meilisearch/milli/src/update/words_level_positions.rs

use std::cmp;
use std::convert::TryFrom;
use std::fs::File;
use std::num::NonZeroU32;

use grenad::{CompressionType, Reader, Writer, FileFuse};
use heed::types::{DecodeIgnore, Str};
use heed::{BytesEncode, Error};
use log::debug;
use roaring::RoaringBitmap;

use crate::heed_codec::{StrLevelPositionCodec, CboRoaringBitmapCodec};
use crate::update::index_documents::WriteMethod;
use crate::update::index_documents::{create_writer, writer_into_reader, write_into_lmdb_database};
use crate::{Index, TreeLevel};

pub struct WordsLevelPositions<'t, 'u, 'i> {
    wtxn: &'t mut heed::RwTxn<'i, 'u>,
    index: &'i Index,
    pub(crate) chunk_compression_type: CompressionType,
    pub(crate) chunk_compression_level: Option<u32>,
    pub(crate) chunk_fusing_shrink_size: Option<u64>,
    level_group_size: NonZeroU32,
    min_level_size: NonZeroU32,
    _update_id: u64,
}

impl<'t, 'u, 'i> WordsLevelPositions<'t, 'u, 'i> {
    pub fn new(
        wtxn: &'t mut heed::RwTxn<'i, 'u>,
        index: &'i Index,
        update_id: u64,
    ) -> WordsLevelPositions<'t, 'u, 'i>
    {
        WordsLevelPositions {
            wtxn,
            index,
            chunk_compression_type: CompressionType::None,
            chunk_compression_level: None,
            chunk_fusing_shrink_size: None,
            level_group_size: NonZeroU32::new(4).unwrap(),
            min_level_size: NonZeroU32::new(5).unwrap(),
            _update_id: update_id,
        }
    }

    pub fn level_group_size(&mut self, value: NonZeroU32) -> &mut Self {
        self.level_group_size = NonZeroU32::new(cmp::max(value.get(), 2)).unwrap();
        self
    }

    pub fn min_level_size(&mut self, value: NonZeroU32) -> &mut Self {
        self.min_level_size = value;
        self
    }

    pub fn execute(self) -> anyhow::Result<()> {
        debug!("Computing and writing the word levels positions docids into LMDB on disk...");

        let entries = compute_positions_levels(
            self.wtxn,
            self.index.word_docids.remap_data_type::<DecodeIgnore>(),
            self.index.word_level_position_docids,
            self.chunk_compression_type,
            self.chunk_compression_level,
            self.chunk_fusing_shrink_size,
            self.level_group_size,
            self.min_level_size,
        )?;

        // The previously computed entries also defines the level 0 entries
        // so we can clear the database and append all of these entries.
        self.index.word_level_position_docids.clear(self.wtxn)?;

        write_into_lmdb_database(
            self.wtxn,
            *self.index.word_level_position_docids.as_polymorph(),
            entries,
            |_, _| anyhow::bail!("invalid facet level merging"),
            WriteMethod::Append,
        )?;

        Ok(())
    }
}

/// Returns the next number after or equal to `x` that is divisible by `d`.
fn next_divisible(x: u32, d: u32) -> u32 {
    (x.saturating_sub(1) | (d - 1)) + 1
}

/// Returns the previous number after or equal to `x` that is divisible by `d`,
/// saturates on zero.
fn previous_divisible(x: u32, d: u32) -> u32 {
    match x.checked_sub(d - 1) {
        Some(0) | None => 0,
        Some(x) => next_divisible(x, d),
    }
}

/// Generates all the words positions levels based on the levels zero (including the level zero).
fn compute_positions_levels(
    rtxn: &heed::RoTxn,
    words_db: heed::Database<Str, DecodeIgnore>,
    words_positions_db: heed::Database<StrLevelPositionCodec, CboRoaringBitmapCodec>,
    compression_type: CompressionType,
    compression_level: Option<u32>,
    shrink_size: Option<u64>,
    level_group_size: NonZeroU32,
    min_level_size: NonZeroU32,
) -> anyhow::Result<Reader<FileFuse>>
{
    // It is forbidden to keep a cursor and write in a database at the same time with LMDB
    // therefore we write the facet levels entries into a grenad file before transfering them.
    let mut writer = tempfile::tempfile().and_then(|file| {
        create_writer(compression_type, compression_level, file)
    })?;

    for result in words_db.iter(rtxn)? {
        let (word, ()) = result?;

        let level_0_range = {
            let left = (word, TreeLevel::min_value(), u32::min_value(), u32::min_value());
            let right = (word, TreeLevel::max_value(), u32::max_value(), u32::max_value());
            left..=right
        };

        let first_level_size = words_positions_db.remap_data_type::<DecodeIgnore>()
            .range(rtxn, &level_0_range)?
            .fold(Ok(0u32), |count, result| result.and(count).map(|c| c + 1))?;

        // Groups sizes are always a power of the original level_group_size and therefore a group
        // always maps groups of the previous level and never splits previous levels groups in half.
        let group_size_iter = (1u8..)
            .map(|l| (TreeLevel::try_from(l).unwrap(), level_group_size.get().pow(l as u32)))
            .take_while(|(_, s)| first_level_size / *s >= min_level_size.get());

        // As specified in the documentation, we also write the level 0 entries.
        for result in words_positions_db.range(rtxn, &level_0_range)? {
            let ((word, level, left, right), docids) = result?;
            write_level_entry(&mut writer, word, level, left, right, &docids)?;
        }

        for (level, group_size) in group_size_iter {
            let mut left = 0;
            let mut right = 0;
            let mut group_docids = RoaringBitmap::new();

            for (i, result) in words_positions_db.range(rtxn, &level_0_range)?.enumerate() {
                let ((_word, _level, value, _right), docids) = result?;

                if i == 0 {
                    left = previous_divisible(value, group_size);
                    right = left + (group_size - 1);
                }

                if value > right {
                    // we found the first bound of the next group, we must store the left
                    // and right bounds associated with the docids.
                    write_level_entry(&mut writer, word, level, left, right, &group_docids)?;

                    // We save the left bound for the new group and also reset the docids.
                    group_docids = RoaringBitmap::new();
                    left = previous_divisible(value, group_size);
                    right = left + (group_size - 1);
                }

                // The right bound is always the bound we run through.
                group_docids.union_with(&docids);
            }

            if !group_docids.is_empty() {
                write_level_entry(&mut writer, word, level, left, right, &group_docids)?;
            }
        }
    }

    writer_into_reader(writer, shrink_size)
}

fn write_level_entry(
    writer: &mut Writer<File>,
    word: &str,
    level: TreeLevel,
    left: u32,
    right: u32,
    ids: &RoaringBitmap,
) -> anyhow::Result<()>
{
    let key = (word, level, left, right);
    let key = StrLevelPositionCodec::bytes_encode(&key).ok_or(Error::Encoding)?;
    let data = CboRoaringBitmapCodec::bytes_encode(&ids).ok_or(Error::Encoding)?;
    writer.insert(&key, &data)?;
    Ok(())
}