index.ts

const zbase32 = 'ybndrfg8ejkmcpqxot1uwisza345h769';
const zbase32Reverse = Object.fromEntries(zbase32.split('').map((x, i) => [ x, BigInt(i) ]));

export function encode(buf: Buffer): string {
	let str = '';

	for (let i = 0; i < buf.length; i += 5) {
		// calculate chunk lengths
		const byteLength = Math.min(5, buf.length - i);
		const zbase32Length = Math.ceil(byteLength * 8 / 5);

		// bytes -> bigint
		const n = BigInt(buf.readUintBE(i, byteLength)) << BigInt((5 - byteLength) * 8);

		// bigint -> zbase32
		for (let j = 0; j < zbase32Length; j++) {
			str += zbase32[Number(n >> BigInt((7 - j) * 5)) & 31];
		}
	}

	return str;
}

export function decode(str: string): Buffer {
	const chunks: Buffer[] = [];

	for (let i = 0; i < str.length; i += 8) {
		// calculate chunk lengths
		const zbase32Length = Math.min(8, str.length - i);
		const byteLength = Math.floor(zbase32Length * 5 / 8);

		// zbase32 -> bigint
		let n = 0n;
		for (let j = 0; j < zbase32Length; j++) {
			n <<= 5n;
			n |= zbase32Reverse[str[i + j]];
		}
		n <<= BigInt((8 - zbase32Length) * 5);

		// bigint -> bytes
		const chunk = Buffer.allocUnsafe(byteLength);
		chunk.writeUintBE(Number(n >> BigInt((5 - byteLength) * 8)), 0, byteLength);
		chunks.push(chunk);
	}

	return Buffer.concat(chunks);
}