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Discussion on: Advent of Code 2020 Solution Megathread - Day 8: Handheld Halting

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Mike Gasparelli

Well this felt a lot easier than yesterday... Looking forward to see whether we're going to be expanding on this one in the coming days...

BootSequence

public record Instruction(string Operation, int Value);

public class BootSequence
    {
        List<int> history = new();

        public int Accumulator { get; private set; }

        public bool Run(Instruction[] instructions)
        {
            int i = 0;
            while(i < instructions.Length)
            {
                if (history.Contains(i))
                {
                    return false;
                }

                switch (instructions[i].Operation)
                {
                    case "nop":
                        history.Add(i);
                        i++;
                        break;
                    case "acc":
                        history.Add(i);
                        Accumulator += instructions[i].Value;
                        i++;
                        break;
                    case "jmp":
                        history.Add(i);
                        i += instructions[i].Value;
                        break;
                    default:
                        break;
                }
            }

            return true;
        }
    }
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Part1

public class Part1 : Puzzle<IEnumerable<Instruction>, int>
    {
        public override int SampleAnswer => 5;

        public override IEnumerable<Instruction> ParseInput(string rawInput)
            => rawInput
                .Split(Environment.NewLine)
                .Where(line => line.Length > 0)
                .Select(ParseInstruction);

        Instruction ParseInstruction(string line)
        {
            string op = line[..3];
            int.TryParse(line[4..], out int val);

            return new Instruction(op, val);
        }

        public override int Solve(IEnumerable<Instruction> input)
        {
            var bootSeq = new BootSequence();
            bootSeq.Run(input.ToArray());
            return bootSeq.Accumulator;
        }
    }
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Part 2

    public class Part2 : Part1
    {
        public override int SampleAnswer => 8;

        public override int Solve(IEnumerable<Instruction> input)
        {
            int count = input.Count();
            for (int i = 0; i < count; i++)
            {
                var aInput = input.ToArray();
                var bootSeq = new BootSequence();

                SwapOp(ref aInput[i]);

                if (bootSeq.Run(aInput))
                {
                    return bootSeq.Accumulator;
                };
            }

            throw new Exception("no solution found!");
        }

        private void SwapOp(ref Instruction instruction)
        {
            instruction = instruction with { Operation = instruction.Operation == "nop" ? "jmp" : "nop" };
        }
    }
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