parquet/bloom_filter/

mod.rs

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// specific language governing permissions and limitations
// under the License.

//! Bloom filter implementation specific to Parquet, as described
//! in the [spec][parquet-bf-spec].
//!
//! # Bloom Filter Size
//!
//! Parquet uses the [Split Block Bloom Filter][sbbf-paper] (SBBF) as its bloom filter
//! implementation. For each column upon which bloom filters are enabled, the offset and length of an SBBF
//! is stored in  the metadata for each row group in the parquet file. The size of each filter is
//! initialized using a calculation based on the desired number of distinct values (NDV) and false
//! positive probability (FPP). The FPP for a SBBF can be approximated as<sup>[1][bf-formulae]</sup>:
//!
//! ```text
//! f = (1 - e^(-k * n / m))^k
//! ```
//!
//! Where, `f` is the FPP, `k` the number of hash functions, `n` the NDV, and `m` the total number
//! of bits in the bloom filter. This can be re-arranged to determine the total number of bits
//! required to achieve a given FPP and NDV:
//!
//! ```text
//! m = -k * n / ln(1 - f^(1/k))
//! ```
//!
//! SBBFs use eight hash functions to cleanly fit in SIMD lanes<sup>[2][sbbf-paper]</sup>, therefore
//! `k` is set to 8. The SBBF will spread those `m` bits accross a set of `b` blocks that
//! are each 256 bits, i.e., 32 bytes, in size. The number of blocks is chosen as:
//!
//! ```text
//! b = NP2(m/8) / 32
//! ```
//!
//! Where, `NP2` denotes *the next power of two*, and `m` is divided by 8 to be represented as bytes.
//!
//! Here is a table of calculated sizes for various FPP and NDV:
//!
//! | NDV       | FPP       | b       | Size (KB) |
//! |-----------|-----------|---------|-----------|
//! | 10,000    | 0.1       | 256     | 8         |
//! | 10,000    | 0.01      | 512     | 16        |
//! | 10,000    | 0.001     | 1,024   | 32        |
//! | 10,000    | 0.0001    | 1,024   | 32        |
//! | 100,000   | 0.1       | 4,096   | 128       |
//! | 100,000   | 0.01      | 4,096   | 128       |
//! | 100,000   | 0.001     | 8,192   | 256       |
//! | 100,000   | 0.0001    | 16,384  | 512       |
//! | 100,000   | 0.00001   | 16,384  | 512       |
//! | 1,000,000 | 0.1       | 32,768  | 1,024     |
//! | 1,000,000 | 0.01      | 65,536  | 2,048     |
//! | 1,000,000 | 0.001     | 65,536  | 2,048     |
//! | 1,000,000 | 0.0001    | 131,072 | 4,096     |
//! | 1,000,000 | 0.00001   | 131,072 | 4,096     |
//! | 1,000,000 | 0.000001  | 262,144 | 8,192     |
//!
//! [parquet-bf-spec]: https://github.com/apache/parquet-format/blob/master/BloomFilter.md
//! [sbbf-paper]: https://arxiv.org/pdf/2101.01719
//! [bf-formulae]: http://tfk.mit.edu/pdf/bloom.pdf

use crate::data_type::AsBytes;
use crate::errors::ParquetError;
use crate::file::metadata::ColumnChunkMetaData;
use crate::file::reader::ChunkReader;
use crate::format::{
    BloomFilterAlgorithm, BloomFilterCompression, BloomFilterHash, BloomFilterHeader,
    SplitBlockAlgorithm, Uncompressed, XxHash,
};
use crate::thrift::{TCompactSliceInputProtocol, TSerializable};
use bytes::Bytes;
use std::hash::Hasher;
use std::io::Write;
use std::sync::Arc;
use thrift::protocol::{TCompactOutputProtocol, TOutputProtocol};
use twox_hash::XxHash64;

/// Salt as defined in the [spec](https://github.com/apache/parquet-format/blob/master/BloomFilter.md#technical-approach).
const SALT: [u32; 8] = [
    0x47b6137b_u32,
    0x44974d91_u32,
    0x8824ad5b_u32,
    0xa2b7289d_u32,
    0x705495c7_u32,
    0x2df1424b_u32,
    0x9efc4947_u32,
    0x5c6bfb31_u32,
];

/// Each block is 256 bits, broken up into eight contiguous "words", each consisting of 32 bits.
/// Each word is thought of as an array of bits; each bit is either "set" or "not set".
#[derive(Debug, Copy, Clone)]
struct Block([u32; 8]);
impl Block {
    const ZERO: Block = Block([0; 8]);

    /// takes as its argument a single unsigned 32-bit integer and returns a block in which each
    /// word has exactly one bit set.
    fn mask(x: u32) -> Self {
        let mut result = [0_u32; 8];
        for i in 0..8 {
            // wrapping instead of checking for overflow
            let y = x.wrapping_mul(SALT[i]);
            let y = y >> 27;
            result[i] = 1 << y;
        }
        Self(result)
    }

    #[inline]
    #[cfg(target_endian = "little")]
    fn to_le_bytes(self) -> [u8; 32] {
        self.to_ne_bytes()
    }

    #[inline]
    #[cfg(not(target_endian = "little"))]
    fn to_le_bytes(self) -> [u8; 32] {
        self.swap_bytes().to_ne_bytes()
    }

    #[inline]
    fn to_ne_bytes(self) -> [u8; 32] {
        // SAFETY: [u32; 8] and [u8; 32] have the same size and neither has invalid bit patterns.
        unsafe { std::mem::transmute(self.0) }
    }

    #[inline]
    #[cfg(not(target_endian = "little"))]
    fn swap_bytes(mut self) -> Self {
        self.0.iter_mut().for_each(|x| *x = x.swap_bytes());
        self
    }

    /// setting every bit in the block that was also set in the result from mask
    fn insert(&mut self, hash: u32) {
        let mask = Self::mask(hash);
        for i in 0..8 {
            self[i] |= mask[i];
        }
    }

    /// returns true when every bit that is set in the result of mask is also set in the block.
    fn check(&self, hash: u32) -> bool {
        let mask = Self::mask(hash);
        for i in 0..8 {
            if self[i] & mask[i] == 0 {
                return false;
            }
        }
        true
    }
}

impl std::ops::Index<usize> for Block {
    type Output = u32;

    #[inline]
    fn index(&self, index: usize) -> &Self::Output {
        self.0.index(index)
    }
}

impl std::ops::IndexMut<usize> for Block {
    #[inline]
    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
        self.0.index_mut(index)
    }
}

/// A split block Bloom filter.
///
/// The creation of this structure is based on the [`crate::file::properties::BloomFilterProperties`]
/// struct set via [`crate::file::properties::WriterProperties`] and is thus hidden by default.
#[derive(Debug, Clone)]
pub struct Sbbf(Vec<Block>);

pub(crate) const SBBF_HEADER_SIZE_ESTIMATE: usize = 20;

/// given an initial offset, and a byte buffer, try to read out a bloom filter header and return
/// both the header and the offset after it (for bitset).
pub(crate) fn chunk_read_bloom_filter_header_and_offset(
    offset: u64,
    buffer: Bytes,
) -> Result<(BloomFilterHeader, u64), ParquetError> {
    let (header, length) = read_bloom_filter_header_and_length(buffer)?;
    Ok((header, offset + length))
}

/// given a [Bytes] buffer, try to read out a bloom filter header and return both the header and
/// length of the header.
#[inline]
pub(crate) fn read_bloom_filter_header_and_length(
    buffer: Bytes,
) -> Result<(BloomFilterHeader, u64), ParquetError> {
    let total_length = buffer.len();
    let mut prot = TCompactSliceInputProtocol::new(buffer.as_ref());
    let header = BloomFilterHeader::read_from_in_protocol(&mut prot)
        .map_err(|e| ParquetError::General(format!("Could not read bloom filter header: {e}")))?;
    Ok((header, (total_length - prot.as_slice().len()) as u64))
}

pub(crate) const BITSET_MIN_LENGTH: usize = 32;
pub(crate) const BITSET_MAX_LENGTH: usize = 128 * 1024 * 1024;

#[inline]
fn optimal_num_of_bytes(num_bytes: usize) -> usize {
    let num_bytes = num_bytes.min(BITSET_MAX_LENGTH);
    let num_bytes = num_bytes.max(BITSET_MIN_LENGTH);
    num_bytes.next_power_of_two()
}

// see http://algo2.iti.kit.edu/documents/cacheefficientbloomfilters-jea.pdf
// given fpp = (1 - e^(-k * n / m)) ^ k
// we have m = - k * n / ln(1 - fpp ^ (1 / k))
// where k = number of hash functions, m = number of bits, n = number of distinct values
#[inline]
fn num_of_bits_from_ndv_fpp(ndv: u64, fpp: f64) -> usize {
    let num_bits = -8.0 * ndv as f64 / (1.0 - fpp.powf(1.0 / 8.0)).ln();
    num_bits as usize
}

impl Sbbf {
    /// Create a new [Sbbf] with given number of distinct values and false positive probability.
    /// Will return an error if `fpp` is greater than or equal to 1.0 or less than 0.0.
    pub(crate) fn new_with_ndv_fpp(ndv: u64, fpp: f64) -> Result<Self, ParquetError> {
        if !(0.0..1.0).contains(&fpp) {
            return Err(ParquetError::General(format!(
                "False positive probability must be between 0.0 and 1.0, got {fpp}"
            )));
        }
        let num_bits = num_of_bits_from_ndv_fpp(ndv, fpp);
        Ok(Self::new_with_num_of_bytes(num_bits / 8))
    }

    /// Create a new [Sbbf] with given number of bytes, the exact number of bytes will be adjusted
    /// to the next power of two bounded by [BITSET_MIN_LENGTH] and [BITSET_MAX_LENGTH].
    pub(crate) fn new_with_num_of_bytes(num_bytes: usize) -> Self {
        let num_bytes = optimal_num_of_bytes(num_bytes);
        let bitset = vec![0_u8; num_bytes];
        Self::new(&bitset)
    }

    pub(crate) fn new(bitset: &[u8]) -> Self {
        let data = bitset
            .chunks_exact(4 * 8)
            .map(|chunk| {
                let mut block = Block::ZERO;
                for (i, word) in chunk.chunks_exact(4).enumerate() {
                    block[i] = u32::from_le_bytes(word.try_into().unwrap());
                }
                block
            })
            .collect::<Vec<Block>>();
        Self(data)
    }

    /// Write the bloom filter data (header and then bitset) to the output. This doesn't
    /// flush the writer in order to boost performance of bulk writing all blocks. Caller
    /// must remember to flush the writer.
    pub(crate) fn write<W: Write>(&self, mut writer: W) -> Result<(), ParquetError> {
        let mut protocol = TCompactOutputProtocol::new(&mut writer);
        let header = self.header();
        header.write_to_out_protocol(&mut protocol).map_err(|e| {
            ParquetError::General(format!("Could not write bloom filter header: {e}"))
        })?;
        protocol.flush()?;
        self.write_bitset(&mut writer)?;
        Ok(())
    }

    /// Write the bitset in serialized form to the writer.
    fn write_bitset<W: Write>(&self, mut writer: W) -> Result<(), ParquetError> {
        for block in &self.0 {
            writer
                .write_all(block.to_le_bytes().as_slice())
                .map_err(|e| {
                    ParquetError::General(format!("Could not write bloom filter bit set: {e}"))
                })?;
        }
        Ok(())
    }

    /// Create and populate [`BloomFilterHeader`] from this bitset for writing to serialized form
    fn header(&self) -> BloomFilterHeader {
        BloomFilterHeader {
            // 8 i32 per block, 4 bytes per i32
            num_bytes: self.0.len() as i32 * 4 * 8,
            algorithm: BloomFilterAlgorithm::BLOCK(SplitBlockAlgorithm {}),
            hash: BloomFilterHash::XXHASH(XxHash {}),
            compression: BloomFilterCompression::UNCOMPRESSED(Uncompressed {}),
        }
    }

    /// Read a new bloom filter from the given offset in the given reader.
    pub(crate) fn read_from_column_chunk<R: ChunkReader>(
        column_metadata: &ColumnChunkMetaData,
        reader: Arc<R>,
    ) -> Result<Option<Self>, ParquetError> {
        let offset: u64 = if let Some(offset) = column_metadata.bloom_filter_offset() {
            offset
                .try_into()
                .map_err(|_| ParquetError::General("Bloom filter offset is invalid".to_string()))?
        } else {
            return Ok(None);
        };

        let buffer = match column_metadata.bloom_filter_length() {
            Some(length) => reader.get_bytes(offset, length as usize),
            None => reader.get_bytes(offset, SBBF_HEADER_SIZE_ESTIMATE),
        }?;

        let (header, bitset_offset) =
            chunk_read_bloom_filter_header_and_offset(offset, buffer.clone())?;

        match header.algorithm {
            BloomFilterAlgorithm::BLOCK(_) => {
                // this match exists to future proof the singleton algorithm enum
            }
        }
        match header.compression {
            BloomFilterCompression::UNCOMPRESSED(_) => {
                // this match exists to future proof the singleton compression enum
            }
        }
        match header.hash {
            BloomFilterHash::XXHASH(_) => {
                // this match exists to future proof the singleton hash enum
            }
        }

        let bitset = match column_metadata.bloom_filter_length() {
            Some(_) => buffer.slice((bitset_offset - offset) as usize..),
            None => {
                let bitset_length: usize = header.num_bytes.try_into().map_err(|_| {
                    ParquetError::General("Bloom filter length is invalid".to_string())
                })?;
                reader.get_bytes(bitset_offset, bitset_length)?
            }
        };

        Ok(Some(Self::new(&bitset)))
    }

    #[inline]
    fn hash_to_block_index(&self, hash: u64) -> usize {
        // unchecked_mul is unstable, but in reality this is safe, we'd just use saturating mul
        // but it will not saturate
        (((hash >> 32).saturating_mul(self.0.len() as u64)) >> 32) as usize
    }

    /// Insert an [AsBytes] value into the filter
    pub fn insert<T: AsBytes + ?Sized>(&mut self, value: &T) {
        self.insert_hash(hash_as_bytes(value));
    }

    /// Insert a hash into the filter
    fn insert_hash(&mut self, hash: u64) {
        let block_index = self.hash_to_block_index(hash);
        self.0[block_index].insert(hash as u32)
    }

    /// Check if an [AsBytes] value is probably present or definitely absent in the filter
    pub fn check<T: AsBytes>(&self, value: &T) -> bool {
        self.check_hash(hash_as_bytes(value))
    }

    /// Check if a hash is in the filter. May return
    /// true for values that was never inserted ("false positive")
    /// but will always return false if a hash has not been inserted.
    fn check_hash(&self, hash: u64) -> bool {
        let block_index = self.hash_to_block_index(hash);
        self.0[block_index].check(hash as u32)
    }

    /// Return the total in memory size of this bloom filter in bytes
    pub(crate) fn estimated_memory_size(&self) -> usize {
        self.0.capacity() * std::mem::size_of::<Block>()
    }
}

// per spec we use xxHash with seed=0
const SEED: u64 = 0;

#[inline]
fn hash_as_bytes<A: AsBytes + ?Sized>(value: &A) -> u64 {
    let mut hasher = XxHash64::with_seed(SEED);
    hasher.write(value.as_bytes());
    hasher.finish()
}

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

    #[test]
    fn test_hash_bytes() {
        assert_eq!(hash_as_bytes(""), 17241709254077376921);
    }

    #[test]
    fn test_mask_set_quick_check() {
        for i in 0..1_000_000 {
            let result = Block::mask(i);
            assert!(result.0.iter().all(|&x| x.is_power_of_two()));
        }
    }

    #[test]
    fn test_block_insert_and_check() {
        for i in 0..1_000_000 {
            let mut block = Block::ZERO;
            block.insert(i);
            assert!(block.check(i));
        }
    }

    #[test]
    fn test_sbbf_insert_and_check() {
        let mut sbbf = Sbbf(vec![Block::ZERO; 1_000]);
        for i in 0..1_000_000 {
            sbbf.insert(&i);
            assert!(sbbf.check(&i));
        }
    }

    #[test]
    fn test_with_fixture() {
        // bloom filter produced by parquet-mr/spark for a column of i64 f"a{i}" for i in 0..10
        let bitset: &[u8] = &[
            200, 1, 80, 20, 64, 68, 8, 109, 6, 37, 4, 67, 144, 80, 96, 32, 8, 132, 43, 33, 0, 5,
            99, 65, 2, 0, 224, 44, 64, 78, 96, 4,
        ];
        let sbbf = Sbbf::new(bitset);
        for a in 0..10i64 {
            let value = format!("a{a}");
            assert!(sbbf.check(&value.as_str()));
        }
    }

    /// test the assumption that bloom filter header size should not exceed SBBF_HEADER_SIZE_ESTIMATE
    /// essentially we are testing that the struct is packed with 4 i32 fields, each can be 1-5 bytes
    /// so altogether it'll be 20 bytes at most.
    #[test]
    fn test_bloom_filter_header_size_assumption() {
        let buffer: &[u8; 16] = &[21, 64, 28, 28, 0, 0, 28, 28, 0, 0, 28, 28, 0, 0, 0, 99];
        let (
            BloomFilterHeader {
                algorithm,
                compression,
                hash,
                num_bytes,
            },
            read_length,
        ) = read_bloom_filter_header_and_length(Bytes::copy_from_slice(buffer)).unwrap();
        assert_eq!(read_length, 15);
        assert_eq!(
            algorithm,
            BloomFilterAlgorithm::BLOCK(SplitBlockAlgorithm {})
        );
        assert_eq!(
            compression,
            BloomFilterCompression::UNCOMPRESSED(Uncompressed {})
        );
        assert_eq!(hash, BloomFilterHash::XXHASH(XxHash {}));
        assert_eq!(num_bytes, 32_i32);
        assert_eq!(20, SBBF_HEADER_SIZE_ESTIMATE);
    }

    #[test]
    fn test_optimal_num_of_bytes() {
        for (input, expected) in &[
            (0, 32),
            (9, 32),
            (31, 32),
            (32, 32),
            (33, 64),
            (99, 128),
            (1024, 1024),
            (999_000_000, 128 * 1024 * 1024),
        ] {
            assert_eq!(*expected, optimal_num_of_bytes(*input));
        }
    }

    #[test]
    fn test_num_of_bits_from_ndv_fpp() {
        for (fpp, ndv, num_bits) in &[
            (0.1, 10, 57),
            (0.01, 10, 96),
            (0.001, 10, 146),
            (0.1, 100, 577),
            (0.01, 100, 968),
            (0.001, 100, 1460),
            (0.1, 1000, 5772),
            (0.01, 1000, 9681),
            (0.001, 1000, 14607),
            (0.1, 10000, 57725),
            (0.01, 10000, 96815),
            (0.001, 10000, 146076),
            (0.1, 100000, 577254),
            (0.01, 100000, 968152),
            (0.001, 100000, 1460769),
            (0.1, 1000000, 5772541),
            (0.01, 1000000, 9681526),
            (0.001, 1000000, 14607697),
            (1e-50, 1_000_000_000_000, 14226231280773240832),
        ] {
            assert_eq!(*num_bits, num_of_bits_from_ndv_fpp(*ndv, *fpp) as u64);
        }
    }
}