use v6.d;
enum opcode (
    halt => 99,
    add => 1,
    multiply => 2
);
class X::Halt is Exception {}

class IntcodeVM is export {
    has @.program;
    has @.memory;
    # Program counter
    has $!pc = 0;

    submethod new(Str $program) {
        my @program = $program.split(',').map(*.Int);
        self.bless(:@program, :memory(@program));
    }

    method run {
        loop {
            CATCH {
                when X::Halt { last }
            }
            self!run-next-command;
        }
    }

    method reset-memory {
        @!memory = @.program;
        $!pc = 0;
    }

    method !run-next-command {
        given self!peek-opcode {
            when halt {
                die X::Halt.new;
            }
            when add {
                self!add();
                $!pc += 4;
            }
            when multiply {
                self!multiply();
                $!pc += 4;
            }
            default { die "Unknwon opcode {self!peek-opcode()}"; }
        }
    }

    method !peek-opcode(--> Int) {
        die "$!pc exceeds memory {@!memory.elems}" if $!pc >= @!memory.elems;
        @!memory[$!pc]
    }

    method !program-chunk(Range $r) {
        @!memory[$r + $!pc]
    }

    method !add {
        my ($opcode, $mem1, $mem2, $memresult) = self!program-chunk(^4);
        @!memory[$memresult] =  @!memory[$mem1] + @!memory[$mem2];
    }

    method !multiply {
        my ($opcode, $mem1, $mem2, $memresult) = self!program-chunk(^4);
        @!memory[$memresult] =  @!memory[$mem1] * @!memory[$mem2];
    }

    method output {
        @!memory[0]
    }
}

sub try-inputs(IntcodeVM $vm, $noun, $verb) {
    $vm.reset-memory();
    $vm.memory[1] = $noun;
    $vm.memory[2] = $verb;
    $vm.run;
    say $vm.memory[^3];
    return $vm.memory[0];
}

#| First problem of day 2
sub day2a($input-filename) is export {
    my $program = slurp($input-filename);
    my $vm = IntcodeVM.new($program);
    # Replace bits
    $vm.memory[1] = 12;
    $vm.memory[2] = 2;
    $vm.run;

    say "Value at position 0: {$vm.output}"
}

sub day2b($input-filename) is export {
    my $program = slurp($input-filename);
    my $vm = IntcodeVM.new($program);

    for ((^100) X (^100)) -> $p {
        $vm.memory[1] = $p[0];
        $vm.memory[2] = $p[1];
        $vm.run;
        my $output = $vm.output;
        if $output == 19690720 {
            say "100 * noun + verb: {100 * $p[0] + $p[1]}";
            last;
        }
        $vm.reset-memory;
    }
}