MeiliSearch/milli/src/update/index_documents/extract/extract_facet_string_docids.rs

use std::fs::File;
use std::io;

use heed::BytesEncode;

use super::helpers::{create_sorter, sorter_into_reader, try_split_array_at, GrenadParameters};
use crate::heed_codec::facet::{FacetGroupKey, FacetGroupKeyCodec, StrRefCodec};
use crate::update::index_documents::merge_cbo_roaring_bitmaps;
use crate::{FieldId, Result};

/// Extracts the facet string and the documents ids where this facet string appear.
///
/// Returns a grenad reader with the list of extracted facet strings and
/// documents ids from the given chunk of docid facet string positions.
#[logging_timer::time]
pub fn extract_facet_string_docids<R: io::Read + io::Seek>(
    docid_fid_facet_string: grenad::Reader<R>,
    indexer: GrenadParameters,
) -> Result<grenad::Reader<File>> {
    let max_memory = indexer.max_memory_by_thread();

    let mut facet_string_docids_sorter = create_sorter(
        grenad::SortAlgorithm::Stable,
        merge_cbo_roaring_bitmaps, // TODO: check that it is correct
        indexer.chunk_compression_type,
        indexer.chunk_compression_level,
        indexer.max_nb_chunks,
        max_memory,
    );

    let mut cursor = docid_fid_facet_string.into_cursor()?;
    while let Some((key, _original_value_bytes)) = cursor.move_on_next()? {
        let (field_id_bytes, bytes) = try_split_array_at(key).unwrap();
        let field_id = FieldId::from_be_bytes(field_id_bytes);

        // document_id_bytes is a big-endian u32
        // merge_cbo_roaring_bitmap works with native endian u32s
        // that is a problem, I think

        let (document_id_bytes, normalized_value_bytes) =
            try_split_array_at::<_, 4>(bytes).unwrap();
        let document_id = u32::from_be_bytes(document_id_bytes);

        let normalised_value = std::str::from_utf8(normalized_value_bytes)?;
        let key = FacetGroupKey { field_id, level: 0, left_bound: normalised_value };
        let key_bytes = FacetGroupKeyCodec::<StrRefCodec>::bytes_encode(&key).unwrap();

        facet_string_docids_sorter.insert(&key_bytes, &document_id.to_ne_bytes())?;
    }

    sorter_into_reader(facet_string_docids_sorter, indexer)
}