day11 part2 fast

src/main.rs

@@ -1,4 +1,4 @@
 fn main() {
     // aoc_2018::tasks::day11::task1();
-    aoc_2018::tasks::day11::task2();
+    aoc_2018::tasks::day11::task2_fast();
 }

src/tasks/day11.rs

@@ -19,6 +19,17 @@ pub fn task1() {
 
 pub fn task2() {
     let serial = 6392;
+
+    let mut values: Vec<Vec<i32>> = Vec::with_capacity(301);
+    for x in 0..=300 {
+        let mut v: Vec<i32> = Vec::with_capacity(301);
+        for y in 0..=300 {
+            v.push(fuel_level(x, y, serial));
+        }
+        values.push(v);
+    }
+    let values = &values;
+
     let result = (1..300)
         .map(|square_size| {
             let result = (1..=300 - square_size)
@@ -27,7 +38,7 @@ pub fn task2() {
                     let power: i32 = (x..x + square_size)
                         .map(|x| {
                             (y..y + square_size)
-                                .map(move |y| fuel_level(x, y, serial))
+                                .map(move |y| values[x as usize][y as usize])
                                 .sum::<i32>()
                         })
                         .sum();
@@ -41,6 +52,56 @@ pub fn task2() {
     println!("{:?}", result);
 }
 
+pub fn task2_fast() {
+    let serial = 6392;
+
+    let mut cache: Vec<Vec<i32>> = Vec::with_capacity(301);
+    for x in 0 as usize..=300 {
+        let mut v: Vec<i32> = Vec::with_capacity(301);
+        for y in 0 as usize..=300 {
+            if x == 0 || y == 0 {
+                v.push(0);
+            } else {
+                v.push(
+                    fuel_level(x as i32, y as i32, serial) + v[v.len() - 1] + cache[x - 1][y]
+                        - cache[x - 1][y - 1],
+                )
+            }
+        }
+        cache.push(v);
+    }
+    let values = &cache;
+
+    for x in 0 as usize..=300 {
+        for y in 0 as usize..=300 {
+            if x != 0 && y != 0 {
+                let cached = area_sum(&values, x, y, 1);
+                let calc = fuel_level(x as i32, y as i32, serial);
+                //println!("{},{}: {} ({})", x, y, values[x][y], calc);
+                assert_eq!(calc, cached);
+            }
+        }
+    }
+
+    let result = (1..=300)
+        .map(|square_size| {
+            let result = (1..=301 - square_size)
+                .flat_map(|x: usize| (1..=301 - square_size).map(move |y: usize| (x, y)))
+                .map(|(x, y)| (x, y, area_sum(&values, x, y, square_size)))
+                .max_by_key(|(_, _, value)| *value)
+                .unwrap();
+            (result.0, result.1, result.2, square_size)
+        })
+        .max_by_key(|result| result.2);
+    println!("{:?}", result);
+}
+
+fn area_sum(values: &Vec<Vec<i32>>, x: usize, y: usize, length: usize) -> i32 {
+    values[x + length - 1][y + length - 1] + values[x - 1][y - 1]
+        - values[x - 1][y + length - 1]
+        - values[x + length - 1][y - 1]
+}
+
 fn fuel_level(x: i32, y: i32, serial: i32) -> i32 {
     let rack_id = x + 10;
     let mut power = rack_id * y;