From a56cc8016912385f41584ff0c0f88f491705d24a Mon Sep 17 00:00:00 2001
From: Thraix <thraix@hotmail.com>
Date: Fri, 4 Sep 2026 20:54:17 +0200
Subject: [PATCH] Add solution for 2016 Day 25

---
 src/2016/Day25.cpp | 180 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 173 insertions(+), 7 deletions(-)

diff --git a/src/2016/Day25.cpp b/src/2016/Day25.cpp
index dc4a26b..61e7f96 100644
--- a/src/2016/Day25.cpp
+++ b/src/2016/Day25.cpp
@@ -2,30 +2,196 @@
 
 namespace y2016::day25
 {
-  REGISTER_DAY(2016, Day25, std::vector<int>, int);
+  enum class Operator
+  {
+    Cpy,
+    Inc,
+    Dec,
+    Jnz,
+    Out,
+  };
 
-  REGISTER_TEST_EXAMPLE(2016, Day25, ExampleInput, 1, 0);
-  REGISTER_TEST(2016, Day25, Input, 1, 0);
-  REGISTER_TEST_EXAMPLE(2016, Day25, ExampleInput, 2, 0);
+  struct Instruction
+  {
+    Operator op;
+    std::variant<char, int> arg1;
+    std::variant<char, int> arg2;
+  };
+
+  REGISTER_DAY(2016, Day25, std::vector<Instruction>, int);
+
+  REGISTER_TEST(2016, Day25, Input, 1, 182);
   REGISTER_TEST(2016, Day25, Input, 2, 0);
 
+  std::variant<char, int> ReadArg(const std::string& str, int arg)
+  {
+    size_t pos = str.find(' ');
+    int i = 0;
+    while (i < arg)
+    {
+      pos = str.find(' ', pos + 1);
+      i++;
+    }
+
+    if (str[pos + 1] >= 'a' && str[pos + 1] <= 'd')
+      return str[pos + 1];
+
+    return (int)std::atoi(str.c_str() + pos + 1);
+  }
+
   READ_INPUT(input)
   {
-    std::vector<int> vec;
+    std::vector<Instruction> vec;
     std::string str;
     while (getline(input, str))
     {
+      Instruction instruction;
+      if (str[0] == 'c')
+      {
+        instruction.op = Operator::Cpy;
+        instruction.arg1 = ReadArg(str, 0);
+        instruction.arg2 = ReadArg(str, 1);
+      }
+      else if (str[0] == 'i')
+      {
+        instruction.op = Operator::Inc;
+        instruction.arg1 = ReadArg(str, 0);
+      }
+      else if (str[0] == 'd')
+      {
+        instruction.op = Operator::Dec;
+        instruction.arg1 = ReadArg(str, 0);
+      }
+      else if (str[0] == 'j')
+      {
+        instruction.op = Operator::Jnz;
+        instruction.arg1 = ReadArg(str, 0);
+        instruction.arg2 = ReadArg(str, 1);
+      }
+      else if (str[0] == 'o')
+      {
+        instruction.op = Operator::Out;
+        instruction.arg1 = ReadArg(str, 0);
+      }
+      vec.emplace_back(instruction);
     }
     return vec;
   }
 
+  void Toggle(Instruction& instruction)
+  {
+    switch (instruction.op)
+    {
+      case Operator::Cpy:
+        instruction.op = Operator::Jnz;
+        break;
+      case Operator::Inc:
+        instruction.op = Operator::Dec;
+        break;
+      case Operator::Dec:
+        instruction.op = Operator::Inc;
+        break;
+      case Operator::Jnz:
+        instruction.op = Operator::Cpy;
+        break;
+      case Operator::Out:
+        instruction.op = Operator::Inc;
+        break;
+    }
+  }
+
+  bool IsValid(const Instruction& instruction)
+  {
+    switch (instruction.op)
+    {
+      case Operator::Cpy:
+        return std::holds_alternative<char>(instruction.arg2);
+      case Operator::Inc:
+        return std::holds_alternative<char>(instruction.arg1);
+      case Operator::Dec:
+        return std::holds_alternative<char>(instruction.arg1);
+      case Operator::Jnz:
+        return true;
+      case Operator::Out:
+        return true;
+    }
+    return false;
+  }
+
+  int GetRegisterIndex(const std::variant<char, int>& c)
+  {
+    return std::get<char>(c) - 'a';
+  }
+
+  int GetValue(const std::vector<int>& registers, const std::variant<char, int>& data)
+  {
+    if (std::holds_alternative<int>(data))
+      return std::get<int>(data);
+    else
+      return registers[GetRegisterIndex(data)];
+  }
+
+  bool Run(std::vector<Instruction> instructions, std::vector<int> registers)
+  {
+    int count = 0;
+    for (size_t pc = 0; pc < instructions.size(); pc++)
+    {
+      // From analysis of the input, it loops the same output every time we get to pc = 8.
+      // As such, if we get there, and count is > 0, we know that the signal alternates, since we've already returned
+      // false if the output to this point is not alternating
+      if (pc == 8 && count > 0)
+        return true;
+      switch (instructions[pc].op)
+      {
+        case Operator::Cpy:
+        {
+          int number = GetValue(registers, instructions[pc].arg1);
+          registers[GetRegisterIndex(instructions[pc].arg2)] = number;
+          break;
+        }
+        case Operator::Inc:
+        {
+          registers[GetRegisterIndex(instructions[pc].arg1)]++;
+          break;
+        }
+        case Operator::Dec:
+        {
+          registers[GetRegisterIndex(instructions[pc].arg1)]--;
+          break;
+        }
+        case Operator::Jnz:
+        {
+          int value = GetValue(registers, instructions[pc].arg1);
+          if (value != 0)
+            pc += GetValue(registers, instructions[pc].arg2) - 1;
+          break;
+        }
+        case Operator::Out:
+        {
+          if (GetValue(registers, instructions[pc].arg1) != count % 2)
+            return false;
+          count++;
+        }
+        break;
+      }
+    }
+    return false;
+  }
+
   OUTPUT1(input)
   {
-    return 0;
+    int i = -1;
+    while (!Run(input, std::vector<int>{++i, 0, 0, 0}))
+      ;
+    return i;
   }
 
   OUTPUT2(input)
   {
-    return 0;
+    std::cout << "  ___                      ___" << std::endl;
+    std::cout << " (o o)                    (o o)" << std::endl;
+    std::cout << "(  V  ) MERRY CHRISTMAS! (  V  )" << std::endl;
+    std::cout << "--m-m----------------------m-m--" << std::endl;
+    return Run(input, std::vector<int>{12, 0, 0, 0});
   }
 }