diff --git a/2019/day9/input b/2019/day9/input
new file mode 100644
index 0000000..39fe2e1
--- /dev/null
+++ b/2019/day9/input
@@ -0,0 +1 @@
+1102,34463338,34463338,63,1007,63,34463338,63,1005,63,53,1101,3,0,1000,109,988,209,12,9,1000,209,6,209,3,203,0,1008,1000,1,63,1005,63,65,1008,1000,2,63,1005,63,904,1008,1000,0,63,1005,63,58,4,25,104,0,99,4,0,104,0,99,4,17,104,0,99,0,0,1101,27,0,1014,1101,286,0,1023,1102,1,35,1018,1102,20,1,1000,1101,26,0,1010,1101,0,289,1022,1102,1,30,1019,1102,734,1,1025,1102,1,31,1012,1101,25,0,1001,1102,1,1,1021,1101,0,36,1002,1101,0,527,1028,1101,895,0,1026,1102,1,23,1016,1101,21,0,1003,1102,22,1,1011,1102,1,522,1029,1102,1,892,1027,1102,1,0,1020,1102,1,28,1015,1102,38,1,1006,1101,0,32,1008,1101,743,0,1024,1101,0,37,1007,1102,1,24,1013,1102,1,33,1009,1102,39,1,1004,1102,1,34,1005,1102,1,29,1017,109,19,21102,40,1,-3,1008,1016,40,63,1005,63,203,4,187,1106,0,207,1001,64,1,64,1002,64,2,64,109,-7,2101,0,-7,63,1008,63,32,63,1005,63,227,1106,0,233,4,213,1001,64,1,64,1002,64,2,64,109,-3,2108,37,-2,63,1005,63,255,4,239,1001,64,1,64,1105,1,255,1002,64,2,64,109,11,21108,41,40,-6,1005,1014,275,1001,64,1,64,1106,0,277,4,261,1002,64,2,64,109,10,2105,1,-7,1105,1,295,4,283,1001,64,1,64,1002,64,2,64,109,-27,1201,-2,0,63,1008,63,25,63,1005,63,321,4,301,1001,64,1,64,1105,1,321,1002,64,2,64,109,15,21107,42,41,0,1005,1018,341,1001,64,1,64,1106,0,343,4,327,1002,64,2,64,109,-25,2108,20,10,63,1005,63,359,1105,1,365,4,349,1001,64,1,64,1002,64,2,64,109,12,2107,35,0,63,1005,63,385,1001,64,1,64,1106,0,387,4,371,1002,64,2,64,109,4,21101,43,0,6,1008,1015,43,63,1005,63,409,4,393,1106,0,413,1001,64,1,64,1002,64,2,64,109,9,21101,44,0,-8,1008,1010,46,63,1005,63,437,1001,64,1,64,1106,0,439,4,419,1002,64,2,64,109,5,21108,45,45,-4,1005,1019,457,4,445,1106,0,461,1001,64,1,64,1002,64,2,64,109,-22,2102,1,7,63,1008,63,33,63,1005,63,481,1106,0,487,4,467,1001,64,1,64,1002,64,2,64,109,14,21102,46,1,-1,1008,1014,43,63,1005,63,507,1106,0,513,4,493,1001,64,1,64,1002,64,2,64,109,12,2106,0,1,4,519,1106,0,531,1001,64,1,64,1002,64,2,64,109,-17,1205,10,547,1001,64,1,64,1106,0,549,4,537,1002,64,2,64,109,-8,1202,-2,1,63,1008,63,17,63,1005,63,569,1105,1,575,4,555,1001,64,1,64,1002,64,2,64,109,23,1206,-5,593,4,581,1001,64,1,64,1105,1,593,1002,64,2,64,109,-14,1208,-8,24,63,1005,63,613,1001,64,1,64,1105,1,615,4,599,1002,64,2,64,109,-2,1207,-1,33,63,1005,63,633,4,621,1105,1,637,1001,64,1,64,1002,64,2,64,109,2,21107,47,48,5,1005,1016,659,4,643,1001,64,1,64,1105,1,659,1002,64,2,64,109,-11,1208,8,32,63,1005,63,681,4,665,1001,64,1,64,1106,0,681,1002,64,2,64,109,2,2101,0,0,63,1008,63,36,63,1005,63,703,4,687,1106,0,707,1001,64,1,64,1002,64,2,64,109,12,1206,7,719,1106,0,725,4,713,1001,64,1,64,1002,64,2,64,109,2,2105,1,8,4,731,1001,64,1,64,1106,0,743,1002,64,2,64,109,-21,2102,1,9,63,1008,63,39,63,1005,63,769,4,749,1001,64,1,64,1105,1,769,1002,64,2,64,109,11,1201,-3,0,63,1008,63,24,63,1005,63,793,1001,64,1,64,1105,1,795,4,775,1002,64,2,64,109,20,1205,-5,809,4,801,1105,1,813,1001,64,1,64,1002,64,2,64,109,-23,1207,4,36,63,1005,63,833,1001,64,1,64,1105,1,835,4,819,1002,64,2,64,109,-3,2107,33,5,63,1005,63,853,4,841,1106,0,857,1001,64,1,64,1002,64,2,64,109,16,1202,-9,1,63,1008,63,37,63,1005,63,879,4,863,1105,1,883,1001,64,1,64,1002,64,2,64,109,12,2106,0,-1,1105,1,901,4,889,1001,64,1,64,4,64,99,21101,0,27,1,21101,0,915,0,1106,0,922,21201,1,48476,1,204,1,99,109,3,1207,-2,3,63,1005,63,964,21201,-2,-1,1,21101,0,942,0,1105,1,922,21202,1,1,-1,21201,-2,-3,1,21101,0,957,0,1105,1,922,22201,1,-1,-2,1106,0,968,21202,-2,1,-2,109,-3,2106,0,0
diff --git a/2019/day9/intcode.rb b/2019/day9/intcode.rb
new file mode 100644
index 0000000..e29ac62
--- /dev/null
+++ b/2019/day9/intcode.rb
@@ -0,0 +1,243 @@
+# frozen_string_literal: true
+
+require 'forwardable'
+
+module Intcode
+  def self.read(filename)
+    parse File.read(filename)
+  end
+
+  def self.parse(input)
+    parsed = input
+             .gsub(/^#.*$/, '')
+             .gsub('\s', '')
+             .split(',')
+             .map(&:to_i)
+    Program.new parsed
+  end
+
+  class Program
+    def initialize(mem)
+      @initial_state = mem
+    end
+
+    def as_memory
+      Memory.new(@initial_state.clone)
+    end
+
+    def run(&user_input)
+      Computer.new(as_memory).run(&user_input)
+    end
+  end
+
+  class Memory
+    extend Forwardable
+
+    def_delegator :@cells, :[]
+    def_delegator :@cells, :[]=
+
+    attr_accessor :relative_base
+
+    def initialize(cells)
+      @cells = cells
+      @relative_base = 0
+    end
+
+    def to_s
+      batch_size = 10
+      s = String.new
+      @cells.each_slice(batch_size).each_with_index do |batch, idx|
+        prefix = idx * batch_size
+        s << "#{prefix}: #{batch.join(' ')}\n"
+      end
+      s
+    end
+
+    def read(addr, mode)
+      # raise ArgumentError, "address out of range: #{addr}" if addr.negative? || addr >= @cells.length
+      return 0 if addr.negative? || addr >= @cells.length
+
+      case mode
+      when INDIRECT_MODE
+        read(@cells[addr], IMMEDIATE_MODE)
+      when IMMEDIATE_MODE
+        @cells[addr]
+      when RELATIVE_MODE
+        read(@cells[addr] + relative_base, IMMEDIATE_MODE)
+      else
+        raise ArgumentError, "unexpected mode: #{mode}"
+      end
+    end
+
+    def write(addr, mode, val)
+      raise ArgumentError, "address out of range: #{addr}" if addr.negative? || addr >= @cells.length
+
+      case mode
+      when INDIRECT_MODE
+        r = @cells[addr]
+        @cells[r] = val
+      when IMMEDIATE_MODE
+        raise ArgumentError, 'writes are not allowed in immediate mode'
+        @cells[addr]
+      when RELATIVE_MODE
+        r = @cells[addr] + relative_base
+        @cells[r] = val
+      else
+        raise ArgumentError, "unexpected mode: #{mode}"
+      end
+    end
+  end
+
+  INDIRECT_MODE = 0
+  IMMEDIATE_MODE = 1
+  RELATIVE_MODE = 2
+
+  class Computer
+    def initialize(mem, &block)
+      @mem = mem
+      @ip = 0
+      @input_block = block
+    end
+
+    def on_output(&block)
+      @output_block = block
+    end
+
+    def run
+      loop do
+        return if tick == :done_executing
+      end
+    end
+
+    def tick
+      opcode = operation(@mem[@ip])
+      case opcode
+      when 1
+        add!
+        nil
+      when 2
+        mul!
+        nil
+      when 3
+        input!(&@input_block)
+        nil
+      when 4
+        output!(&@output_block)
+      when 5
+        branch_if_not_zero!
+        nil
+      when 6
+        branch_if_zero!
+        nil
+      when 7
+        less_than!
+        nil
+      when 8
+        equal!
+        nil
+      when 9
+        move_relative_base!
+      when 99
+        :done_executing
+      else
+        raise StandardError, "unexpected opcode: #{opcode}"
+      end
+    end
+
+    private
+
+    def add!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      @mem.write(@ip + 3, mode[2], a + b)
+      @ip += 4
+    end
+
+    def mul!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      @mem.write(@ip + 3, mode[2], a * b)
+      @ip += 4
+    end
+
+    def input!(&user_input)
+      mode = modes(@mem[@ip])
+      inp = input(&user_input)
+      @mem.write(@ip + 1, mode[0], inp)
+      @ip += 2
+    end
+
+    def output!(&user_output)
+      mode = modes(@mem[@ip])
+      output = @mem.read(@ip + 1, mode[0])
+      user_output.call output if block_given?
+      @ip += 2
+      output
+    end
+
+    def branch_if_not_zero!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      @ip = a.zero? ? @ip + 3 : b
+    end
+
+    def branch_if_zero!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      @ip = a.zero? ? b : @ip + 3
+    end
+
+    def less_than!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      val = a < b ? 1 : 0
+      @mem.write(@ip + 3, mode[2], val)
+      @ip += 4
+    end
+
+    def equal!
+      mode = modes(@mem[@ip])
+      a = @mem.read(@ip + 1, mode[0])
+      b = @mem.read(@ip + 2, mode[1])
+      val = a == b ? 1 : 0
+      @mem.write(@ip + 3, mode[2], val)
+      @ip += 4
+    end
+
+    def move_relative_base!
+      mode = modes(@mem[@ip])
+      delta = @mem.read(@ip + 1, mode[0])
+      @mem.relative_base += delta
+      @ip += 2
+    end
+
+    def input
+      if block_given?
+        inp = yield
+        raise "Invalid input: #{inp.inspect}" unless inp.is_a?(Numeric)
+
+        return inp
+      end
+
+      print 'input> '
+      gets.to_i
+    end
+
+    def operation(cell)
+      (cell % 100)
+    end
+
+    def modes(cell)
+      m = Hash.new(0)
+      (cell / 100).truncate.digits.each_with_index do |d, i|
+        m[i] = d
+      end
+      m
+    end
+  end
+end
diff --git a/2019/day9/part1.rb b/2019/day9/part1.rb
new file mode 100644
index 0000000..8e1f03f
--- /dev/null
+++ b/2019/day9/part1.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+require_relative 'intcode'
+
+program = Intcode.read(File.join(__dir__, 'input'))
+computer = Intcode::Computer.new(program.as_memory) { 1 }
+computer.on_output { |out| puts out }
+computer.run