1 files changed, 47 insertions, 0 deletions
diff --git a/2017/src/bin/day_17.rs b/2017/src/bin/day_17.rs
new file mode 100644
index 0000000..09d6fdc
--- /dev/null
+++ b/2017/src/bin/day_17.rs
@@ -0,0 +1,47 @@
+extern crate advent_of_code_2017;
+use advent_of_code_2017::*;
+
+fn main() {
+    let args = AdventArgs::init();
+    let step_size: usize = args.input[0].parse().unwrap();
+
+    let inserts = if args.part == 1 {
+        2017
+    } else {
+        50_000_000
+    };
+
+    let mut buffer = Vec::with_capacity(inserts + 1);
+    buffer.push(0);
+    let mut position = 0;
+
+    for i in 0..inserts as u32 {
+        let to_insert = i+1;
+        // the +1 is because they want it to insert AFTER the element
+        // that adding position ends on
+        position = ((position + step_size) % buffer.len()) + 1;
+        if args.part == 2 && position != 1 {
+            // for big vectors, push is MUCH more efficient than
+            // insert (O(C) vs O(n)). In part 2, we want the element
+            // after 0, which will always be index 1. It only needs to
+            // be inserted into the right place if it's actually going
+            // to be in position 1.
+            //
+            // If I wasn't meshing the solution with part 1, there
+            // probably wouldn't even be a vector, just tracking the
+            // length and index 1.
+            buffer.push(to_insert);
+        } else {
+            buffer.insert(position, to_insert);
+        }
+    }
+
+    let answer_position = if args.part == 1 {
+        (position+1)%buffer.len()
+    } else {
+        1
+    };
+    
+    let answer = buffer[answer_position];
+    println!("{}", answer);
+}