1 files changed, 61 insertions, 0 deletions
diff --git a/day15/part2.rb b/day15/part2.rb
new file mode 100644
index 0000000..cb3219d
--- /dev/null
+++ b/day15/part2.rb
@@ -0,0 +1,61 @@
+class Sensor
+  def initialize(sx, sy, bx, by)
+    @x = sx
+    @y = sy
+    @range = dist(bx, by)
+  end
+
+  attr_reader :x, :y, :range
+
+  def dist(x, y)
+    (@x - x).abs + (@y - y).abs
+  end
+
+  def cover?(x, y)
+    dist(x, y) <= @range
+  end
+
+  def edge
+    edges = []
+    at = [@x, @y - (@range + 1)]
+    until at[1] == @y
+      edges << at
+      at[0] += 1
+      at[1] += 1
+    end
+    until at[0] == @x
+      edges << at
+      at[0] -= 1
+      at[1] += 1
+    end
+    until at[1] == @y
+      edges << at
+      at[0] -= 1
+      at[1] -= 1
+    end
+    until at[0] == @x
+      edges << at
+      at[0] += 1
+      at[1] -= 1
+    end
+    edges
+  end
+end
+
+sensors = []
+$stdin.readlines.each do |line|
+  c = line.scan(/x=(-?\d+), y=(-?\d+)/).flatten.map(&:to_i)
+  sensors << Sensor.new(*c)
+end
+
+max = 4_000_000
+
+coord = sensors.map { |s|
+  s.edge.reject { |e|
+    e[0] < 0 || e[0] > max ||
+    e[1] < 0 || e[1] > max ||
+    sensors.any? { |s2| s2.cover?(*e) }
+  }
+}.flatten(1).uniq[0]
+
+puts coord[0] * 4_000_000 + coord[1]