Day 2

1 files changed, 97 insertions, 0 deletions

diff --git a/src/bin/2.rs b/src/bin/2.rs
new file mode 100644
index 0000000..a674c9a
--- /dev/null
+++ b/src/bin/2.rs
@@ -0,0 +1,97 @@
+use std::{
+    error::Error,
+    fs::File,
+    io::{BufRead, BufReader},
+    num::ParseIntError,
+};
+
+#[derive(PartialEq, Eq)]
+enum Dir {
+    Ascending,
+    Descending,
+}
+
+impl Dir {
+    fn characterise(a: i16, b: i16) -> Option<Self> {
+        let dir = if a < b {
+            Self::Ascending
+        } else {
+            Self::Descending
+        };
+        if dir.diff_ok(a, b) {
+            Some(dir)
+        } else {
+            None
+        }
+    }
+    fn diff_ok(&self, a: i16, b: i16) -> bool {
+        let diff = match *self {
+            Self::Ascending => b - a,
+            Self::Descending => a - b,
+        };
+        diff >= 1 && diff <= 3
+    }
+}
+
+fn p1cond(report: &[i16]) -> bool {
+    if report.len() <= 1 {
+        return true;
+    }
+    let Some(dir) = Dir::characterise(report[0], report[1]) else {
+        return false;
+    };
+    for win in report[1..].windows(2) {
+        if !dir.diff_ok(win[0], win[1]) {
+            return false;
+        }
+    }
+    return true;
+}
+
+fn p2cond(report: &[i16]) -> bool {
+    if report.len() <= 2 {
+        // One element can always be removed to make a safe report
+        return true;
+    }
+    'outer: for skip_i in 0..report.len() {
+        let it =
+            report
+                .iter()
+                .copied()
+                .enumerate()
+                .filter_map(|(i, e)| if i != skip_i { Some(e) } else { None });
+        let mut pairs = it.clone().zip(it.skip(1));
+        let Some((a, b)) = pairs.next() else {
+            continue;
+        };
+        let Some(dir) = Dir::characterise(a, b) else {
+            continue;
+        };
+        for (a, b) in pairs {
+            if !dir.diff_ok(a, b) {
+                continue 'outer;
+            }
+        }
+        return true;
+    }
+    return false;
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let f = File::open("input/2")?;
+    let f = BufReader::new(f);
+    let mut input = Vec::new();
+    for line in f.lines() {
+        let elems = line?
+            .trim_end()
+            .split(' ')
+            .map(str::parse::<i16>)
+            .collect::<Result<Vec<_>, ParseIntError>>()?
+            .into_boxed_slice();
+        input.push(elems);
+    }
+    let input = input.into_boxed_slice();
+    println!("{}", input.iter().filter(|e| p1cond(*e)).count());
+    println!("{}", input.iter().filter(|e| p2cond(*e)).count());
+    Ok(())
+}