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Day 17

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Part 2 was a doozy. I sought some help online for understanding what the program was doing and how to feed it an input to get the output we were after.
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Chris Pickett
2024-12-17 10:13:16 -06:00
parent 3c4ce451b9
commit 12b9f7fdf5
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using System.Diagnostics;
using System.Numerics;
namespace aoc2024;
internal class Day17 : Day
{
[DebuggerDisplay("A={A}, B={B}, C={C}")]
class Registers
{
public long A = 0;
public long B = 0;
public long C = 0;
public Registers()
{
}
public Registers(long a, long b, long c)
{
A = a;
B = b;
C = c;
}
public Registers(Registers other)
{
A = other.A;
B = other.B;
C = other.C;
}
}
class ComputeResult
{
public int? InstructionPointer;
public int? Output;
}
private readonly Registers InitRegisters = new();
private int[] Program = [];
private int[] outputScratch = [];
internal override void Parse()
{
var lines = Util.Parsing.ReadAllLines($"{GetDay()}").ToList();
InitRegisters.A = int.Parse(lines[0].Split(' ')[2]);
InitRegisters.B = int.Parse(lines[1].Split(' ')[2]);
InitRegisters.C = int.Parse(lines[2].Split(' ')[2]);
Program = lines[4].Split(' ')[1].Split(',').Select(int.Parse).ToArray();
outputScratch = new int[Program.Length];
RunTests();
}
private void RunTests()
{
{
var r = new Registers{C = 9};
int[] program = [2, 6];
RunProgram(program, r);
Debug.Assert(r.B == 1);
r = new Registers{C = 9};
RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(r.B == 1);
}
{
var r = new Registers{A = 10};
int[] program = [5, 0, 5, 1, 5, 4];
var output = RunProgram(program, r);
Debug.Assert(output.SequenceEqual([0, 1, 2]));
r = new Registers{A = 10};
var o2 = RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(o2.SequenceEqual([0,1,2]));
}
{
var r = new Registers{A = 2024};
int[] program = [0,1,5,4,3,0];
var output = RunProgram(program, r);
Debug.Assert(output.SequenceEqual([4,2,5,6,7,7,7,7,3,1,0]));
Debug.Assert(r.A == 0);
r = new Registers{A = 2024};
var o2 = RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(o2.SequenceEqual([4,2,5,6,7,7,7,7,3,1,0]));
Debug.Assert(r.A == 0);
}
{
var r = new Registers{B = 29};
int[] program = [1,7];
RunProgram(program, r);
Debug.Assert(r.B == 26);
r = new Registers{B = 29};
RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(r.B == 26);
}
{
var r = new Registers{B = 2024, C = 43690};
int[] program = [4,0];
RunProgram(program, r);
Debug.Assert(r.B == 44354);
r = new Registers{B = 2024, C = 43690};
RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(r.B == 44354);
}
{
var r = new Registers{A = 117440};
int[] program = [0,3,5,4,3,0];
var output = RunProgram(program, r);
Debug.Assert(output.SequenceEqual([0,3,5,4,3,0]));
r = new Registers{A = 117440};
var o2 = RunProgramOptimized(program, ref r.A, ref r.B, ref r.C);
Debug.Assert(o2.SequenceEqual([0,3,5,4,3,0]));
}
}
private static long GetCombo(int operand, Registers registers)
{
return operand switch
{
0 or 1 or 2 or 3 => operand,
4 => registers.A,
5 => registers.B,
6 => registers.C,
_ => throw new Exception("invalid operand value for combo")
};
}
private static ComputeResult? Compute(int instruction, int operand, Registers registers)
{
switch (instruction)
{
// adv
case 0:
{
div(operand, registers, out registers.A);
break;
}
// bdv
case 6:
{
div(operand, registers, out registers.B);
break;
}
// cdv
case 7:
{
div(operand, registers, out registers.C);
break;
}
// bxl
case 1:
{
var v1 = registers.B;
var v2 = operand;
var result = v1 ^ v2;
registers.B = result;
break;
}
// bst
case 2:
{
var v1 = GetCombo(operand, registers);
var result = v1 % 8;
registers.B = result;
break;
}
// jnz
case 3:
{
if (registers.A == 0)
{
break;
}
return new ComputeResult {InstructionPointer = operand};
}
// bxc
case 4:
{
var v1 = registers.B;
var v2 = registers.C;
var result = v1 ^ v2;
registers.B = result;
break;
}
// out
case 5:
{
var v1 = GetCombo(operand, registers);
var result = v1 % 8;
var truncated = (int) result;
return new ComputeResult {Output = truncated};
}
}
return null;
static void div(int op, Registers regs, out long store)
{
var numer = regs.A;
var denom = BigInteger.Pow(2, (int)GetCombo(op, regs));
var result = numer / denom;
int truncated = (int)(result % int.MaxValue);
store = truncated;
}
}
private static List<int> RunProgram(int[] program, Registers registers)
{
List<int> output = [];
int instructionPointer = 0;
while (instructionPointer >= 0 && instructionPointer < program.Length - 1)
{
int instruction = program[instructionPointer];
int operand = program[instructionPointer + 1];
var result = Compute(instruction, operand, registers);
if (result?.InstructionPointer.HasValue == true)
{
instructionPointer = result.InstructionPointer.Value;
}
else
{
instructionPointer += 2;
}
if (result?.Output != null)
{
output.Add(result.Output.Value);
}
}
return output;
}
internal override string Part1()
{
var registers = new Registers(InitRegisters);
var output = RunProgram(Program, registers);
return $"Output: <+white>{string.Join(',', output)}";
}
private ReadOnlySpan<int> RunProgramOptimized(ReadOnlySpan<int> program, ref long a, ref long b, ref long c)
{
int instructionPointer = 0;
int lengthCurr = 0;
var val = new[] { 0, 1, 2, 3, a, b, c, 0 };
while (instructionPointer < program.Length)
{
(a, b, c) = (val[4], val[5], val[6]);
var instruction = program[instructionPointer];
var operand = program[instructionPointer + 1];
(instructionPointer, var result) = (instruction != 3 || a == 0 ? instructionPointer + 2 : operand, (int)val[operand]);
(val[4], val[5], val[6]) = instruction switch
{
0 => (a >> result, b, c),
1 => (a, b ^ operand, c),
2 => (a, val[operand] & 7, c),
4 => (a, b ^ c, c),
6 => (a, a >> result, c),
7 => (a, b, a >> result),
_ => (a, b, c)
};
if (instruction == 5)
{
outputScratch[lengthCurr++] = result & 7;
}
}
(a, b, c) = (val[4], val[5], val[6]);
return outputScratch.AsSpan()[..lengthCurr];
}
internal override string Part2()
{
long a = 0;
for (int i = 0; i < Program.Length; i++)
{
for (a <<= 3;; ++a)
{
long regA = a;
long regB = 0;
long regC = 0;
var outputSpan = RunProgramOptimized(Program, ref regA, ref regB, ref regC);
if (outputSpan[..(i + 1)].SequenceEqual(Program.AsSpan()[^(i + 1)..]))
{
break;
}
}
}
return $"Value of A to repeat the program: <+white>{a}";
}
}