Function to_index

pub fn to_index(len: usize, i: i64) -> usize

Converts an integer to a valid index within a bounded length.

For non-negative input i, it returns the minimum of i or len - 1, ensuring the result does not exceed the upper bound of the available length. For negative i, it computes an index by subtracting the negated value of i from the total length using saturating operations to avoid overflow, effectively wrapping around from the end of the array-like sequence. This function handles out-of-bounds indices gracefully, ensuring the resulting index always falls within the valid range of 0 to len - 1.