Add solution for day 3.

common.cr

@@ -0,0 +1,22 @@
+module Enumerable(T)
+  def count_each(others)
+    count_map = {} of T => Int32
+    each do |other|
+      count_map[other] = (count_map[other]? || 0) + 1
+    end
+    return count_map
+  end
+end
+
+class Rectangle
+  def initialize(@x : Int32, @y : Int32, @width : Int32, @height : Int32)
+  end
+
+  def each_coord(&block)
+    (@x...(@x + @width)).each do |x|
+      (@y...(@y + @height)).each do |y|
+        yield x, y
+      end
+    end
+  end
+end

day3.cr

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+require "./common.cr"
+
+lines = File.read("day3").split("\n")
+lines.pop
+
+REGEX = /#([0-9]+) @ ([0-9]+),([0-9]+): ([0-9]+)x([0-9]+)/
+
+claims = {} of Int32 => Rectangle
+lines.each do |line|
+  match = line.match(REGEX).not_nil!
+  claim = match[1].to_i32
+  x = match[2].to_i32
+  y = match[3].to_i32
+  width = match[4].to_i32
+  height = match[5].to_i32
+  claims[claim] = Rectangle.new(x, y, width, height)
+end
+
+claimed_coords = {} of Tuple(Int32, Int32) => Int32
+claims.values.each do |claim|
+  claim.each_coord do |x, y|
+    coord = {x, y}
+    claimed_coords[coord] = (claimed_coords[coord]? || 0) + 1
+  end
+end
+
+puts "Overlapping: #{claimed_coords.values.count &.>(1)}"
+
+claims.each do |k, v|
+  count_overlapped = 0
+  v.each_coord do |x, y|
+    count_overlapped += 1 if claimed_coords[{x, y}] > 1
+  end
+  if count_overlapped == 0
+    puts "No overlaps: #{k}"
+    exit
+  end
+end