vortex-array 0.75.0

// SPDX-License-Identifier: Apache-2.0
// SPDX-FileCopyrightText: Copyright the Vortex contributors

//! The 16-byte view struct stored in variable-length binary vectors.

use std::fmt;
use std::hash::Hash;
use std::hash::Hasher;
use std::ops::Range;

use static_assertions::assert_eq_align;
use static_assertions::assert_eq_size;
use vortex_error::VortexExpect;

/// A view over a variable-length binary value.
///
/// Either an inlined representation (for values <= 12 bytes) or a reference
/// to an external buffer (for values > 12 bytes).
#[derive(Clone, Copy)]
#[repr(C, align(16))]
pub union BinaryView {
    /// Numeric representation. This is logically `u128`, but we split it into the high and low
    /// bits to preserve the alignment.
    pub(crate) le_bytes: [u8; 16],

    /// Inlined representation: strings <= 12 bytes
    pub(crate) inlined: Inlined,

    /// Reference type: strings > 12 bytes.
    pub(crate) _ref: Ref,
}

assert_eq_align!(BinaryView, u128);
assert_eq_size!(BinaryView, [u8; 16]);
assert_eq_size!(Inlined, [u8; 16]);
assert_eq_size!(Ref, [u8; 16]);

/// Variant of a [`BinaryView`] that holds an inlined value.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
#[repr(C, align(8))]
pub struct Inlined {
    /// The size of the full value.
    pub size: u32,
    /// The full inlined value.
    pub data: [u8; BinaryView::MAX_INLINED_SIZE],
}

impl Inlined {
    /// Creates a new inlined representation from the provided value of constant size.
    fn new<const N: usize>(value: &[u8]) -> Self {
        debug_assert_eq!(value.len(), N);
        let mut inlined = Self {
            size: N.try_into().vortex_expect("inlined size must fit in u32"),
            data: [0u8; BinaryView::MAX_INLINED_SIZE],
        };
        inlined.data[..N].copy_from_slice(&value[..N]);
        inlined
    }

    /// Returns the full inlined value.
    #[inline]
    pub fn value(&self) -> &[u8] {
        &self.data[0..(self.size as usize)]
    }
}

/// Variant of a [`BinaryView`] that holds a reference to an external buffer.
#[derive(Clone, Copy, Debug)]
#[repr(C, align(8))]
pub struct Ref {
    /// The size of the full value.
    pub size: u32,
    /// The prefix bytes of the value (first 4 bytes).
    pub prefix: [u8; 4],
    /// The index of the buffer where the full value is stored.
    pub buffer_index: u32,
    /// The offset within the buffer where the full value starts.
    pub offset: u32,
}

impl Ref {
    /// Returns the range within the buffer where the full value is stored.
    #[inline]
    pub fn as_range(&self) -> Range<usize> {
        self.offset as usize..(self.offset + self.size) as usize
    }

    /// Replaces the buffer index and offset of the reference, returning a new `Ref`.
    #[inline]
    pub fn with_buffer_and_offset(&self, buffer_index: u32, offset: u32) -> Ref {
        Self {
            size: self.size,
            prefix: self.prefix,
            buffer_index,
            offset,
        }
    }
}

impl BinaryView {
    /// Maximum size of an inlined binary value.
    pub const MAX_INLINED_SIZE: usize = 12;

    /// Create a view from a value, block and offset
    ///
    /// Depending on the length of the provided value either a new inlined
    /// or a reference view will be constructed.
    ///
    /// Adapted from arrow-rs <https://github.com/apache/arrow-rs/blob/f4fde769ab6e1a9b75f890b7f8b47bc22800830b/arrow-array/src/builder/generic_bytes_view_builder.rs#L524>
    /// Explicitly enumerating inlined view produces code that avoids calling generic `ptr::copy_non_interleave` that's slower than explicit stores
    #[inline(never)]
    pub fn make_view(value: &[u8], block: u32, offset: u32) -> Self {
        match value.len() {
            0 => Self {
                inlined: Inlined::new::<0>(value),
            },
            1 => Self {
                inlined: Inlined::new::<1>(value),
            },
            2 => Self {
                inlined: Inlined::new::<2>(value),
            },
            3 => Self {
                inlined: Inlined::new::<3>(value),
            },
            4 => Self {
                inlined: Inlined::new::<4>(value),
            },
            5 => Self {
                inlined: Inlined::new::<5>(value),
            },
            6 => Self {
                inlined: Inlined::new::<6>(value),
            },
            7 => Self {
                inlined: Inlined::new::<7>(value),
            },
            8 => Self {
                inlined: Inlined::new::<8>(value),
            },
            9 => Self {
                inlined: Inlined::new::<9>(value),
            },
            10 => Self {
                inlined: Inlined::new::<10>(value),
            },
            11 => Self {
                inlined: Inlined::new::<11>(value),
            },
            12 => Self {
                inlined: Inlined::new::<12>(value),
            },
            _ => Self::new_ref(
                u32::try_from(value.len()).vortex_expect("value length must fit in u32"),
                value[0..4]
                    .try_into()
                    .ok()
                    .vortex_expect("prefix must be exactly 4 bytes"),
                block,
                offset,
            ),
        }
    }

    /// Create a new empty view
    #[inline]
    pub fn empty_view() -> Self {
        Self { le_bytes: [0; 16] }
    }

    /// Create a reference view directly from its components, without inspecting the value.
    ///
    /// `size` must be greater than [`MAX_INLINED_SIZE`], and `prefix` must hold the first four
    /// bytes of the value. This is the fast path for bulk view construction where the caller has
    /// already established that the value is too long to inline; it assembles the 16-byte view as a
    /// single `u128` so the compiler can emit one wide store per view.
    ///
    /// [`MAX_INLINED_SIZE`]: Self::MAX_INLINED_SIZE
    #[inline]
    pub fn new_ref(size: u32, prefix: [u8; 4], buffer_index: u32, offset: u32) -> Self {
        debug_assert!(size as usize > Self::MAX_INLINED_SIZE);
        // Matches the little-endian field order of `Ref` (size, prefix, buffer_index, offset),
        // consistent with `le_bytes` and the `From<u128>`/`as_u128` representation.
        Self::from(
            u128::from(size)
                | (u128::from(u32::from_le_bytes(prefix)) << 32)
                | (u128::from(buffer_index) << 64)
                | (u128::from(offset) << 96),
        )
    }

    /// Create a new inlined binary view
    ///
    /// # Panics
    ///
    /// Panics if the provided string is too long to inline.
    #[inline]
    pub fn new_inlined(value: &[u8]) -> Self {
        assert!(
            value.len() <= Self::MAX_INLINED_SIZE,
            "expected inlined value to be <= 12 bytes, was {}",
            value.len()
        );

        Self::make_view(value, 0, 0)
    }

    /// Returns the length of the binary value.
    #[inline]
    pub fn len(&self) -> u32 {
        unsafe { self.inlined.size }
    }

    /// Returns true if the binary value is empty.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns true if the binary value is inlined.
    #[inline]
    #[expect(
        clippy::cast_possible_truncation,
        reason = "MAX_INLINED_SIZE is a small constant"
    )]
    pub fn is_inlined(&self) -> bool {
        self.len() <= (Self::MAX_INLINED_SIZE as u32)
    }

    /// Returns the inlined representation of the binary value.
    pub fn as_inlined(&self) -> &Inlined {
        debug_assert!(self.is_inlined());
        unsafe { &self.inlined }
    }

    /// Returns the reference representation of the binary value.
    pub fn as_view(&self) -> &Ref {
        debug_assert!(!self.is_inlined());
        unsafe { &self._ref }
    }

    /// Returns a mutable reference to the reference representation of the binary value.
    pub fn as_view_mut(&mut self) -> &mut Ref {
        unsafe { &mut self._ref }
    }

    /// Returns the binary view as u128 representation.
    pub fn as_u128(&self) -> u128 {
        // SAFETY: binary view always safe to read as u128 LE bytes
        unsafe { u128::from_le_bytes(self.le_bytes) }
    }
}

impl From<u128> for BinaryView {
    fn from(value: u128) -> Self {
        BinaryView {
            le_bytes: value.to_le_bytes(),
        }
    }
}

impl From<Ref> for BinaryView {
    fn from(value: Ref) -> Self {
        BinaryView { _ref: value }
    }
}

impl PartialEq for BinaryView {
    fn eq(&self, other: &Self) -> bool {
        let a = unsafe { std::mem::transmute::<&BinaryView, &u128>(self) };
        let b = unsafe { std::mem::transmute::<&BinaryView, &u128>(other) };
        a == b
    }
}
impl Eq for BinaryView {}

impl Hash for BinaryView {
    fn hash<H: Hasher>(&self, state: &mut H) {
        unsafe { std::mem::transmute::<&BinaryView, &u128>(self) }.hash(state);
    }
}

impl Default for BinaryView {
    fn default() -> Self {
        Self::make_view(&[], 0, 0)
    }
}

impl fmt::Debug for BinaryView {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut s = f.debug_struct("BinaryView");
        if self.is_inlined() {
            s.field("inline", &self.as_inlined());
        } else {
            s.field("ref", &self.as_view());
        }
        s.finish()
    }
}

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

    #[rstest::rstest]
    // Just past the inline boundary, typical, and large values.
    #[case(13, 7, 42)]
    #[case(20, 7, 42)]
    #[case(255, 7, 42)]
    #[case(4096, 7, 42)]
    // Zero buffer index/offset and the `u32` extremes, to confirm the `u128` field assembly does
    // not overflow into neighbouring fields.
    #[case(13, 0, 0)]
    #[case(13, u32::MAX, u32::MAX)]
    fn new_ref_matches_make_view(#[case] len: u32, #[case] buffer_index: u32, #[case] offset: u32) {
        // `new_ref` assembles the reference view as a `u128`; it must be byte-identical to the
        // value-inspecting `make_view` for any value longer than the inline limit.
        let value: Vec<u8> = (0..len)
            .map(|i| u8::try_from(i % 251).vortex_expect("i % 251 fits in u8"))
            .collect();
        let prefix = [value[0], value[1], value[2], value[3]];
        let made = BinaryView::make_view(&value, buffer_index, offset);
        let built = BinaryView::new_ref(len, prefix, buffer_index, offset);
        assert_eq!(made.as_u128(), built.as_u128(), "mismatch at len {len}");
        assert!(!built.is_inlined());
        let r = built.as_view();
        assert_eq!(r.size, len);
        assert_eq!(r.prefix, prefix);
        assert_eq!(r.buffer_index, buffer_index);
        assert_eq!(r.offset, offset);
    }
}