2018 day 11

Author: ictrobot

crates/year2018/src/day11.rs

@@ -0,0 +1,70 @@
+use utils::prelude::*;
+
+/// Locating the optimal square subregion.
+///
+/// See <https://en.wikipedia.org/wiki/Summed-area_table>, which allows computing the sum of any
+/// square in constant time with only 4 lookups.
+#[derive(Clone, Debug)]
+pub struct Day11 {
+    summed_area_table: Vec<i32>,
+}
+
+impl Day11 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let serial_num = parser::number_range(0..=9999).parse_complete(input)?;
+
+        let mut table = vec![0i32; 301 * 301];
+        for y in 1..301 {
+            for x in 1..301 {
+                let rack_id = x + 10;
+                let power_level = ((rack_id * y + serial_num) * rack_id) / 100 % 10 - 5;
+                let index = (y * 301 + x) as usize;
+                table[index] =
+                    power_level + table[index - 1] + table[index - 301] - table[index - 302];
+            }
+        }
+
+        Ok(Self {
+            summed_area_table: table,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> String {
+        let (_, x, y) = self.largest_total_power(3);
+        format!("{x},{y}")
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> String {
+        let (size, (_, x, y)) = (1..=300)
+            .map(|s| (s, self.largest_total_power(s)))
+            .max_by_key(|&(_, (total, _, _))| total)
+            .unwrap();
+        format!("{x},{y},{size}")
+    }
+
+    fn largest_total_power(&self, size: usize) -> (i32, u32, u32) {
+        let (mut max_total, mut max_x, mut max_y) = (i32::MIN, 0, 0);
+        for y in 0..301 - size {
+            for x in 0..301 - size {
+                let index = y * 301 + x;
+                let total = self.summed_area_table[index]
+                    + self.summed_area_table[index + 302 * size]
+                    - self.summed_area_table[index + size]
+                    - self.summed_area_table[index + 301 * size];
+                if total > max_total {
+                    max_total = total;
+                    max_x = x as u32 + 1;
+                    max_y = y as u32 + 1;
+                }
+            }
+        }
+        (max_total, max_x, max_y)
+    }
+}
+
+examples!(Day11 -> (&'static str, &'static str) [
+    {input: "18", part1: "33,45", part2: "90,269,16"},
+    {input: "42", part1: "21,61", part2: "232,251,12"},
+]);

crates/year2018/src/lib.rs

@@ -12,4 +12,5 @@ utils::year!(2018 => year2018, ${
     8 => day08::Day08,
     9 => day09::Day09,
     10 => day10::Day10,
+    11 => day11::Day11,
 });