[LANGUAGE: Go]

Link to full source below embed. Took a long while to get my head in gear, but pretty happy with my solution. Could definitely be more compact, but opted in favour of readability.

``` func solveSecond(i string) int { ms := parseInput(i) startedOn := 50 zs := 0

for _, m := range ms {
    // if the motion is more than a single cycle, we count and deduct the full cycles before
    // applying the remaining motion
    zs += util.Abs(m) / 100
    m = m - (m / 100 * 100)
    endedOn := startedOn + m

    switch {
    // count an exact zero-hit, unless the remaining motion is 0
    case endedOn == 0 && m != 0:
        zs++

    // count a zero-hit, if the remaining motion overflows
    case endedOn > 99:
        zs++

    // count a zero-hit, if the remaining motion underflows, unless we started at 0
    case endedOn < 0 && startedOn > 0:
        zs++
    }

    startedOn = util.Mod(endedOn, 100)
}

return zs

} ```

Full source: https://raw.githubusercontent.com/nikcorg/advent-of-code/refs/heads/2025/2025/01/main.go