diff --git a/2020/src/day17.rs b/2020/src/day17.rs
index 8919807..8859a73 100644
--- a/2020/src/day17.rs
+++ b/2020/src/day17.rs
@@ -1,12 +1,111 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
 use std::io::Read;
+use std::mem::swap;
 
+use crate::common::Lines;
 use crate::Solution;
 
+type Space = HashSet<(i32, i32, i32)>;
+
+fn read_input(input: &mut dyn Read) -> Space {
+    let mut space = HashSet::new();
+
+    for (y, line) in Lines::new(input).enumerate() {
+        space.extend(line.chars().enumerate().filter_map(|(x, c)| {
+            if c == '#' {
+                Some((x as i32, y as i32, 0))
+            } else {
+                None
+            }
+        }))
+    }
+
+    space
+}
+
+fn advance(current: &Space, next: &mut Space) {
+    let mut live_count = HashMap::new();
+
+    for &(x, y, z) in current {
+        for nx in (x - 1)..=(x + 1) {
+            for ny in (y - 1)..=(y + 1) {
+                for nz in (z - 1)..=(z + 1) {
+                    if x != nx || y != ny || nz != z {
+                        *live_count.entry((nx, ny, nz)).or_insert(0) += 1;
+                    }
+                }
+            }
+        }
+    }
+
+    next.clear();
+
+    for (pos, neighbours) in live_count {
+        if (neighbours == 2 && current.contains(&pos)) || neighbours == 3 {
+            next.insert(pos);
+        }
+    }
+}
+
+#[allow(unused)]
+fn print_state(space: &Space) {
+    let mut xmin = i32::max_value();
+    let mut xmax = i32::min_value();
+    let mut ymin = xmin;
+    let mut ymax = xmax;
+    let mut zmin = xmin;
+    let mut zmax = xmax;
+
+    for &(x, y, z) in space {
+        xmax = xmax.max(x);
+        xmin = xmin.min(x);
+        ymax = ymax.max(y);
+        ymin = ymin.min(y);
+        zmax = zmax.max(z);
+        zmin = zmin.min(z);
+    }
+
+    for z in zmin..=zmax {
+        println!("z={}", z);
+        for y in ymin..=ymax {
+            let line: String = (xmin..=xmax)
+                .map(|x| if space.contains(&(x, y, z)) { '#' } else { '.' })
+                .collect();
+
+            println!("{}", line);
+        }
+        println!();
+    }
+}
+
 #[derive(Default)]
 pub struct Day17;
 
 impl Solution for Day17 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let mut space = read_input(input);
+        let mut scratch_pad = Space::new();
+
+        for _ in 0..6 {
+            advance(&space, &mut scratch_pad);
+            swap(&mut space, &mut scratch_pad);
+        }
+
+        space.len().to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/17.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day17, 1, SAMPLE, 112);
     }
 }