2015, day 18

Author: alokmenghrajani

src/main.rs

@@ -48,6 +48,7 @@ fn main() {
         (2015, 15) => year_2015::day15::run(inputs::read(year, day)),
         (2015, 16) => year_2015::day16::run(inputs::read(year, day)),
         (2015, 17) => year_2015::day17::run(inputs::read(year, day)),
+        (2015, 18) => year_2015::day18::run(inputs::read(year, day)),
         (2015, _) => println!("work in progress..."),
 
         // 2016

src/year_2015/day18.rs

@@ -0,0 +1,88 @@
+pub fn run(input: Vec<String>) {
+    let part1 = solve(&input, 100, false);
+    println!("part 1: {}", part1);
+    assert_eq!(part1, 1061);
+
+    let part2 = solve(&input, 100, true);
+    println!("part 2: {}", part2);
+    assert_eq!(part2, 1006);
+}
+
+fn solve(input: &Vec<String>, iterations: u64, sticky: bool) -> u64 {
+    let mut grid = [[false; 100]; 100];
+
+    // process each line of input
+    for (y, line) in input.iter().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            if c == '#' {
+                grid[y][x] = true;
+            }
+        }
+    }
+
+    // run Conway's game of life iterations number of times
+    for _ in 0..iterations {
+        // create a copy of the existing grid
+        let mut grid2 = [[false; 100]; 100];
+        for y in 0..100 {
+            for x in 0..100 {
+                grid2[y][x] = grid[y][x];
+            }
+        }
+
+        for y in 0..100 {
+            for x in 0..100 {
+                // count neighbors
+                let mut c = 0;
+                for dy in -1..2 {
+                    for dx in -1..2 {
+                        if (dx == 0) && (dy == 0) {
+                            continue;
+                        }
+                        if y + dy < 0 {
+                            continue;
+                        }
+                        if y + dy >= 100 {
+                            continue;
+                        }
+                        if x + dx < 0 {
+                            continue;
+                        }
+                        if x + dx >= 100 {
+                            continue;
+                        }
+
+                        if grid2[(y + dy) as usize][(x + dx) as usize] {
+                            c += 1;
+                        }
+                    }
+                }
+
+                // change state on grid but by looking up in grid2
+                if grid2[y as usize][x as usize] {
+                    grid[y as usize][x as usize] = (c == 2) || (c == 3);
+                } else {
+                    grid[y as usize][x as usize] = c == 3;
+                }
+            }
+        }
+
+        if sticky {
+            grid[0][0] = true;
+            grid[99][0] = true;
+            grid[0][99] = true;
+            grid[99][99] = true;
+        }
+    }
+
+    // count how many lights are lit
+    let mut lit = 0;
+    for x in grid.iter() {
+        for y in x.iter() {
+            if *y {
+                lit += 1;
+            }
+        }
+    }
+    return lit;
+}

src/year_2015/mod.rs

@@ -15,3 +15,4 @@ pub mod day14;
 pub mod day15;
 pub mod day16;
 pub mod day17;
+pub mod day18;