From ab3c6d20bc7cb9a9f161f8d89a3a9e4bd04fb981 Mon Sep 17 00:00:00 2001
From: Daniel Flanagan <daniel@lyte.dev>
Date: Mon, 2 Dec 2024 11:14:00 -0600
Subject: [PATCH] Day 2

---
 2024/rust/src/day2.rs | 139 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 2024/rust/src/day2.rs

diff --git a/2024/rust/src/day2.rs b/2024/rust/src/day2.rs
new file mode 100644
index 0000000..967b25d
--- /dev/null
+++ b/2024/rust/src/day2.rs
@@ -0,0 +1,139 @@
+mod prelude;
+pub use crate::prelude::*;
+
+fn main() {
+    let data: Data = day_input(2).parse().unwrap();
+    show_answers(data.num_safe(), data.num_safe_with_dampener());
+}
+
+#[derive(Default, Debug)]
+struct Report {
+    levels: Vec<i64>,
+}
+
+impl FromStr for Report {
+    type Err = ParseIntError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let levels = s
+            .split(" ")
+            .filter(|s| !s.is_empty())
+            .map(|s| s.parse())
+            .collect::<Result<Vec<i64>, ParseIntError>>()?;
+        Ok(Self { levels })
+    }
+}
+
+#[derive(PartialEq, Eq)]
+enum LevelSafeKind {
+    Increasing,
+    Decreasing,
+}
+
+impl Report {
+    fn is_safe(&self) -> bool {
+        Self::is_level_set_safe(&self.levels)
+    }
+
+    fn is_level_set_safe(levels: &[i64]) -> bool {
+        let mut iter = levels.windows(2);
+        let mut level_safe_kind: Option<LevelSafeKind> = None;
+
+        while let Some(&[a, b]) = iter.next() {
+            if !Self::safe_level_pair(a, b, &level_safe_kind) {
+                return false;
+            }
+            if level_safe_kind == None {
+                if a > b {
+                    level_safe_kind = Some(LevelSafeKind::Increasing)
+                } else {
+                    level_safe_kind = Some(LevelSafeKind::Decreasing)
+                }
+            }
+        }
+        return true;
+    }
+
+    fn is_safe_with_any_single_level_removed(&self) -> bool {
+        // TODO: this allocates for each level combination and so is suboptimal
+        // best solution I think is to update is_level_set_safe to take index pairs or something and slide those in such a way that we skip one of the indices
+        for i in 0..self.levels.len() {
+            let levels: Vec<i64> = self
+                .levels
+                .clone()
+                .iter()
+                .enumerate()
+                .filter(|&(index, _val)| i != index)
+                .map(|(_index, val)| *val)
+                .collect();
+            if Self::is_level_set_safe(&levels) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    fn safe_level_pair(a: i64, b: i64, kind: &Option<LevelSafeKind>) -> bool {
+        if a == b {
+            return false;
+        }
+        if (a - b).abs() > 3 {
+            return false;
+        }
+        if a - b < 0 && *kind == Some(LevelSafeKind::Increasing) {
+            return false;
+        }
+        if a - b > 0 && *kind == Some(LevelSafeKind::Decreasing) {
+            return false;
+        }
+        return true;
+    }
+}
+
+#[derive(Default)]
+struct Data {
+    reports: Vec<Report>,
+}
+
+impl FromStr for Data {
+    type Err = ParseIntError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let reports = s
+            .lines()
+            .map(|s| s.parse())
+            .collect::<Result<Vec<Report>, ParseIntError>>()?;
+        Ok(Self { reports })
+    }
+}
+
+impl Data {
+    fn num_safe(&self) -> usize {
+        self.reports.iter().filter(|r| r.is_safe()).count()
+    }
+
+    fn num_safe_with_dampener(&self) -> usize {
+        self.reports
+            .iter()
+            .filter(|r| r.is_safe_with_any_single_level_removed())
+            .count()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let mut report: Data = "7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9"
+            .parse()
+            .unwrap();
+        assert_eq!(report.num_safe(), 2);
+        assert_eq!(report.num_safe_with_dampener(), 4);
+        // assert_eq!(report.(), 31);
+    }
+}