Advent of Functional PHP: Day 8

Submitted by Larry on 20 December 2021 - 10:06am

Advent of Code Day 8 was, to put it mildly, a pain in the ass. There's a couple of reasons for that. It's a naturally tricky problem, it's hard to genericize, and it's explained fairly badly. It took a while but with some help from others I was finally able to figure out (and refactor to) a good, functional solution to it. So let's dive in.

The problem boils down to one of encryption. Our input is several lines that all look like this:

acedgfb cdfbe gcdfa fbcad dab cefabd cdfgeb eafb cagedb ab | cdfeb fcadb cdfeb cdbaf

Where each letter corresponds to one segment in an LED display for a number. Each number appears once on the left side, which is enough for you to figure out what letter corresponds to what segment. Then we need to use that knowledge to decode the numbers on the right and figure out what the number is.

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