duct-tape | little Java 8 library providing an implementation

but what is the purpose of scanf then?

An excellent question.

Is it simply an useless broken function that should never be used?

It is almost useless. It is, arguably, quite broken. It should almost never be used.

Why is it in the libraries to begin with then?

My personal belief is that it was an experiment. It tries to be the opposite of printf. But that turned out not to be such a good idea in practice, and the function never got used very much, and pretty much fell out of favor, except for one particular use case...

This seems really absurd, especially considering all beginners are taught to use scanf...

You're absolutely right. It is really quite absurd.

There's a reason all beginners are taught to use scanf. During week 1 of your first C programming class, you might write the little program

ReadOnlySpan could work.

Have a look at All About Span: Exploring a New .NET Mainstay

A second variant of Span, called System.ReadOnlySpan, enables read-only access. This type is just like Span, except its indexer takes advantage of a new C# 7.2 feature to return a “ref readonly T” instead of a “ref T,” enabling it to work with immutable data types like System.String. ReadOnlySpan makes it very efficient to slice strings without allocating or copying, as shown here:

You can solve the issue by adding the following exclusion in the dependencies (of pom.xml) that caused conflict.

There is no need for any looping at all here. You have effectively two different line_mask functions. Neither needs to be looped explicitly, but you would probably get a significant speedup just from rewriting it with a pair of for loops in an if and else, rather than an if and else in a for loop, which gets evaluated many many times.

The really numpythonic thing to do is to properly vectorize your code to operate on entire arrays without any loops. Here is a vectorized version of line_mask: