How 'MOD' the binary operation works with floor function?

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I was looking for some example on MOD and floor function from Concrete Mathematics book(1). But, the solution of each example is very confusing for me.

The general formula of x mod y is;

x mod y = x - y(lfloor (x / y) rfloor) [for y is not equal to 0]

Here, lfloor means left floor and rfloor means right floor

I know, how it works when value of x and y is positive. But, I get confused when value of x or y or both becomes negative.

Examples from the book(1):

5 mod -3 = 5 - (-3)(lfloor (5 / (-3)) rfloor) = -1

-5 mod 3 = -5 -3(lfloor (-5 / 3) rfloor) = 1

-5 mod -3 = -5 - (-3)(lfloor (-5 / (-3)) rfloor) = -2

Resource:

(1) Book - Concrete Mathematics See page 95