diff --git a/input/12-test b/input/12-test
new file mode 100644
index 0000000..c5bec3a
--- /dev/null
+++ b/input/12-test
@@ -0,0 +1,6 @@
+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1
\ No newline at end of file
diff --git a/src/bin/12.rs b/src/bin/12.rs
new file mode 100644
index 0000000..5445abe
--- /dev/null
+++ b/src/bin/12.rs
@@ -0,0 +1,110 @@
+use aoc2023::*;
+use std::collections::VecDeque;
+
+const INPUT: &str = include_str!("../../input/12");
+
+fn main() {
+    assert_example!(part1, "12-test", 21);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "12-test", 0);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    input
+        .lines()
+        .map(Row::parse)
+        .map(Row::possible_arrangements)
+        .sum()
+}
+
+fn part2(input: &str) -> usize {
+    0
+}
+
+#[derive(Clone, Debug)]
+struct Row {
+    springs: VecDeque<Spring>,
+    groups: VecDeque<usize>,
+}
+
+impl Row {
+    fn parse(line: &str) -> Self {
+        let (springs, groups) = line.split_once(' ').unwrap();
+        let springs = springs.chars().map(Spring::from).collect();
+        let groups = groups.split(',').map(|n| n.parse().unwrap()).collect();
+        Self { springs, groups }
+    }
+
+    fn possible_arrangements(mut self) -> usize {
+        // Pop all operational springs on the front and then get the first different one.
+        let first = loop {
+            match self.springs.pop_front() {
+                None => return if self.groups.is_empty() { 1 } else { 0 },
+                Some(Spring::Operational) => continue,
+                Some(Spring::Broken) => break InterestingSpring::Broken,
+                Some(Spring::Unknown) => break InterestingSpring::Unknown,
+            }
+        };
+
+        match first {
+            InterestingSpring::Broken => {
+                // A broken spring requires a run that matches the group.
+                let Some(mut group) = self.groups.pop_front() else {
+                    // There was a broken spring but no group
+                    return 0;
+                };
+
+                // We already encountered the first broken spring.
+                group -= 1;
+
+                // Pop all required springs or return if impossible.
+                loop {
+                    match self.springs.pop_front() {
+                        None if group > 0 => return 0,
+                        None if group == 0 => break,
+                        Some(Spring::Broken | Spring::Unknown) if group > 0 => group -= 1,
+                        Some(Spring::Broken) if group == 0 => return 0,
+                        Some(Spring::Operational) if group > 0 => return 0,
+                        Some(Spring::Operational | Spring::Unknown) => break,
+                        other => panic!("group detection weird case {other:?}, group={group}"),
+                    }
+                }
+
+                self.possible_arrangements()
+            }
+            InterestingSpring::Unknown => {
+                // We don't know what kind of spring is here, recursively try both options.
+                let mut a = self.clone();
+                let mut b = self;
+                a.springs.push_front(Spring::Broken);
+                b.springs.push_front(Spring::Operational);
+                a.possible_arrangements() + b.possible_arrangements()
+            }
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+enum InterestingSpring {
+    Broken,
+    Unknown,
+}
+
+#[derive(Copy, Clone, Debug)]
+enum Spring {
+    Broken,
+    Unknown,
+    Operational,
+}
+
+impl From<char> for Spring {
+    fn from(value: char) -> Self {
+        match value {
+            '#' => Self::Broken,
+            '.' => Self::Operational,
+            '?' => Self::Unknown,
+            other => panic!("unknown spring '{other}'"),
+        }
+    }
+}