2 files changed, 154 insertions(+), 1 deletions(-)

A data/day12
M src/bin/day12.rs

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

@@ 1,1 1,120 @@
-fn main() {}
+//static INPUT: &str = "data/day12";
+
+static INPUT: &[u8] = b"#.......##.###.#.#..##..##..#.#.###..###..##.#.#..##....#####..##.#.....########....#....##.#..##...";
+
+fn rule(m: &[u8]) -> u8 {
+    match m {
+        b"#####" => b'#',
+        b"####." => b'#',
+        b"###.#" => b'#',
+        b"###.." => b'.',
+        b"##.##" => b'.',
+        b"##.#." => b'#',
+        b"##..#" => b'#',
+        b"##..." => b'#',
+        b"#.###" => b'.',
+        b"#.##." => b'.',
+        b"#.#.#" => b'#',
+        b"#.#.." => b'.',
+        b"#..##" => b'.',
+        b"#..#." => b'#',
+        b"#...#" => b'#',
+        b"#...." => b'.',
+        b".####" => b'.',
+        b".###." => b'#',
+        b".##.#" => b'#',
+        b".##.." => b'.',
+        b".#.##" => b'#',
+        b".#.#." => b'.',
+        b".#..#" => b'.',
+        b".#..." => b'#',
+        b"..###" => b'.',
+        b"..##." => b'.',
+        b"..#.#" => b'#',
+        b"..#.." => b'.',
+        b"...##" => b'.',
+        b"...#." => b'#',
+        b"....#" => b'.',
+        b"....." => b'.',
+        _ => panic!("invalid input"),
+    }
+}
+
+fn last(l: &[u8]) -> [u8; 4] {
+    let n = l.len();
+    [rule(&[l[n - 4], l[n - 3], l[n - 2], l[n - 1], b'.']),
+     rule(&[l[n - 3], l[n - 2], l[n - 1], b'.', b'.']),
+     rule(&[l[n - 2], l[n - 1], b'.', b'.', b'.']),
+     rule(&[l[n - 1], b'.', b'.', b'.', b'.'])]
+}
+
+fn generation(line: Vec<u8>, zero: usize) -> (Vec<u8>, usize) {
+    let first = [rule(&[b'.', b'.', b'.', b'.', line[0]]),
+                 rule(&[b'.', b'.', b'.', line[0], line[1]]),
+                 rule(&[b'.', b'.', line[0], line[1], line[2]]),
+                 rule(&[b'.', line[0], line[1], line[2], line[3]])];
+    let first = if first[0] == b'.' {
+        if first[1] == b'.' {
+            &first[2..]
+        } else {
+            &first[1..]
+        }
+    } else {
+        &first[..]
+    };
+
+    let next = line.windows(5).map(rule);
+    let last = last(&line);
+    let last = if let Some(p) = last.iter().rposition(|&b| b == b'#') {
+        &last[0..p+1]
+    } else {
+        &[]
+    };
+    (first.iter().cloned().chain(next).chain(last.iter().cloned()).collect::<Vec<_>>(),
+     zero + first.len() - 2)
+}
+
+fn plant_sum(line: &[u8], zero: usize) -> isize {
+    line.iter().enumerate().map(|(i, &e)| if e == b'#' {
+        (i as isize - zero as isize)
+    } else {
+        0
+    }).sum()
+}
+
+fn main() {
+    let mut line = Vec::from(INPUT);
+    let mut zero = 0;
+
+    for _ in 0..20 {
+        let gen = generation(line, zero);
+        line = gen.0;
+        zero = gen.1;
+    }
+
+    let mut sum = plant_sum(&line, zero);
+    println!("  1: {}", sum);
+
+    // Assumption, difference stabilizes eventually
+    let mut heuristic = 0;
+    let mut diff = 0;
+    let mut end_gen = 20;
+    loop {
+        end_gen += 1;
+        let gen = generation(line, zero);
+        line = gen.0;
+        zero = gen.1;
+        let prev = sum;
+        sum = plant_sum(&line, zero);
+        if diff == sum-prev {
+            heuristic += 1;
+        } else {
+            heuristic = 0;
+        }
+        if heuristic > 1000 {
+            break;
+        }
+        diff = sum - prev;
+    };
+    println!("  2: {}", sum + (50_000_000_000 - end_gen) * diff);
+}