I made some modifications to the guessing game from the Rust book and attempted to use binary search to give the player a hint about where the correct answer might be. In my first approach, I created three immutable variables—start, end, and hint. My intention was for the binary search function to return updated values for these three variables in each iteration, which I would then reassign to the original variables using shadowing.

However, I noticed that after the first iteration, the values stopped updating. I eventually resolved the issue by making the variables mutable when initializing them and updating them directly within the binary search function instead of reassigning them.

I want to understand why my initial version, which used immutable variables and shadowing, did not work as expected.

use std::{cmp::Ordering, io};
use rand::Rng;

fn bisect(num: &i32, start: i32, end: i32) -> (i32, i32, String){
    let mid  = (start + end) / 2;
    match mid.cmp(num){
        Ordering::Equal => return (start, mid, String::from(format!("Try Again: The correct answer is between {start} and {mid}"))),
        Ordering::Greater => return (start, mid, String::from(format!("Try Again: The correct answer is between {start} and {mid}"))),
        Ordering::Less => return (mid, end, String::from(format!("Try Again: The correct answer is between {mid} and {end}")))
    }
}

fn main() {
    // use this
    let (mut start, mut end, mut hint) = (1, 100, String::from("Enter a number between zero and hundred"));
    /*
     this won't work
     let (start, end, hint) = (1, 100, String::from("Enter a number between zero and hundred")); 
    */
    
    println!("Welcome to guess the number: {hint}");
    let secret_number: i32 = rand::rng().random_range(start..=end);
    loop{
        let mut guess = String::new();
        io::stdin()
        .read_line(&mut guess)
        .expect("Failed to readline");
        let guess: i32 = match guess.trim().parse(){
            Ok(num) => num,
            Err(_) => continue
        };
        // use this 
        (start, end, hint) = bisect(&secret_number, start, end);
        /* 
        this won't work
        let (start, end, hint) = bisect(&secret_number, start, end);
        */
        
        match guess.cmp(&secret_number){
            Ordering::Equal => {println!("{guess} is the correct answer"); break},
            Ordering::Greater => println!("{hint}"),
            Ordering::Less => println!("{hint}"),
        }
        if (end - start) <= 1{
            println!("Game Over! You couldn't take the hint");
            break;
        }
    }
    
  
}

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