Our goal is to turn our letter through ninety degrees. As explained in the background chapter we are going to do this by rotating the box. This way the picture will be drawn turned automatically.

Here we see our goal box on the right, and the reference box on the left. Let's figure out the relations between the new box fields a, b and c and the reference box fields a, b, c.

Notice that for the target box the a field is the sum of the original box a and b field.

The new b is the original c and the new c is the original b negated.

Open src/canvas/mod.rs and find the turn_box function. At the moment it is implemented by calling the identity function on its argument

identity(&bx)

We need to change that and return a turned box. Box has a constructor called new. When provided with three Vector arguments it will a create a box with the corresponding fields.

Take a look at the documentation for Vector in order to figure out how to do vector addition and vector negation.

Having done that, try to implement the turn_box function correctly. You can test your function by running geometry binary.

cargo run --bin geometry

This should produce an output.svg and if you implemented turn_box correctly should show the letter d turned.