day15 solution 2

1 files changed, 74 insertions, 5 deletions

diff --git a/src/bin/day15.rs b/src/bin/day15.rs
index 648534a..2b3b04a 100644
--- a/src/bin/day15.rs
+++ b/src/bin/day15.rs
@@ -1,16 +1,18 @@
+use std::collections::VecDeque;
+
 static DAY: u8 = 15;
 
 fn main() {
     let input = advent::read_file(DAY);
     println!("{DAY}a: {}", hash_sum(&input));
-    println!("{DAY}b: {}", 0);
+    println!("{DAY}b: {}", focusing_power(&input));
 }
 
-fn hash(input: &str) -> u32 {
+fn hash(input: &str) -> usize {
     let mut value = 0;
 
     for c in input.chars() {
-        value += c as u32;
+        value += c as usize;
         value *= 17;
         value %= 256;
     }
@@ -18,10 +20,76 @@ fn hash(input: &str) -> u32 {
     value
 }
 
-fn hash_sum(input: &str) -> u32 {
+#[derive(Clone, Debug)]
+struct Lens {
+    label: String,
+    focal_length: usize,
+}
+
+enum Operation {
+    Assign(Lens),
+    Remove(String),
+}
+
+impl Operation {
+    fn from(input: &str) -> Operation {
+        if let Some((label, value)) = input.split_once('=') {
+            Operation::Assign(Lens {
+                label: label.to_string(),
+                focal_length: value.parse().unwrap()
+            })
+        } else if let Some((label, _)) = input.split_once('-') {
+            Operation::Remove(label.to_string())
+        } else {
+            panic!("invalid operation");
+        }
+    }
+
+    fn label(&self) -> String {
+        match self {
+            Operation::Assign(lens) => lens.label.clone(),
+            Operation::Remove(label) => label.to_string(),
+        }
+    }
+}
+
+fn hash_sum(input: &str) -> usize {
     let input = input.trim_end();
     input.split(',')
-         .map(|x| hash(x))
+         .map(hash)
+         .sum()
+}
+
+fn init_sequence(boxes: &mut [VecDeque::<Lens>], operations: &[Operation]) {
+    for operation in operations {
+        let box_nr = hash(&operation.label());
+        match operation {
+            Operation::Assign(lens) => {
+                if let Some(boxed_lens) = boxes[box_nr].iter_mut().find(|l| l.label == lens.label) {
+                    boxed_lens.focal_length = lens.focal_length;
+                } else {
+                    boxes[box_nr].push_back(lens.clone());
+                }
+            },
+            Operation::Remove(label) => {
+                boxes[box_nr].retain(|lens| lens.label != *label);
+            }
+        }
+    }
+}
+
+fn focusing_power(input: &str) -> usize {
+    let input = input.trim_end();
+    let operations = input.split(',')
+                          .map(Operation::from)
+                          .collect::<Vec<_>>();
+
+    let mut boxes = vec![VecDeque::<Lens>::new(); 256];
+    init_sequence(&mut boxes, &operations);
+
+    boxes.iter()
+         .enumerate()
+         .map(|(box_nr, lenses)| lenses.iter().enumerate().map(|(slot, lens)| (box_nr + 1) * (slot + 1) * lens.focal_length).sum::<usize>())
          .sum()
 }
 
@@ -34,5 +102,6 @@ mod tests {
         let input = "rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7";
         assert_eq!(hash("HASH"), 52);
         assert_eq!(hash_sum(input), 1320);
+        assert_eq!(focusing_power(input), 145);
     }
 }