diff --git a/2025/Cargo.lock b/2025/Cargo.lock
index f153b5a..fa6ab88 100644
--- a/2025/Cargo.lock
+++ b/2025/Cargo.lock
@@ -190,6 +190,21 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "day-07"
+version = "0.1.0"
+dependencies = [
+ "divan",
+ "itertools",
+ "miette",
+ "nom",
+ "rstest",
+ "test-log",
+ "thiserror",
+ "tracing",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "divan"
 version = "0.1.21"
diff --git a/2025/day-07/src/part1.rs b/2025/day-07/src/part1.rs
index 1639d01..0f2cad4 100644
--- a/2025/day-07/src/part1.rs
+++ b/2025/day-07/src/part1.rs
@@ -1,9 +1,65 @@
+use std::collections::HashSet;
+use tracing::info;
+
+type Column = usize;
+type Row = usize;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+struct Splitter {
+    column: Column,
+    row: Row,
+}
+
+#[derive(Debug)]
+struct SimulationState {
+    active_positions: HashSet<Column>,
+    splitters: HashSet<Splitter>,
+}
+
+impl SimulationState {
+    fn new(start_column: Column) -> Self {
+        let mut active_positions = HashSet::new();
+        active_positions.insert(start_column);
+        Self {
+            active_positions,
+            splitters: HashSet::new(),
+        }
+    }
+
+    fn process_row(&mut self, row: Row, line: &str) {
+        let mut next_positions = HashSet::new();
+
+        for col in &self.active_positions {
+            if line.as_bytes()[*col] == b'^' {
+                info!(?col, "split at");
+                next_positions.insert(col - 1);
+                next_positions.insert(col + 1);
+                next_positions.insert(col + 1);
+                self.splitters.insert(Splitter { column: *col, row });
+            } else {
+                next_positions.insert(*col);
+            }
+        }
+
+        self.active_positions = next_positions;
+    }
+}
+
 #[tracing::instrument]
 #[allow(clippy::missing_panics_doc)]
 #[allow(clippy::missing_errors_doc)]
 pub fn process(input: &str) -> miette::Result<usize> {
-    todo!("day xx - part 1");
-    Ok(0)
+    let mut lines = input.lines().enumerate();
+    let (_, first_line) = lines.next().unwrap();
+    let start_column = first_line.chars().position(|ch| ch == 'S').unwrap();
+
+    let mut state = SimulationState::new(start_column);
+
+    for (row, line) in lines {
+        state.process_row(row, line);
+    }
+
+    Ok(state.splitters.len())
 }
 
 #[cfg(test)]
@@ -12,9 +68,24 @@ mod tests {
 
     #[test]
     fn test_process() -> miette::Result<()> {
-        let input = "";
-        todo!("haven't built test yet");
-        let result = 0;
+        let input = ".......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............
+";
+        let result = 21;
         assert_eq!(process(input)?, result);
         Ok(())
     }