Module day23

Coprocessor Conflagration

Just like Day 18 reverse engineering the code is essential. The entire input can be reduced to only the very first number.

    set b $NUMBER       if a == 0 {
    set c b                 b = $NUMBER;
    jnz a 2                 c = b;
    jnz 1 5             } else {
    mul b 100               b = 100000 + 100 * $NUMBER;
    sub b -100000           c = b + 17000;
    set c b             }
    sub c -17000
    set f 1             for b in (b..=c).step_by(17) {
    set d 2                 f = 1;
    set e 2                 for d in 2..b {
    set g d                     for e in 2..b {
    mul g e                         if d * e == b {
    sub g b                             f = 0;
    jnz g 2                         }
    set f 0
    sub e -1
    set g e
    sub g b
    jnz g -8                    }
    sub d -1
    set g d
    sub g b
    jnz g -13               }
    jnz f 2
    sub h -1                if f == 0 {
    set g b                     h += 1;
    sub g c                 }
    jnz g 2
    jnz 1 3
    sub b -17
    jnz 1 -23           }

Part One

The number of mul operations is the product of the two inner loops from 2 to n exclusive.

Part Two

Counts the number of composite numbers starting from 100,000 + 100 * n checking the next 1,000 numbers in steps of 17. The raw code take O(n²) complexity for each number so emulating this directly would take at least 10⁵.10⁵.10³ = 10¹³ = 10,000,000,000,000 steps.

Functions

• composite 🔒
  Simple prime number check of all factors from 2 to √n inclusive.
• parse
  We only need the vrey first number from the input.
• part1
  The number of mul operations is (n - 2)²
• part2
  Count the number of composite numbers in a range calculated from the input number.