import { BufferWriter } from '@node-dlc/bufio';

/**
 * Converts an IPv6 address in string notation to the
 * byte representation.
 */
export function ipv6StringToBuffer(host: string): Buffer {
  // replace start or end expansion with single part to retain correct
  // number of parts (8) that are used in the remainder of the logic.
  // ie: ::1:2:3:4:5:6:7 would split to ['','',1,2,3,4,5,6,7] and
  // result in overflows
  if (host.startsWith('::')) host = host.substr(1);
  else if (host.endsWith('::')) host = host.substr(0, host.length - 1);

  const parts = host.split(':');

  const writer = new BufferWriter(Buffer.alloc(16));

  const expandBy = 8 - parts.length;
  let needsExpansion = expandBy > 0;

  for (const part of parts) {
    if (needsExpansion && part === '') {
      const b = Buffer.alloc((expandBy + 1) * 2);
      writer.writeBytes(b);
      needsExpansion = false;
    } else {
      const b = Buffer.from(expandZeros(part), 'hex');
      writer.writeBytes(b);
    }
  }

  return writer.toBuffer();
}

function expandZeros(part: string): string {
  return part.padStart(4, '0');
}