    }

    let output_index_hex = output_index_to_hex(0);
    println!("{output_index_hex}");
}

Python:

def output_index_to_hex(index: int) -> str:
    return index.to_bytes(2, "little").hex()

print(output_index_to_hex(0))

JS:

// Works in browser and Node.js
function outputIndexToHex(number) {
    let buf = new ArrayBuffer(2);
    const view = new DataView(buf);
    view.setUint16(0, number, true)
    return [...new Uint8Array(buf)].map(x => x.toString(16).padStart(2, '0'))
        .join('')
}
console.log(outputIndexToHex(0))

// Cleaner version, but Node.js only
function outputIndexToHexNodeJs(number) {
    let buf = Buffer.allocUnsafe(2);
    buf.writeInt16LE(number)
    return buf.toString('hex')
}
console.log(outputIndexToHexNodeJs(0))

The reverse operation in JS:

function hexOutputIndexToInt(number) {
    const numberString = number.toString();
    let chunks = [];
    for (let i = 0, charsLength = numberString.length; i < charsLength; i += 2) {
        chunks.push(numberString.substring(i, i + 2));
    }
    const separated = chunks.map(n => parseInt(n, 16))
    const buf = Uint8Array.from(separated).buffer;
    const view = new DataView(buf);
    return view.getUint16(0, true)
}

Answer 2

Qt/C++ will be

to get the transaction id:

auto trpay=Payload::Transaction(essence,unlocks);
auto transactionId=trpay->get_id();
auto outputId=transactionId.append(0)
//auto outputId=transactionId.append(128)
qDebug()<<outputId.toHex();

To get the hex string:

qblocks::c_array output_index_to_hex(quint16 index)
{
    c_array var;
    var.append(index);
    return var;
}
qDebug()<<output_index_to_hex(0).toHex();
qDebug()<<output_index_to_hex(1).toHex();
qDebug()<<output_index_to_hex(2).toHex();
qDebug()<<output_index_to_hex(42).toHex();
qDebug()<<output_index_to_hex(128).toHex();

But I Have never used the later code.