Year 2015 Day 13

Author: maneatingape

README.md

@@ -248,3 +248,4 @@ pie
 | 10 | [Elves Look, Elves Say](https://adventofcode.com/2015/day/10) | [Source](src/year2015/day10.rs) | 14 |
 | 11 | [Corporate Policy](https://adventofcode.com/2015/day/11) | [Source](src/year2015/day11.rs) | 1 |
 | 12 | [JSAbacusFramework.io](https://adventofcode.com/2015/day/12) | [Source](src/year2015/day12.rs) | 80 |
+| 13 | [Knights of the Dinner Table](https://adventofcode.com/2015/day/13) | [Source](src/year2015/day13.rs) | 39 |

benches/benchmark.rs

@@ -48,6 +48,7 @@ mod year2015 {
     benchmark!(year2015, day10);
     benchmark!(year2015, day11);
     benchmark!(year2015, day12);
+    benchmark!(year2015, day13);
 }
 
 mod year2019 {

input/year2015/day13.txt

@@ -0,0 +1,56 @@
+Alice would lose 57 happiness units by sitting next to Bob.
+Alice would lose 62 happiness units by sitting next to Carol.
+Alice would lose 75 happiness units by sitting next to David.
+Alice would gain 71 happiness units by sitting next to Eric.
+Alice would lose 22 happiness units by sitting next to Frank.
+Alice would lose 23 happiness units by sitting next to George.
+Alice would lose 76 happiness units by sitting next to Mallory.
+Bob would lose 14 happiness units by sitting next to Alice.
+Bob would gain 48 happiness units by sitting next to Carol.
+Bob would gain 89 happiness units by sitting next to David.
+Bob would gain 86 happiness units by sitting next to Eric.
+Bob would lose 2 happiness units by sitting next to Frank.
+Bob would gain 27 happiness units by sitting next to George.
+Bob would gain 19 happiness units by sitting next to Mallory.
+Carol would gain 37 happiness units by sitting next to Alice.
+Carol would gain 45 happiness units by sitting next to Bob.
+Carol would gain 24 happiness units by sitting next to David.
+Carol would gain 5 happiness units by sitting next to Eric.
+Carol would lose 68 happiness units by sitting next to Frank.
+Carol would lose 25 happiness units by sitting next to George.
+Carol would gain 30 happiness units by sitting next to Mallory.
+David would lose 51 happiness units by sitting next to Alice.
+David would gain 34 happiness units by sitting next to Bob.
+David would gain 99 happiness units by sitting next to Carol.
+David would gain 91 happiness units by sitting next to Eric.
+David would lose 38 happiness units by sitting next to Frank.
+David would gain 60 happiness units by sitting next to George.
+David would lose 63 happiness units by sitting next to Mallory.
+Eric would gain 23 happiness units by sitting next to Alice.
+Eric would lose 69 happiness units by sitting next to Bob.
+Eric would lose 33 happiness units by sitting next to Carol.
+Eric would lose 47 happiness units by sitting next to David.
+Eric would gain 75 happiness units by sitting next to Frank.
+Eric would gain 82 happiness units by sitting next to George.
+Eric would gain 13 happiness units by sitting next to Mallory.
+Frank would gain 77 happiness units by sitting next to Alice.
+Frank would gain 27 happiness units by sitting next to Bob.
+Frank would lose 87 happiness units by sitting next to Carol.
+Frank would gain 74 happiness units by sitting next to David.
+Frank would lose 41 happiness units by sitting next to Eric.
+Frank would lose 99 happiness units by sitting next to George.
+Frank would gain 26 happiness units by sitting next to Mallory.
+George would lose 63 happiness units by sitting next to Alice.
+George would lose 51 happiness units by sitting next to Bob.
+George would lose 60 happiness units by sitting next to Carol.
+George would gain 30 happiness units by sitting next to David.
+George would lose 100 happiness units by sitting next to Eric.
+George would lose 63 happiness units by sitting next to Frank.
+George would gain 57 happiness units by sitting next to Mallory.
+Mallory would lose 71 happiness units by sitting next to Alice.
+Mallory would lose 28 happiness units by sitting next to Bob.
+Mallory would lose 10 happiness units by sitting next to Carol.
+Mallory would gain 44 happiness units by sitting next to David.
+Mallory would gain 22 happiness units by sitting next to Eric.
+Mallory would gain 79 happiness units by sitting next to Frank.
+Mallory would lose 16 happiness units by sitting next to George.

src/lib.rs

@@ -172,6 +172,7 @@ pub mod year2015 {
     pub mod day10;
     pub mod day11;
     pub mod day12;
+    pub mod day13;
 }
 
 /// # Rescue Santa from deep space with a solar system adventure.

src/main.rs

@@ -88,6 +88,7 @@ fn all_solutions() -> Vec<Solution> {
         solution!(year2015, day10),
         solution!(year2015, day11),
         solution!(year2015, day12),
+        solution!(year2015, day13),
         // 2019
         solution!(year2019, day01),
         solution!(year2019, day02),

src/year2015/day13.rs

@@ -0,0 +1,84 @@
+//! # Knights of the Dinner Table
+//!
+//! This problem is very similar to [`Day 9`] and we solve it in almost exactly the same way by
+//! computing an adjacency matrix of happiness then permuting the order of the diners.
+//!
+//! For part one we reduce the permutatations from 8! = 40,320 permutations to 7! = 5,040
+//! permutations by arbitrarily choosing one of the diners as the start.
+//!
+//! We solve part two at the same time by noticing that by inserting yourself between two diners
+//! you set the value of their mutual link to zero. Keeping tracking of the weakest link
+//! then subtracting that from the value for part one gives the result for part two at almost
+//! no additional cost.
+//!
+//! [`Day 9`]: crate::year2015::day09
+use crate::util::hash::*;
+use crate::util::parse::*;
+use crate::util::slice::*;
+
+type Input = (i32, i32);
+
+pub fn parse(input: &str) -> Input {
+    // Assign each diner an index on a first come first served basis.
+    let lines: Vec<Vec<_>> = input.lines().map(|line| line.split([' ', '.']).collect()).collect();
+    let mut indices = FastMap::new();
+
+    for tokens in &lines {
+        let size = indices.len();
+        indices.entry(tokens[0]).or_insert(size);
+
+        let size = indices.len();
+        indices.entry(tokens[10]).or_insert(size);
+    }
+
+    // Calculate the happiness values. Note that the values are not reciprocal a => b != b => a.
+    let stride = indices.len();
+    let mut happiness = vec![0; stride * stride];
+
+    for tokens in &lines {
+        let start = indices[tokens[0]];
+        let end = indices[tokens[10]];
+        let sign = if tokens[2] == "gain" { 1 } else { -1 };
+        let value: i32 = tokens[3].signed();
+
+        // Add the values together to make the mutual link reciprocal
+        happiness[stride * start + end] += sign * value;
+        happiness[stride * end + start] += sign * value;
+    }
+
+    // Solve both parts simultaneously.
+    let mut part_one = 0;
+    let mut part_two = 0;
+    let mut indices: Vec<_> = (1..stride).collect();
+
+    indices.permutations(|slice| {
+        let mut sum = 0;
+        let mut weakest_link = i32::MAX;
+
+        let mut link = |from, to| {
+            let value = happiness[stride * from + to];
+            sum += value;
+            weakest_link = weakest_link.min(value);
+        };
+
+        link(0, slice[0]);
+        link(0, slice[slice.len() - 1]);
+
+        for i in 1..slice.len() {
+            link(slice[i], slice[i - 1]);
+        }
+
+        part_one = part_one.max(sum);
+        part_two = part_two.max(sum - weakest_link);
+    });
+
+    (part_one, part_two)
+}
+
+pub fn part1(input: &Input) -> i32 {
+    input.0
+}
+
+pub fn part2(input: &Input) -> i32 {
+    input.1
+}

tests/test.rs

@@ -41,6 +41,7 @@ mod year2015 {
     mod day10_test;
     mod day11_test;
     mod day12_test;
+    mod day13_test;
 }
 
 mod year2019 {

tests/year2015/day13_test.rs

@@ -0,0 +1,27 @@
+use aoc::year2015::day13::*;
+
+const EXAMPLE: &str = "\
+Alice would gain 54 happiness units by sitting next to Bob.
+Alice would lose 79 happiness units by sitting next to Carol.
+Alice would lose 2 happiness units by sitting next to David.
+Bob would gain 83 happiness units by sitting next to Alice.
+Bob would lose 7 happiness units by sitting next to Carol.
+Bob would lose 63 happiness units by sitting next to David.
+Carol would lose 62 happiness units by sitting next to Alice.
+Carol would gain 60 happiness units by sitting next to Bob.
+Carol would gain 55 happiness units by sitting next to David.
+David would gain 46 happiness units by sitting next to Alice.
+David would lose 7 happiness units by sitting next to Bob.
+David would gain 41 happiness units by sitting next to Carol.";
+
+#[test]
+fn part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 330);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), 286);
+}