fn flags(a: &[i64]) -> i64 {
    let size = a.len();
    let mut peaks = vec![false; size];
    for i in 1..size {
        let next = a.get(i + 1).copied().unwrap_or(0);
        peaks[i] = a[i - 1] < a[i] && a[i] > next;
    }

    let mut next_peak = vec![-1i64; size];
    if size > 0 {
        next_peak[size - 1] = -1;
        for i in (0..size - 1).rev() {
            next_peak[i] = if peaks[i] { i as i64 } else { next_peak[i + 1] };
        }
    }

    let mut i = 1i64;
    let mut result = 0i64;
    while i * (i - 1) <= size as i64 {
        let mut pos = 0i64;
        let mut num = 0i64;
        while pos < size as i64 && num < i {
            pos = next_peak[pos as usize];
            if pos == -1 {
                break;
            }
            num += 1;
            pos += i;
        }
        i += 1;
        result = result.max(num);
    }

    result
}