Day 15 - Part 2

Signed-off-by: Anthony Oteri <anthony.oteri@gmail.com>

day-15/src/part2.rs

@@ -1,8 +1,129 @@
+use itertools::Itertools;
+
 use crate::error::AocError;
+use nom::{
+    bytes::complete::{is_a, tag},
+    character::complete::{self, alpha1},
+    combinator::opt,
+    multi::separated_list1,
+    IResult,
+};
+
+#[derive(Debug)]
+struct Instruction<'a> {
+    hash: usize,
+    label: &'a str,
+    operation: Operation,
+}
+
+#[derive(Debug, Clone)]
+enum Operation {
+    Insert(u8),
+    Remove,
+}
+
+#[derive(Debug, Clone)]
+struct Lens<'a> {
+    label: &'a str,
+    power: u8,
+}
+
+#[derive(Debug)]
+struct LightBox<'a> {
+    lenses: Vec<Lens<'a>>,
+}
+
+fn parse_instruction(input: &str) -> IResult<&str, Instruction> {
+    let (input, label) = alpha1(input)?;
+    let (input, _) = is_a("=-")(input)?;
+    let (input, power) = opt(complete::u8)(input)?;
+
+    let operation = {
+        match power {
+            Some(p) => Operation::Insert(p),
+            None => Operation::Remove,
+        }
+    };
+    let instruction = Instruction {
+        hash: hash_value(label),
+        label,
+        operation,
+    };
+
+    Ok((input, instruction))
+}
+
+fn parse(input: &str) -> IResult<&str, Vec<Instruction>> {
+    separated_list1(tag(","), parse_instruction)(input)
+}
+
+#[tracing::instrument]
+fn hash_value(input: &str) -> usize {
+    let ascii_codes = input.chars().map(|c| c as u8).collect_vec();
+
+    let mut sum: usize = 0;
+
+    for code in ascii_codes {
+        sum += code as usize;
+        sum *= 17;
+        sum %= 256;
+    }
+
+    sum
+}
 
 #[tracing::instrument]
 pub fn process(input: &str) -> miette::Result<u64, AocError> {
-    Ok(0)
+    let (input, instructions) = parse(input).unwrap();
+    debug_assert!(input.is_empty(), "Failed to completely parse input");
+
+    let boxes = (0..u8::MAX)
+        .map(|_| LightBox { lenses: Vec::new() })
+        .collect_vec();
+
+    let boxes = instructions.iter().fold(boxes, |mut boxes, instruction| {
+        match instruction.operation {
+            Operation::Insert(power) => {
+                let idx = boxes[instruction.hash]
+                    .lenses
+                    .iter()
+                    .position(|l| l.label == instruction.label);
+                match idx {
+                    Some(idx) => {
+                        boxes[instruction.hash].lenses[idx].power = power;
+                    }
+                    None => {
+                        let lens = Lens {
+                            label: instruction.label,
+                            power,
+                        };
+                        boxes[instruction.hash].lenses.push(lens);
+                    }
+                }
+            }
+            Operation::Remove => boxes[instruction.hash]
+                .lenses
+                .retain(|l| l.label != instruction.label),
+        };
+
+        boxes
+    });
+
+    let power: u64 = boxes
+        .iter()
+        .enumerate()
+        .flat_map(|(box_index, b)| {
+            b.lenses.iter().enumerate().map(move |(lens_index, l)| {
+                let box_part = box_index + 1;
+                let slot_part = lens_index + 1;
+                let focal_length = l.power as usize;
+
+                box_part as u64 * slot_part as u64 * focal_length as u64
+            })
+        })
+        .sum();
+
+    Ok(power)
 }
 
 #[cfg(test)]
@@ -12,7 +133,7 @@ mod tests {
     #[test_log::test]
     fn test_process() -> miette::Result<()> {
         let input = include_str!("../test-input.txt");
-        assert_eq!(0, process(input)?);
+        assert_eq!(145, process(input)?);
         Ok(())
     }
 }