Function wrapping_geom_series 

pub fn wrapping_geom_series<T, N>(r: T, n: N) -> T
where
    T: PrimInt + Unsigned + ConstOne + ConstZero + WrappingMul + WrappingAdd + WrappingSub + AddAssign + MulAssign,
    N: PrimInt + Unsigned + ConstOne + ConstZero + BitAnd,

Calculate geometric series

That is, calculate the geometric series:

1 + r + r^2 + r^3 + … r^(n-1)

summed to n terms, with the natural modulo of the unsigned integer type T (ie, with wrapping).

It makes use of the fact that the series can pair up terms:

(1 + r) + (1 + r) r^2 + (1 + r) r^4 + … + (1 + r) (r^2)^(n/2-1) + [ r^(n-1) if n is odd ] (1 + r) (1 + r^2 + r^4 + … + (r^2)^(n/2-1)) + [ r^(n-1) if n is odd ]

Which can easily be calculated by recursion, with time order O(log n), and stack depth O(log n). However that stack depth isn’t good, so a non-recursive implementation is preferable. This implementation is by a loop, not recursion, with time order O(log n) and stack depth O(1).

use simplerandom::maths::wrapping_geom_series;
let result = wrapping_geom_series(12345_u32, 1500000_u32);
assert_eq!(result, 57634016_u32);