// here the FnMut is being returned. So the vars (zz and dd)
// need to live longer than the stack frame that contains them
// to get this, "move" copies the variables from the stack to the
// heap.
fn main_2() {
    let mut r = make_fn(2);
    println!("{}", r());
    println!("{}", r());
    println!("{}", r());
 }
 fn make_fn(inp:i32) -> impl FnMut() -> i32 {
    let mut zz = vec![inp];
    let mut dd = inp + 4;
    let zp: u64 = &zz as *const Vec<i32> as u64;
    println!("zp (stack) {zp}");
    let f = move || { 
        zz.push(dd);
        dd += 2;
        let zp: u64 = &zz as *const Vec<i32> as u64;
        println!("zp (heap) {zp}   {:?}", zz);
        return zz.len() as i32;
    };
    //println!("zz {:?}", zz);
    f
 }




// create a FnMut and pass it to another function
// can pass the closure here because the function in which the 
// the closed variables exists remains on the stack, so those vars
// exist at least as long as the function
fn main_1() {
    let mut x: usize = 1;
    println!("Before {x}");
    let add_two_to_x = || x += 2;
    do_repeatedly(add_two_to_x, 5);
    println!("After  {x}");
}

fn do_repeatedly(mut func: impl FnMut(), count:usize)
{
    for i in 0..count {
        func();
    }
}

fn main() {
    main_2();
}