From 5111d633b76d1e81769b2c47bc4311e8103c69a6 Mon Sep 17 00:00:00 2001
From: Thraix <thraix@hotmail.com>
Date: Wed, 26 Aug 2026 23:13:22 +0200
Subject: [PATCH] Add solution for 2016 Day 15

---
 res/2016/day15/test_input.txt |  3 +++
 src/2016/Day15.cpp            | 43 ++++++++++++++++++++++++++++-------
 src/common/lib/Helper.h       | 34 ++++++++++++++++++++-------
 3 files changed, 64 insertions(+), 16 deletions(-)

diff --git a/res/2016/day15/test_input.txt b/res/2016/day15/test_input.txt
index e69de29..e52aa7a 100644
--- a/res/2016/day15/test_input.txt
+++ b/res/2016/day15/test_input.txt
@@ -0,0 +1,3 @@
+Disc #1 has 5 positions; at time=0, it is at position 4.
+Disc #2 has 2 positions; at time=0, it is at position 1.
+Disc #2 has 3 positions; at time=0, it is at position 2.
diff --git a/src/2016/Day15.cpp b/src/2016/Day15.cpp
index f9c2032..8a1443b 100644
--- a/src/2016/Day15.cpp
+++ b/src/2016/Day15.cpp
@@ -2,30 +2,57 @@
 
 namespace y2016::day15
 {
-  REGISTER_DAY(2016, Day15, std::vector<int>, int);
+  struct Disc
+  {
+    int positions;
+    int position;
+  };
 
-  REGISTER_TEST_EXAMPLE(2016, Day15, ExampleInput, 1, 0);
-  REGISTER_TEST(2016, Day15, Input, 1, 0);
-  REGISTER_TEST_EXAMPLE(2016, Day15, ExampleInput, 2, 0);
-  REGISTER_TEST(2016, Day15, Input, 2, 0);
+  REGISTER_DAY(2016, Day15, std::vector<Disc>, int64_t);
+
+  REGISTER_TEST_EXAMPLE(2016, Day15, ExampleInput, 1, 25);
+  REGISTER_TEST(2016, Day15, Input, 1, 400589);
+  REGISTER_TEST_EXAMPLE(2016, Day15, ExampleInput, 2, 205);
+  REGISTER_TEST(2016, Day15, Input, 2, 3045959);
 
   READ_INPUT(input)
   {
-    std::vector<int> vec;
+    std::vector<Disc> vec;
     std::string str;
     while (getline(input, str))
     {
+      std::stringstream ss{str};
+      int i;
+      Disc disc;
+      ss >> "Disc #" >> i >> "has " >> disc.positions >> "positions; at time=" >> i >> ", it is at position " >>
+        disc.position;
+      vec.emplace_back(disc);
     }
     return vec;
   }
 
+  int64_t Solve(const std::vector<Disc>& discs)
+  {
+    std::vector<int64_t> starts;
+    std::vector<int64_t> mods;
+    std::vector<int64_t> remainders(discs.size(), 0);
+    for (int j = 0; j < discs.size(); j++)
+    {
+      starts.emplace_back(discs[j].position + j + 1);
+      mods.emplace_back(discs[j].positions);
+    }
+    return Helper::ChineseRemainderTheorem(mods, remainders, starts);
+  }
+
   OUTPUT1(input)
   {
-    return 0;
+    return Solve(input);
   }
 
   OUTPUT2(input)
   {
-    return 0;
+    auto discs = input;
+    discs.emplace_back(Disc{11, 0});
+    return Solve(discs);
   }
 }
diff --git a/src/common/lib/Helper.h b/src/common/lib/Helper.h
index 76af015..fba0459 100644
--- a/src/common/lib/Helper.h
+++ b/src/common/lib/Helper.h
@@ -600,12 +600,15 @@ struct Helper
     return Index2D{-1, -1};
   }
 
-  static int64_t ChineseRemainderTheoremTwo(int64_t mod1, int64_t mod2, int64_t res1, int64_t res2)
+  // Solve for N:
+  // remainder1 = N % mod1
+  // remainder2 = N % mod2
+  static int64_t ChineseRemainderTheoremTwo(int64_t mod1, int64_t mod2, int64_t remainder1, int64_t remainder2)
   {
-    int64_t i = res1;
+    int64_t i = remainder1;
     while (true)
     {
-      if (i % mod2 == res2)
+      if (i % mod2 == remainder2)
         return i;
 
       i += mod1;
@@ -613,17 +616,32 @@ struct Helper
     return 0;
   }
 
-  static int64_t ChineseRemainderTheorem(const std::vector<int64_t>& mods, const std::vector<int64_t>& results)
+  // Solve for N:
+  // remainders = N % mods
+  static int64_t ChineseRemainderTheorem(const std::vector<int64_t>& mods, const std::vector<int64_t>& remainders)
   {
     int64_t currentMod = mods.front();
-    int64_t currentRes = results.front();
+    int64_t currentRem = remainders.front();
     for (int i = 1; i < mods.size(); i++)
     {
-      int64_t result = ChineseRemainderTheoremTwo(currentMod, mods[i], currentRes, results[i]);
-      currentRes = result;
+      int64_t result = ChineseRemainderTheoremTwo(currentMod, mods[i], currentRem, remainders[i]);
+      currentRem = result;
       currentMod = currentMod * mods[i];
     }
-    return currentRes;
+    return currentRem;
+  }
+
+  // Solve for N:
+  // remainders = (start + N) % mods
+  static int64_t ChineseRemainderTheorem(const std::vector<int64_t>& mods,
+                                         std::vector<int64_t> remainders,
+                                         const std::vector<int64_t>& starts)
+  {
+    for (int i = 0; i < mods.size(); i++)
+    {
+      remainders[i] = ((remainders[i] - starts[i]) % mods[i] + mods[i]) % mods[i];
+    }
+    return ChineseRemainderTheorem(mods, remainders);
   }
 
   static uint64_t FastExponentiation(uint64_t start, uint64_t multiplication, uint64_t power, uint64_t mod)