2017 day 3

Author: ictrobot

crates/year2017/src/day03.rs

@@ -0,0 +1,96 @@
+use utils::prelude::*;
+
+/// Calculating spiral patterns.
+#[derive(Clone, Debug)]
+pub struct Day03 {
+    input: u32,
+}
+
+impl Day03 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        Ok(Self {
+            input: parser::u32().parse_complete(input)?,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        let ring = (self.input as f64).sqrt().ceil() as u32 / 2;
+        let side_length = ring * 2 + 1;
+        let bottom_right = side_length * side_length;
+        let middles = [
+            bottom_right - ring,       // Bottom
+            bottom_right - (ring * 3), // Left
+            bottom_right - (ring * 5), // Top
+            bottom_right - (ring * 7), // Right
+        ];
+        let offset = middles
+            .iter()
+            .map(|m| m.abs_diff(self.input))
+            .min()
+            .unwrap();
+        ring + offset
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        // To cover 0..=u32::MAX only the first 150 values in the sequence are necessary. The 150th
+        // value is in the 6th ring, meaning a grid of 13x13 is required, or 15x15 with an extra
+        // ring around the edge to avoid needing bounds checks.
+        const LEN: usize = 15;
+
+        let mut grid = [[0; LEN]; LEN];
+        let (mut x, mut y) = (LEN / 2, LEN / 2);
+        grid[x][y] = 1;
+
+        // Store the number of turns and remaining steps until the next turn. The number of previous
+        // turns gives you the current direction and next number of steps by following the pattern:
+        //  1x Right
+        //  1x Up
+        //  2x Left
+        //  2x Down
+        //  3x Right
+        //  3x Up
+        //  4x Left
+        //  ...
+        let mut turns = 0;
+        let mut steps = 1;
+
+        while grid[x][y] < self.input as u64 {
+            if steps == 0 {
+                turns += 1;
+                steps = (turns / 2) + 1;
+            }
+            steps -= 1;
+            match turns % 4 {
+                0 => x += 1,
+                1 => y += 1,
+                2 => x -= 1,
+                3 => y -= 1,
+                _ => unreachable!(),
+            }
+
+            grid[x][y] = grid[x - 1][y - 1]
+                + grid[x - 1][y]
+                + grid[x - 1][y + 1]
+                + grid[x][y - 1]
+                + grid[x][y + 1]
+                + grid[x + 1][y - 1]
+                + grid[x + 1][y]
+                + grid[x + 1][y + 1];
+        }
+
+        grid[x][y]
+    }
+}
+
+examples!(Day03 -> (u32, u64) [
+    {input: "1", part1: 0},
+    {input: "12", part1: 3},
+    {input: "23", part1: 2},
+    {input: "1024", part1: 31},
+    // Custom examples
+    {input: "100", part2: 122},
+    {input: "200", part2: 304},
+    {input: "4294967295", part2: 4429173742},
+]);

crates/year2017/src/lib.rs

@@ -4,4 +4,5 @@
 utils::year!(2017 => year2017, ${
     1 => day01::Day01<'_>,
     2 => day02::Day02,
+    3 => day03::Day03,
 });