Add day 5 solution.

day5.cr

@@ -0,0 +1,55 @@
+INPUT = File.read("day5.txt").lines.map(&.chomp) 
+
+def partition(choices, list)
+  b = 0
+  e = list.size
+  choices.chars.each do |c|
+    b = (b+e)//2 if c == 'B' || c == 'R'
+    e = (b+e)//2 if c == 'F' || c == 'L'
+  end
+
+  return {b,e}
+end
+
+def get_id(str)
+  col = str[0..6]
+  row = str[7..]
+  {partition(col, (0..127).to_a)[0], partition(row, (0..7).to_a)[0]}
+end
+
+def id(t)
+  r, c = t
+  r * 8 + c
+end
+
+def part1
+  input = INPUT.clone
+  max = input.max_of do |line|
+    r, c = get_id(line)
+    r * 8 + c
+  end
+  puts max
+end
+
+def part2
+  input = INPUT.clone
+  seen = Set(Int32).new
+  max = input.each do |line|
+    seen << id(get_id(line))
+  end
+  candidates = [] of Int32
+  128.times do |r|
+    next if r == 0 || r == 127
+    8.times do |c|
+      next if seen.includes? id({r, c})
+      candidates << id({r,c})
+    end
+  end
+  candidates = candidates.select do |i|
+    seen.includes?(i-1) && seen.includes?(i+1)
+  end
+  puts candidates[0]
+end
+
+part1
+part2