Year 2018 Day 2

Author: maneatingape

README.md

@@ -238,6 +238,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | Day | Problem | Solution | Benchmark (μs) |
 | --- | --- | --- | --: |
 | 1 | [Chronal Calibration](https://adventofcode.com/2018/day/1) | [Source](src/year2018/day01.rs) | 15 |
+| 2 | [Inventory Management System](https://adventofcode.com/2018/day/2) | [Source](src/year2018/day02.rs) | 77 |
 
 ## 2017
 

benches/benchmark.rs

@@ -130,6 +130,7 @@ mod year2017 {
 
 mod year2018 {
     benchmark!(year2018, day01);
+    benchmark!(year2018, day02);
 }
 
 mod year2019 {

src/lib.rs

@@ -110,9 +110,10 @@ pub mod year2017 {
     pub mod day25;
 }
 
-/// # Travel through time to restore the seasonal timeline.
+/// # Travel through time to restore the festive timeline.
 pub mod year2018 {
     pub mod day01;
+    pub mod day02;
 }
 
 /// # Rescue Santa from deep space with a solar system adventure.

src/main.rs

@@ -178,7 +178,7 @@ fn year2017() -> Vec<Solution> {
 }
 
 fn year2018() -> Vec<Solution> {
-    vec![solution!(year2018, day01)]
+    vec![solution!(year2018, day01), solution!(year2018, day02)]
 }
 
 fn year2019() -> Vec<Solution> {

src/year2018/day02.rs

@@ -0,0 +1,72 @@
+//! # Inventory Management System
+use crate::util::hash::*;
+
+pub fn parse(input: &str) -> Vec<&[u8]> {
+    input.lines().map(str::as_bytes).collect()
+}
+
+pub fn part1(input: &[&[u8]]) -> u32 {
+    let mut total_twos = 0;
+    let mut total_threes = 0;
+
+    for &id in input {
+        // Ids are lowercase ASCII only with cardinality of 26.
+        let mut freq = [0; 26];
+        let mut twos = 0;
+        let mut threes = 0;
+
+        for &b in id {
+            let index = (b - b'a') as usize;
+            let current = freq[index];
+
+            match current {
+                0 => (),
+                1 => twos += 1,
+                2 => {
+                    twos -= 1;
+                    threes += 1;
+                }
+                _ => threes -= 1,
+            }
+
+            freq[index] += 1;
+        }
+
+        if twos > 0 {
+            total_twos += 1;
+        }
+        if threes > 0 {
+            total_threes += 1;
+        }
+    }
+
+    total_twos * total_threes
+}
+
+pub fn part2(input: &[&[u8]]) -> String {
+    let width = input[0].len();
+
+    let mut seen = FastSet::with_capacity(input.len());
+    let mut buffer = [0; 32];
+
+    // Use a set to check for duplicates after replacing a single character with '*' in each column.
+    for column in 0..width {
+        for &id in input {
+            buffer[0..width].copy_from_slice(id);
+            buffer[column] = b'*';
+
+            if !seen.insert(buffer) {
+                // Convert to String
+                return buffer
+                    .iter()
+                    .filter(|&&b| b.is_ascii_lowercase())
+                    .map(|&b| b as char)
+                    .collect();
+            }
+        }
+
+        seen.clear();
+    }
+
+    unreachable!()
+}

tests/test.rs

@@ -114,6 +114,7 @@ mod year2017 {
 
 mod year2018 {
     mod day01_test;
+    mod day02_test;
 }
 
 mod year2019 {

tests/year2018/day02_test.rs

@@ -0,0 +1,31 @@
+use aoc::year2018::day02::*;
+
+const FIRST_EXAMPLE: &str = "\
+abcdef
+bababc
+abbcde
+abcccd
+aabcdd
+abcdee
+ababab";
+
+const SECOND_EXAMPLE: &str = "\
+abcde
+fghij
+klmno
+pqrst
+fguij
+axcye
+wvxyz";
+
+#[test]
+fn part1_test() {
+    let input = parse(FIRST_EXAMPLE);
+    assert_eq!(part1(&input), 12);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(SECOND_EXAMPLE);
+    assert_eq!(part2(&input), "fgij");
+}