1 files changed, 60 insertions, 0 deletions
diff --git a/src/bin/day20.rs b/src/bin/day20.rs
new file mode 100644
index 0000000..79ab0f9
--- /dev/null
+++ b/src/bin/day20.rs
@@ -0,0 +1,60 @@
+static DAY: u8 = 20;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", grove_coordinates(&input));
+    println!("{DAY}b: {}", 0);
+}
+
+fn grove_coordinates(input: &[String]) -> isize {
+    let mut input = input.iter()
+                         .map(|x| (x.parse::<isize>().unwrap(), false))
+                         .collect::<Vec<_>>();
+
+    loop {
+        let pos = match input.iter().position(|x| !x.1) {
+            None => break,
+            Some(p) => p,
+        };
+        let val = input[pos];
+        input.remove(pos);
+
+        let input_len = input.len() as isize;
+
+        let pos_diff = val.0 % input_len;
+        let mut new_pos = (pos as isize + pos_diff) % input_len;
+        if new_pos.is_negative() {
+            new_pos += input_len;
+        }
+
+        if new_pos == 0 {
+            input.push((val.0 as isize, true));
+        } else {
+            input.insert(new_pos as usize, (val.0 as isize, true));
+        }
+    }
+
+    let pos = input.iter().position(|x| x.0 == 0).unwrap();
+    let n = ((pos + 1000) % input.len(), (pos + 2000) % input.len(), (pos + 3000) % input.len());
+    input[n.0].0 + input[n.1].0 + input[n.2].0
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "1",
+            "2",
+            "-3",
+            "3",
+            "-2",
+            "0",
+            "4",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+
+        assert_eq!(grove_coordinates(&input), 3);
+    }
+}