diff --git a/2023/day21.rb b/2023/day21.rb
new file mode 100644
index 0000000..379d0e8
--- /dev/null
+++ b/2023/day21.rb
@@ -0,0 +1,100 @@
+# frozen_string_literal: true
+# typed: true
+
+require 'set'
+
+require 'sorbet-runtime'
+require 'sorbet-struct-comparable'
+
+extend T::Sig
+
+class Cell < T::Struct
+  extend T::Sig
+  include T::Struct::ActsAsComparable
+
+  const :r, Integer
+  const :c, Integer
+
+  class << self
+    extend T::Sig
+
+    sig { params(r: Integer, c: Integer).returns(Cell) }
+    def at(r, c) = Cell::new(r: r, c: c)
+  end
+
+  sig { returns(Cell) }
+  def north = Cell::at(r - 1, c)
+
+  sig { returns(Cell) }
+  def south = Cell::at(r + 1, c)
+
+  sig { returns(Cell) }
+  def west  = Cell::at(r, c - 1)
+
+  sig { returns(Cell) }
+  def east  = Cell::at(r, c + 1)
+
+  sig { returns(T::Array[Cell]) }
+  def neighbors = [north, south, west, east]
+end
+
+class Garden
+  extend T::Sig
+
+  sig { returns(Cell) }
+  attr_reader :start
+
+  def initialize(text)
+    @grid = T.let({}, T::Hash[Cell, String])
+
+    text.strip.each_line.each_with_index do |line, r|
+      line.strip.each_char.each_with_index do |char, c|
+        @grid[Cell::at(r, c)] = char
+        @start = Cell::at(r, c) if char == ?S
+      end
+    end
+  end
+
+  sig { params(cell: Cell).returns(T.nilable(String)) }
+  def char(cell) = @grid[cell]
+
+  sig { params(cell: Cell).returns(T::Boolean) }
+  def is_plot?(cell)
+    sym = char(cell) or return false
+    sym != '#'
+  end
+end
+
+class Gardener
+  extend T::Sig
+
+  sig { params(garden: Garden).void }
+  def initialize(garden)
+    @garden = garden
+  end
+
+  sig { params(starts: T::Set[Cell]).returns(T::Set[Cell]) }
+  def step_hypothetically(starts)
+    dests = Set::new
+    starts.each do |start|
+      dests.merge start.neighbors.select { |c| @garden.is_plot?(c) }
+    end
+    dests
+  end
+end
+
+sig { params(path: String).returns(Integer) }
+def part1(path)
+  text = File.read(path)
+  garden = Garden::new(text)
+  gardener = Gardener::new(garden)
+
+  pos = Set[garden.start]
+  64.times do
+    pos = gardener.step_hypothetically(pos)
+  end
+  pos.size
+end
+
+puts part1 File.join(__dir__, 'input/test.txt')
+puts part1 File.join(__dir__, 'input/day21.txt')