From 9784fa357e88647573087f81e1d4cf6d37b6c6c2 Mon Sep 17 00:00:00 2001
From: Quentin Boyer <quentin+dev@familleboyer.net>
Date: Sat, 14 Dec 2024 18:15:15 +0100
Subject: [PATCH] Day 13

---
 src/bin/day13.rs | 159 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 159 insertions(+)
 create mode 100644 src/bin/day13.rs

diff --git a/src/bin/day13.rs b/src/bin/day13.rs
new file mode 100644
index 0000000..740c620
--- /dev/null
+++ b/src/bin/day13.rs
@@ -0,0 +1,159 @@
+use std::time::Instant;
+
+use aoc_2024::{load, print_res};
+use bstr::BString;
+use color_eyre::eyre::{bail, Context};
+
+#[derive(Debug)]
+struct Point {
+    x: i64,
+    y: i64,
+}
+
+#[derive(Debug)]
+pub struct Crane {
+    a: Point,
+    b: Point,
+    prize: Point,
+}
+
+type Parsed = Vec<Crane>;
+
+#[inline(never)]
+pub fn parsing(input: &BString) -> color_eyre::Result<Parsed> {
+    fn parse_point(operation: char, s: &str) -> color_eyre::Result<Point> {
+        let Some((x, y)) = s.split_once(", ") else {
+            bail!("Missing separator in `{s}`")
+        };
+
+        let Some(x) = x.strip_prefix('X').and_then(|s| s.strip_prefix(operation)) else {
+            bail!("Missing X in `${x}`")
+        };
+        let Some(y) = y.strip_prefix("Y").and_then(|s| s.strip_prefix(operation)) else {
+            bail!("Missing Y in `${y}`")
+        };
+
+        Ok(Point {
+            x: x.parse().with_context(|| format!("Invalid X: `{x}`"))?,
+            y: y.parse().with_context(|| format!("Invalid Y: `{y}`"))?,
+        })
+    }
+
+    std::str::from_utf8(input)?
+        .split("\n\n")
+        .map(|desc| {
+            let Some((a, rest)) = desc.trim().split_once('\n') else {
+                bail!("Missing first linebreak")
+            };
+            let Some((b, prize)) = rest.split_once('\n') else {
+                bail!("Missing second linebreak")
+            };
+
+            let Some(a) = a.strip_prefix("Button A: ") else {
+                bail!("Invalid button A: `{a}`")
+            };
+            let Some(b) = b.strip_prefix("Button B: ") else {
+                bail!("Invalid button B: `{b}`")
+            };
+            let Some(prize) = prize.strip_prefix("Prize: ") else {
+                bail!("Invalid prize: `{prize}`")
+            };
+
+            Ok(Crane {
+                a: parse_point('+', a)?,
+                b: parse_point('+', b)?,
+                prize: parse_point('=', prize)?,
+            })
+        })
+        .collect()
+}
+
+impl Crane {
+    pub fn presses(&self) -> Option<(i64, i64)> {
+        // X = a * AX + b * BX
+        // Y = a * AY + b * BY
+        //
+        // M = ( AX BX ) T = ( X ) B = ( a )
+        //     ( AY BY )     ( Y )     ( b )
+        //
+        // M * B = ( a * AX + b * BX ) = T
+        //         ( a * AY + b * BY )
+        // B = M^-1 * T
+
+        let det = self.a.x * self.b.y - self.b.x * self.a.y;
+        if det == 0 {
+            todo!()
+        }
+
+        // M^-1 = 1/det * (  BY -BX )
+        //                ( -AY  AX )
+        //
+        // M^-1 * T = 1/det * ( BY * X - BX * Y )
+        //                    ( -AY * X + AX * Y )
+
+        let raw_a = det.signum() * (self.b.y * self.prize.x - self.b.x * self.prize.y);
+        let raw_b = det.signum() * (-self.a.y * self.prize.x + self.a.x * self.prize.y);
+
+        if raw_a < 0 || raw_a % det != 0 {
+            return None;
+        }
+
+        if raw_b < 0 || raw_b % det != 0 {
+            return None;
+        }
+
+        Some((raw_a / det.abs(), raw_b / det.abs()))
+    }
+}
+
+#[inline(never)]
+pub fn part1(input: Parsed) {
+    let mut cost = 0;
+    for crane in input {
+        let Some((a, b)) = crane.presses() else {
+            continue;
+        };
+
+        cost += a * 3 + b;
+    }
+
+    print_res!("Total cost: {cost}");
+}
+
+#[inline(never)]
+pub fn part2(input: Parsed) {
+    let mut cost = 0;
+    for mut crane in input {
+        crane.prize.x += 10000000000000;
+        crane.prize.y += 10000000000000;
+
+        let Some((a, b)) = crane.presses() else {
+            continue;
+        };
+
+        cost += a * 3 + b;
+    }
+
+    print_res!("Total cost: {cost}");
+}
+
+pub fn main() -> color_eyre::Result<()> {
+    let context = load()?;
+
+    let start = Instant::now();
+    let parsed = parsing(&context.input)?;
+    let elapsed = humantime::format_duration(start.elapsed());
+
+    let start = Instant::now();
+    if context.part == 1 {
+        part1(parsed);
+    } else {
+        part2(parsed);
+    }
+    let elapsed_part = humantime::format_duration(start.elapsed());
+
+    println!("  Parsing: {elapsed}");
+    println!("  Solving: {elapsed_part}");
+
+    Ok(())
+}