Advent of Code F# – Day 10

The source code for this post (both Part 1 and Part 2) is available here and you can click here to see my solutions for the other Advent of Code challenges.

Description for today’s challenge is here.


Day 10 is nice and easy, and the solution I came up with is less than 20 lines of code, so I’m gonna leave it here and walk you through it:


So the bulk of the way happens in the fold inside the read function, which, starting with the empty list as the accumulated state, the following happened:

  1. saw ‘3’, acc state became [ (1, ‘3’) ]
  2. saw ‘1’, head of acc state is on ‘3’, so acc sate became [ (1, ‘1’); (1, ‘3’) ]
  3. saw ‘1’, head of acc state is also on ‘1’, so acc state became [ (2, ‘1’); (1, ‘3’) ]
  4. saw ‘3’, head of acc state is on ‘1’, so acc state became [ (1, ‘3’); (2, ‘1’); (1, ‘3’) ]

After the fold, we still need to reverse the list, and then construct the output string as “132123…”.

To answer the challenge, we just iteratively apply the read function (I used a fold here, but you can equally just write it as a recursive function), and voila!

Part 2 is identical, with the only difference being that read is invoked 50 times instead of 40.

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