long straightforward(long n) {
    return (0..n).sum(Long.&bitCount)
}
assert (0..10).collect(this.&straightforward) == [0, 1, 2, 4, 5, 7, 9, 12, 13, 15, 17]
 
long separateDigits(long n) {
    ++n
    long acc = 0
    long mask = 0
    for (long scanbit = 1; scanbit < n; scanbit <<= 1) {
        mask |= scanbit
        long remainder = n & mask
        acc += (n - remainder) >> 1
        if (n & scanbit)
            acc += remainder - scanbit
    }
    return acc
}
(0..100).each { assert separateDigits(it) == straightforward(it) }

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