Function parallel_map 

pub fn parallel_map<T, R>(
    items: &[T],
    f: impl Fn(&T) -> R + Send + Sync + 'static,
) -> Vec<R>
where
    T: Send + Sync + Clone + 'static,
    R: Send + 'static,

Apply f to every item in items in parallel and return the results.

Uses std::thread::spawn to fan out work across chunks, then collects results in the original order.

Type constraints

• T: Send + Sync – items must be shareable across threads.
• R: Send – results must be sendable back to the main thread.
• f: Fn(&T) -> R + Send + Sync – the closure must be thread-safe.