2023: Day 13, part 1

2023/day13/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day13"
+version = "0.1.0"

2023/day13/Cargo.toml

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+[package]
+name = "day13"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2023/day13/src/main.rs

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+use std::string::FromUtf8Error;
+use std::{env, fs};
+
+type BoxError = Box<dyn std::error::Error>;
+
+fn main() -> Result<(), BoxError> {
+    let input_path = env::args_os()
+        .skip(1)
+        .next()
+        .ok_or("missing input file path")?;
+    let input = fs::read_to_string(&input_path)?;
+
+    let mut vertical = Vec::new();
+    let mut horizontal = Vec::new();
+
+    for pattern in input.split("\n\n") {
+        if pattern.is_empty() {
+            continue;
+        }
+
+        println!("{pattern}");
+        let lines = pattern.lines().collect::<Vec<_>>();
+        if let Some(i) = find_mirror_point(&lines) {
+            horizontal.push(i);
+            continue;
+        }
+
+        // transpose the lines to try to a vertical mirror point
+        let transposed = transpose(&lines)?;
+        if let Some(i) = find_mirror_point(&transposed) {
+            vertical.push(i);
+        } else {
+            return Err("no vertical or horizontal split".into());
+        }
+        println!()
+    }
+
+    let part1 = vertical.iter().sum::<usize>() + horizontal.iter().map(|h| *h * 100).sum::<usize>();
+    println!("Part 1: {part1}");
+
+    Ok(())
+}
+
+fn find_mirror_point<T: AsRef<str>>(lines: &[T]) -> Option<usize> {
+    let mut prev: Option<&str> = None;
+    for (i, line) in lines.iter().enumerate() {
+        match prev {
+            Some(prev) if prev == line.as_ref() => {
+                print!("possible mirror point at {i}: ");
+                // split the array in half, see if they're the same
+                let (first, second) = lines.split_at(i);
+                // ensure they're the same length
+                let len = first.len().min(second.len());
+                let start = first.len() - len;
+                let (first, second) = (&first[start..], &second[..len]);
+                if first
+                    .iter()
+                    .rev()
+                    .zip(second.iter())
+                    .all(|(l, r)| l.as_ref() == r.as_ref())
+                {
+                    println!("seems to be mirror point");
+                    return Some(i);
+                } else {
+                    println!("no match");
+                }
+            }
+            Some(_) | None => {}
+        }
+        prev = Some(line.as_ref())
+    }
+    None
+}
+
+fn transpose(lines: &[&str]) -> Result<Vec<String>, FromUtf8Error> {
+    let mut transposed = Vec::new();
+    for i in 0..lines[0].len() {
+        let mut col = Vec::new();
+        for line in lines.iter() {
+            col.push(line.as_bytes()[i])
+        }
+        transposed.push(String::from_utf8(col)?);
+    }
+    Ok(transposed)
+}

2023/input/day13.sample

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+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#
+