diff --git a/2021/day1/vhdl/.gitignore b/2021/day1/vhdl/.gitignore
new file mode 100644
index 0000000..11bf01b
--- /dev/null
+++ b/2021/day1/vhdl/.gitignore
@@ -0,0 +1,2 @@
+day1
+workdir/
diff --git a/2021/day1/vhdl/day1.vhdl b/2021/day1/vhdl/day1.vhdl
new file mode 100644
index 0000000..32b0501
--- /dev/null
+++ b/2021/day1/vhdl/day1.vhdl
@@ -0,0 +1,11 @@
+configuration day1 of sim is
+	for aoc_stdio
+		for dut_inst : dut
+			use entity work.top generic map (
+				MAX_INPUT_DIGITS => 6,
+				OUTPUT_WIDTH => OUTPUT_WIDTH,
+				STEP => STEP
+			);
+		end for;
+	end for;
+end configuration;
diff --git a/2021/day1/vhdl/run.sh b/2021/day1/vhdl/run.sh
new file mode 100755
index 0000000..917b1ab
--- /dev/null
+++ b/2021/day1/vhdl/run.sh
@@ -0,0 +1,8 @@
+#!/bin/bash
+
+source "$COMMON_DIR/vhdl_run.sh"
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+DUT_OUTPUT_WIDTH=12
+test_sim day1 top.vhdl
diff --git a/2021/day1/vhdl/sim.gtkw b/2021/day1/vhdl/sim.gtkw
new file mode 100644
index 0000000..8f774df
--- /dev/null
+++ b/2021/day1/vhdl/sim.gtkw
@@ -0,0 +1,55 @@
+[*]
+[*] GTKWave Analyzer v3.3.109 (w)1999-2020 BSI
+[*] Wed Dec  1 13:50:21 2021
+[*]
+[dumpfile] "/home/xiretza/dev/advent-of-code/2021/day1/vhdl/workdir/sim1.ghw"
+[dumpfile_mtime] "Wed Dec  1 13:48:36 2021"
+[dumpfile_size] 235598
+[savefile] "/home/xiretza/dev/advent-of-code/2021/day1/vhdl/sim.gtkw"
+[timestart] 11375200000
+[size] 1920 1035
+[pos] -1 -1
+*-26.627251 11673700000 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
+[treeopen] top.
+[treeopen] top.sim.
+[treeopen] top.sim.dut_inst.
+[treeopen] top.sim.dut_inst.window.
+[sst_width] 221
+[signals_width] 173
+[sst_expanded] 1
+[sst_vpaned_height] 296
+@28
+top.sim.clk
+top.sim.reset
+@420
+top.sim.char_in
+@200
+-
+@420
+top.sim.dut_inst.window[0][0]
+top.sim.dut_inst.window[0][1]
+top.sim.dut_inst.window[0][2]
+top.sim.dut_inst.window[0][3]
+top.sim.dut_inst.window[0][4]
+top.sim.dut_inst.window[0][5]
+@200
+-
+@420
+top.sim.dut_inst.window[1][0]
+top.sim.dut_inst.window[1][1]
+top.sim.dut_inst.window[1][2]
+@421
+top.sim.dut_inst.window[1][3]
+@420
+top.sim.dut_inst.window[1][4]
+top.sim.dut_inst.window[1][5]
+@200
+-
+@28
+top.sim.input_valid
+@22
+#{top.sim.output[11:0]} top.sim.output[11] top.sim.output[10] top.sim.output[9] top.sim.output[8] top.sim.output[7] top.sim.output[6] top.sim.output[5] top.sim.output[4] top.sim.output[3] top.sim.output[2] top.sim.output[1] top.sim.output[0]
+@28
+top.sim.output_valid
+[pattern_trace] 1
+[pattern_trace] 0
diff --git a/2021/day1/vhdl/top.vhdl b/2021/day1/vhdl/top.vhdl
new file mode 100644
index 0000000..d922850
--- /dev/null
+++ b/2021/day1/vhdl/top.vhdl
@@ -0,0 +1,71 @@
+library ieee;
+use ieee.std_logic_1164.all,
+	ieee.numeric_std.all;
+
+use work.util.all;
+
+entity top is
+	generic (
+		MAX_INPUT_DIGITS : positive;
+		OUTPUT_WIDTH : positive;
+		STEP : natural range 1 to 2
+	);
+	port (
+		clk : in std_logic;
+		reset : in std_logic;
+
+		char  : in character;
+		input_valid : in std_logic;
+
+		output : out unsigned(OUTPUT_WIDTH-1 downto 0);
+		output_valid : out std_logic
+	);
+end entity;
+
+architecture arch of top is
+	function get_window_size return positive is
+	begin
+		if STEP = 1 then
+			return 1;
+		else
+			return 3;
+		end if;
+	end function;
+
+	constant WINDOW_SIZE : positive := get_window_size;
+
+	type number_t is array(MAX_INPUT_DIGITS-1 downto 0) of digit_t;
+	type window_t is array(0 to WINDOW_SIZE) of number_t;
+
+	signal window : window_t;
+
+	signal window_countdown : natural range WINDOW_SIZE downto 0 := 0;
+begin
+	process(clk)
+	begin
+		if rising_edge(clk) then
+			if reset then
+				output <= (others => '0');
+				output_valid <= '1';
+				window_countdown <= 0;
+			elsif input_valid then
+				output_valid <= '0';
+
+				if char = LF then
+					if window_countdown = WINDOW_SIZE then
+						if window(0) > window(window'high) then
+							output <= output + 1;
+						end if;
+						output_valid <= '1';
+					else
+						window_countdown <= window_countdown + 1;
+					end if;
+
+					window <= number_t'(others => 0) & window(0 to window'high-1);
+				else
+					window(0) <= window(0)(MAX_INPUT_DIGITS-2 downto 0) & char_to_digit(char);
+				end if;
+			end if;
+		end if;
+	end process;
+end architecture;