Learning by Example: checksum

In this lesson, you will learn how to generate the checksum of a string.

We'll cover the following

ChecksumAccumulator

To understand the concepts of the next lesson, we will be using Martin Ordersky’s (inventor of Scala) ChecksumAccumulator program. Before we move on, let’s briefly go over checksum.

Checksum

A checksum is a string of characters consisting of numbers and letters. It acts as a fingerprint for files and messages and is used to ensure the file or message you have received is genuine and error free. To do so, you create a checksum of the file you have received and compare it to the checksum of the source file. Even the most minor of changes will result in a completely different checksum.

For the scope of this course, we will be generating a checksum of a string.

The ChecksumAccumulator Class

The ChecksumAccumulator class has a single private field value sum, which is initialized to 0. It further has two methods, namely add, which is adding the bytes of a string, and checksum which is calculating the final checksum. checksum first uses the bitwise operator & with the final sum and the hexadecimal 0xFF. It then takes the result of the & operator and applies another bitwise operator to it ~. Finally, it adds a 1 to the result of the ~ operator, resulting in the final checksum value.

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class ChecksumAccumulator {
  private var sum = 0
  def add(b: Byte) = sum += b
  def checksum() = ~(sum & 0xFF) + 1
}

This is a predefined algorithm which we are implementing. If you do not understand the algorithm, it’s okay. You just need to know that it is used to generate a checksum.

In the next lesson, we will use ChecksumAccumulator to introduce singleton objects.