Is there a standard way of cyclically rotating mutable variables in Rust? - rust

Question

A very common operation in implementing algorithms is the cyclic rotate: given, say, 3 variables a, b, c change them to the effect of
t ⇽ c
c ⇽ b
b ⇽ a
a ⇽ t
Given that everything is bitwise swappable, cyclic rotation should be an area where Rust excels more than any other language I know of.
For comparison, in C++ the most efficient generic way to rotate N elements is performing n+1 std::move operations, which in turn roughly leads to (for a typical move constructor implementation) 3 (n+1) sizeof(T) word assignments (this can be improved for PODs via template specializing rotate, but requires work).
In Rust, the language makes it possible to implement rotate with only (n+1) size_of(T) word assignments. To my surprise, I could not find standard library support for rotation. (No rotate method in std::mem). It would probably look like this:
pub fn rotate<T>(x: &mut T, y: &mut T, z: &mut T) {
unsafe {
let mut t: T = std::mem::uninitialized();
std::ptr::copy_nonoverlapping(&*z, &mut t, 1);
std::ptr::copy_nonoverlapping(&*y, z, 1);
std::ptr::copy_nonoverlapping(&*x, y, 1);
std::ptr::copy_nonoverlapping(&t, x, 1);
std::mem::forget(t);
}
}
For clarification on why rotation cannot be implemented efficiently in C++, consider:
struct String {
char *data1;
char *data2;
String(String &&other) : data1(other.data1), data2(other.data2)
{ other.data1 = other.data2 = nullptr;}
String &operator=(String &&other)
{ std::swap(data1, other.data1); std::swap(data2, other.data2);
return *this; }
~String() { delete [] data1; delete [] data2; }
};
Here an operation like s2 = std::move(s1); will take 3 pointer assignments for each member field, totaling to 6 assignments since pointer swap requires 3 assignments (1 into temp, 1 out of temp, one across operands)

Answer 1

Is there a standard way of cyclically rotating mutable variables in Rust?
No.
I'd just swap the variables twice, no need for unsafe:
use std::mem;
pub fn rotate<T>(x: &mut T, y: &mut T, z: &mut T) {
mem::swap(x, y);
mem::swap(y, z);
}
fn main() {
let mut a = 1;
let mut b = 2;
let mut c = 3;
println!("{}, {}, {}", a, b, c);
// 1, 2, 3
rotate(&mut a, &mut b, &mut c);
println!("{}, {}, {}", a, b, c);
// 2, 3, 1
}
This produces 7 movl instructions (Rust 1.35.0, Release, x86_64, Linux)
playground::rotate:
movl (%rdi), %eax
movl (%rsi), %ecx
movl %ecx, (%rdi)
movl %eax, (%rsi)
movl (%rdx), %ecx
movl %ecx, (%rsi)
movl %eax, (%rdx)
retq
As opposed to the original 6 movl instructions:
playground::rotate_original:
movl (%rdx), %eax
movl (%rsi), %ecx
movl %ecx, (%rdx)
movl (%rdi), %ecx
movl %ecx, (%rsi)
movl %eax, (%rdi)
retq
I'm OK giving up that single instruction for purely safe code that is also easier to reason about.
In "real" code, I'd make use of the fact that all the variables are the same type and that slice::rotate_left and slice::rotate_right exist:
fn main() {
let mut vals = [1, 2, 3];
let [a, b, c] = &vals;
println!("{}, {}, {}", a, b, c);
// 1, 2, 3
vals.rotate_left(1);
let [a, b, c] = &vals;
println!("{}, {}, {}", a, b, c);
// 2, 3, 1
}