2017 day 17

Author: ictrobot

crates/year2017/src/day17.rs

@@ -0,0 +1,55 @@
+use utils::prelude::*;
+
+/// Simulating a circular buffer.
+#[derive(Clone, Debug)]
+pub struct Day17 {
+    step_size: u32,
+}
+
+impl Day17 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        Ok(Self {
+            step_size: parser::u32().parse_complete(input)?,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        let mut buffer = Vec::with_capacity(2018);
+        buffer.push(0);
+
+        let mut i = 0;
+        for iteration in 1..=2017u16 {
+            i = (i + self.step_size as usize) % buffer.len();
+            buffer.insert(i + 1, iteration);
+            i += 1;
+        }
+
+        buffer[(i + 1) % buffer.len()] as u32
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u32 {
+        let mut result = 0u32;
+
+        // 0 is always at the start of the buffer, so the answer is the last value in position 1,
+        // which is the last value inserted when the spinlock is at 0.
+        let (mut i, mut iteration) = (0, 1);
+        while iteration <= 50_000_000 {
+            if i == 0 {
+                result = iteration;
+            }
+
+            // Skip iterations until the spinlock wraps around to the start of the buffer again
+            let skip_iterations = (iteration - i).div_ceil(self.step_size + 1);
+            iteration += skip_iterations;
+            i = (i + skip_iterations * (self.step_size + 1)) % iteration;
+        }
+
+        result
+    }
+}
+
+examples!(Day17 -> (u32, u32) [
+    {input: "3", part1: 638},
+]);

crates/year2017/src/lib.rs

@@ -20,4 +20,5 @@ utils::year!(2017 => year2017, ${
     14 => day14::Day14,
     15 => day15::Day15,
     16 => day16::Day16,
+    17 => day17::Day17,
 });