2023: Day 12, part 1

2023/day12/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day12"
+version = "0.1.0"

2023/day12/Cargo.toml

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+[package]
+name = "day12"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2023/day12/src/main.rs

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+use std::env;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+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 file = BufReader::new(File::open(input_path)?);
+
+    let mut state = Vec::new();
+    let mut combinations = Vec::new();
+    let mut part1 = 0;
+    for line in file.lines() {
+        let line = line?;
+        if line.is_empty() {
+            continue;
+        }
+        let (pattern, info) = line.split_once(' ').ok_or("unable to split line")?;
+
+        let info = info
+            .split(',')
+            .map(|n| n.parse::<u32>())
+            .collect::<Result<Vec<_>, _>>()?;
+
+        // generate all combinations of wildcards
+        combinations.clear();
+        state.clear();
+        generate(pattern.as_bytes(), &mut state, &mut combinations, &info);
+        // dbg!(&combinations);
+        part1 += combinations.len();
+    }
+
+    println!("Part 1: {part1}");
+
+    Ok(())
+}
+
+fn matches(s: &str, pattern: &str) -> bool {
+    s.chars()
+        .zip(pattern.chars())
+        .all(|(a, b)| a == b || b == '?')
+}
+
+fn generate(
+    pattern: &[u8],
+    state: &mut Vec<u8>,
+    combinations: &mut Vec<String>,
+    contiguous: &[u32],
+) {
+    if pattern.is_empty() {
+        let candidate = String::from_utf8(state.clone()).unwrap();
+        let candidate_info = count_contiguous(&candidate);
+        if candidate_info == contiguous {
+            combinations.push(candidate);
+        }
+        return;
+    }
+
+    match pattern[0] {
+        b'.' | b'#' => {
+            state.push(pattern[0]);
+            generate(&pattern[1..], state, combinations, contiguous);
+            state.pop().unwrap();
+        }
+        b'?' => {
+            state.push(b'.');
+            generate(&pattern[1..], state, combinations, contiguous);
+            state.pop().unwrap();
+            state.push(b'#');
+            generate(&pattern[1..], state, combinations, contiguous);
+            state.pop().unwrap();
+        }
+        _ => unreachable!("unexpected char"),
+    }
+}
+
+fn count_contiguous(s: &str) -> Vec<u32> {
+    let mut counts = Vec::new();
+    let mut in_contiguous = false;
+    let mut count: u32 = 0;
+    for c in s.chars() {
+        match c {
+            '#' if in_contiguous => count += 1,
+            '#' => {
+                in_contiguous = true;
+                count = 1;
+            }
+            '.' if in_contiguous => {
+                counts.push(count);
+                in_contiguous = false;
+            }
+            _ => {}
+        }
+    }
+    if in_contiguous {
+        counts.push(count);
+    }
+    counts
+}

2023/input/day12.sample

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+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1