找到 0 和无符号整数与有符号整数之和之间的最大值

Question

I'm trying to write a function which will take an X position and an offset. This offset can be either negative or positive so I need it to be signed. So if I pass it X = 0 and ofs = -3 I need the function to set X to 0 , also, or if I pass it X = 0 , ofs = 6 and uppr_lmt = 5 I need it set it to 5

fn test(x: &mut u32, ofs: i32, uppr_lmt: u32) {
    use std::cmp::max;

    if ofs < 0 {
        x = max(0, x + ofs);
    } else {
        x = max(uppr_lmt, x + ofs);
    }
}

However, in this code rust complains about adding an i32 to an &mut u32 and 0 and uppr_lmt not being of type &mut u32 . How can I go about fixing this? I'm still fairly new to Rust and I'm not used to type conversions.

Answer 1

The below code is returning a u32 rather than mutating x value. Note that we are having extra conversions to i64 because in an i32, the -3 is going to be considered greater than 0 because of how negative numbers are stored (MSB is 1 for negatives while MSB is taken as 2 power for u32). Note that I have used min here for else case because otherwise, for 0,5,6 passed to function, it returns 6 because max of 5 and 6 is 6 which is not what you expected from your question

 fn test(x: u32, ofs: i32, uppr_lmt: u32) -> u32 {
        use std::cmp::{max,min};
        let y;
        if ofs < 0 {
            y = max(0, x as i64 + ofs as i64);
        } else {
            y = min(uppr_lmt as i64, x as i64 + ofs as i64);
        }
        y as u32
 }

Instead of i64, you can also use i32 like this assuming inputs are in i32 range and not u32 range (if the number plus offset is less than 2^31 - 1 only)

fn test(x: u32, ofs: i32, uppr_lmt: u32) -> u32 {
    use std::cmp::{max,min};
    let y;
    if ofs < 0 {
        y = max(0, x as i32+ ofs);
    } else {
        y = min(uppr_lmt as i32, x as i32+ ofs);
    }
    y as u32
}