> For the complete documentation index, see [llms.txt](https://cs61b-2.gitbook.io/cs61b-textbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cs61b-2.gitbook.io/cs61b-textbook/38.-compression-and-complexity/38.8-exercises.md).

# 38.8 Exercises

## Factual

1. Suppose we build a Shannon Fano or Huffman code for the text of this question including spaces and punctuation characters. Which characters would have the longest code?
2. What two ways could we represent a Huffman code for characters in Java?

<details>

<summary>Problem 1</summary>

`?` and `.`, since both are only used once in the above sentence.

</details>

<details>

<summary>Problem 2</summary>

A `HashMap<Character, BitSequence>` or a `BitSequence[]`. Note that the two are equivalent in Java because a Character is a number.

</details>

## Procedural

1. Suppose we have a string `abcdefg` which repeats 1000 times. How many bits would be in the compressed bitstream?

<details>

<summary>Problem 1</summary>

Since all 8 characters are equal in frequency, we get a balanced binary tree as our Huffman encoding, so all codewords are 3 bits long. 1000 \* 8 \* 3 = 24000 bits.

</details>

## Metacognitive

1. Using the idea of self-extracting bits, come up with an encoding for the sequence `abdefg` repeated 1000 times that uses less than 2000 bits.&#x20;

<details>

<summary>Problem 1</summary>

The idea of self-extracting bits includes writing code or an interpreter that can generate the original uncompressed sequence. This can be done with the following code:

```java
public class Sequence {
    public static void main(String[] args) {
        for (int i = 0; i < 1000; i++) {
            for (int j = 0; j < 8; j++) {
                System.out.print(String.format("%c", 'a' + j));
            }
        }
    }
}
```

This code uses exactly 239 characters, or 1912 bits. This demonstrates the power of the self-extracting bits model: compare this to the 24000 bits required for a Huffman code.

</details>


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