From 0838646973a1cd62d0c0c6286d8eebff21d2536f Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 13 Dec 2023 18:21:19 +0100
Subject: [PATCH] Implement 2023 day 13 part 1

---
 2023/src/common.rs      |  8 ++++
 2023/src/day13.rs       | 92 ++++++++++++++++++++++++++++++++++++++++-
 2023/src/samples/13.txt | 15 +++++++
 3 files changed, 113 insertions(+), 2 deletions(-)
 create mode 100644 2023/src/samples/13.txt

diff --git a/2023/src/common.rs b/2023/src/common.rs
index 1ee763d..a291dce 100644
--- a/2023/src/common.rs
+++ b/2023/src/common.rs
@@ -1,6 +1,8 @@
 //! Common helper utilities to all days
 
 use std::cmp::Ordering;
+use std::fmt;
+use std::fmt::Display;
 use std::ops::Add;
 use std::ops::Div;
 use std::ops::Index;
@@ -292,3 +294,9 @@ impl<'a> Index<usize> for Grid<'a> {
         &self.data[offset..(offset + self.width())]
     }
 }
+
+impl Display for Grid<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", String::from_utf8_lossy(self.data))
+    }
+}
diff --git a/2023/src/day13.rs b/2023/src/day13.rs
index 7c1760f..3fdd3a8 100644
--- a/2023/src/day13.rs
+++ b/2023/src/day13.rs
@@ -1,7 +1,95 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use crate::common::Grid;
+
+#[derive(Debug)]
+enum Symmetry {
+    Horizontal(u32),
+    Vertical(u32),
+}
+
+impl Symmetry {
+    fn value(self) -> u32 {
+        match self {
+            Symmetry::Horizontal(value) => 100 * value,
+            Symmetry::Vertical(value) => value,
+        }
+    }
+}
+
+fn find_symmetry(grid: Grid<'_>) -> Option<Symmetry> {
+    // Attempt to find a vertical line of reflection first
+    for c in 1..grid.width() {
+        if grid
+            .rows()
+            .all(|row| row[..c].iter().rev().zip(&row[c..]).all(|(a, b)| a == b))
+        {
+            return Some(Symmetry::Vertical(c as u32));
+        }
+    }
+
+    for r in 1..grid.height() {
+        if (0..r)
+            .rev()
+            .zip(r..grid.height())
+            .all(|(a, b)| grid[a] == grid[b])
+        {
+            return Some(Symmetry::Horizontal(r as u32));
+        }
+    }
+
+    println!("Suspiciously did not find an axis of symmetry:\n\n{grid}\n");
+    None
+}
+
+fn parse_grids(input: &[u8]) -> anyhow::Result<Vec<Grid<'_>>> {
+    let mut result = Vec::new();
+    let mut start = 0;
+    let mut last_newline = 0;
+
+    for i in input
+        .iter()
+        .enumerate()
+        .filter_map(|(i, c)| (*c == b'\n').then_some(i))
+    {
+        if last_newline == i - 1 {
+            result.push(Grid::new(&input[start..i])?);
+            start = i + 1;
+        }
+        last_newline = i;
+    }
+
+    result.push(Grid::new(&input[start..])?);
+
+    Ok(result)
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let grids = parse_grids(input)?;
+
+    let sum: u32 = grids
+        .into_iter()
+        .filter_map(find_symmetry)
+        .map(Symmetry::value)
+        .sum();
+    Ok(sum.to_string())
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/13.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("405", part1(SAMPLE).unwrap());
+    }
+
+    // #[test]
+    // fn sample_part2() {
+    //     assert_eq!("525152", part2(SAMPLE).unwrap());
+    // }
+}
diff --git a/2023/src/samples/13.txt b/2023/src/samples/13.txt
new file mode 100644
index 0000000..3b6b5cc
--- /dev/null
+++ b/2023/src/samples/13.txt
@@ -0,0 +1,15 @@
+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#