From 1f49f710683533d337b5e23d4386d7daf64cbf3d Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Thu, 12 Dec 2024 21:30:15 +0100
Subject: [PATCH] Solve 12.1

---
 src/bin/04.rs | 13 +------
 src/bin/10.rs | 11 +-----
 src/bin/12.rs | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs    | 21 ++++++++++++
 4 files changed, 118 insertions(+), 22 deletions(-)
 create mode 100644 src/bin/12.rs

diff --git a/src/bin/04.rs b/src/bin/04.rs
index 1aae2b5..33ec3dd 100644
--- a/src/bin/04.rs
+++ b/src/bin/04.rs
@@ -56,7 +56,7 @@ fn check_x_mas(start: IVec2, field: &Field) -> usize {
 }
 
 fn count_xmas_at(start: IVec2, field: &Field) -> usize {
-    DIRECTIONS
+    DIRECTIONS8
         .iter()
         .filter(|&&dir| check_xmas(start, dir, field))
         .count()
@@ -69,17 +69,6 @@ fn check_xmas(start: IVec2, direction: IVec2, field: &Field) -> bool {
         && field.get(start + direction * 3) == 'S'
 }
 
-const DIRECTIONS: [IVec2; 8] = [
-    IVec2::new(1, 0),
-    IVec2::new(1, 1),
-    IVec2::new(0, 1),
-    IVec2::new(-1, 1),
-    IVec2::new(-1, 0),
-    IVec2::new(-1, -1),
-    IVec2::new(0, -1),
-    IVec2::new(1, -1),
-];
-
 struct Field {
     data: Vec<Vec<char>>,
 }
diff --git a/src/bin/10.rs b/src/bin/10.rs
index a0f6fa4..20e3aec 100644
--- a/src/bin/10.rs
+++ b/src/bin/10.rs
@@ -22,15 +22,6 @@ fn part2(input: &str) -> usize {
     map.trailheads().map(|t| map.distinct_trails(t)).sum()
 }
 
-fn directions() -> [IVec2; 4] {
-    [
-        IVec2::new(1, 0),
-        IVec2::new(-1, 0),
-        IVec2::new(0, 1),
-        IVec2::new(0, -1),
-    ]
-}
-
 #[derive(Debug)]
 struct Map {
     height: HashMap<IVec2, i64>,
@@ -67,7 +58,7 @@ impl Map {
     }
 
     fn climb_up(&self, from: IVec2, from_height: i64) -> Vec<IVec2> {
-        directions()
+        DIRECTIONS4
             .into_iter()
             .filter_map(|dir| {
                 let pos = from + dir;
diff --git a/src/bin/12.rs b/src/bin/12.rs
new file mode 100644
index 0000000..93dcbd5
--- /dev/null
+++ b/src/bin/12.rs
@@ -0,0 +1,95 @@
+use aoc::*;
+use glam::IVec2;
+use std::collections::{HashMap, HashSet};
+use std::iter;
+
+const INPUT: &str = include_str!("../../input/12");
+
+fn main() {
+    assert_example!(part1, "12-test", 140);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "12-test", 0);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    parse_regions(input).iter().map(Region::fence_cost).sum()
+}
+fn part2(_input: &str) -> usize {
+    0
+}
+
+fn parse_regions(input: &str) -> Vec<Region> {
+    let mut map = parse_map(input);
+    let mut regions = Vec::new();
+    loop {
+        let Some((&start_pos, &plant)) = map.iter().next() else {
+            break;
+        };
+        let mut visited = HashSet::new();
+        let mut region = Vec::new();
+        let mut check = vec![start_pos];
+        let mut perimeter = 0;
+        while let Some(next) = check.pop() {
+            if visited.contains(&next) {
+                continue;
+            }
+            visited.insert(next);
+            let Some(&p) = map.get(&next) else { continue };
+            if p != plant {
+                continue;
+            }
+
+            region.push(next);
+            map.remove(&next);
+
+            // Adjust perimeter
+            let empty_sides = DIRECTIONS4
+                .into_iter()
+                .map(|dir| dir + next)
+                .filter(|pos| !region.contains(pos))
+                .count();
+            let taken_sides = 4 - empty_sides;
+            perimeter = perimeter + empty_sides - taken_sides;
+
+            let neighbors = DIRECTIONS4.into_iter().map(|dir| dir + next);
+            check.extend(neighbors);
+        }
+        regions.push(Region {
+            _plant: plant,
+            fields: region,
+            perimeter,
+        });
+    }
+    regions
+}
+
+#[derive(Debug)]
+struct Region {
+    _plant: char,
+    fields: Vec<IVec2>,
+    perimeter: usize,
+}
+
+impl Region {
+    fn area(&self) -> usize {
+        self.fields.len()
+    }
+
+    fn fence_cost(&self) -> usize {
+        self.perimeter * self.area()
+    }
+}
+
+fn parse_map(input: &str) -> HashMap<IVec2, char> {
+    // TODO Reading a grid with coordinates should be a helper function.
+    input
+        .lines()
+        .enumerate()
+        .flat_map(|(y, line)| line.chars().enumerate().zip(iter::repeat(y)))
+        .map(|((x, c), y)| {
+            let pos = IVec2::new(x as i32, y as i32);
+            (pos, c)
+        })
+        .collect()
+}
diff --git a/src/lib.rs b/src/lib.rs
index 9d7408b..2520767 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,6 @@
 //! This library contains useful helper functions that may be useful in several problems.
 
+use glam::IVec2;
 use std::{
     fmt::{Debug, Display},
     ops::{Div, Mul, Rem},
@@ -106,3 +107,23 @@ macro_rules! assert_example {
         )
     };
 }
+
+/// 4 directions. Start pointing right and go CCW.
+pub const DIRECTIONS4: [IVec2; 4] = [
+    IVec2::new(1, 0),
+    IVec2::new(0, 1),
+    IVec2::new(-1, 0),
+    IVec2::new(0, -1),
+];
+
+/// 8 directions. Start pointing right and go CCW.
+pub const DIRECTIONS8: [IVec2; 8] = [
+    IVec2::new(1, 0),
+    IVec2::new(1, 1),
+    IVec2::new(0, 1),
+    IVec2::new(-1, 1),
+    IVec2::new(-1, 0),
+    IVec2::new(-1, -1),
+    IVec2::new(0, -1),
+    IVec2::new(1, -1),
+];