README.md

Note: deprecated.

See instead:

• https://github.com/sroccaserra/advent-of-code

Learnings

See also:

• https://github.com/sroccaserra/aoc2018#learnings
• https://github.com/sroccaserra/aoc2019#learnings
• https://github.com/sroccaserra/aoc2020#learnings
• https://github.com/sroccaserra/aoc2021#learnings
• https://github.com/sroccaserra/aoc2022#learnings

Python

• Use defaultdict(int) to count elements:
  • d = defaultdict(int) # then d[k] += 1, values default to zero
  • beware, in a defaultdict even a read can invalidate iterators
  • defaultdict(bool) might be useful too, but a set() might be better
• There are useful implementations in Python's itertools doc: https://docs.python.org/3/library/itertools.html (see "Itertools Recipes" section)

Maths & Algorithms

• The sum of a number's divisors is equal to the product of (1 + p + p^2 + ... + p^k) where p is a prime divisor and k is its exponent in n's factorisation. S(144) = S(2^4.3^2) = S(2^4).S(3^2) Also, 1 + p + p^2 + . . . + p^k = (p^(k+1) - 1) / (p-1) - https://www2.math.upenn.edu/~deturck/m170/wk3/lecture/sumdiv.html

• https://en.wikipedia.org/wiki/Knapsack_problem

• https://en.wikipedia.org/wiki/Stars_and_bars_%28combinatorics%29

• If I make a map of values to list of values (or a dict or hashtable of values to arrays of values, same idea), I am probably working with a tree or graph like structure. There are probably classic algorithms I can use to solve the problem, and classical structures I can use to represent it (I can switch to adjacenty list or matrix if it works better for example).

Haskell

• concatMap f . g is g >=> f (see day 10)