From 5adc85d5cd69f5e1ac017aed6fae4a13c63ef966 Mon Sep 17 00:00:00 2001
From: skasch <romain.mondoncancel@gmail.com>
Date: Tue, 26 Dec 2023 19:58:48 +0100
Subject: [PATCH 1/3] Fix rounding error

---
 day-24/part-2/skasch.cpp | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/day-24/part-2/skasch.cpp b/day-24/part-2/skasch.cpp
index f2d22c8..8042d86 100644
--- a/day-24/part-2/skasch.cpp
+++ b/day-24/part-2/skasch.cpp
@@ -2,6 +2,7 @@
 #include <cstddef>
 #include <iostream>
 #include <optional>
+#include <ostream>
 #include <span>
 #include <sstream>
 #include <stdexcept>
@@ -29,6 +30,14 @@ __int128_t gcd(__int128_t a, __int128_t b) {
   return b;
 }
 
+std::ostream& operator<<(std::ostream& o, const __int128& x) {
+  if (x == std::numeric_limits<__int128>::min())
+    return o << "-170141183460469231731687303715884105728";
+  if (x < 0) return o << "-" << -x;
+  if (x < 10) return o << (char)(x + '0');
+  return o << x / 10 << (char)(x % 10 + '0');
+}
+
 struct Vec {
   __int128_t x;
   __int128_t y;
@@ -45,6 +54,10 @@ struct Vec {
   Vec operator^(const Vec& o) const {
     return {y * o.z - z * o.y, z * o.x - x * o.z, x * o.y - y * o.x};
   }
+
+  friend std::ostream& operator<<(std::ostream& out, const Vec& v) {
+    return out << "(" << v.x << "," << v.y << "," << v.z << ")";
+  }
 };
 
 struct Hail {
@@ -57,8 +70,8 @@ std::optional<std::pair<__int128_t, __int128_t>> solve(
     __int128_t b1, __int128_t b2) {
   __int128_t delta = x11 * x22 - x21 * x12;
   if (delta == 0) return std::nullopt;
-  return std::make_pair(x22 * b1 / delta - x12 * b2 / delta,
-                        -x21 * b1 / delta + x11 * b2 / delta);
+  return std::make_pair((x22 * b1 - x12 * b2) / delta,
+                        -(x21 * b1 + x11 * b2) / delta);
 }
 
 static std::array<Hail, kNHails> kHails;

From 95db80789b3689b80a8ed19cefb6c1491284fb0b Mon Sep 17 00:00:00 2001
From: skasch <romain.mondoncancel@gmail.com>
Date: Tue, 26 Dec 2023 20:00:19 +0100
Subject: [PATCH 2/3] Remove ostream helpers

---
 day-24/part-2/skasch.cpp | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/day-24/part-2/skasch.cpp b/day-24/part-2/skasch.cpp
index 8042d86..e0ad47a 100644
--- a/day-24/part-2/skasch.cpp
+++ b/day-24/part-2/skasch.cpp
@@ -2,7 +2,6 @@
 #include <cstddef>
 #include <iostream>
 #include <optional>
-#include <ostream>
 #include <span>
 #include <sstream>
 #include <stdexcept>
@@ -30,14 +29,6 @@ __int128_t gcd(__int128_t a, __int128_t b) {
   return b;
 }
 
-std::ostream& operator<<(std::ostream& o, const __int128& x) {
-  if (x == std::numeric_limits<__int128>::min())
-    return o << "-170141183460469231731687303715884105728";
-  if (x < 0) return o << "-" << -x;
-  if (x < 10) return o << (char)(x + '0');
-  return o << x / 10 << (char)(x % 10 + '0');
-}
-
 struct Vec {
   __int128_t x;
   __int128_t y;
@@ -54,10 +45,6 @@ struct Vec {
   Vec operator^(const Vec& o) const {
     return {y * o.z - z * o.y, z * o.x - x * o.z, x * o.y - y * o.x};
   }
-
-  friend std::ostream& operator<<(std::ostream& out, const Vec& v) {
-    return out << "(" << v.x << "," << v.y << "," << v.z << ")";
-  }
 };
 
 struct Hail {

From 2866b2cc5d5469004ddcc68025a312de178864a1 Mon Sep 17 00:00:00 2001
From: skasch <romain.mondoncancel@gmail.com>
Date: Tue, 26 Dec 2023 20:01:59 +0100
Subject: [PATCH 3/3] Simplify solver

---
 day-24/part-2/skasch.cpp | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/day-24/part-2/skasch.cpp b/day-24/part-2/skasch.cpp
index e0ad47a..b9c0070 100644
--- a/day-24/part-2/skasch.cpp
+++ b/day-24/part-2/skasch.cpp
@@ -52,13 +52,11 @@ struct Hail {
   Vec v;
 };
 
-std::optional<std::pair<__int128_t, __int128_t>> solve(
-    __int128_t x11, __int128_t x12, __int128_t x21, __int128_t x22,
-    __int128_t b1, __int128_t b2) {
+std::optional<__int128_t> solve(__int128_t x11, __int128_t x12, __int128_t x21,
+                                __int128_t x22, __int128_t b1, __int128_t b2) {
   __int128_t delta = x11 * x22 - x21 * x12;
   if (delta == 0) return std::nullopt;
-  return std::make_pair((x22 * b1 - x12 * b2) / delta,
-                        -(x21 * b1 + x11 * b2) / delta);
+  return (x22 * b1 - x12 * b2) / delta;
 }
 
 static std::array<Hail, kNHails> kHails;
@@ -156,7 +154,7 @@ std::string Run(const std::string& input) {
       auto solution = solve(v1.x - vx, -v2.x + vx, v1.y - vy, -v2.y + vy,
                             p2.x - p1.x, p2.y - p1.y);
       if (!solution.has_value()) continue;
-      const auto& [t1, t2] = *solution;
+      __int128 t1 = *solution;
       for (int vz = -kMaxVelocity; vz < kMaxVelocity; ++vz) {
         if (invalid_vz.contains(vz)) continue;
         Vec v(vx, vy, vz);