Day 13 solutions

Cadiac · Dec 13, 2024 · 9d1e453 · 9d1e453
1 parent 7737e99
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diff --git a/aoc-solver/src/y2024/day13.rs b/aoc-solver/src/y2024/day13.rs
@@ -0,0 +1,208 @@
+use itertools::Itertools;
+
+use crate::solution::{AocError, Solution};
+
+#[derive(Debug, PartialEq)]
+struct Machine {
+    a: (i64, i64),
+    b: (i64, i64),
+    price: (i64, i64),
+}
+
+fn parse(input: &str) -> Result<Vec<Machine>, AocError> {
+    let machines = input
+        .trim()
+        .split("\n\n")
+        .map(|machine_input| {
+            let (a_input, b_input, price_input) = machine_input
+                .lines()
+                .next_tuple()
+                .ok_or_else(|| AocError::parse(machine_input, "Invalid machine"))?;
+
+            let a = a_input
+                .strip_prefix("Button A: X+")
+                .and_then(|a| a.split_once(", Y+"))
+                .and_then(|(x_str, y_str)| {
+                    let x = x_str.parse::<i64>().ok()?;
+                    let y = y_str.parse::<i64>().ok()?;
+                    Some((x, y))
+                })
+                .ok_or_else(|| AocError::parse(a_input, "Invalid button A"))?;
+
+            let b = b_input
+                .strip_prefix("Button B: X+")
+                .and_then(|a| a.split_once(", Y+"))
+                .and_then(|(x_str, y_str)| {
+                    let x = x_str.parse::<i64>().ok()?;
+                    let y = y_str.parse::<i64>().ok()?;
+                    Some((x, y))
+                })
+                .ok_or_else(|| AocError::parse(b_input, "Invalid button B"))?;
+
+            let price = price_input
+                .strip_prefix("Prize: X=")
+                .and_then(|a| a.split_once(", Y="))
+                .and_then(|(x_str, y_str)| {
+                    let x = x_str.parse::<i64>().ok()?;
+                    let y = y_str.parse::<i64>().ok()?;
+                    Some((x, y))
+                })
+                .ok_or_else(|| AocError::parse(b_input, "Invalid price"))?;
+
+            Ok(Machine { a, b, price })
+        })
+        .try_collect()?;
+
+    Ok(machines)
+}
+
+fn find_fewest_tokens(
+    Machine { price, a, b }: &Machine,
+    unit_conversion_error: bool,
+) -> Option<i64> {
+    let price = if !unit_conversion_error {
+        (price.0 + 10000000000000, price.1 + 10000000000000)
+    } else {
+        *price
+    };
+
+    // Solving `a_presses` and `b_presses`
+    // from a system of two equations, accepting only integer solutions:
+    // a_presses * a.x + b_presses * b.x = price.x
+    // a_presses * a.y + b_presses * b.y = price.y
+
+    let dividend_b = price.1 * a.0 - a.1 * price.0;
+    let divisor_b = a.0 * b.1 - b.0 * a.1;
+
+    let b_presses = dividend_b / divisor_b;
+    let a_presses = (price.0 - b_presses * b.0) / a.0;
+
+    let is_integer_a = (price.0 - b_presses * b.0) % a.0 == 0;
+    let is_integer_b = dividend_b % divisor_b == 0;
+
+    (is_integer_a && is_integer_b).then_some(3 * a_presses + b_presses)
+}
+
+pub struct Day13;
+impl Solution for Day13 {
+    type A = i64;
+    type B = i64;
+
+    fn default_input(&self) -> &'static str {
+        include_str!("../../../inputs/2024/day13.txt")
+    }
+
+    fn part_1(&self, input: &str) -> Result<i64, AocError> {
+        let machines = parse(input)?;
+
+        let fewest = machines
+            .iter()
+            .flat_map(|machine| find_fewest_tokens(machine, true))
+            .sum();
+
+        Ok(fewest)
+    }
+
+    fn part_2(&self, input: &str) -> Result<i64, AocError> {
+        let machines = parse(input)?;
+
+        let fewest = machines
+            .iter()
+            .flat_map(|machine| find_fewest_tokens(machine, false))
+            .sum();
+
+        Ok(fewest)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_parses_input() {
+        assert_eq!(
+            parse(
+                "Button A: X+94, Y+34\n\
+                 Button B: X+22, Y+67\n\
+                 Prize: X=8400, Y=5400\n\
+                 \n\
+                 Button A: X+26, Y+66\n\
+                 Button B: X+67, Y+21\n\
+                 Prize: X=12748, Y=12176"
+            ),
+            Ok(vec![
+                Machine {
+                    a: (94, 34),
+                    b: (22, 67),
+                    price: (8400, 5400)
+                },
+                Machine {
+                    a: (26, 66),
+                    b: (67, 21),
+                    price: (12748, 12176)
+                }
+            ])
+        )
+    }
+
+    #[test]
+    fn it_solves_part1_example_1() {
+        assert_eq!(
+            Day13.part_1(
+                "Button A: X+94, Y+34\n\
+                 Button B: X+22, Y+67\n\
+                 Prize: X=8400, Y=5400\n\
+                 \n\
+                 Button A: X+26, Y+66\n\
+                 Button B: X+67, Y+21\n\
+                 Prize: X=12748, Y=12176\n\
+                 \n\
+                 Button A: X+17, Y+86\n\
+                 Button B: X+84, Y+37\n\
+                 Prize: X=7870, Y=6450\n\
+                 \n\
+                 Button A: X+69, Y+23\n\
+                 Button B: X+27, Y+71\n\
+                 Prize: X=18641, Y=10279"
+            ),
+            Ok(480)
+        );
+    }
+
+    #[test]
+    fn it_solves_part1_example_single() {
+        assert_eq!(
+            Day13.part_1(
+                "Button A: X+94, Y+34\n\
+                 Button B: X+22, Y+67\n\
+                 Prize: X=8400, Y=5400"
+            ),
+            Ok(280)
+        );
+    }
+
+    #[test]
+    fn it_solves_part2_example_1() {
+        assert_eq!(
+            Day13.part_2(
+                "Button A: X+94, Y+34\n\
+                 Button B: X+22, Y+67\n\
+                 Prize: X=8400, Y=5400\n\
+                 \n\
+                 Button A: X+26, Y+66\n\
+                 Button B: X+67, Y+21\n\
+                 Prize: X=12748, Y=12176\n\
+                 \n\
+                 Button A: X+17, Y+86\n\
+                 Button B: X+84, Y+37\n\
+                 Prize: X=7870, Y=6450\n\
+                 \n\
+                 Button A: X+69, Y+23\n\
+                 Button B: X+27, Y+71\n\
+                 Prize: X=18641, Y=10279"
+            ),
+            Ok(875318608908)
+        );
+    }
+}
diff --git a/aoc-solver/src/y2024/mod.rs b/aoc-solver/src/y2024/mod.rs
@@ -12,7 +12,7 @@ pub mod day09;
 pub mod day10;
 pub mod day11;
 pub mod day12;
-// pub mod day13;
+pub mod day13;
 // pub mod day14;
 // pub mod day15;
 // pub mod day16;
@@ -26,7 +26,7 @@ pub mod day12;
 // pub mod day24;
 // pub mod day25;
 
-pub const MAX_DAYS: u8 = 12;
+pub const MAX_DAYS: u8 = 13;
 
 pub struct Y2024;
 
@@ -45,7 +45,7 @@ impl Solver for Y2024 {
             10 => day10::Day10.run(input, 10, 2024),
             11 => day11::Day11.run(input, 11, 2024),
             12 => day12::Day12.run(input, 12, 2024),
-            // 13 => day13::Day13.run(input, 13, 2024),
+            13 => day13::Day13.run(input, 13, 2024),
             // 14 => day14::Day14.run(input, 14, 2024),
             // 15 => day15::Day15.run(input, 15, 2024),
             // 16 => day16::Day16.run(input, 16, 2024),
@@ -87,7 +87,7 @@ impl Solver for Y2024 {
             10 => include_str!("./day10.rs"),
             11 => include_str!("./day11.rs"),
             12 => include_str!("./day12.rs"),
-            // 13 => include_str!("./day13.rs"),
+            13 => include_str!("./day13.rs"),
             // 14 => include_str!("./day14.rs"),
             // 15 => include_str!("./day15.rs"),
             // 16 => include_str!("./day16.rs"),

diff --git a/aoc-web/src/header.rs b/aoc-web/src/header.rs
@@ -31,6 +31,7 @@ pub fn header(props: &HeaderProps) -> Html {
                                     <NavLink route={Route::Solution { year: 2024, day: 10 }} current={props.route.clone()} text={"10"}/>
                                     <NavLink route={Route::Solution { year: 2024, day: 11 }} current={props.route.clone()} text={"11"}/>
                                     <NavLink route={Route::Solution { year: 2024, day: 12 }} current={props.route.clone()} text={"12"}/>
+                                    <NavLink route={Route::Solution { year: 2024, day: 13 }} current={props.route.clone()} text={"13"}/>
                                 </>
                             }
                         },