/// Finds the unique number in a slice where every other element appears twice.

///

/// This function uses the XOR bitwise operation. Since XOR has the property that

/// `a ^ a = 0` and `a ^ 0 = a`, all paired numbers cancel out, leaving only the

/// unique number.

///

/// # Arguments

///

/// * `arr` - A slice of integers where all elements except one appear exactly twice

///

/// # Returns

///

/// * `Ok(i32)` - The unique number that appears only once

/// * `Err(String)` - An error message if the input is empty

///

/// # Examples

///

/// ```

/// # use the_algorithms_rust::bit_manipulation::find_unique_number;

/// assert_eq!(find_unique_number(&[1, 1, 2, 2, 3]).unwrap(), 3);

/// assert_eq!(find_unique_number(&[4, 5, 4, 6, 6]).unwrap(), 5);

/// assert_eq!(find_unique_number(&[7]).unwrap(), 7);

/// assert_eq!(find_unique_number(&[10, 20, 10]).unwrap(), 20);

/// assert!(find_unique_number(&[]).is_err());

/// ```

pub fn find_unique_number(arr: &[i32]) -> Result<i32, String> {

if arr.is_empty() {

return Err("input list must not be empty".to_string());

}

let result = arr.iter().fold(0, |acc, &num| acc ^ num);

Ok(result)

}

#[cfg(test)]

mod tests {

use super::*;

#[test]

fn test_basic_case() {

assert_eq!(find_unique_number(&[1, 1, 2, 2, 3]).unwrap(), 3);

}

#[test]

fn test_different_order() {

assert_eq!(find_unique_number(&[4, 5, 4, 6, 6]).unwrap(), 5);

}

#[test]

fn test_single_element() {

assert_eq!(find_unique_number(&[7]).unwrap(), 7);

}

#[test]

fn test_three_elements() {

assert_eq!(find_unique_number(&[10, 20, 10]).unwrap(), 20);

}

#[test]

fn test_empty_array() {

assert!(find_unique_number(&[]).is_err());

assert_eq!(

find_unique_number(&[]).unwrap_err(),

"input list must not be empty"

);

}

#[test]

fn test_negative_numbers() {

assert_eq!(find_unique_number(&[-1, -1, -2, -2, -3]).unwrap(), -3);

}

#[test]

fn test_large_numbers() {

assert_eq!(

find_unique_number(&[1000, 2000, 1000, 3000, 3000]).unwrap(),

2000

);

}

#[test]

fn test_zero() {

assert_eq!(find_unique_number(&[0, 1, 1]).unwrap(), 0);

}

}