Add day24 solution.

day24.cr

@@ -0,0 +1,78 @@
+require "advent"
+
+INPUT = input(2020, 24).lines.map &.tiles.map { |t| MATCHES[t] }
+MATCHES = { "se" => {0.5, -1.0}, "sw" => {-0.5, -1.0}, "ne" => {0.5, 1.0}, "nw" => {-0.5, 1.0}, "e" => {1.0, 0.0}, "w" => {-1.0, 0.0} }
+
+class String
+  def tiles
+    i = 0
+    tiles = [] of String
+    while i < size
+      if self[i] == 's' || self[i] == 'n'
+        tiles << self[i..i+1] 
+        i += 2
+      else
+        tiles << self[i..i]
+        i += 1
+      end
+    end
+    tiles
+  end
+end
+
+class Array(T)
+  def pos
+    curr = {0.0, 0.0}
+    each do |c|
+      curr = curr.add c
+    end
+    curr
+  end
+end
+
+def part1(input)
+  counts = Hash({Float64,Float64}, Int32).new(0)
+  input.each do |i|
+    counts[i.pos] += 1
+  end
+  counts.count &.[1].%(2).==(1)
+end
+
+struct Tuple(*T)
+  def neighbors
+    MATCHES.values.map &.add(self)
+  end
+end
+
+def part2(input)
+  state = Hash({Float64,Float64}, Bool).new(false)
+  counts = Hash({Float64,Float64}, Int32).new(0)
+
+  input.each do |i|
+    state[i.pos] ^= true
+  end
+
+  100.times do |i|
+    counts.clear
+    state.each do |t, f|
+      next unless f
+      t.neighbors.each do |n|
+        counts[n] += 1
+      end
+    end
+
+    new_state = state.clone.clear
+    state.each do |k, v|
+      next unless v
+      new_state[k] = !(counts[k] == 0 || (counts[k] > 2))
+    end
+    counts.each do |k, v|
+      new_state[k] = true if (!state[k]) && v == 2
+    end
+    state = new_state
+  end
+  state.count &.[1]
+end
+
+puts part1(INPUT.clone)
+puts part2(INPUT.clone)