1 files changed, 59 insertions, 0 deletions
diff --git a/src/bin/day9.rs b/src/bin/day9.rs
new file mode 100644
index 0000000..f6a116d
--- /dev/null
+++ b/src/bin/day9.rs
@@ -0,0 +1,59 @@
+use std::collections::VecDeque;
+
+static DAY: u8 = 9;
+
+fn main() {
+    let input = advent::read_file(DAY);
+    println!("{DAY}a: {}", fs_checksum(&input));
+    println!("{DAY}b: {}", 0);
+}
+
+fn fs_checksum(map: &str) -> usize {
+    let counts = map.trim_end().chars().map(|x| x.to_digit(10).unwrap() as u8).collect::<Vec<_>>();
+
+    let mut file_ids = VecDeque::new();
+    let mut block_sizes = VecDeque::new();
+    let mut holes = VecDeque::new();
+    for (id, c) in counts.chunks(2).enumerate() {
+        let file = c[0];
+        for _ in 0 .. file {
+            file_ids.push_back(id);
+        }
+        block_sizes.push_back(file);
+        if c.len() == 2 {
+            let free = c[1];
+            holes.push_back(free);
+        }
+    }
+
+    let mut disk = Vec::new();
+    disk.reserve(map.len() * 10);
+    while !holes.is_empty() && !file_ids.is_empty() {
+        let blocks_front = block_sizes.pop_front().unwrap() as usize;
+        for _ in 0 .. blocks_front.min(file_ids.len()) {
+            let front = file_ids.pop_front().unwrap();
+            disk.push(front);
+        }
+        let holes_front = holes.pop_front().unwrap() as usize;
+        for _ in 0 .. holes_front.min(file_ids.len()) {
+            let back = file_ids.pop_back().unwrap();
+            disk.push(back);
+        }
+    }
+
+    disk.iter()
+        .enumerate()
+        .map(|(i, block)| i * block)
+        .sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = "2333133121414131402";
+        assert_eq!(fs_checksum(input), 1928);
+    }
+}