这是求 $1 \sim n$ 的数位之和的代码，但是很神奇看不懂，求大佬解答一下

#include<bits/stdc++.h>
using namespace std;
int main() {
	auto f2 = []( unsigned n ) -> unsigned {
		register unsigned tot = 0;
		while( n ) {
			tot += n % 10;
			n /= 10;
		}
		return tot;
	};
	unsigned n;
	cin >> n;
	vector<unsigned> tot( n );
	for( unsigned i = 0, j; i < n; ++i ) {
		cin >> j;
		for( register unsigned k = 1; k <= j; ++k )
			tot[ i ] += f2( k );
	}
	for( register unsigned i = 0; i < n; ++i )
		cout << tot[ i ] << endl;
}