diff --git a/DIRECTORY.md b/DIRECTORY.md index af47f388d1c..9645d01eda3 100644 --- a/DIRECTORY.md +++ b/DIRECTORY.md @@ -20,6 +20,7 @@ * [Binary Coded Decimal](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/binary_coded_decimal.rs) * [Counting Bits](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/counting_bits.rs) * [Highest Set Bit](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/highest_set_bit.rs) + * [Is Power of Two](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/is_power_of_two.rs) * [N Bits Gray Code](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/n_bits_gray_code.rs) * [Previous Power of Two](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/find_previous_power_of_two.rs) * [Reverse Bits](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/reverse_bits.rs) diff --git a/src/bit_manipulation/is_power_of_two.rs b/src/bit_manipulation/is_power_of_two.rs new file mode 100644 index 00000000000..a036c715f9a --- /dev/null +++ b/src/bit_manipulation/is_power_of_two.rs @@ -0,0 +1,240 @@ +//! Power of Two Check +//! +//! This module provides a function to determine if a given positive integer is a power of two +//! using efficient bit manipulation. +//! +//! # Algorithm +//! +//! The algorithm uses the property that powers of two have exactly one bit set in their +//! binary representation. When we subtract 1 from a power of two, all bits after the single +//! set bit become 1, and the set bit becomes 0: +//! +//! ```text +//! n = 0..100..00 (power of 2) +//! n - 1 = 0..011..11 +//! n & (n - 1) = 0 (no intersections) +//! ``` +//! +//! For example: +//! - 8 in binary: 1000 +//! - 7 in binary: 0111 +//! - 8 & 7 = 0000 = 0 ✓ +//! +//! Author: Alexander Pantyukhin +//! Date: November 1, 2022 + +/// Determines if a given number is a power of two. +/// +/// This function uses bit manipulation to efficiently check if a number is a power of two. +/// A number is a power of two if it has exactly one bit set in its binary representation. +/// The check `number & (number - 1) == 0` leverages this property. +/// +/// # Arguments +/// +/// * `number` - An integer to check (must be non-negative) +/// +/// # Returns +/// +/// A `Result` containing: +/// - `Ok(true)` - If the number is a power of two (including 0 and 1) +/// - `Ok(false)` - If the number is not a power of two +/// - `Err(String)` - If the number is negative +/// +/// # Examples +/// +/// ``` +/// use the_algorithms_rust::bit_manipulation::is_power_of_two; +/// +/// assert_eq!(is_power_of_two(0).unwrap(), true); +/// assert_eq!(is_power_of_two(1).unwrap(), true); +/// assert_eq!(is_power_of_two(2).unwrap(), true); +/// assert_eq!(is_power_of_two(4).unwrap(), true); +/// assert_eq!(is_power_of_two(8).unwrap(), true); +/// assert_eq!(is_power_of_two(16).unwrap(), true); +/// +/// assert_eq!(is_power_of_two(3).unwrap(), false); +/// assert_eq!(is_power_of_two(6).unwrap(), false); +/// assert_eq!(is_power_of_two(17).unwrap(), false); +/// +/// // Negative numbers return an error +/// assert!(is_power_of_two(-1).is_err()); +/// ``` +/// +/// # Errors +/// +/// Returns an error if the input number is negative. +/// +/// # Time Complexity +/// +/// O(1) - The function performs a constant number of operations regardless of input size. +pub fn is_power_of_two(number: i32) -> Result { + if number < 0 { + return Err("number must not be negative".to_string()); + } + + // Convert to u32 for safe bit operations + let num = number as u32; + + // Check if number & (number - 1) == 0 + // For powers of 2, this will always be true + Ok(num & num.wrapping_sub(1) == 0) +} + +#[cfg(test)] +mod tests { + use super::*; + + #[test] + fn test_zero() { + // 0 is considered a power of 2 by the algorithm (2^(-∞) interpretation) + assert!(is_power_of_two(0).unwrap()); + } + + #[test] + fn test_one() { + // 1 = 2^0 + assert!(is_power_of_two(1).unwrap()); + } + + #[test] + fn test_powers_of_two() { + assert!(is_power_of_two(2).unwrap()); // 2^1 + assert!(is_power_of_two(4).unwrap()); // 2^2 + assert!(is_power_of_two(8).unwrap()); // 2^3 + assert!(is_power_of_two(16).unwrap()); // 2^4 + assert!(is_power_of_two(32).unwrap()); // 2^5 + assert!(is_power_of_two(64).unwrap()); // 2^6 + assert!(is_power_of_two(128).unwrap()); // 2^7 + assert!(is_power_of_two(256).unwrap()); // 2^8 + assert!(is_power_of_two(512).unwrap()); // 2^9 + assert!(is_power_of_two(1024).unwrap()); // 2^10 + assert!(is_power_of_two(2048).unwrap()); // 2^11 + assert!(is_power_of_two(4096).unwrap()); // 2^12 + assert!(is_power_of_two(8192).unwrap()); // 2^13 + assert!(is_power_of_two(16384).unwrap()); // 2^14 + assert!(is_power_of_two(32768).unwrap()); // 2^15 + assert!(is_power_of_two(65536).unwrap()); // 2^16 + } + + #[test] + fn test_non_powers_of_two() { + assert!(!is_power_of_two(3).unwrap()); + assert!(!is_power_of_two(5).unwrap()); + assert!(!is_power_of_two(6).unwrap()); + assert!(!is_power_of_two(7).unwrap()); + assert!(!is_power_of_two(9).unwrap()); + assert!(!is_power_of_two(10).unwrap()); + assert!(!is_power_of_two(11).unwrap()); + assert!(!is_power_of_two(12).unwrap()); + assert!(!is_power_of_two(13).unwrap()); + assert!(!is_power_of_two(14).unwrap()); + assert!(!is_power_of_two(15).unwrap()); + assert!(!is_power_of_two(17).unwrap()); + assert!(!is_power_of_two(18).unwrap()); + } + + #[test] + fn test_specific_non_powers() { + assert!(!is_power_of_two(6).unwrap()); + assert!(!is_power_of_two(17).unwrap()); + assert!(!is_power_of_two(100).unwrap()); + assert!(!is_power_of_two(1000).unwrap()); + } + + #[test] + fn test_large_powers_of_two() { + assert!(is_power_of_two(131072).unwrap()); // 2^17 + assert!(is_power_of_two(262144).unwrap()); // 2^18 + assert!(is_power_of_two(524288).unwrap()); // 2^19 + assert!(is_power_of_two(1048576).unwrap()); // 2^20 + } + + #[test] + fn test_numbers_near_powers_of_two() { + // One less than powers of 2 + assert!(!is_power_of_two(3).unwrap()); // 2^2 - 1 + assert!(!is_power_of_two(7).unwrap()); // 2^3 - 1 + assert!(!is_power_of_two(15).unwrap()); // 2^4 - 1 + assert!(!is_power_of_two(31).unwrap()); // 2^5 - 1 + assert!(!is_power_of_two(63).unwrap()); // 2^6 - 1 + assert!(!is_power_of_two(127).unwrap()); // 2^7 - 1 + assert!(!is_power_of_two(255).unwrap()); // 2^8 - 1 + + // One more than powers of 2 + assert!(!is_power_of_two(3).unwrap()); // 2^1 + 1 + assert!(!is_power_of_two(5).unwrap()); // 2^2 + 1 + assert!(!is_power_of_two(9).unwrap()); // 2^3 + 1 + assert!(!is_power_of_two(17).unwrap()); // 2^4 + 1 + assert!(!is_power_of_two(33).unwrap()); // 2^5 + 1 + assert!(!is_power_of_two(65).unwrap()); // 2^6 + 1 + assert!(!is_power_of_two(129).unwrap()); // 2^7 + 1 + } + + #[test] + fn test_negative_number_returns_error() { + let result = is_power_of_two(-1); + assert!(result.is_err()); + assert_eq!(result.unwrap_err(), "number must not be negative"); + } + + #[test] + fn test_multiple_negative_numbers() { + assert!(is_power_of_two(-1).is_err()); + assert!(is_power_of_two(-2).is_err()); + assert!(is_power_of_two(-4).is_err()); + assert!(is_power_of_two(-8).is_err()); + assert!(is_power_of_two(-100).is_err()); + } + + #[test] + fn test_all_powers_of_two_up_to_30() { + // Test 2^0 through 2^30 + for i in 0..=30 { + let power = 1u32 << i; // 2^i + assert!( + is_power_of_two(power as i32).unwrap(), + "2^{i} = {power} should be a power of 2" + ); + } + } + + #[test] + fn test_range_verification() { + // Test that between consecutive powers of 2, only the powers return true + for i in 1..10 { + let power = 1 << i; // 2^i + assert!(is_power_of_two(power).unwrap()); + + // Check numbers between this power and the next + let next_power = 1 << (i + 1); + for num in (power + 1)..next_power { + assert!( + !is_power_of_two(num).unwrap(), + "{num} should not be a power of 2" + ); + } + } + } + + #[test] + fn test_bit_manipulation_correctness() { + // Verify the bit manipulation logic for specific examples + // For 8: 1000 & 0111 = 0000 ✓ + assert_eq!(8 & 7, 0); + assert!(is_power_of_two(8).unwrap()); + + // For 16: 10000 & 01111 = 00000 ✓ + assert_eq!(16 & 15, 0); + assert!(is_power_of_two(16).unwrap()); + + // For 6: 110 & 101 = 100 ✗ + assert_ne!(6 & 5, 0); + assert!(!is_power_of_two(6).unwrap()); + } + + #[test] + fn test_edge_case_max_i32_power_of_two() { + // Largest power of 2 that fits in i32: 2^30 = 1073741824 + assert!(is_power_of_two(1073741824).unwrap()); + } +} diff --git a/src/bit_manipulation/mod.rs b/src/bit_manipulation/mod.rs index 13f649207c2..95bbb755123 100644 --- a/src/bit_manipulation/mod.rs +++ b/src/bit_manipulation/mod.rs @@ -2,6 +2,7 @@ mod binary_coded_decimal; mod counting_bits; mod find_previous_power_of_two; mod highest_set_bit; +mod is_power_of_two; mod n_bits_gray_code; mod reverse_bits; mod sum_of_two_integers; @@ -12,6 +13,7 @@ pub use self::binary_coded_decimal::binary_coded_decimal; pub use self::counting_bits::count_set_bits; pub use self::find_previous_power_of_two::find_previous_power_of_two; pub use self::highest_set_bit::find_highest_set_bit; +pub use self::is_power_of_two::is_power_of_two; 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;