diff --git a/day16.cr b/day16.cr
new file mode 100644
index 0000000..2a1a719
--- /dev/null
+++ b/day16.cr
@@ -0,0 +1,101 @@
+require "./common.cr"
+
+lines = File.read("day16_input").split "\n"
+lines.pop
+count = 0
+end_index = 0
+start_index = 0
+
+lines.each_with_index do |line, i|
+  if line.size == 0
+    count += 1 
+  else
+    count = 0
+  end
+
+  if count == 3
+    end_index = i - 3
+    start_index = i + 1 
+  end
+end
+
+registers = Array(Int32).new(4, 0)
+OPCODES = {
+  "addr" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] + regs[b] },
+  "addi" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] + b },
+  "mulr" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] * regs[b] },
+  "muli" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] * b },
+  "banr" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] & regs[b] },
+  "bani" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] & b },
+  "borr" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] | regs[b] },
+  "bori" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] | b },
+  "setr" => ->(regs : Array(Int32), a : Int32, b : Int32) { regs[a] },
+  "seti" => ->(regs : Array(Int32), a : Int32, b : Int32) { a },
+  "gtir" => ->(regs : Array(Int32), a : Int32, b : Int32) { (a > regs[b]) ? 1 : 0 },
+  "gtri" => ->(regs : Array(Int32), a : Int32, b : Int32) { (regs[a] > b) ? 1 : 0 },
+  "gtrr" => ->(regs : Array(Int32), a : Int32, b : Int32) { (regs[a] > regs[b]) ? 1 : 0 },
+  "eqir" => ->(regs : Array(Int32), a : Int32, b : Int32) { (a == regs[b]) ? 1 : 0 },
+  "eqri" => ->(regs : Array(Int32), a : Int32, b : Int32) { (regs[a] == b) ? 1 : 0 },
+  "eqrr" => ->(regs : Array(Int32), a : Int32, b : Int32) { (regs[a] == regs[b]) ? 1 : 0 },
+}
+
+def find_possible(instr, before, after)
+  OPCODES.select do |name, code|
+    sample = before.dup
+    sample[instr[3]] = code.call(sample, instr[1], instr[2])
+    sample == after
+  end
+end
+
+ARRAY_REGEX = /[^:]+:\s*\[([^\]]*)\]/
+
+class String
+  def extract_array
+    self.match(ARRAY_REGEX).not_nil![1].split(", ").map &.to_i32
+  end
+end
+
+index = 0
+count_more_three = 0
+possibilities = Array(Set(String)).new(16) { Set(String).new }
+while index < end_index
+  start_array = lines[index].extract_array
+  end_array = lines[index + 2].extract_array
+  instruction = lines[index + 1].split(" ").map &.to_i32
+
+  possible = find_possible(instruction, start_array, end_array)
+  count_more_three += 1 if possible.keys.size >= 3
+
+  opcode = instruction[0]
+  old_set = possibilities[opcode]
+  if old_set.empty?
+    old_set.concat possible.keys
+  else
+    old_set.each do |it|
+      old_set.delete it unless possible.has_key? it
+    end
+  end
+
+  index += 4
+end
+puts count_more_three
+
+sure_possibilities = {} of Int32 => Proc(Array(Int32), Int32, Int32, Int32)
+while sure_possibilities.keys.size != 16
+  possibilities.each_with_index do |set, index|
+    next unless set.size == 1
+    first = set.first
+    sure_possibilities[index] = OPCODES[first]
+    possibilities.each &.delete(first)
+  end
+end
+
+registers = [0, 0, 0, 0]
+index = start_index
+while index < lines.size
+  instruction = lines[index].split(" ").map &.to_i32
+  registers[instruction[3]] = sure_possibilities[instruction[0]].call(registers, instruction[1], instruction[2])
+  index += 1
+end
+puts registers[0]
+# opcodes = {} of Int32 => Proc(Int32, Int32, Int32) # 0 => ->(x : Int32, y : Int32) { x + y }