diff --git a/inputs/12.txt b/inputs/12.txt
new file mode 100644
index 0000000..be4e79e
--- /dev/null
+++ b/inputs/12.txt
@@ -0,0 +1,34 @@
+initial state: ##.##.##..#..#.#.#.#...#...#####.###...#####.##..#####.#..#.##..#..#.#...#...##.##...#.##......####.
+
+##.#. => #
+#.#.. => #
+##... => .
+...## => #
+###.# => #
+#.##. => #
+#.### => #
+####. => #
+.#..# => #
+...#. => .
+#..#. => .
+#.#.# => .
+.##.# => .
+..#.. => .
+.#.## => #
+..##. => .
+.#.#. => #
+#..## => #
+..#.# => #
+#.... => .
+..### => .
+#...# => .
+##### => #
+###.. => #
+....# => .
+##.## => #
+.#### => .
+..... => .
+##..# => #
+.##.. => .
+.###. => .
+.#... => #
diff --git a/inputs/12_test.txt b/inputs/12_test.txt
new file mode 100644
index 0000000..864fa0c
--- /dev/null
+++ b/inputs/12_test.txt
@@ -0,0 +1,16 @@
+initial state: #..#.#..##......###...###
+
+...## => #
+..#.. => #
+.#... => #
+.#.#. => #
+.#.## => #
+.##.. => #
+.#### => #
+#.#.# => #
+#.### => #
+##.#. => #
+##.## => #
+###.. => #
+###.# => #
+####. => #
diff --git a/src/day12.rs b/src/day12.rs
new file mode 100644
index 0000000..87a9f4b
--- /dev/null
+++ b/src/day12.rs
@@ -0,0 +1,265 @@
+extern crate regex;
+
+use std::collections::{HashMap, VecDeque};
+use std::error::Error;
+use std::fmt;
+use std::fs;
+use std::result;
+use std::str::FromStr;
+
+use regex::Regex;
+
+type Result<T> = result::Result<T, Box<Error>>;
+
+const INPUT: &str = "inputs/12.txt";
+
+#[derive(Debug, PartialEq)]
+struct Pots(VecDeque<bool>);
+
+#[derive(Debug, PartialEq)]
+struct SpreadRules(HashMap<[bool; 5], bool>);
+
+#[derive(Debug, PartialEq)]
+struct GrowthSimulation {
+    pots: Pots,
+    spread_rules: SpreadRules,
+    generations: usize,
+}
+
+impl fmt::Display for Pots {
+    #[allow(clippy::write_with_newline)]
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for pot in self.0.iter() {
+            if *pot {
+                write!(f, "#")?;
+            } else {
+                write!(f, ".")?;
+            }
+        }
+        write!(f, "\n")?;
+        Ok(())
+    }
+}
+
+impl fmt::Display for SpreadRules {
+    #[allow(clippy::write_with_newline)]
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for (pattern, result) in self.0.iter() {
+            for pot in pattern {
+                if *pot {
+                    write!(f, "#")?;
+                } else {
+                    write!(f, ".")?;
+                }
+            }
+            write!(f, " => ")?;
+            if *result {
+                write!(f, "#")?;
+            } else {
+                write!(f, ".")?;
+            }
+            write!(f, "\n")?;
+        }
+        write!(f, "\n")?;
+        Ok(())
+    }
+}
+
+impl fmt::Display for GrowthSimulation {
+    #[allow(clippy::write_with_newline)]
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.pots)?;
+        write!(f, "\n")?;
+        write!(f, "{}", self.spread_rules)?;
+        Ok(())
+    }
+}
+
+impl FromStr for GrowthSimulation {
+    type Err = Box<Error>;
+
+    fn from_str(s: &str) -> Result<GrowthSimulation> {
+        let sections: Vec<&str> = s.split("\n\n").collect();
+        let (initial_state_str, spread_rules_str) = (sections[0], sections[1]);
+        let mut pots = Pots(VecDeque::new());
+        let mut spread_rules = SpreadRules(HashMap::new());
+
+        lazy_static! {
+            static ref initial_state_regex: Regex =
+                Regex::new(r"initial state: (?P<initial_state>[#.]+)").unwrap();
+            static ref spread_rules_regex: Regex =
+                Regex::new(r"(?P<pattern>[#.]{5}) => (?P<result>[#.])").unwrap();
+        }
+
+        let initial_state_captures = match initial_state_regex.captures(initial_state_str) {
+            None => {
+                return Err(From::from(
+                    "Malformed initial state, no fields could be found",
+                ));
+            }
+            Some(captures) => captures,
+        };
+
+        for pot in initial_state_captures["initial_state"].chars() {
+            pots.0.push_back(pot == '#');
+        }
+
+        for rule in spread_rules_str.lines() {
+            let spread_rules_captures = match spread_rules_regex.captures(rule) {
+                None => {
+                    return Err(From::from(
+                        "Malformed spread rules, no fields could be found",
+                    ));
+                }
+                Some(captures) => captures,
+            };
+
+            let mut pattern = [false; 5];
+            let pattern_vec: Vec<bool> = spread_rules_captures["pattern"]
+                .chars()
+                .map(|c| c == '#')
+                .collect();
+            pattern.copy_from_slice(&pattern_vec);
+            let result = &spread_rules_captures["result"] == "#";
+            spread_rules.0.insert(pattern, result);
+        }
+
+        Ok(GrowthSimulation { pots, spread_rules, generations: 0 })
+    }
+}
+
+impl GrowthSimulation {
+    fn advance_generation(&mut self) {
+        let mut next_generation = VecDeque::new();
+        let padding = &[false; 4];
+        let pots_slice = self.pots.0.as_slices();
+        let pots_slice = [padding, pots_slice.0, pots_slice.1, padding].concat();
+        for (index, pot_window) in pots_slice.windows(5).enumerate() {
+            match self.spread_rules.0.get(pot_window) {
+                Some(result) => {
+                    next_generation.push_back(*result);
+                },
+                None => {
+                    next_generation.push_back(pots_slice[2]);
+                },
+            }
+        }
+        self.pots = Pots(next_generation);
+        self.generations += 1;
+    }
+
+    fn sum_plant_indices(&self) -> i32 {
+        let mut sum: i32 = 0;
+        for (index, pot) in self.pots.0.iter().enumerate() {
+            if *pot {
+                let shifted_index = index as i32 - self.generations as i32 * 2;
+                sum += shifted_index;
+            }
+        }
+        sum
+    }
+}
+
+fn read_initial_state_and_rules(filename: &str) -> Result<GrowthSimulation> {
+    let input = fs::read_to_string(filename)?;
+    Ok(input.parse()?)
+}
+
+pub fn solve_part1() -> Result<i32> {
+    let mut growth_sim = read_initial_state_and_rules(INPUT)?;
+    for _ in 0..20 {
+        growth_sim.advance_generation();
+    }
+    Ok(growth_sim.sum_plant_indices())
+}
+
+pub fn solve_part2() -> Result<i32> {
+    let mut growth_sim = read_initial_state_and_rules(INPUT)?;
+    for _ in 0..50_000_000_000_i64 {
+        growth_sim.advance_generation();
+    }
+    Ok(growth_sim.sum_plant_indices())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = "inputs/12_test.txt";
+    fn test_growth_sim() -> GrowthSimulation {
+        GrowthSimulation {
+            pots: Pots(VecDeque::from(vec![
+                true, false, false, true, false, true, false, false, true, true, false, false,
+                false, false, false, false, true, true, true, false, false, false, true, true,
+                true,
+            ])),
+            spread_rules: SpreadRules([
+                ([true, true, true, true, false], true),
+                ([true, true, false, true, true], true),
+                ([false, false, true, false, false], true),
+                ([false, true, false, false, false], true),
+                ([true, true, false, true, false], true),
+                ([false, true, true, true, true], true),
+                ([true, false, true, true, true], true),
+                ([true, true, true, false, true], true),
+                ([false, true, false, true, true], true),
+                ([true, false, true, false, true], true),
+                ([false, true, false, true, false], true),
+                ([false, true, true, false, false], true),
+                ([true, true, true, false, false], true),
+                ([false, false, false, true, true], true),
+            ]
+            .iter()
+            .cloned()
+            .collect()),
+            generations: 0,
+        }
+    }
+
+    #[test]
+    fn reads_initial_state_and_rules_file() {
+        let growth_sim = read_initial_state_and_rules(TEST_INPUT).unwrap();
+        assert_eq!(growth_sim, test_growth_sim());
+    }
+
+    #[test]
+    fn displays_growth_simulation() {
+        assert_eq!(
+            format!("{}", test_growth_sim()).lines().collect::<Vec<&str>>().sort(),
+            vec![
+                "#..#.#..##......###...###",
+                "",
+                "####. => #",
+                "##.## => #",
+                "..#.. => #",
+                ".#... => #",
+                "##.#. => #",
+                ".#### => #",
+                "#.### => #",
+                "###.# => #",
+                ".#.## => #",
+                "#.#.# => #",
+                ".#.#. => #",
+                ".##.. => #",
+                "###.. => #",
+                "...## => #",
+            ].sort()
+        );
+    }
+
+    #[test]
+    fn advances_simulation_by_one_generation() {
+        let mut growth_sim = test_growth_sim();
+        growth_sim.advance_generation();
+        assert_eq!(format!("{}", growth_sim.pots), "..#...#....#.....#..#..#..#..\n");
+    }
+
+    #[test]
+    fn returns_correct_sum_after_20_generations() {
+        let mut growth_sim = test_growth_sim();
+        for _ in 0..20 {
+            growth_sim.advance_generation();
+        }
+        assert_eq!(growth_sim.sum_plant_indices(), 325);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 8fbf22c..9800da6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -14,6 +14,7 @@ mod day8;
 mod day9;
 mod day10;
 mod day11;
+mod day12;
 
 fn main() {
     // println!("Day 1:");
@@ -47,10 +48,12 @@ fn main() {
     // let parts = day10::solve_parts().unwrap();
     // println!("{}", parts.0);
     // println!("{}", parts.1);
-    println!("Day 11:");
-    let part1 = day11::solve_part1().unwrap();
-    println!("{},{}", part1.coord.x, part1.coord.y);
-    let part2 = day11::solve_part2().unwrap();
-    println!("{},{},{}", part2.coord.x, part2.coord.y, part2.size);
-    // println!("{}", parts.1);
+    // println!("Day 11:");
+    // let part1 = day11::solve_part1().unwrap();
+    // println!("{},{}", part1.coord.x, part1.coord.y);
+    // let part2 = day11::solve_part2().unwrap();
+    // println!("{},{},{}", part2.coord.x, part2.coord.y, part2.size);
+    println!("Day 12:");
+    println!("{}", day12::solve_part1().unwrap());
+    println!("{}", day12::solve_part2().unwrap());
 }