Dictionary encoding

Dictionary encoding is a method of [[ /lossless compression|lossless compression]] where we replace repeated data with a reference to a single copy of that data, stored in a sort of ‘dictionary’ that maps the reference to the original data.

For example, if we had the string ABCDABCDABCD, we could store that as:

Dictionary:
1: ABCD

Data:
1 1 1

That is, we have a dictionary that maps the reference 1 to the string ABCD, and then in our data, we just store the reference 1 three times to indicate that we want to repeat the string ABCD three times.

Benefits of dictionary encoding

• The general benefits of lossless compression
• It can be very effective for certain types of data, like text with lots of repeated words or phrases, or images with lots of repeated patterns.
• It can be more efficient than run-length encoding for certain types of data, as it doesn’t require the data to all be repeated in a row, and we can have a pattern as a dictionary entry, rather than just a single character or pixel.
• A variation of dictionary encoding can be used for things like PNG images, where we can store a pattern of pixels in the dictionary, and then reference that pattern multiple times in the image data, which can save a lot of space.

Drawbacks of dictionary encoding

• It can be more complex to implement than run-length encoding, as we need to manage the dictionary and the references to it.
• We have to actually store the dictionary itself. Sometimes that can make the file size bigger!
• In certain cases, it can be less efficient than [[ /run length encoding|run length encoding]].