Add two's complement implementation (TheAlgorithms#968)

Author: AliAlimohammadi

DIRECTORY.md

@@ -24,6 +24,7 @@
     * [Reverse Bits](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/reverse_bits.rs)
     * [Sum of Two Integers](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/sum_of_two_integers.rs)
     * [Swap Odd and Even Bits](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/swap_odd_even_bits.rs)
+    * [Two's Complement](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/twos_complement.rs)
   * Ciphers
     * [AES](https://github.com/TheAlgorithms/Rust/blob/master/src/ciphers/aes.rs)
     * [Another ROT13](https://github.com/TheAlgorithms/Rust/blob/master/src/ciphers/another_rot13.rs)

src/bit_manipulation/mod.rs

@@ -5,11 +5,13 @@ mod n_bits_gray_code;
 mod reverse_bits;
 mod sum_of_two_integers;
 mod swap_odd_even_bits;
+mod twos_complement;
 
-pub use binary_coded_decimal::binary_coded_decimal;
-pub use counting_bits::count_set_bits;
-pub use highest_set_bit::find_highest_set_bit;
-pub use n_bits_gray_code::generate_gray_code;
-pub use reverse_bits::reverse_bits;
-pub use sum_of_two_integers::add_two_integers;
-pub use swap_odd_even_bits::swap_odd_even_bits;
+pub use self::binary_coded_decimal::binary_coded_decimal;
+pub use self::counting_bits::count_set_bits;
+pub use self::highest_set_bit::find_highest_set_bit;
+pub use self::n_bits_gray_code::generate_gray_code;
+pub use self::reverse_bits::reverse_bits;
+pub use self::sum_of_two_integers::add_two_integers;
+pub use self::swap_odd_even_bits::swap_odd_even_bits;
+pub use self::twos_complement::twos_complement;

src/bit_manipulation/twos_complement.rs

@@ -0,0 +1,137 @@
+//! Two's Complement Representation
+//!
+//! Two's complement is a mathematical operation on binary numbers and a binary signed
+//! number representation. It is widely used in computing as the most common method of
+//! representing signed integers on computers.
+//!
+//! For more information: <https://en.wikipedia.org/wiki/Two%27s_complement>
+
+/// Takes a negative integer and returns its two's complement binary representation.
+///
+/// The two's complement of a negative number is calculated by finding the binary
+/// representation that, when added to the positive value with the same magnitude,
+/// equals 2^n (where n is the number of bits).
+///
+/// # Arguments
+///
+/// * `number` - A non-positive integer (0 or negative)
+///
+/// # Returns
+///
+/// A `Result` containing:
+/// - `Ok(String)` - The two's complement representation with "0b" prefix
+/// - `Err(String)` - An error message if the input is positive
+///
+/// # Examples
+///
+/// ```
+/// use the_algorithms_rust::bit_manipulation::twos_complement;
+///
+/// assert_eq!(twos_complement(0).unwrap(), "0b0");
+/// assert_eq!(twos_complement(-1).unwrap(), "0b11");
+/// assert_eq!(twos_complement(-5).unwrap(), "0b1011");
+/// assert_eq!(twos_complement(-17).unwrap(), "0b101111");
+/// assert_eq!(twos_complement(-207).unwrap(), "0b100110001");
+///
+/// // Positive numbers return an error
+/// assert!(twos_complement(1).is_err());
+/// ```
+///
+/// # Errors
+///
+/// Returns an error if the input number is positive.
+pub fn twos_complement(number: i32) -> Result<String, String> {
+    if number > 0 {
+        return Err("input must be a negative integer".to_string());
+    }
+
+    if number == 0 {
+        return Ok("0b0".to_string());
+    }
+
+    // Calculate the number of bits needed for the binary representation
+    // (excluding the sign bit in the original representation)
+    let binary_number_length = format!("{:b}", number.abs()).len();
+
+    // Calculate two's complement value
+    // This is equivalent to: abs(number) - 2^binary_number_length
+    let twos_complement_value = (number.abs() as i64) - (1_i64 << binary_number_length);
+
+    // Format as binary string (removing the negative sign)
+    let mut twos_complement_str = format!("{:b}", twos_complement_value.abs());
+
+    // Add leading zeros if necessary
+    let padding_zeros = binary_number_length.saturating_sub(twos_complement_str.len());
+    if padding_zeros > 0 {
+        twos_complement_str = format!("{}{twos_complement_str}", "0".repeat(padding_zeros));
+    }
+
+    // Add leading '1' to indicate negative number in two's complement
+    Ok(format!("0b1{twos_complement_str}"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_zero() {
+        assert_eq!(twos_complement(0).unwrap(), "0b0");
+    }
+
+    #[test]
+    fn test_negative_one() {
+        assert_eq!(twos_complement(-1).unwrap(), "0b11");
+    }
+
+    #[test]
+    fn test_negative_five() {
+        assert_eq!(twos_complement(-5).unwrap(), "0b1011");
+    }
+
+    #[test]
+    fn test_negative_seventeen() {
+        assert_eq!(twos_complement(-17).unwrap(), "0b101111");
+    }
+
+    #[test]
+    fn test_negative_two_hundred_seven() {
+        assert_eq!(twos_complement(-207).unwrap(), "0b100110001");
+    }
+
+    #[test]
+    fn test_negative_small_values() {
+        assert_eq!(twos_complement(-2).unwrap(), "0b110");
+        assert_eq!(twos_complement(-3).unwrap(), "0b101");
+        assert_eq!(twos_complement(-4).unwrap(), "0b1100");
+    }
+
+    #[test]
+    fn test_negative_larger_values() {
+        assert_eq!(twos_complement(-128).unwrap(), "0b110000000");
+        assert_eq!(twos_complement(-255).unwrap(), "0b100000001");
+        assert_eq!(twos_complement(-1000).unwrap(), "0b10000011000");
+    }
+
+    #[test]
+    fn test_positive_number_returns_error() {
+        let result = twos_complement(1);
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "input must be a negative integer");
+    }
+
+    #[test]
+    fn test_large_positive_number_returns_error() {
+        let result = twos_complement(100);
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "input must be a negative integer");
+    }
+
+    #[test]
+    fn test_edge_case_negative_powers_of_two() {
+        assert_eq!(twos_complement(-8).unwrap(), "0b11000");
+        assert_eq!(twos_complement(-16).unwrap(), "0b110000");
+        assert_eq!(twos_complement(-32).unwrap(), "0b1100000");
+        assert_eq!(twos_complement(-64).unwrap(), "0b11000000");
+    }
+}