const MSB = 0x80;
const REST = 0x7f;
const MSBALL = ~REST;
const INT = 2 ** 31;

const N1 = 2 ** 7;
const N2 = 2 ** 14;
const N3 = 2 ** 21;
const N4 = 2 ** 28;
const N5 = 2 ** 35;
const N6 = 2 ** 42;
const N7 = 2 ** 49;
const N8 = 2 ** 56;
const N9 = 2 ** 63;

/**
 * Encodes a varint
 * @param num Number to encode
 * @param buf Buffer to write on
 * @param offset Starting position on the buffer
 * @returns The amount of bytes written
 */
export const encode = (num: number, buf: Uint8Array | number[], offset = 0): number => {
	if (num > Number.MAX_SAFE_INTEGER) {
		throw new RangeError('could not encode varint');
	}

	const start = offset;

	while (num >= INT) {
		buf[offset++] = (num & 0xff) | MSB;
		num /= 128;
	}

	while (num & MSBALL) {
		buf[offset++] = (num & 0xff) | MSB;
		num >>>= 7;
	}

	buf[offset] = num | 0;
	return offset - start + 1;
};

/**
 * Decodes a varint
 * @param buf Buffer to read from
 * @param offset Starting position on the buffer
 * @returns A tuple containing the resulting number, and the amount of bytes read
 */
export const decode = (buf: Uint8Array | number[], offset = 0): [num: number, read: number] => {
	// deno-lint-ignore prefer-const
	let l = buf.length;

	let res = 0;
	let shift = 0;
	let counter = offset;
	let b: number;

	do {
		if (counter >= l) {
			throw new RangeError('could not decode varint');
		}

		b = buf[counter++];
		res += shift < 28 ? (b & REST) << shift : (b & REST) * Math.pow(2, shift);
		shift += 7;
	} while (b >= MSB);

	return [res, counter - offset];
};

/**
 * Returns encoding length
 * @param num The number to encode
 * @returns Amount of bytes needed for encoding
 */
export const encodingLength = (num: number): number => {
	return num < N1
		? 1
		: num < N2
			? 2
			: num < N3
				? 3
				: num < N4
					? 4
					: num < N5
						? 5
						: num < N6
							? 6
							: num < N7
								? 7
								: num < N8
									? 8
									: num < N9
										? 9
										: 10;
};