From 41a6478127089ee73d9548bbec762e34f04b0a44 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sat, 24 Dec 2022 14:30:29 -0800
Subject: [PATCH] 2022 Day 23 Part 1

---
 2022/src/bin/day23.rs | 227 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 227 insertions(+)
 create mode 100644 2022/src/bin/day23.rs

diff --git a/2022/src/bin/day23.rs b/2022/src/bin/day23.rs
new file mode 100644
index 0000000..7adc1f6
--- /dev/null
+++ b/2022/src/bin/day23.rs
@@ -0,0 +1,227 @@
+use std::collections::{HashMap, HashSet};
+
+const INPUT: &str = include_str!("../../input/day23.txt");
+
+fn main() {
+    let grove = Grove::parse(INPUT);
+
+    println!("{}", part1(grove));
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+struct RC {
+    r: i64,
+    c: i64,
+}
+
+impl RC {
+    const ORIGIN: Self = Self { r: 0, c: 0 };
+
+    fn new(r: i64, c: i64) -> Self {
+        Self { r, c }
+    }
+
+    fn unsigned(r: usize, c: usize) -> Self {
+        Self::new(r.try_into().unwrap(), c.try_into().unwrap())
+    }
+
+
+    #[rustfmt::skip]
+    fn neighbors() -> Vec<(i64, i64)> {
+        vec![
+            (-1, -1), (-1,  0), (-1,  1),
+            ( 0, -1),           ( 0,  1),
+            ( 1, -1), ( 1,  0), ( 1,  1),
+        ]
+    }
+
+    fn north() -> Vec<(i64, i64)> {
+        vec![(-1, -1), (-1, 0), (-1, 1)]
+    }
+
+    fn south() -> Vec<(i64, i64)> {
+        vec![(1, -1), (1, 0), (1, 1)]
+    }
+
+    fn west() -> Vec<(i64, i64)> {
+        vec![(-1, -1), (0, -1), (1, -1)]
+    }
+
+    fn east() -> Vec<(i64, i64)> {
+        vec![(-1, 1), (0, 1), (1, 1)]
+    }
+
+    fn toward_north(&self) -> RC {
+        RC::new(self.r - 1, self.c)
+    }
+
+    fn toward_south(&self) -> RC {
+        RC::new(self.r + 1, self.c)
+    }
+
+    fn toward_east(&self) -> RC {
+        RC::new(self.r, self.c + 1)
+    }
+
+    fn toward_west(&self) -> RC {
+        RC::new(self.r, self.c - 1)
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Grove {
+    grid: HashSet<RC>,
+}
+
+impl Grove {
+    fn parse(input: &str) -> Self {
+        let mut grid = HashSet::new();
+
+        for (r, line) in input.lines().enumerate() {
+            for (c, fill) in line.chars().enumerate() {
+                match fill {
+                    '.' => continue,
+                    '#' => grid.insert(RC::unsigned(r, c)),
+                    other => panic!("{:?}", other),
+                };
+            }
+        }
+
+        Self { grid }
+    }
+
+    fn propose(&self, from: RC, round: usize) -> Option<RC> {
+        if !self.has_elves(&from, &RC::neighbors()) {
+            return None;
+        }
+
+        let mut props: Vec<(RC, Vec<(i64, i64)>)> = vec![
+            (from.toward_north(), RC::north()),
+            (from.toward_south(), RC::south()),
+            (from.toward_west(), RC::west()),
+            (from.toward_east(), RC::east()),
+        ];
+
+        props.rotate_left(round % 4);
+
+        for (dest, neighbors) in props {
+            if !self.has_elves(&from, &neighbors) {
+                return Some(dest);
+            }
+        }
+
+        None
+    }
+
+    fn has_elf(&self, loc: &RC) -> bool {
+        self.grid.contains(loc)
+    }
+
+    fn has_elves(&self, RC { r, c }: &RC, deltas: &[(i64, i64)]) -> bool {
+        deltas
+            .iter()
+            .any(|&(dr, dc)| self.has_elf(&RC::new(r + dr, c + dc)))
+    }
+
+    fn apply(&self, moves: Vec<Move>) -> Self {
+        let mut grid = self.grid.clone();
+
+        for Move { from, to } in moves {
+            let was_present = grid.remove(&from);
+            assert!(was_present);
+
+            let was_empty = grid.insert(to);
+            assert!(was_empty);
+        }
+
+        Self { grid }
+    }
+}
+
+#[derive(Debug, Copy, Clone)]
+struct Move {
+    from: RC,
+    to: RC,
+}
+
+fn part1(mut grove: Grove) -> usize {
+    println!("== Initial State ==\n{}", grove);
+    for round in 0..10 {
+        let mut proposals: HashMap<RC, Vec<RC>> = HashMap::new();
+        for &src in &grove.grid {
+            let Some(dest) = grove.propose(src, round) else { continue };
+            proposals
+                .entry(dest)
+                .and_modify(|v| v.push(src))
+                .or_insert_with(|| vec![src]);
+        }
+
+        let mut moves: Vec<Move> = vec![];
+        for (dest, sources) in proposals {
+            if sources.len() != 1 {
+                continue;
+            }
+
+            let src = sources[0];
+            moves.push(Move {
+                from: src,
+                to: dest,
+            });
+        }
+
+        grove = grove.apply(moves);
+
+        println!("== End of Round {} ==\n{}", round + 1, grove);
+    }
+
+    grove.empty_tiles()
+}
+
+impl Grove {
+    fn empty_tiles(&self) -> usize {
+        let (lo, hi) = self.bounds();
+
+        let rows: usize = (1 + hi.r - lo.r).try_into().unwrap();
+        let cols: usize = (1 + hi.c - lo.c).try_into().unwrap();
+
+        (rows * cols) - self.grid.len()
+    }
+
+    fn bounds(&self) -> (RC, RC) {
+        let (lo, hi) = self
+            .grid
+            .iter()
+            .fold((RC::ORIGIN, RC::ORIGIN), |(lo, hi), RC { r, c }| {
+                (
+                    RC::new(lo.r.min(*r), lo.c.min(*c)),
+                    RC::new(hi.r.max(*r), hi.c.max(*c)),
+                )
+            });
+        (lo, hi)
+    }
+}
+
+impl std::fmt::Display for Grove {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let (lo, hi) = self.bounds();
+
+        for r in lo.r..=hi.r {
+            for c in lo.c..=hi.c {
+                let is_origin = (r, c) == (0, 0);
+                let is_elf = self.has_elf(&RC { r, c });
+
+                let chr = match (is_origin, is_elf) {
+                    (true, true) => 'X',
+                    (true, false) => 'O',
+                    (false, true) => '#',
+                    (false, false) => '.',
+                };
+
+                write!(f, "{}", chr)?;
+            }
+            writeln!(f)?;
+        }
+
+        Ok(())
+    }
+}