const SHORT_TO_HEX: { [key: number]: string } = {};
const HEX_TO_SHORT: Record<string, number> = {};

for (let i = 0; i < 256; i++) {
  let encodedByte = i.toString(16).toLowerCase();
  if (encodedByte.length === 1) {
    encodedByte = `0${encodedByte}`;
  }

  SHORT_TO_HEX[i] = encodedByte;
  HEX_TO_SHORT[encodedByte] = i;
}

export function fromHex(encoded: string): Uint8Array {
  if (encoded.length % 2 !== 0) {
    throw new Error("Hex encoded strings must have an even number length");
  }

  const out = new Uint8Array(encoded.length / 2);
  for (let i = 0; i < encoded.length; i += 2) {
    const encodedByte = encoded.slice(i, i + 2).toLowerCase();
    if (encodedByte in HEX_TO_SHORT) {
      out[i / 2] = HEX_TO_SHORT[encodedByte];
    } else {
      throw new Error(`Cannot decode unrecognized sequence ${encodedByte} as hexadecimal`);
    }
  }

  return out;
}