DataAppender.ts

function DataAppender(array = new Uint8Array(8)) {
	let length = array.length;
	let offset = 0;
	let view = new DataView(array.buffer);

	const textEncoder = new TextEncoder();

	const grow = () => {
		length = array.length * 2;
		const newArray = new Uint8Array(length);
		newArray.set(array);
		array = newArray;
		view = new DataView(array.buffer);
	};

	const max8 = 2 ** 8 - 1;
	const max16 = 2 ** 16 - 1;
	const max32 = 2 ** 32 - 1;

	return {
		array: () => array.subarray(0, offset),
		u8: (n: number) => {
			if (n > max8)
				throw new RangeError(`${n} > ${max8}`);
			if (offset + 1 > length)
				grow();
			view.setUint8(offset, n);
			offset += 1;
		},
		u16: (n: number) => {
			if (n > max16)
				throw new RangeError(`${n} > ${max16}`);
			if (offset + 2 > length)
				grow();
			view.setUint16(offset, n);
			offset += 2;
		},
		u32: (n: number) => {
			if (n > max16)
				throw new RangeError(`${n} > ${max32}`);
			if (offset + 4 > length)
				grow();
			view.setUint32(offset, n);
			offset += 4;
		},
		u64: (n: number) => {
			if (offset + 8 > length)
				grow();
			view.setBigUint64(offset, BigInt(n));
			offset += 8;
		},
		str: (s: string) => {
			const bytes = textEncoder.encode(s);
			while (offset + bytes.length > length)
				grow();
			array.set(bytes, offset);
			offset += bytes.length;
		}
	};
}