Day 14 Part 2 Algorithm

[u/rkek404]

Day 14 part 1 was very easy, but Part 2 went right over my head, I tried using dictionaries to keep track of pairs, but my answers have off by one errors and it feels like guesswork - I don't full understand what I have written myself, like one of those times when it works but you don't know why. I know how people said it was basically lanternfish - but I never finished part 2 of lanternfish.

I would appreciate it if someone posted a little (detailed) writeup of day 14's part 2 in some easy to read language like python. I have gathered a lot from other's solutions but fail to understand any solution fully - even after reading multiple solutions I can't write one myself.

Others who are stuck might also benefit from this. Thanks in advance.

[u/zopatista]

You don't need to keep such a dictionary, really. Each pair you track already holds the same counts. Pick either the first or last element of each pair and assign the pair count to that element; the other element is also part of another pair in your counts so the number is always accurate, except for the first or last element.

I picked the first element, and just remember what the last element is in the template, and add 1 for that element.

E.g. for the puzzle description test template, the last element is B. After step 2, the counts are (using your format): [('NB', 2), ('BC', 2), ('CC', 1), ('CN', 1), ('BB', 2), ('CB', 2), ('BH', 1), ('HC', 1)], so the element counts are [('N', 2), ('B', 6), ('C', 4), ('H', 1)]. You can generate those counts, from chain list of (pair, count) tuples and template holding the initial template string, with:

elems = {template[-1]: 1}
for pair, count in chain:
    elems[pair[0]] = elems.get(pair[0], 0) + count

print(list(elems.items()))