Code shared from the Rust Playground

playground.rs

fn smart_8_to_5(n: u8) -> u8 {
    (((n as u32) * 31 + 30) / 255) as u8
}

fn dumb_8_to_5(n: u8) -> u8 {
    (((n as u32) * 31) / 255) as u8
}

fn just_5_to_8(n: u8) -> u8 {
    (((n as u32) * 255) / 31) as u8
}

fn balanced_5_to_8(n: u8) -> u8 {
    ((n as u32) * 8 + 3) as u8   
}

fn balanced_8_to_5(n: u8) -> u8 {
    n / 8
}

type F = fn(u8) -> u8;

#[derive(Debug, Copy, Clone)]
struct Sample {
    n8: u8,
    n5: u8,
    n8b: u8,
    n5b: u8,
}

impl Sample {
    fn se8(&self) -> f64 {
        (self.n8b as f64 - self.n8 as f64).powi(2)
    }
    
    fn se5(&self) -> f64 {
        (self.n5b as f64 - self.n5 as f64).powi(2)
    }
}

fn main() {

    for (name, f_8_to_5, f_5_to_8) in [
        ("dumb", dumb_8_to_5 as F, just_5_to_8 as F),
        ("smart", smart_8_to_5 as F, just_5_to_8 as F),
        ("balanced", balanced_8_to_5 as F, balanced_5_to_8 as F),
    ]
    .iter()
    {
        println!("{}", name);
        
        let mut last_n5: u8 = 255;
    
        let samples: Vec<Sample> = (0u8..=255u8).map(|n8| {
            let n5: u8 = f_8_to_5(n8);
            let n8b: u8 = f_5_to_8(n5);
            let n5b: u8 = f_8_to_5(n8b);
            
            let oops = n5 != n5b;

            if n5 != last_n5 {
                println!();
                last_n5 = n5;
            }

            println!(
                "{:4} {:3} -> {:2} -> {:3} -> {:2}",
                if oops { "oops" } else { "" },
                n8,
                n5,
                n8b,
                n5b
            );
            
            Sample {
                n8,
                n5,
                n8b,
                n5b,
            }
        }).collect();
        
        let sse8: f64 = samples.iter().map(Sample::se8).sum();
        let mse8: f64 = sse8 / samples.len() as f64;
        
        let sse5: f64 = samples.iter().map(Sample::se5).sum();
        let mse5: f64 = sse5 / samples.len() as f64;
        
        println!();
        
        println!("mse8 {:2.3}", mse8);
        println!("mse5 {:2.3}", mse5);
        
        println!();
    }
}