day(15): add solution

Signed-off-by: Matej Focko <me@mfocko.xyz>

samples/day15.txt

@@ -0,0 +1 @@
+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7

src/bin/day15.rs

@@ -0,0 +1,118 @@
+use std::str::FromStr;
+
+use aoc_2023::*;
+
+type Output1 = i32;
+type Output2 = Output1;
+
+fn hash_algorithm(s: &str) -> i32 {
+    s.chars().fold(0, |mut h, c| {
+        let mut b = [0];
+
+        c.encode_utf8(&mut b);
+
+        h += b[0] as i32;
+        h *= 17;
+        h %= 256;
+
+        h
+    })
+}
+
+enum Step {
+    Del(String),
+    Ins(String, i32),
+}
+
+impl FromStr for Step {
+    type Err = &'static str;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (label, focal_length) = s.split_once(['=', '-']).unwrap();
+
+        Ok(match focal_length {
+            "" => Step::Del(label.to_owned()),
+            _ => Step::Ins(label.to_owned(), focal_length.parse().unwrap()),
+        })
+    }
+}
+
+impl Step {
+    fn hash_step(&self) -> i32 {
+        let original = match self {
+            Step::Del(label) => format!("{}-", label),
+            Step::Ins(label, length) => format!("{}={}", label, length),
+        };
+
+        hash_algorithm(&original)
+    }
+
+    fn label(&self) -> &str {
+        match self {
+            Step::Del(l) | Step::Ins(l, _) => l,
+        }
+    }
+
+    fn hash_label(&self) -> i32 {
+        hash_algorithm(self.label())
+    }
+}
+
+struct Day15 {
+    steps: Vec<Step>,
+}
+impl Solution<Output1, Output2> for Day15 {
+    fn new<P: AsRef<Path>>(pathname: P) -> Self {
+        Self {
+            steps: file_to_string(pathname)
+                .trim_end()
+                .split(',')
+                .map(|s| s.parse().unwrap())
+                .collect_vec(),
+        }
+    }
+
+    fn part_1(&mut self) -> Output1 {
+        self.steps.iter().map(|s| s.hash_step()).sum()
+    }
+
+    fn part_2(&mut self) -> Output2 {
+        let mut boxes: Vec<Vec<(&str, i32)>> = vec![];
+        boxes.resize_with(256, Vec::new);
+
+        for step in &self.steps {
+            let h = step.hash_label();
+
+            let b: &mut Vec<(&str, i32)> = &mut boxes[h as usize];
+
+            match step {
+                Step::Del(_) => {
+                    if let Some(idx) = b.iter().position(|&(l, _)| l == step.label()) {
+                        b.remove(idx);
+                    }
+                }
+                Step::Ins(label, l) => match b.iter().position(|&(l, _)| l == step.label()) {
+                    Some(idx) => b[idx] = (label, *l),
+                    None => b.push((label, *l)),
+                },
+            }
+        }
+
+        boxes
+            .iter()
+            .enumerate()
+            .map(|(i, b)| {
+                b.iter()
+                    .enumerate()
+                    .map(|(j, (_, l))| (i as i32 + 1) * (j as i32 + 1) * l)
+                    .sum::<i32>()
+            })
+            .sum()
+    }
+}
+
+fn main() -> Result<()> {
+    Day15::main()
+}
+
+test_sample!(day_15, Day15, 1320, 145);