> 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/38.1-introduction-to-compression.md).

# 38.1 Introduction to Compression

As an introduction to compression, consider the processes of creating and unzipping a zip file.

<pre class="language-bash"><code class="lang-bash">$ zip mobydick.zip mobydick.txt 
adding: mobydick.txt (deflated 59%)

$ ls -l
<strong>-rw-rw-r-- 1 jug jug 643207 Apr 24 10:55 mobydick.txt
</strong>-rw-rw-r-- 1 jug jug 261375 Apr 24 10:55 mobydick.zip
</code></pre>

Note that before and after unzipping, the file size changes!

## Compression Model 1: Algorithms on Bits

In our first model of compression, we consider compression as applying a *compression algorithm* on a sequence of bits. To reverse the compression, we apply the inverse *decompression algorithm.*

<figure><img src="/files/FKqiw1XiggxMh6UEzyAa" alt=""><figcaption><p>Compression and decompression.</p></figcaption></figure>

Say you had a text file called `example.txt`. If you were to zip that text file, you'd get `example.zip`, a zip file with a size much lesser than the original `example.txt` file. This is the main idea behind compression--a technique used to reduce file size.

Then, if you were to unzip `example.zip` into a file called `unzippedexample.txt`, you would notice no difference between `example.txt` and `unzippedexample.txt.` This is an indicator of **lossless** compression, where no information is lost.