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use super::bitmask::BitMask;
use super::EMPTY;
use core::{mem, ptr};
// Use the native word size as the group size. Using a 64-bit group size on
// a 32-bit architecture will just end up being more expensive because
// shifts and multiplies will need to be emulated.
cfg_if! {
if #[cfg(any(
target_pointer_width = "64",
target_arch = "aarch64",
target_arch = "x86_64",
target_arch = "wasm32",
))] {
type GroupWord = u64;
type NonZeroGroupWord = core::num::NonZeroU64;
} else {
type GroupWord = u32;
type NonZeroGroupWord = core::num::NonZeroU32;
}
}
pub(crate) type BitMaskWord = GroupWord;
pub(crate) type NonZeroBitMaskWord = NonZeroGroupWord;
pub(crate) const BITMASK_STRIDE: usize = 8;
// We only care about the highest bit of each byte for the mask.
#[allow(clippy::cast_possible_truncation, clippy::unnecessary_cast)]
pub(crate) const BITMASK_MASK: BitMaskWord = 0x8080_8080_8080_8080_u64 as GroupWord;
pub(crate) const BITMASK_ITER_MASK: BitMaskWord = !0;
/// Helper function to replicate a byte across a `GroupWord`.
#[inline]
fn repeat(byte: u8) -> GroupWord {
GroupWord::from_ne_bytes([byte; Group::WIDTH])
}
/// Abstraction over a group of control bytes which can be scanned in
/// parallel.
///
/// This implementation uses a word-sized integer.
#[derive(Copy, Clone)]
pub(crate) struct Group(GroupWord);
// We perform all operations in the native endianness, and convert to
// little-endian just before creating a BitMask. The can potentially
// enable the compiler to eliminate unnecessary byte swaps if we are
// only checking whether a BitMask is empty.
#[allow(clippy::use_self)]
impl Group {
/// Number of bytes in the group.
pub(crate) const WIDTH: usize = mem::size_of::<Self>();
/// Returns a full group of empty bytes, suitable for use as the initial
/// value for an empty hash table.
///
/// This is guaranteed to be aligned to the group size.
#[inline]
pub(crate) const fn static_empty() -> &'static [u8; Group::WIDTH] {
#[repr(C)]
struct AlignedBytes {
_align: [Group; 0],
bytes: [u8; Group::WIDTH],
}
const ALIGNED_BYTES: AlignedBytes = AlignedBytes {
_align: [],
bytes: [EMPTY; Group::WIDTH],
};
&ALIGNED_BYTES.bytes
}
/// Loads a group of bytes starting at the given address.
#[inline]
#[allow(clippy::cast_ptr_alignment)] // unaligned load
pub(crate) unsafe fn load(ptr: *const u8) -> Self {
Group(ptr::read_unaligned(ptr.cast()))
}
/// Loads a group of bytes starting at the given address, which must be
/// aligned to `mem::align_of::<Group>()`.
#[inline]
#[allow(clippy::cast_ptr_alignment)]
pub(crate) unsafe fn load_aligned(ptr: *const u8) -> Self {
// FIXME: use align_offset once it stabilizes
debug_assert_eq!(ptr as usize & (mem::align_of::<Self>() - 1), 0);
Group(ptr::read(ptr.cast()))
}
/// Stores the group of bytes to the given address, which must be
/// aligned to `mem::align_of::<Group>()`.
#[inline]
#[allow(clippy::cast_ptr_alignment)]
pub(crate) unsafe fn store_aligned(self, ptr: *mut u8) {
// FIXME: use align_offset once it stabilizes
debug_assert_eq!(ptr as usize & (mem::align_of::<Self>() - 1), 0);
ptr::write(ptr.cast(), self.0);
}
/// Returns a `BitMask` indicating all bytes in the group which *may*
/// have the given value.
///
/// This function may return a false positive in certain cases where
/// the byte in the group differs from the searched value only in its
/// lowest bit. This is fine because:
/// - This never happens for `EMPTY` and `DELETED`, only full entries.
/// - The check for key equality will catch these.
/// - This only happens if there is at least 1 true match.
/// - The chance of this happening is very low (< 1% chance per byte).
#[inline]
pub(crate) fn match_byte(self, byte: u8) -> BitMask {
// This algorithm is derived from
// https://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
let cmp = self.0 ^ repeat(byte);
BitMask((cmp.wrapping_sub(repeat(0x01)) & !cmp & repeat(0x80)).to_le())
}
/// Returns a `BitMask` indicating all bytes in the group which are
/// `EMPTY`.
#[inline]
pub(crate) fn match_empty(self) -> BitMask {
// If the high bit is set, then the byte must be either:
// 1111_1111 (EMPTY) or 1000_0000 (DELETED).
// So we can just check if the top two bits are 1 by ANDing them.
BitMask((self.0 & (self.0 << 1) & repeat(0x80)).to_le())
}
/// Returns a `BitMask` indicating all bytes in the group which are
/// `EMPTY` or `DELETED`.
#[inline]
pub(crate) fn match_empty_or_deleted(self) -> BitMask {
// A byte is EMPTY or DELETED iff the high bit is set
BitMask((self.0 & repeat(0x80)).to_le())
}
/// Returns a `BitMask` indicating all bytes in the group which are full.
#[inline]
pub(crate) fn match_full(self) -> BitMask {
self.match_empty_or_deleted().invert()
}
/// Performs the following transformation on all bytes in the group:
/// - `EMPTY => EMPTY`
/// - `DELETED => EMPTY`
/// - `FULL => DELETED`
#[inline]
pub(crate) fn convert_special_to_empty_and_full_to_deleted(self) -> Self {
// Map high_bit = 1 (EMPTY or DELETED) to 1111_1111
// and high_bit = 0 (FULL) to 1000_0000
//
// Here's this logic expanded to concrete values:
// let full = 1000_0000 (true) or 0000_0000 (false)
// !1000_0000 + 1 = 0111_1111 + 1 = 1000_0000 (no carry)
// !0000_0000 + 0 = 1111_1111 + 0 = 1111_1111 (no carry)
let full = !self.0 & repeat(0x80);
Group(!full + (full >> 7))
}
}
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