Solve day 8 of AoC 2024

advent-of-code/2024/day_08/README.md

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+# Day 8: Resonant Collinearity
+
+[Advent of Code 2024](https://adventofcode.com/2024/day/8)
+
+You find yourselves on the roof of a top-secret Easter Bunny installation.
+
+While The Historians do their thing, you take a look at the familiar huge antenna.
+Much to your surprise, it seems to have been reconfigured to emit a signal that makes people 0.1% more likely to buy
+Easter Bunny brand Imitation Mediocre Chocolate as a Christmas gift! Unthinkable!
+
+Scanning across the city, you find that there are actually many such antennas.
+Each antenna is tuned to a specific frequency indicated by a single lowercase letter, uppercase letter, or digit.
+You create a map (your puzzle input) of these antennas. For example:
+
+```
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............
+```
+
+The signal only applies its nefarious effect at specific antinodes based on the resonant frequencies of the antennas.
+In particular, an antinode occurs at any point that is perfectly in line with two antennas of the same frequency - but only when
+one of the antennas is twice as far away as the other. This means that for any pair of antennas with the same frequency,
+there are two antinodes, one on either side of them.
+
+So, for these two antennas with frequency a, they create the two antinodes marked with #:
+
+```
+..........
+...#......
+..........
+....a.....
+..........
+.....a....
+..........
+......#...
+..........
+..........
+```
+
+Adding a third antenna with the same frequency creates several more antinodes. It would ideally add four antinodes,
+but two are off the right side of the map, so instead it adds only two:
+
+```
+..........
+...#......
+#.........
+....a.....
+........a.
+.....a....
+..#.......
+......#...
+..........
+..........
+```
+
+Antennas with different frequencies don't create antinodes; A and a count as different frequencies.
+However, antinodes can occur at locations that contain antennas. In this diagram, the lone antenna with frequency capital A
+creates no antinodes but has a lowercase-a-frequency antinode at its location:
+
+```
+..........
+...#......
+#.........
+....a.....
+........a.
+.....a....
+..#.......
+......A...
+..........
+..........
+```
+
+The first example has antennas with two different frequencies, so the antinodes they create look like this,
+plus an antinode overlapping the topmost A-frequency antenna:
+
+```
+......#....#
+...#....0...
+....#0....#.
+..#....0....
+....0....#..
+.#....A.....
+...#........
+#......#....
+........A...
+.........A..
+..........#.
+..........#.
+```
+
+Because the topmost A-frequency antenna overlaps with a 0-frequency antinode, there are 14 total unique locations
+that contain an antinode within the bounds of the map.
+
+Calculate the impact of the signal. How many unique locations within the bounds of the map contain an antinode?
+
+
+## Part Two
+
+Watching over your shoulder as you work, one of The Historians asks if you took the effects of resonant harmonics into your calculations.
+
+Whoops!
+
+After updating your model, it turns out that an antinode occurs at any grid position exactly in line with at least two antennas
+of the same frequency, regardless of distance. This means that some of the new antinodes will occur at the position
+of each antenna (unless that antenna is the only one of its frequency).
+
+So, these three T-frequency antennas now create many antinodes:
+
+```
+T....#....
+...T......
+.T....#...
+.........#
+..#.......
+..........
+...#......
+..........
+....#.....
+..........
+```
+
+In fact, the three T-frequency antennas are all exactly in line with two antennas, so they are all also antinodes!
+This brings the total number of antinodes in the above example to 9.
+
+The original example now has 34 antinodes, including the antinodes that appear on every antenna:
+
+```
+##....#....#
+.#.#....0...
+..#.#0....#.
+..##...0....
+....0....#..
+.#...#A....#
+...#..#.....
+#....#.#....
+..#.....A...
+....#....A..
+.#........#.
+...#......##
+```
+
+Calculate the impact of the signal using this updated model. How many unique locations within the bounds of the map contain an antinode?

advent-of-code/2024/day_08/input.txt

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+..........M..........j.............y.....O........
+...B...............q......m........lGO............
+....................q......2.l.GQ...O.............
+.....X.......................................4....
+.....................q............................
+....M......P...............xl.K.............2.....
+....F.........L.......C.K..............m..........
+..........FM......P....jy......m..........o...r...
+..X.......P.....RL..............G..x..........4...
+............L..........NC.....q...................
+.....C.X...............K....y..........4..........
+........S...R.............j.x.....V...4...........
+.....................R..x.....V..i......m.........
+...........................R.V......N.......X.....
+.....F.........M......N......E....................
+................v................T.......F......O.
+.............................N...V.......Q........
+...v.....................C.....i..................
+......c.....W..n.w........................E.......
+3...................c.....................Q..6....
+...........h......................j...............
+.......n.0......h.................E..............2
+.v.............7.......120.....c..................
+......n.0............w...........D.t.........E...r
+....8..3......0.w.hP....z...D..T...............r..
+.................f........T........G......eQ......
+......f.n.....7..p................................
+.....Y..7.......f......I......D......K............
+............Uf....T..W.....D..r...i...............
+......I...............................Z...........
+....5....B.......b..............s..............Z..
+..........d...W..Uwh.............c..........i.....
+..I.3..Y......................e...................
+.....p.b..........k......7........................
+p...........k....I..b..........s..................
+.....k.......o...........W........................
+.A..Y..........U.................a........6.......
+..A...Y.p...................................6.....
+B......k..........................Z............u..
+...3.....................s..............a.........
+......A.........................g.....a...........
+.......A....8...b.U......H....sS..................
+.........................S1.............t.........
+.....................9z..e.....5..1.g.u...........
+.......................z....d....g....H.J....o.6..
+........B................d.....u....9.J.H.........
+.8........S.................u9.............J.....H
+.....................Z5.............t1...........a
+.....................e..v...................o..t..
+.....8...............L.....z.............J........

advent-of-code/2024/day_08/lib.rb

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+class Vec2
+  attr_accessor :x, :y
+
+  def initialize(x, y)
+    @x = x
+    @y = y
+  end
+end
+
+def count_antinodes(matrix, limit_length)
+  station_coords = extract_stations(matrix)
+
+  # Create zero matrix.
+  antinodes = Array.new(matrix.length) { Array.new(matrix[0].length, 0) }
+
+  station_coords.each_value do |loc|
+    (0..loc.length - 2).each do |i|
+      (i + 1..loc.length - 1).each do |j|
+        x_diff = loc[j].x - loc[i].x
+        y_diff = loc[j].y - loc[i].y
+
+        x1 = loc[i].x - x_diff
+        y1 = loc[i].y - y_diff
+        while in_range?(matrix, x1, y1)
+          antinodes[x1][y1] += 1
+          break if limit_length
+
+          x1 -= x_diff
+          y1 -= y_diff
+        end
+
+        x2 = loc[j].x + x_diff
+        y2 = loc[j].y + y_diff
+        while in_range?(matrix, x2, y2)
+          antinodes[x2][y2] += 1
+          break if limit_length
+
+          x2 += x_diff
+          y2 += y_diff
+        end
+      end
+    end
+  end
+
+  unless limit_length
+    station_coords.each_value do |locations|
+      locations.each do |loc|
+        antinodes[loc.x][loc.y] += 1
+      end
+    end
+  end
+
+  count = 0
+  antinodes.each do |row|
+    row.each do |value|
+      next if value.zero?
+
+      count += 1
+    end
+  end
+
+  count
+end
+
+def in_range?(matrix, x, y)
+  x >= 0 and y >= 0 and x < matrix.length and y < matrix[x].length
+end
+
+def extract_stations(matrix)
+  station_coords = {}
+
+  (0..matrix.length - 1).each do |x|
+    (0..matrix[x].length - 1).each do |y|
+      ch = matrix[x][y]
+      next if ch == '.'
+
+      station_coords[ch] = [] unless station_coords.include?(ch)
+      station_coords[ch].push(Vec2.new(x, y))
+    end
+  end
+
+  station_coords
+end

advent-of-code/2024/day_08/main.rb

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+require_relative "lib.rb"
+
+input = File.readlines('input.txt', chomp: true)
+res1 = count_antinodes(input, true)
+puts("Part 1: #{res1}")
+
+res2 = count_antinodes(input, false)
+puts("Part 2: #{res2}")

advent-of-code/2024/day_08/test.rb

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+require_relative "lib.rb"
+require "test/unit"
+
+class TestAntinodeCounter < Test::Unit::TestCase
+  def test_part_1
+    input = File.readlines("test.txt", chomp: true)
+    assert_equal(14, count_antinodes(input, true))
+  end
+
+  def test_part_2
+    input = File.readlines("test.txt", chomp: true)
+    assert_equal(34, count_antinodes(input, false))
+  end
+end

advent-of-code/2024/day_08/test.txt

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+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............