Discussion on: Advent of Code 2020 Solution Megathread - Day 9: Encoding Error

[Derk-Jan Karrenbeld]

This is the first day I did not OOP it up, because I felt lazy. Part 2 is still nicely linear, but didn't bother to optimise Part 1. Here's Ruby:

require 'benchmark'

def find_sum_of_uniques(candidates, check)
  skip = check / 2.to_f

  candidates.each do |x|
    next if x == skip

    candidates.each do |y|
      next if y == skip
      return true if x + y == check
    end
  end

  false
end

def find_invalid(numbers, preamble: 25)
  pointer = preamble
  candidates = numbers[0...preamble]


  while pointer < numbers.length
    check = numbers[pointer]

    unless find_sum_of_uniques(candidates, check)
      return check
    end

    candidates.shift
    candidates.push(numbers[pointer])

    pointer += 1
  end
end

def find_sum_set(numbers, match)
  pointer_left = 0
  pointer_right = 1

  sum = numbers[pointer_left..pointer_right].sum

  checks = 0

  while sum != match
    checks += 1

    # puts "#{match} != #{numbers[pointer_left]} + (...#{[pointer_right - pointer_left - 2, 0].max} numbers) + #{numbers[pointer_right]}"

    if sum < match
      pointer_right += 1
      sum += numbers[pointer_right]
    elsif sum > match
      sum -= numbers[pointer_left]
      pointer_left += 1
    end
  end

  numbers[pointer_left...pointer_right].tap do |x|
    # puts "#{match} == #{numbers[pointer_left..pointer_right].sort.join(" + ")}"
    puts "\nChecked #{checks} sets to find the sum set"
  end
end

numbers = File.readlines('input.txt').map(&:to_i)

Benchmark.bmbm do |b|
  b.report do
    invalid = find_invalid(numbers, preamble: 25)
    set = find_sum_set(numbers, invalid)

    puts "xmas: #{set.minmax.sum}"
  end
end