simd_kernels/kernels/bitmask/

mod.rs

// Copyright Peter Bower 2025. All Rights Reserved.
// Licensed under Mozilla Public License (MPL) 2.0.

//! # **Bitmask Kernels Module** - *High-Performance Null-Aware Bitmask Operations*
//!
//! SIMD-optimised bitmask operations for Arrow-compatible nullable array processing with efficient null handling.
//!
//! ## Overview
//! 
//! This module provides the foundational bitmask operations that enable null-aware and bit-packed boolean computing 
//! throughout the minarrow ecosystem, but can be applied to any bitmasking contenxt. 
//! These kernels handle bitwise logical operations, set membership tests, equality comparisons,
//! and population counts on Arrow-format bitmasks with optimal performance characteristics.
//!
//! ## Architecture
//! 
//! The bitmask module follows a three-tier architecture:
//! - **Dispatch layer**: Smart runtime selection between SIMD and scalar implementations
//! - **SIMD kernels**: Vectorised implementations using `std::simd` with portable lane counts
//! - **Scalar kernels**: High-performance but non-SIMD fallback implementations for compatibility
//!
//! ## Modules
//! - **`dispatch`**: Runtime dispatch layer selecting SIMD vs scalar implementations based on feature flags
//! - **`simd`**: SIMD-accelerated implementations using vectorised bitwise operations with configurable lane counts
//! - **`std`**: Scalar fallback implementations for word-level operations on 64-bit boundaries
//!
//! ## Core Operations
//! 
//! ### **Logical Operations**
//! - **`and_masks`**: Bitwise AND across two bitmasks for intersection operations
//! - **`or_masks`**: Bitwise OR across two bitmasks for union operations  
//! - **`xor_masks`**: Bitwise XOR across two bitmasks for symmetric difference
//! - **`not_mask`**: Bitwise NOT for complement operations
//!
//! ### **Set Membership**
//! - **`in_mask`**: Set inclusion tests - output bits indicate membership of LHS values in RHS set
//! - **`not_in_mask`**: Set exclusion tests - complement of inclusion operations
//!
//! ### **Equality Testing**
//! - **`eq_mask`**: Element-wise equality comparison producing result bitmask
//! - **`ne_mask`**: Element-wise inequality comparison producing result bitmask
//! - **`all_eq`**: Bulk equality test across entire bitmask windows
//! - **`all_ne`**: Bulk inequality test across entire bitmask windows
//!
//! ### **Population Analysis**
//! - **`popcount_mask`**: Fast population count (number of set bits) using SIMD reduction
//! - **`all_true_mask`**: Test if all bits in bitmask are set to 1
//! - **`all_false_mask`**: Test if all bits in bitmask are set to 0
//!
//! ## Arrow Compatibility
//! 
//! All operations maintain full compatibility with Apache Arrow's bitmask format:
//! - **LSB bit ordering**: Bit 0 is the least significant bit in each byte
//! - **Byte-packed storage**: 8 bits per byte with proper alignment handling
//! - **Trailing bit management**: Automatic masking of unused bits in final bytes
//! - **64-bit word alignment**: Optimised for modern CPU architectures

pub mod dispatch;
#[cfg(feature = "simd")]
pub mod simd;
#[cfg(not(feature = "simd"))]
pub mod std;

use core::mem;
use minarrow::{Bitmask, BitmaskVT};

/// Fundamental word type for bitmask operations on 64-bit architectures.
///
/// Defines the basic unit of bitmask storage and manipulation. All bitmask
/// operations are optimised around 64-bit word boundaries for maximum
/// performance on modern CPU architectures.
///
/// # Architecture Alignment
/// This type is chosen to align with:
/// - 64-bit CPU registers: Native register width on x86-64 and AArch64
/// - Cache line efficiency: Optimal memory access patterns
/// - SIMD compatibility: Natural alignment for vectorised operations
pub type Word = u64;

/// Number of bits in a `Word` for bit-level bitmask calculations.
///
/// This constant determines the fundamental granularity of bitmask operations,
/// enabling efficient bit manipulation algorithms that operate on word
/// boundaries.
pub const WORD_BITS: usize = mem::size_of::<Word>() * 8;

/// Helper to compute number of u64 words required for bitmask of `len` bits.
#[inline(always)]
pub fn words_for(len: usize) -> usize {
    (len + WORD_BITS - 1) / WORD_BITS
}

/// Cast &[u8] to &[u64] for word-wise access.
#[inline(always)]
pub unsafe fn mask_bits_as_words(bits: &[u8]) -> *const Word {
    bits.as_ptr() as *const Word
}

/// Cast &mut [u8] to &mut [u64] for word-wise mutation.
#[inline(always)]
pub unsafe fn mask_bits_as_words_mut(bits: &mut [u8]) -> *mut Word {
    bits.as_mut_ptr() as *mut Word
}

/// Create zeroed bitmask of length `len` bits.
#[inline(always)]
pub fn new_mask(len: usize) -> Bitmask {
    Bitmask::new_set_all(len, false)
}

/// Return window into bitmask's bits slice covering offset..offset+len bits.
#[inline(always)]
pub fn bitmask_window_bytes(mask: &Bitmask, offset: usize, len: usize) -> &[u8] {
    let start = offset / 8;
    let end = (offset + len + 7) / 8;
    &mask.bits[start..end]
}

/// Return mutable window into bitmask's bits slice covering offset..offset+len bits.
/// Enables efficient in-place modification of bitmask regions.
#[inline(always)]
pub fn bitmask_window_bytes_mut(mask: &mut Bitmask, offset: usize, len: usize) -> &mut [u8] {
    let start = offset / 8;
    let end = (offset + len + 7) / 8;
    &mut mask.bits[start..end]
}

/// Zero all slack bits ≥ `bm.len`.
#[inline(always)]
pub fn clear_trailing_bits(bm: &mut Bitmask) {
    if bm.len == 0 {
        return;
    }
    let used = bm.len & 7;
    if used != 0 {
        let last = bm.bits.last_mut().unwrap();
        *last &= (1u8 << used) - 1;
    }
}

/// Quick population count of true bits
#[inline(always)]
pub fn popcount_bits(m: BitmaskVT<'_>) -> usize {
    #[cfg(feature = "simd")]
    {
        // Use a default SIMD width if not otherwise specified
        use crate::kernels::bitmask::simd::popcount_mask_simd;
        popcount_mask_simd::<8>(m)
    }
    #[cfg(not(feature = "simd"))]
    {
        use crate::kernels::bitmask::std::popcount_mask;
        popcount_mask(m)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use minarrow::Bitmask;

    #[test]
    fn test_words_for() {
        assert_eq!(words_for(0), 0);
        assert_eq!(words_for(1), 1);
        assert_eq!(words_for(63), 1);
        assert_eq!(words_for(64), 1);
        assert_eq!(words_for(65), 2);
        assert_eq!(words_for(128), 2);
        assert_eq!(words_for(129), 3);
    }

    #[test]
    fn test_mask_bits_as_words_and_mut() {
        // Test reading words from bits
        let mut mask = Bitmask::new_set_all(128, false);
        // Write known pattern into mask
        mask.set(0, true);
        mask.set(63, true);
        mask.set(64, true);
        mask.set(127, true);
        let bits = &mask.bits;
        unsafe {
            let words = mask_bits_as_words(bits);
            assert_eq!(*words, 1u64 | (1u64 << 63));
            assert_eq!(*words.add(1), 1u64 | (1u64 << 63));
        }

        // Test writing via mask_bits_as_words_mut
        let mut mask = Bitmask::new_set_all(128, false);
        let bits = &mut mask.bits;
        unsafe {
            let words_mut = mask_bits_as_words_mut(bits);
            *words_mut = 0xDEADBEEFDEADBEEF;
            *words_mut.add(1) = 0xCAFEBABECAFEBABE;
        }
        assert_eq!(mask.bits[0], 0xEF);
        assert_eq!(mask.bits[7], 0xDE);
        assert_eq!(mask.bits[8], 0xBE);
        assert_eq!(mask.bits[15], 0xCA);
    }

    #[test]
    fn test_bitmask_window_bytes() {
        let mut mask = Bitmask::new_set_all(24, false);
        mask.set(0, true);
        mask.set(7, true);
        mask.set(8, true);
        mask.set(15, true);
        mask.set(16, true);
        mask.set(23, true);
        // Window: bytes 1..3 should cover bits 8..24
        let bytes = bitmask_window_bytes(&mask, 8, 16);
        assert_eq!(bytes.len(), 2); // 16 bits = 2 bytes
        assert_eq!(bytes[0], 0b10000001); // bits 8 and 15 set
        assert_eq!(bytes[1], 0b10000001); // bits 16 and 23 set
    }

    #[test]
    fn test_bitmask_window_bytes_mut() {
        let mut mask = Bitmask::new_set_all(16, false);
        {
            let window = bitmask_window_bytes_mut(&mut mask, 0, 16);
            window[0] = 0xAA;
            window[1] = 0x55;
        }
        assert_eq!(mask.bits[0], 0xAA);
        assert_eq!(mask.bits[1], 0x55);
    }

    #[test]
    fn test_clear_trailing_bits() {
        let mut mask = Bitmask::new_set_all(10, true);
        // The last byte should have only the low 2 bits set after clearing trailing bits
        clear_trailing_bits(&mut mask);
        // 10 bits => 2 bytes, last byte should have only bits 0 and 1 set (0b00000011 == 0x03)
        assert_eq!(mask.bits[1], 0x03);
        // The remaining bits should still be set
        for i in 0..10 {
            assert!(mask.get(i));
        }
        // Any bits beyond 10 should be cleared
        for i in 10..16 {
            assert!(!mask.get(i));
        }
    }
}