day12: done

Author: tung

Cargo.lock

@@ -22,6 +22,7 @@ members = [
     "day10pt2",
     "day11",
     "day11pt2",
+    "day12",
 ]
 
 [profile.dev]

Cargo.toml

@@ -0,0 +1,8 @@
+[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]

day12/Cargo.toml

@@ -0,0 +1,65 @@
+fn count_arrangements(s: &str, damage_seqs: &[usize]) -> i32 {
+    let s = s.trim_start_matches('.');
+    let count = s
+        .strip_prefix('?')
+        .map(|s| count_arrangements(s, damage_seqs))
+        .unwrap_or(0);
+    if let Some((&d, damage_seqs)) = damage_seqs.split_first() {
+        let mut d = d;
+        let mut s = s;
+        while d > 0 {
+            if let Some(s_suffix) = s.strip_prefix(&['#', '?']) {
+                s = s_suffix;
+                d -= 1;
+            } else {
+                break;
+            }
+        }
+        if d > 0 {
+            // not enough damage to match
+            count
+        } else if !damage_seqs.is_empty() {
+            if let Some(s_suffix) = s.strip_prefix(&['.', '?']) {
+                count + count_arrangements(s_suffix, damage_seqs)
+            } else {
+                // a gap must exist between damage sequences
+                count
+            }
+        } else {
+            count + count_arrangements(s, damage_seqs)
+        }
+    } else if s.is_empty() {
+        // good match: damage_seqs and s are both empty
+        count + 1
+    } else {
+        // no match: damage_seqs is empty, but s is not empty
+        count
+    }
+}
+
+fn main() {
+    let sum: i32 = std::io::stdin()
+        .lines()
+        .map(Result::unwrap)
+        .map(|line| {
+            let (row, line) = line.split_once(" ").unwrap();
+            let damage_seqs = line
+                .split(",")
+                .map(|d| d.parse::<usize>().expect("damage usize"))
+                .collect::<Vec<usize>>();
+            count_arrangements(row, &damage_seqs[..])
+        })
+        .sum();
+    println!("{sum}");
+}
+
+#[test]
+#[rustfmt::skip]
+fn test_count_arrangements() {
+    assert_eq!(1, count_arrangements("???.###", &[1, 1, 3]));
+    assert_eq!(4, count_arrangements(".??..??...?##", &[1, 1, 3]));
+    assert_eq!(1, count_arrangements("?#?#?#?#?#?#?#?", &[1, 3, 1, 6]));
+    assert_eq!(1, count_arrangements("????.#...#...", &[4, 1, 1]));
+    assert_eq!(4, count_arrangements("????.######..#####.", &[1, 6, 5]));
+    assert_eq!(10, count_arrangements("?###????????", &[3, 2, 1]));
+}