Implement part 1 of day 12.

Part two is going to take 31 hours of computation, so I might need to
improve that.

2018/inputs/12.txt

@@ -0,0 +1,34 @@
+initial state: ####....#...######.###.#...##....#.###.#.###.......###.##..##........##..#.#.#..##.##...####.#..##.#
+
+..#.. => .
+#.#.# => #
+#.### => #
+.##.. => .
+#.#.. => #
+.#.#. => #
+.###. => #
+.#### => #
+##... => #
+#.##. => #
+#..## => #
+....# => .
+###.# => .
+##### => #
+..... => .
+..#.# => .
+.#... => #
+##.#. => .
+.#.## => #
+..##. => .
+#...# => .
+##.## => #
+...#. => .
+#..#. => .
+..### => .
+.##.# => .
+#.... => .
+.#..# => #
+####. => .
+...## => #
+##..# => .
+###.. => .

2018/src/day12.rs

@@ -1,25 +1,151 @@
+use std::io::BufRead;
+use std::io::BufReader;
 use std::io::Read;
 
 use common::Solution;
 
-#[derive(Default)]
-pub struct Day12 {}
+#[derive(Default, Debug)]
+pub struct Day12 {
+    productions: [bool; 32],
+}
+
+fn char_bool(c: char) -> bool {
+    match c {
+        '#' => true,
+        '.' => false,
+        _ => panic!("Invalid input {}", c)
+    }
+}
+
+fn print_state(state: &[(i32, bool)]) {
+    for &(_, b) in state {
+        if b {
+            print!("#");
+        } else {
+            print!(".");
+        }
+    }
+    println!();
+}
 
 impl Day12 {
     pub fn new() -> Self {
         Default::default()
     }
+
+    fn read_input(&mut self, input: &mut Read) -> Vec<(i32, bool)> {
+        let mut state = Vec::new();
+        let mut reader = BufReader::new(input);
+        {
+            let mut line = String::new();
+            reader.read_line(&mut line).unwrap();
+
+            for (idx, c) in line.trim().chars().skip("initial state: ".len()).enumerate() {
+                state.push((idx as i32, char_bool(c)));
+            }
+        }
+
+        for line in reader.lines() {
+            let line = line.unwrap();
+
+            if line.is_empty() {
+                continue;
+            }
+
+            let mut index = 0;
+            for c in line.chars().take(5) {
+                index <<= 1;
+                if char_bool(c) {
+                    index |= 1;
+                }
+            }
+
+            self.productions[index] = char_bool(line.chars().last().unwrap());
+        }
+
+        state
+    }
+
+    fn simulate(&self, state: &[(i32, bool)]) -> Vec<(i32, bool)> {
+        let mut new_state = Vec::new();
+        let mut index = 0;
+        let mut last_idx = None;
+        for &(idx, b) in state {
+            index = (index << 1) & 0x1f;
+            if b {
+                index |= 1;
+            }
+
+            if self.productions[index] {
+                new_state.push((idx - 2, true));
+            } else if !new_state.is_empty() {
+                new_state.push((idx - 2, false));
+            }
+            last_idx = Some(idx);
+        }
+        if let Some(idx) = last_idx {
+            for i in 1..=4 {
+                index = (index << 1) & 0x1f;
+
+                if self.productions[index] {
+                    new_state.push((idx + i - 2, true));
+                } else if !new_state.is_empty() {
+                    new_state.push((idx + i - 2, false));
+                }
+            }
+        }
+
+        let new_len = new_state.len() - new_state.iter().rev()
+            .take_while(|&&(_, x)| !x).count();
+        new_state.truncate(new_len);
+
+        new_state
+    }
 }
 
 impl Solution for Day12 {
-    fn part1(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+    fn part1(&mut self, input: &mut Read) -> String {
+        let mut state = self.read_input(input);
+        print_state(&state);
+        for _ in 1..=20 {
+            state = self.simulate(&state);
         }
 
-    fn part2(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+        let total: i32 = state.iter()
+            .filter(|&&(_, x)| x)
+            .map(|&(i, _)| i)
+            .sum();
+
+        format!("{}", total)
+    }
+
+    fn part2(&mut self, input: &mut Read) -> String {
+        // Note: this is way too slow
+        let mut state = self.read_input(input);
+        print_state(&state);
+        for _ in 1..=50000000000i64 {
+            state = self.simulate(&state);
+        }
+
+        let total: i32 = state.iter()
+            .filter(|&&(_, x)| x)
+            .map(|&(i, _)| i)
+            .sum();
+
+        format!("{}", total)
     }
 }
 
 #[cfg(test)]
-mod tests {}
+mod tests {
+    use common::Solution;
+    use day12::Day12;
+
+    const SAMPLE_INPUT: &[u8] = include_bytes!("samples/12.txt");
+
+    #[test]
+    fn sample_part1() {
+        let mut instance = Day12::new();
+        assert_eq!("325", instance.part1(&mut SAMPLE_INPUT));
+    }
+}

2018/src/samples/12.txt

@@ -0,0 +1,16 @@
+initial state: #..#.#..##......###...###
+
+...## => #
+..#.. => #
+.#... => #
+.#.#. => #
+.#.## => #
+.##.. => #
+.#### => #
+#.#.# => #
+#.### => #
+##.#. => #
+##.## => #
+###.. => #
+###.# => #
+####. => #