Day 08 - Handheld Halting

• This puzzle is taken from: https://adventofcode.com/2020/day/8

Your flight to the major airline hub reaches cruising altitude without incident. While you consider checking the in-flight menu for one of those drinks that come with a little umbrella, you are interrupted by the kid sitting next to you.

Their handheld game console won't turn on! They ask if you can take a look.

You narrow the problem down to a strange infinite loop in the boot code (your puzzle input) of the device. You should be able to fix it, but first you need to be able to run the code in isolation.

The boot code is represented as a text file with one instruction per line of text. Each instruction consists of an operation (acc, jmp, or nop) and an argument (a signed number like +4 or -20).

• acc increases or decreases a single global value called the accumulator by the value given in the argument. For example, acc +7 would increase the accumulator by 7. The accumulator starts at 0. After an acc instruction, the instruction immediately below it is executed next.
• jmp jumps to a new instruction relative to itself. The next instruction to execute is found using the argument as an offset from the jmp instruction; for example, jmp +2 would skip the next instruction, jmp +1 would continue to the instruction immediately below it, and jmp -20 would cause the instruction 20 lines above to be executed next.
• nop stands for No OPeration - it does nothing. The instruction immediately below it is executed next.

For example, consider the following program:

nop +0
acc +1
jmp +4
acc +3
jmp -3
acc -99
acc +1
jmp -4
acc +6

These instructions are visited in this order:

nop +0  | 1
acc +1  | 2, 8(!)
jmp +4  | 3
acc +3  | 6
jmp -3  | 7
acc -99 |
acc +1  | 4
jmp -4  | 5
acc +6  |

First, the nop +0 does nothing. Then, the accumulator is increased from 0 to 1 (acc +1) and jmp +4 sets the next instruction to the other acc +1 near the bottom. After it increases the accumulator from 1 to 2, jmp -4 executes, setting the next instruction to the only acc +3. It sets the accumulator to 5, and jmp -3 causes the program to continue back at the first acc +1.

This is an infinite loop: with this sequence of jumps, the program will run forever. The moment the program tries to run any instruction a second time, you know it will never terminate.

Immediately before the program would run an instruction a second time, the value in the accumulator is 5.

Run your copy of the boot code. Immediately before any instruction is executed a second time, what value is in the accumulator?

Instruction is a grammar that parses the instruction & seperates operation from argument.

# Instruction grammar seperates the operation & argument.
grammar Instruction {
    rule TOP { <operation> <argument> }
    token operation { acc|jmp|nop }
    token argument { .* }
}

execute-from takes an index & executes instructions from that point. It returns the accumulator value. If the instruction has already been executed then it will stop & avoid infinite loop.

sub execute-from (
    @instructions, Int $idx --> Int
) {
    my Int $acc = 0;

    return $acc unless @instructions[$idx];
    my $entry = @instructions[$idx];

    $repeated++ if $entry<executed>;
    return $acc if $entry<executed>;

    if Instruction.parse($entry<instruction>) -> $match {
        $entry<executed> = 1;
        given $match<operation> {
            when 'acc' {
                $acc += $match<argument>;
                $acc += execute-from(@instructions, $idx + 1);
            }
            when 'jmp' {
                $acc += execute-from(@instructions, $idx + $match<argument>);
            }
            when 'nop' {
                $acc += execute-from(@instructions, $idx + 1);
            }
        }
    }
    return $acc;
}

Each instruction is pushed to @instructions along with executed value which indicates the number of time that instruction has been executed. It'll be useful to avoid infinite loops.

@instructions's 0th index is set to a dummy instruction to make it easier for execute-from to run the operations. The operations are written around instruction file which is numbered from 1. It's easier to add a dummy instruction than to make execute-from understand this.

sub MAIN (
    Int $part where * == 1|2 = 1 #= part to run (1 or 2)
) {
    # To match the numbers in instructions with array we just push an
    # element at 0th index.
    my @instructions = %(instruction => "", executed => 0),;

    for "input".IO.lines {
        push @instructions, %(
            instruction => $_.Str,
            executed => 0,
        );
    }

    my Int $acc;
    if $part == 1 {
        ...
    } elsif $part == 2 {
        ...
    }
    say "Part $part: ", $acc;
}

For part 1, we just call execute-from & print the $acc value.

$acc = execute-from(@instructions, 1) if $part == 1;