export class Hex {
  static readonly HEX_TABLE = '0123456789abcdef';
  static textEncoder = new TextEncoder();

  static encodeLength(n: number): number {
    return n * 2;
  }

  static encode(src: Uint8Array): Uint8Array {
    const dst = new Uint8Array(Hex.encodeLength(src.length));
    let j = 0;
    for (let i = 0; i < src.length; i++) {
      dst[j] = Hex.HEX_TABLE[src[i] >> 4].charCodeAt(0);
      dst[j + 1] = Hex.HEX_TABLE[src[i] & 0x0f].charCodeAt(0);
      j += 2;
    }
    return dst;
  }

  static decodeString(s: string): Uint8Array {
    return Hex.decode(s);
  }
  static fromHexChar(c: number): number {
    if ('0'.charCodeAt(0) <= c && c <= '9'.charCodeAt(0)) {
      return c - '0'.charCodeAt(0);
    } else if ('a'.charCodeAt(0) <= c && c <= 'f'.charCodeAt(0)) {
      return c - 'a'.charCodeAt(0) + 10;
    }
    if ('A'.charCodeAt(0) <= c && c <= 'F'.charCodeAt(0)) {
      return c - 'A'.charCodeAt(0) + 10;
    }

    throw new Error(`Invalid byte char ${c}`);
  }

  private static decode(src: string): Uint8Array {
    let i = 0;
    let j = 1;
    const dst: number[] = [];
    for (; j < src.length; j += 2) {
      const a = Hex.fromHexChar(src[j - 1].charCodeAt(0));
      const b = Hex.fromHexChar(src[j].charCodeAt(0));
      dst[i] = (a << 4) | b;
      i++;
    }
    if (src.length % 2 == 1) {
      throw new Error('Invalid hex string');
    }
    return Uint8Array.from(dst);
  }

  static encodeString(b: Uint8Array): string {
    return new TextDecoder().decode(Hex.encode(b));
  }
}