From 8b96dde3ef9b7b37ac8e94f260e1ddc35fa12e41 Mon Sep 17 00:00:00 2001
From: Justin Wernick <justin@worthe-it.co.za>
Date: Fri, 15 Dec 2023 11:02:52 +0200
Subject: Day 15 part 2

---
 2023/src/bin/day_15.rs | 124 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 115 insertions(+), 9 deletions(-)

diff --git a/2023/src/bin/day_15.rs b/2023/src/bin/day_15.rs
index badb34f..429e7f4 100644
--- a/2023/src/bin/day_15.rs
+++ b/2023/src/bin/day_15.rs
@@ -1,30 +1,136 @@
 use std::fs;
 
+use nom::{
+    branch::alt,
+    bytes::complete::tag,
+    character::complete::{alpha1, u32},
+    combinator::{consumed, map, value},
+    multi::separated_list1,
+    sequence::{pair, preceded},
+    IResult,
+};
+
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let input = fs::read_to_string("inputs/day_15.txt")?;
-    let parsed = InitializationInstructions::parse(&input);
+    let parsed = InitializationInstructions::parser(&input).unwrap().1;
     dbg!(&parsed.hash_sum());
+    dbg!(&parsed.final_focusing_power());
 
     Ok(())
 }
 
 #[derive(Debug)]
-struct InitializationInstructions(Vec<String>);
+struct InitializationInstructions(Vec<Instruction>);
+
+#[derive(Debug)]
+struct Instruction {
+    instruction: String,
+    label: String,
+    action: Action,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Action {
+    Remove,
+    Insert(u32),
+}
+
+#[derive(Default, Debug, Clone)]
+struct LensBox {
+    lenses: Vec<Lens>,
+}
+
+#[derive(Debug, Clone)]
+struct Lens {
+    label: String,
+    power: u32,
+}
 
 impl InitializationInstructions {
-    fn parse(input: &str) -> Self {
-        Self(input.trim().split(",").map(|s| s.to_owned()).collect())
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(
+            separated_list1(tag(","), Instruction::parser),
+            InitializationInstructions,
+        )(input)
     }
 
-    fn hash_sum(&self) -> u32 {
-        self.0.iter().map(|s| hash(&s)).sum()
+    fn hash_sum(&self) -> usize {
+        self.0.iter().map(|i| hash(&i.instruction)).sum()
+    }
+
+    fn final_focusing_power(&self) -> u32 {
+        let mut boxes = vec![LensBox::default(); 256];
+        for instruction in &self.0 {
+            let box_index = hash(&instruction.label);
+            let lens_box = &mut boxes[box_index];
+            match instruction.action {
+                Action::Remove => {
+                    lens_box
+                        .lenses
+                        .retain(|lens| lens.label != instruction.label);
+                }
+                Action::Insert(power) => {
+                    let existing_position = lens_box
+                        .lenses
+                        .iter()
+                        .position(|lens| lens.label == instruction.label);
+                    match existing_position {
+                        Some(position) => {
+                            lens_box.lenses[position].power = power;
+                        }
+                        None => {
+                            lens_box.lenses.push(Lens {
+                                label: instruction.label.clone(),
+                                power,
+                            });
+                        }
+                    }
+                }
+            }
+        }
+
+        boxes
+            .into_iter()
+            .enumerate()
+            .flat_map(|(box_index, lens_box)| {
+                lens_box
+                    .lenses
+                    .into_iter()
+                    .enumerate()
+                    .map(move |(lens_index, lens)| {
+                        (box_index as u32 + 1) * (lens_index as u32 + 1) * lens.power
+                    })
+            })
+            .sum()
+    }
+}
+
+impl Instruction {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(
+            consumed(pair(alpha1, Action::parser)),
+            |(instruction, (label, action))| Instruction {
+                instruction: instruction.to_owned(),
+                label: label.to_owned(),
+                action,
+            },
+        )(input)
+    }
+}
+
+impl Action {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        alt((
+            value(Action::Remove, tag("-")),
+            map(preceded(tag("="), u32), Action::Insert),
+        ))(input)
     }
 }
 
-fn hash(input: &str) -> u32 {
-    let mut result: u32 = 0;
+fn hash(input: &str) -> usize {
+    let mut result: usize = 0;
     for c in input.bytes() {
-        result += Into::<u32>::into(c);
+        result += Into::<usize>::into(c);
         result *= 17;
         result %= 256;
     }
-- 
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