vortex-btrblocks 0.74.0

BtrBlocks style compressor
Documentation 
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// SPDX-License-Identifier: Apache-2.0
// SPDX-FileCopyrightText: Copyright the Vortex contributors

//! FSST (Fast Static Symbol Table) string compression.

use vortex_array::ArrayRef;
use vortex_array::Canonical;
use vortex_array::ExecutionCtx;
use vortex_array::IntoArray;
use vortex_array::arrays::PrimitiveArray;
use vortex_array::arrays::VarBinArray;
use vortex_array::arrays::primitive::PrimitiveArrayExt;
use vortex_array::arrays::varbin::VarBinArrayExt;
use vortex_compressor::estimate::CompressionEstimate;
use vortex_compressor::estimate::DeferredEstimate;
use vortex_error::VortexResult;
use vortex_fsst::FSST;
use vortex_fsst::FSSTArrayExt;
use vortex_fsst::fsst_compress;
use vortex_fsst::fsst_train_compressor;

use crate::ArrayAndStats;
use crate::CascadingCompressor;
use crate::CompressorContext;
use crate::Scheme;
use crate::SchemeExt;

/// FSST (Fast Static Symbol Table) compression.
///
/// One of the two string-fragmentation schemes in the default
/// [`crate::ALL_SCHEMES`] (alongside `OnPairScheme`); the sample-based selector
/// keeps whichever is smaller per column. FSST compresses faster, OnPair
/// usually wins on ratio.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub struct FSSTScheme;

impl Scheme for FSSTScheme {
    fn scheme_name(&self) -> &'static str {
        "vortex.string.fsst"
    }

    fn matches(&self, canonical: &Canonical) -> bool {
        canonical.dtype().is_utf8()
    }

    /// Children: lengths=0, code_offsets=1.
    fn num_children(&self) -> usize {
        2
    }

    fn expected_compression_ratio(
        &self,
        _data: &ArrayAndStats,
        _compress_ctx: CompressorContext,
        _exec_ctx: &mut ExecutionCtx,
    ) -> CompressionEstimate {
        CompressionEstimate::Deferred(DeferredEstimate::Sample)
    }

    fn compress(
        &self,
        compressor: &CascadingCompressor,
        data: &ArrayAndStats,
        compress_ctx: CompressorContext,
        exec_ctx: &mut ExecutionCtx,
    ) -> VortexResult<ArrayRef> {
        let utf8 = data.array_as_varbinview().into_owned();
        let compressor_fsst = fsst_train_compressor(&utf8);
        let fsst = fsst_compress(&utf8, utf8.len(), utf8.dtype(), &compressor_fsst, exec_ctx);

        let uncompressed_lengths_primitive = fsst
            .uncompressed_lengths()
            .clone()
            .execute::<PrimitiveArray>(exec_ctx)?
            .narrow(exec_ctx)?;
        let compressed_original_lengths = compressor.compress_child(
            &uncompressed_lengths_primitive.into_array(),
            &compress_ctx,
            self.id(),
            0,
            exec_ctx,
        )?;

        let codes_offsets_primitive = fsst
            .codes()
            .offsets()
            .clone()
            .execute::<PrimitiveArray>(exec_ctx)?
            .narrow(exec_ctx)?;
        let compressed_codes_offsets = compressor.compress_child(
            &codes_offsets_primitive.into_array(),
            &compress_ctx,
            self.id(),
            1,
            exec_ctx,
        )?;
        let compressed_codes = VarBinArray::try_new(
            compressed_codes_offsets,
            fsst.codes().bytes().clone(),
            fsst.codes().dtype().clone(),
            fsst.codes().validity()?,
        )?;

        let fsst = FSST::try_new(
            fsst.dtype().clone(),
            fsst.symbols().clone(),
            fsst.symbol_lengths().clone(),
            compressed_codes,
            compressed_original_lengths,
            exec_ctx,
        )?;

        Ok(fsst.into_array())
    }
}