bitcoinjs-lib/ts_src/bufferutils.ts

// https://github.com/feross/buffer/blob/master/index.js#L1127
function verifuint(value: number, max: number): void {
  if (typeof value !== 'number')
    throw new Error('cannot write a non-number as a number');
  if (value < 0)
    throw new Error('specified a negative value for writing an unsigned value');
  if (value > max) throw new Error('RangeError: value out of range');
  if (Math.floor(value) !== value)
    throw new Error('value has a fractional component');
}

export function readUInt64LE(buffer: Buffer, offset: number): number {
  const a = buffer.readUInt32LE(offset);
  let b = buffer.readUInt32LE(offset + 4);
  b *= 0x100000000;

  verifuint(b + a, 0x001fffffffffffff);
  return b + a;
}

export function writeUInt64LE(
  buffer: Buffer,
  value: number,
  offset: number,
): number {
  verifuint(value, 0x001fffffffffffff);

  buffer.writeInt32LE(value & -1, offset);
  buffer.writeUInt32LE(Math.floor(value / 0x100000000), offset + 4);
  return offset + 8;
}

export function reverseBuffer(buffer: Buffer): Buffer {
  if (buffer.length < 1) return buffer;
  let j = buffer.length - 1;
  let tmp = 0;
  for (let i = 0; i < buffer.length / 2; i++) {
    tmp = buffer[i];
    buffer[i] = buffer[j];
    buffer[j] = tmp;
    j--;
  }
  return buffer;
}