day21: problem 2 solution

not proud of this one either, completely cheated and it does not get exactly the correct error (it was off by one for my input), but the quadratic solver on Wolfram Alpha was able to do it.
day 21: problem 1 solution
2023-12-21 03:17:11 -08:00 · 2023-12-21 00:34:08 -08:00
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+.#........#.......#............##...........##........#.......................#................#....................#...###......#.
+........###..........#..............#.............#..#.#.#............##...............#........#...#........#...................#.
+...#............................#...##.##........#......#..#...................#.#.....#...#.##...............#........#...#.......
+..#...##....................#.....##......#.......#..........#......#.........#...#..............#..............#..................
+.....#..#....................#....###.....#..#.............#...#....#................#.....#.....................#........#........
+.....#.#...#.....#..............##............#....##.#....#.#........##...........#....#.........................#....#......##...
+....#..#.....##...........#.#.#.#............#....#.....#.........#........................##.......#.#..............##...#........
+...............#..............#................................#....#........#.......#.......##......#.#............#......#....#..
+.#......#....##........#.............#..#.#.............#...#..#.........#.........#................#..................#...........
+.........#..............#....#......#.....##......#............#....#.........#....#...................#....#.........#......#..#..
+..............................#....#.#........#.#.....#..#.................#....#...#..........#............##.......#.#.#....#....
+.#..................#....#..#.......#.##..............#...#.......#................#...............................................
+.#..##.........................#.....###......#....#......#.....#............#.#......##......##......##....#..........##.....#....
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+..##.....#.......#....#.........#..........#....................#.........#........###........................#..........#.........
+.#.#...............#..........#.......#.....#....##.#..#.##...............#...#...#.#......#.......#.......#.#.............#....##.
+..................#...............##.........##..#........#..#..........#.....................#............................#.......
+.....#................#...##.......##.........#.....#.#....#.#........#...#..#....#..............#..#....#....#....#...............
+..##.#........#...............#.#..#...................##...#......#.#...#..#.....##.................##...#.#...................#..
+...#...............#......#.....#.......#......#.........#...#......#...#..#....#..#......#.......#.......#...#.##.#...........##..
+..............##..........##..........#..##...............#...........##..#..#..#.#.......#...#.............#......................
+..........#.#........................#...#.#..#.#...##..#.#.#.#.......#....#.#.#..............#...#...#........###..##...........#.
+......................#...#.#..##.....#....#.....###.#..#.....#..........#........#...........##...#..#...........##..#............
+..................................####.#..............#.................#........#.#............##......#.#..........#.............
+..................#..........#.............................#...........#..#..#......#................#.......##...##..#............
+....................#...#....#.#.#.##......#................#.................#...........#....#..........#....#...##.###..........
+.................#.......#...#............###.###..#.......#......#.......#..##......#.#.....#.....................#...............
+.................................................................S.................................................................
+.......##...........#.........###.......#....###.#.#....#.#....#....#........#.......#...#...#.#.........#...................#.....
+.......#...............#............#...#..#.......##...#...#.#............#.............#......##..........#..............#.......
+.......##.........#.....................##...#...#.................#...#.......#..####......#.....#..............#..##.............
+........#..#.............#.#..##..............#...#.............#.....#......#...#.#.......#...............##...##.................
+.........#..#...#..#.#..#..#...................##......#.....#..#...##..............#......#..........#.##...#.#.##.#..............
+..............#..........#....................................#.........#.......#..#............##................#................
+...........#...#.....#.........#....#..###....#...#.......#....#...............##.....#.......#.#.....#.#....#.#..#................
+..#...................#........###..............#.#................#...###...##.............#.###......#...#...#.....#..........#..
+.#.#............#.....#....###......................................#..........................#..................#................
+................#..#...#................#.................##.................##..........#.......#...#.............#...............
+.....##...........#....#.......#......#.#.....#.....#................#.....#.........#..#.................#........#........#......
+.#.#.............#..#.#..............#.....#..#.#.....#..#......#.........#.....#.#......#..#.......#.#......##............#..#....
+...................#............#....#...........#........#...................#.....##..............#........###...............#...
+......###..........###.#.......#.................................................#..#.........#.#...#.........#.#..........#.#.....
+..#.................#..#..#...#............#...#...#..##.#....##..#..................##.....#......#.........................#...#.
+......................#.#.....#.#...#.......##...........#...#....#.............#.......#.............#.#.#.#......................
+......#..#.#..........#....#.........#......#................#.........##.#....#.......#........#.#....................#......#....
+..#....#....#...............#..........#...#.#......#...................#.........#..#.#......#..#.........................##....#.
+..............#..........##......#.##...#...#.#................#...#..#.......#.#.....#........#...###..##.........#..##.........#.
+........##...........................##.......#.#..................#........#.......#.#........#.........................##....#...
+..#.#...........#.......................#.#.....#....#.....#...............####...#..............#...#....................#.#......
+.......#...#....#.#.......#####...##.....#.#.......#....#.#........#................................#.##..........##....#..#.......
+.........#.....#................##.#..................#.....................#...#................#.###.#.......###....#............
+............#.#....#........#....#..................#...##.............#............#.......#.....#..#............##...............
+....#.....#..........#..............#...#...#........##.....#.#....#....#........##.............................#.#.......#........
+..#..#.#.......#.................#................................#......##.#..#..............#.#................#...#.#...........
+.....#.....#........................#..#................#......#.......#.........###.#.#.........#.........#.....#.......#.........
+.......#.......................................#.......#...............#.#..###...#.#.##.......#...........#.#.....#.##........#...
+......##.........#.................................#.........#........##.#................#.#...#.............#...#..#........#....
+......#.....#.#.......#...........#..#...........#.........#..#.#.......#.........##.....#....#.#...............#...#...#....#...#.
+..#...#.#....#....#..............................#..#...#.......#.#.#..##..................#...............#......#..###..#.#......
+.....#...#..#......#.....##...............#..........#..#.#####.......#...........#...................#.....#...#.........#..#.#...
+.................#.......#..................#........#.......##...##.#....#.#.............#.............#....#....#................
+...........####..#.....#....#.............#.......#...#..#.....#.............#...#....#.............#.#....#.#.............#.......
+...#..#..#...#...#.#.....#.......................#....#..###.###.....#.........#....................#....#..#............#.....#...
+.#.##....##...#......#.#.....#......................#.##..................#........#..#................#.#.......#.............#...
+....##........#.......#..#..#............#..#..#..#...#......###..#..#..#...##...............................#.#........#......#.#.
+..#..#..........#..........#..#.#.............#.....#...........#.##.......#............................#...........#...#..........
+..#.#..#...#....#.....#.....#...#................#.....#.....#....#..#...#..#....#....#.....................#....#.................
+.......#........#...#.............#.................#...#.#.....#.#....#..........#......................#..#............#.........
+....#........#........#..............#.........................#...................#..........#.........#..#.........#.............
+................#........#.........#............................#.#.............#..##.......#......#..#.................#......#.#.
+..#.....#.........#...#..#.#.........................#...#...#...........#.....#...........#........#.............#.....#........#.
+....#...#....#......#....#...#..#.................#...##.....#.#............................##...#...#..##.#.....................#.
+...#..##...............#............................#...........#......#.#.....#............#........#.##................#.........
+................#..#.............#..#..............#.#....#...#.#...#...#.#.##.............................#..#...........#........
+.......#.......#....###............#.#.#.....................#.#..#....#.................##.....#.#..........##..#......#..........
+.......#..........##.#.......#.##...#...............##.....#..........#................#............#...#........#........#........
+...###.#.............................#.....#.#............##..#...#.....###..........#...............................#.....#....#..
+....###.#...#.......#.##........#......#..##.#..........#.#...#.#....#.#..............##....#..#...#...........#...#.#..#..........
+.....#....#.#..##.....#..........#..#...##..............#.......#.......#.............#..#.....#.......#.....#...##...###.......#..
+........##...#...##..#.#.....#..........................#.#.#..#..#...................#....#....#..#....#..........#....##.........
+...........##.....#...#........##..#........#....#..........#..#...............................................#......#.#.......#..
+...#..##.........##....#...........##...#.......##.........#........#...........#........#.#.##...........#....#.....###...#.......
+...#...#........................#..#...#..#........................................#......#..#...#.#....#...........#..............
+...#..#.......#..#......##...................#...............#.....#.#.............#.....#.........#.#..##.....#..........#..#.....
+....###....#..#.....#.#........#.##.......##...#...............#...............###.###.#...#............##.....#.................#.
+....#...#.........#.##............#.....#........#............................................##.#......#..##.........#....###..#..
+............#...##..#..........#........#.......#.#..........................#..#.#.......#......#.......................##.#..#...
+....#......#....#....#..#.....#........#......#....#..#.#..............................#..#....#............#....#.................
+..#.............#...#......#...#.....................#.##...............................##...#.......#........................#.##.
+....#....##........#..............#....#.......#.....................................#........#........#...##..........#...........
+....#.#.##..#..#....................#.................#.##.............#....#....#.............###...#...#..##....#.....##.........
+.............#.......#...............#........#.#.....#..#.#................#.....#..##.....#....#...............#....#......#...#.
+...................................................................................................................................
--- a/21/src/main.rs
+++ b/21/src/main.rs
@ -0,0 +1,230 @@
+use core::panic;
+use indicatif::{ProgressBar, ProgressStyle};
+use rayon::prelude::*;
+use std::collections::HashSet;
+use std::fs::File;
+use std::io::{BufRead, BufReader, Lines};
+use std::time::Instant;
+
+// BOILERPLATE
+type InputIter = Lines<BufReader<File>>;
+
+fn get_input() -> InputIter {
+    let f = File::open("input").unwrap();
+    let br = BufReader::new(f);
+    br.lines()
+}
+
+fn main() {
+    let start = Instant::now();
+    let ans1 = problem1(get_input());
+    let duration = start.elapsed();
+    println!("Problem 1 solution: {} [{}s]", ans1, duration.as_secs_f64());
+
+    let start = Instant::now();
+    let ans2 = problem2(get_input());
+    let duration = start.elapsed();
+    println!("Problem 2 solution: {} [{}s]", ans2, duration.as_secs_f64());
+}
+
+// PARSE
+
+type Position = (isize, isize);
+type WrappedPosition = (usize, usize);
+type Offset = (isize, isize);
+
+#[derive(Debug, Clone)]
+struct MapTile {
+    c: char,
+}
+
+impl MapTile {
+    fn new(c: char) -> Self {
+        Self { c }
+    }
+}
+
+struct GardenMap {
+    map: Vec<Vec<MapTile>>,
+    start: Position,
+}
+
+impl<T: BufRead> From<Lines<T>> for GardenMap {
+    fn from(lines: Lines<T>) -> Self {
+        let map = lines
+            .map(|line| line.unwrap().chars().map(|c| MapTile::new(c)).collect())
+            .collect();
+        let mut new = Self { map, start: (0, 0) };
+        new.find_start();
+        new
+    }
+}
+
+const ADJACENCIES: [Offset; 4] = [(-1, 0), (1, 0), (0, -1), (0, 1)];
+
+impl GardenMap {
+    fn wrap_pos(&self, pos: &Position) -> WrappedPosition {
+        let (width, height) = (self.width() as isize, self.height() as isize);
+        (
+            if pos.0 < 0 {
+                (pos.0 + (-pos.0 / width + 1) * width) as usize % self.width()
+            } else {
+                pos.0 as usize % self.width()
+            },
+            if pos.1 < 0 {
+                (pos.1 + (-pos.1 / height + 1) * height) as usize % self.height()
+            } else {
+                pos.1 as usize % self.height()
+            },
+        )
+    }
+    fn width(&self) -> usize {
+        self.map[0].len()
+    }
+    fn height(&self) -> usize {
+        self.map.len()
+    }
+    fn at(&self, pos: &Position) -> &MapTile {
+        let pos = self.wrap_pos(pos);
+        &self.map[pos.1][pos.0]
+    }
+    // return the valid 'moves' from pos
+    fn adjacent_to(&self, pos: &Position) -> Vec<Position> {
+        ADJACENCIES
+            .iter()
+            .filter_map(|ofs| self.offset_pos(pos, ofs))
+            .filter(|pos| self.at(pos).c != '#')
+            .collect()
+    }
+    fn offset_pos(&self, pos: &Position, ofs: &Offset) -> Option<Position> {
+        let new_pos = (pos.0 as isize + ofs.0, pos.1 as isize + ofs.1);
+        return Some((new_pos.0, new_pos.1));
+    }
+    fn find_start(&mut self) {
+        for (y, row) in self.map.iter().enumerate() {
+            for (x, tile) in row.iter().enumerate() {
+                if tile.c == 'S' {
+                    self.start = (x as isize, y as isize);
+                    return;
+                }
+            }
+        }
+        panic!("didn't find the start square!");
+    }
+    fn reachable_after(&self, from: &Position, n: usize) -> u64 {
+        let bar = ProgressBar::new(n as u64).with_style(
+            ProgressStyle::with_template(
+                "[{elapsed_precise}/{eta_precise}] {bar:40.cyan/blue} {pos:>7}/{len:7} {per_sec}",
+            )
+            .unwrap(),
+        );
+        let mut visited_after: Vec<HashSet<Position>> = Vec::new();
+        visited_after.push(HashSet::from([*from]));
+        for i in 1..n+1 {
+            visited_after.push(
+                visited_after[i - 1]
+                    .iter()
+                    .flat_map(|last| self.adjacent_to(last))
+                    .collect(),
+            );
+            bar.inc(1);
+            // if primes::is_prime(i as u64) {
+            //     println!("count after {} steps: {}", i, visited_after[i].len());
+            // }
+        }
+
+        visited_after[n].len() as u64
+    }
+    fn reachable_count_after(&self, from: &Position, n: usize) -> u64 {
+        let dim = self.width() as f64;
+        let target_mod = (n % self.width()) as f64;
+        let x_values:Vec<f64> = vec![target_mod, target_mod + dim, target_mod + 2.*dim];
+        let y_values:Vec<f64> = x_values.iter().map(|n| self.reachable_after(from, *n as usize) as f64).collect();
+
+        let coeffs = polyfit_rs::polyfit_rs::polyfit(
+            &x_values,
+            &y_values,
+            2,
+        ).unwrap();
+        println!("values: x: {:?} y: {:?}", x_values, y_values);
+        println!("coefficients: {:?}", coeffs);
+
+        let f_n= n as f64;
+        let result = coeffs[0] + coeffs[1] * f_n  + coeffs[2] * f_n.powf(2.0);
+
+        result.round() as u64
+    }
+    fn draw_with_bounds(&self, from: &Position, to: &Position) {
+        for row in from.1..to.1 + 1 {
+            for col in from.0..to.0 + 1 {
+                print!("{}", self.at(&(col, row)).c);
+            }
+            println!();
+        }
+    }
+}
+
+// fn print_visited(map: &GardenMap, visited: &Vec<Vec<bool>>) {
+//     for (y, row) in visited.iter().enumerate() {
+//         for (x, cell) in row.iter().enumerate() {
+//             print!("{}", if *cell { 'O' } else { map.at(&(x, y)).c });
+//         }
+//         println!();
+//     }
+// }
+
+// PROBLEM 1 solution
+
+fn problem1_impl<T: BufRead>(input: Lines<T>, n: usize) -> u64 {
+    let map = GardenMap::from(input);
+    // println!("map: {:?} start: {:?}", map.map, &map.start);
+    // map.draw_with_bounds(
+    //     &(-(map.width() as isize), -(map.height() as isize)),
+    //     &(map.width() as isize * 2 + 1, map.height() as isize * 2 + 1),
+    // );
+    map.reachable_count_after(&map.start, n)
+}
+
+fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
+    problem1_impl(input, 64)
+}
+
+// PROBLEM 2 solution
+fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
+    problem1_impl(input, 26501365)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+    use std::io::Cursor;
+    use test_case::test_case;
+
+    const EXAMPLE: &str = &"...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........";
+
+    #[test]
+    fn problem1_example() {
+        let c = Cursor::new(EXAMPLE);
+        assert_eq!(problem1_impl(c.lines(), 6), 16);
+    }
+
+    #[test_case(6, 16)]
+    #[test_case(10, 50)]
+    #[test_case(50, 1594)]
+    #[test_case(100, 6536)]
+    #[test_case(500, 167004)]
+    fn problem2_example(n: usize, expect: u64) {
+        let c = Cursor::new(EXAMPLE);
+        assert_eq!(problem1_impl(c.lines(), n), expect);
+    }
+}