Solve day 8 of AoC 2024
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152
advent-of-code/2024/day_08/README.md
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152
advent-of-code/2024/day_08/README.md
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# Day 8: Resonant Collinearity
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[Advent of Code 2024](https://adventofcode.com/2024/day/8)
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You find yourselves on the roof of a top-secret Easter Bunny installation.
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While The Historians do their thing, you take a look at the familiar huge antenna.
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Much to your surprise, it seems to have been reconfigured to emit a signal that makes people 0.1% more likely to buy
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Easter Bunny brand Imitation Mediocre Chocolate as a Christmas gift! Unthinkable!
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Scanning across the city, you find that there are actually many such antennas.
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Each antenna is tuned to a specific frequency indicated by a single lowercase letter, uppercase letter, or digit.
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You create a map (your puzzle input) of these antennas. For example:
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```
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............
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........0...
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.....0......
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.......0....
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....0.......
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......A.....
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............
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............
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........A...
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.........A..
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............
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............
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```
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The signal only applies its nefarious effect at specific antinodes based on the resonant frequencies of the antennas.
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In particular, an antinode occurs at any point that is perfectly in line with two antennas of the same frequency - but only when
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one of the antennas is twice as far away as the other. This means that for any pair of antennas with the same frequency,
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there are two antinodes, one on either side of them.
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So, for these two antennas with frequency a, they create the two antinodes marked with #:
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```
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..........
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...#......
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..........
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....a.....
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..........
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.....a....
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..........
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......#...
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..........
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..........
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```
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Adding a third antenna with the same frequency creates several more antinodes. It would ideally add four antinodes,
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but two are off the right side of the map, so instead it adds only two:
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```
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..........
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...#......
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#.........
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....a.....
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........a.
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.....a....
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..#.......
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......#...
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..........
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..........
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```
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Antennas with different frequencies don't create antinodes; A and a count as different frequencies.
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However, antinodes can occur at locations that contain antennas. In this diagram, the lone antenna with frequency capital A
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creates no antinodes but has a lowercase-a-frequency antinode at its location:
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```
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..........
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...#......
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#.........
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....a.....
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........a.
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.....a....
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..#.......
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......A...
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..........
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..........
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```
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The first example has antennas with two different frequencies, so the antinodes they create look like this,
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plus an antinode overlapping the topmost A-frequency antenna:
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```
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......#....#
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...#....0...
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....#0....#.
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..#....0....
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....0....#..
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.#....A.....
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...#........
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#......#....
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........A...
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.........A..
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..........#.
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..........#.
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```
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Because the topmost A-frequency antenna overlaps with a 0-frequency antinode, there are 14 total unique locations
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that contain an antinode within the bounds of the map.
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Calculate the impact of the signal. How many unique locations within the bounds of the map contain an antinode?
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## Part Two
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Watching over your shoulder as you work, one of The Historians asks if you took the effects of resonant harmonics into your calculations.
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Whoops!
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After updating your model, it turns out that an antinode occurs at any grid position exactly in line with at least two antennas
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of the same frequency, regardless of distance. This means that some of the new antinodes will occur at the position
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of each antenna (unless that antenna is the only one of its frequency).
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So, these three T-frequency antennas now create many antinodes:
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```
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T....#....
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...T......
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.T....#...
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.........#
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..#.......
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..........
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...#......
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..........
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....#.....
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..........
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```
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In fact, the three T-frequency antennas are all exactly in line with two antennas, so they are all also antinodes!
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This brings the total number of antinodes in the above example to 9.
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The original example now has 34 antinodes, including the antinodes that appear on every antenna:
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```
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##....#....#
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.#.#....0...
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..#.#0....#.
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..##...0....
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....0....#..
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.#...#A....#
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...#..#.....
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#....#.#....
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..#.....A...
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....#....A..
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.#........#.
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...#......##
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```
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Calculate the impact of the signal using this updated model. How many unique locations within the bounds of the map contain an antinode?
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50
advent-of-code/2024/day_08/input.txt
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advent-of-code/2024/day_08/input.txt
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..........M..........j.............y.....O........
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...B...............q......m........lGO............
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....................q......2.l.GQ...O.............
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.....X.......................................4....
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.....................q............................
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....M......P...............xl.K.............2.....
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....F.........L.......C.K..............m..........
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..........FM......P....jy......m..........o...r...
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..X.......P.....RL..............G..x..........4...
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............L..........NC.....q...................
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.....C.X...............K....y..........4..........
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........S...R.............j.x.....V...4...........
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.....................R..x.....V..i......m.........
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...........................R.V......N.......X.....
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.....F.........M......N......E....................
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................v................T.......F......O.
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.............................N...V.......Q........
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...v.....................C.....i..................
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......c.....W..n.w........................E.......
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3...................c.....................Q..6....
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...........h......................j...............
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.......n.0......h.................E..............2
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.v.............7.......120.....c..................
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......n.0............w...........D.t.........E...r
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....8..3......0.w.hP....z...D..T...............r..
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.................f........T........G......eQ......
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......f.n.....7..p................................
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.....Y..7.......f......I......D......K............
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............Uf....T..W.....D..r...i...............
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......I...............................Z...........
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....5....B.......b..............s..............Z..
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..........d...W..Uwh.............c..........i.....
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..I.3..Y......................e...................
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.....p.b..........k......7........................
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p...........k....I..b..........s..................
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.....k.......o...........W........................
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.A..Y..........U.................a........6.......
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..A...Y.p...................................6.....
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B......k..........................Z............u..
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...3.....................s..............a.........
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......A.........................g.....a...........
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.......A....8...b.U......H....sS..................
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.........................S1.............t.........
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.....................9z..e.....5..1.g.u...........
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.......................z....d....g....H.J....o.6..
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........B................d.....u....9.J.H.........
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.8........S.................u9.............J.....H
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.....................Z5.............t1...........a
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.....................e..v...................o..t..
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.....8...............L.....z.............J........
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83
advent-of-code/2024/day_08/lib.rb
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83
advent-of-code/2024/day_08/lib.rb
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class Vec2
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attr_accessor :x, :y
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def initialize(x, y)
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@x = x
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@y = y
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end
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end
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def count_antinodes(matrix, limit_length)
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station_coords = extract_stations(matrix)
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# Create zero matrix.
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antinodes = Array.new(matrix.length) { Array.new(matrix[0].length, 0) }
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station_coords.each_value do |loc|
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(0..loc.length - 2).each do |i|
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(i + 1..loc.length - 1).each do |j|
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x_diff = loc[j].x - loc[i].x
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y_diff = loc[j].y - loc[i].y
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x1 = loc[i].x - x_diff
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y1 = loc[i].y - y_diff
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while in_range?(matrix, x1, y1)
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antinodes[x1][y1] += 1
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break if limit_length
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x1 -= x_diff
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y1 -= y_diff
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end
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x2 = loc[j].x + x_diff
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y2 = loc[j].y + y_diff
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while in_range?(matrix, x2, y2)
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antinodes[x2][y2] += 1
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break if limit_length
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x2 += x_diff
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y2 += y_diff
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end
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end
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end
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end
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unless limit_length
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station_coords.each_value do |locations|
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locations.each do |loc|
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antinodes[loc.x][loc.y] += 1
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end
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end
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end
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count = 0
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antinodes.each do |row|
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row.each do |value|
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next if value.zero?
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count += 1
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end
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end
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count
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end
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def in_range?(matrix, x, y)
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x >= 0 and y >= 0 and x < matrix.length and y < matrix[x].length
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end
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def extract_stations(matrix)
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station_coords = {}
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(0..matrix.length - 1).each do |x|
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(0..matrix[x].length - 1).each do |y|
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ch = matrix[x][y]
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next if ch == '.'
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station_coords[ch] = [] unless station_coords.include?(ch)
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station_coords[ch].push(Vec2.new(x, y))
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end
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end
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station_coords
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end
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8
advent-of-code/2024/day_08/main.rb
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advent-of-code/2024/day_08/main.rb
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require_relative "lib.rb"
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input = File.readlines('input.txt', chomp: true)
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res1 = count_antinodes(input, true)
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puts("Part 1: #{res1}")
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res2 = count_antinodes(input, false)
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puts("Part 2: #{res2}")
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14
advent-of-code/2024/day_08/test.rb
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advent-of-code/2024/day_08/test.rb
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require_relative "lib.rb"
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require "test/unit"
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class TestAntinodeCounter < Test::Unit::TestCase
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def test_part_1
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input = File.readlines("test.txt", chomp: true)
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assert_equal(14, count_antinodes(input, true))
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end
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def test_part_2
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input = File.readlines("test.txt", chomp: true)
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assert_equal(34, count_antinodes(input, false))
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end
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end
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12
advent-of-code/2024/day_08/test.txt
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advent-of-code/2024/day_08/test.txt
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............
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........0...
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.....0......
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.......0....
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....0.......
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......A.....
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............
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............
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........A...
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.........A..
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............
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............
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