structured-zstd 0.0.58

use crate::{
    blocks::block::BlockType,
    common::MAX_BLOCK_SIZE,
    encoding::{
        CompressionLevel, Matcher,
        block_header::BlockHeader,
        blocks::{compress_block, compress_block_with_post_split},
        frame_compressor::CompressState,
        incompressible::{
            block_looks_incompressible, block_looks_incompressible_strict,
            compression_level_allows_raw_fast_path, raw_skip_worth_asking,
        },
        match_generator::MatchGeneratorDriver,
        strategy::StrategyTag,
    },
};
use alloc::vec::Vec;

/// Compresses a single block using the shared compressed-block pipeline.
///
/// Used by all compressed levels (Fastest, Default, Better, Best, and numeric levels). The actual
/// compression quality is determined by the matcher backend in `state`,
/// not by this function.
///
/// # Parameters
/// - `state`: [`CompressState`] so the compressor can refer to data before
///   the start of this block
/// - `last_block`: Whether or not this block is going to be the last block in the frame
///   (needed because this info is written into the block header)
/// - `block_len`: The block is the first `block_len` bytes of the matcher's
///   [`uncommitted_input`](Matcher::uncommitted_input); it is committed here
/// - `output`: As the block is compressed, it's appended to `output`.
// Mirrors the per-block sidecar plumbing of its borrowed sibling
// (`compress_block_encoded_borrowed`): the lsm decompressed-size and
// optional XXH64 checksum out-params push the arg count past the lint's
// threshold. Bundling them into a struct would diverge from the established
// emit-fn shape for no readability gain.
#[allow(clippy::too_many_arguments)]
#[inline]
pub(crate) fn compress_block_encoded<M: Matcher>(
    state: &mut CompressState<M>,
    compression_level: CompressionLevel,
    last_block: bool,
    block_len: usize,
    output: &mut Vec<u8>,
    // Whether a dictionary is primed for this frame. A high-entropy block is
    // not necessarily incompressible when one is, so the raw-skip below raises
    // its bar rather than trusting the block's own bytes.
    dict_active: bool,
    // Per-physical-block decompressed (regenerated) size sidecar, in
    // block-emit order — 1:1 with `FrameEmitInfo.blocks`, same cardinality
    // discipline as the XXH64 checksum sidecar. Captured under `lsm` alone
    // (not gated on `hash`, not opt-in) because `FrameEmitInfo` is always
    // built under `lsm` and every block needs its `decompressed_size`.
    #[cfg(feature = "lsm")] block_decompressed_sizes: Option<&mut Vec<u32>>,
    #[cfg(all(feature = "lsm", feature = "hash"))] block_checksums: Option<&mut Vec<u32>>,
) -> BlockType {
    let block_size = block_len as u32;
    // Classify the block while the bytes are still uncommitted. Every query
    // here is read-only, so the borrow of the matcher's history coexists with
    // the `window_size()` probe.
    let bytes = &state.matcher.uncommitted_input()[..block_len];
    let rle_byte_opt = bytes
        .first()
        .copied()
        .filter(|f| bytes.iter().all(|x| x == f));
    // Order is by cost, cheapest question first, because every one of these
    // runs on every block: an RLE block and a window that forbids the skip are
    // constants; the dict probe is a constant `true` for the conservative
    // backends and a small table probe for Fast; the classifier reads a sample;
    // the repeat grid reads the WHOLE block.
    //
    // The grid is therefore asked last, and only about blocks the classifier
    // already called incompressible — the only blocks a repeat could save from
    // going out raw. That is also why recording there is enough: a block
    // duplicating a COMPRESSIBLE one is compressible itself, so it is never at
    // risk of being skipped and never needs to find its original here.
    let window_size = state.matcher.window_size();
    // An attached dictionary keeps the block on the search, full stop. Asking a
    // sample of thirty-odd positions whether the dictionary is relevant is the
    // wrong shape for a gate that DISCARDS the search: dictionary matches are
    // external to the block, so nothing else in this chain can see them, and a
    // block whose dictionary runs happen to fall between the sampled offsets
    // goes out raw although the search would have coded nearly all of it from
    // the dictionary. The classifier's own sample is safe in a way this one is
    // not — it measures the block against itself, and a repeat is what the grid
    // then catches.
    let dict_rejects_raw = dict_active;
    // The level and the window decide this for the whole frame, so it also says
    // whether the grid is worth keeping at all: nothing it records can be acted
    // on where no block may go out raw, and recording there would take its
    // tables and hash a run of every block for an answer no one asks for.
    let raw_skip_reachable = compression_level_allows_raw_fast_path(compression_level, window_size);
    let classifier_asked = raw_skip_worth_asking(
        state.literal_compression_disabled,
        state.workspace.on_fresh_pages(),
        block_len,
    );
    let looks_incompressible = rle_byte_opt.is_none()
        && !dict_rejects_raw
        && raw_skip_reachable
        && classifier_asked
        && should_emit_raw_fast_path(compression_level, bytes);
    let repeats_earlier_content = if looks_incompressible {
        state
            .seen_content
            .record_and_report_repeat(bytes, window_size as usize)
    } else {
        // Searched, so the matcher will hold it for as long as the window does.
        // It still has to be RECORDED, or a later block made mostly of this one
        // finds nothing on the grid and goes out raw with the match sitting in
        // history. Recording without probing: the probe is what costs.
        //
        // A block of one repeated byte is the exception: anything that
        // duplicates it is itself one repeated byte, and such a block is
        // answered as RLE above without ever asking the grid. Recording it is a
        // key every `RECORD_STEP` bytes for a question nobody puts.
        //
        // Nor is a frame's last block recorded: nothing reads the grid after
        // it, so the keys would be hashed for nobody.
        if raw_skip_reachable && (rle_byte_opt.is_some() || last_block) {
            state.seen_content.skip_recording(bytes.len());
        } else if raw_skip_reachable && !classifier_asked {
            state
                .seen_content
                .record_unclassified(bytes, window_size as usize);
        } else if raw_skip_reachable {
            state
                .seen_content
                .record_searched(bytes, window_size as usize);
        }
        false
    };
    let raw_fast_path = looks_incompressible && !repeats_earlier_content;
    // Hashed once, from the pre-commit view, and reused by whichever branch
    // wins — the compressed branch covers the same bytes as the RLE and raw
    // ones. This is a whole-block pass, so it is skipped whenever nothing will
    // consume it: when no sink collects checksums (the common case), and when
    // the block is headed for the post-split helper, which emits several
    // physical blocks and records a checksum per partition of its own.
    let post_split =
        state.post_split_allowed && post_split_enabled(state.strategy_tag, window_size);
    #[cfg(all(feature = "lsm", feature = "hash"))]
    let post_split_path = rle_byte_opt.is_none() && !raw_fast_path && post_split;
    #[cfg(all(feature = "lsm", feature = "hash"))]
    let precomputed_checksum = block_checksums
        .as_ref()
        .filter(|_| !post_split_path)
        .map(|_| crate::encoding::frame_compressor::xxh64_block_low32(bytes));

    // First check to see if run length encoding can be used for the entire block
    if let Some(rle_byte) = rle_byte_opt {
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            sink.push(precomputed_checksum.expect("checksum is hashed whenever a sink exists"));
        }
        state.matcher.commit_filled(block_len);
        state.matcher.skip_matching_with_hint(Some(false));
        let header = BlockHeader {
            last_block,
            block_type: BlockType::RLE,
            block_size,
        };
        // Write the header, then the block
        header.serialize(output);
        output.push(rle_byte);
        BlockType::RLE
    } else if raw_fast_path {
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            sink.push(precomputed_checksum.expect("checksum is hashed whenever a sink exists"));
        }
        state.matcher.commit_filled(block_len);
        state.matcher.skip_matching_with_hint(Some(true));
        let header = BlockHeader {
            last_block,
            block_type: BlockType::Raw,
            block_size,
        };
        header.serialize(output);
        output.extend_from_slice(state.matcher.get_last_space());
        BlockType::Raw
    } else {
        // Compress as a standard compressed block
        state.matcher.commit_filled(block_len);
        if post_split {
            // This helper may emit multiple physical blocks (compressed or raw)
            // into `output`; the decompressed-size and (if requested) checksum
            // sidecars are pushed per physical block from inside the partition
            // loop so the cardinality matches the decoder's per-block count
            // exactly.
            #[cfg(all(feature = "lsm", feature = "hash"))]
            compress_block_with_post_split(
                state,
                last_block,
                output,
                block_decompressed_sizes,
                block_checksums,
            );
            #[cfg(all(feature = "lsm", not(feature = "hash")))]
            compress_block_with_post_split(state, last_block, output, block_decompressed_sizes);
            #[cfg(not(feature = "lsm"))]
            compress_block_with_post_split(state, last_block, output);
            return BlockType::Compressed;
        }
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            // The pre-commit view covers exactly the bytes the decoder will see
            // for this block, so the hash taken above stands — no second pass
            // over the committed copy.
            sink.push(precomputed_checksum.expect("checksum is hashed whenever a sink exists"));
        }

        // Keep rollback snapshots for the oversize fallback path below:
        // `compress_block` can mutate entropy/history state before we know
        // whether the compressed payload fits `MAX_BLOCK_SIZE`.
        let saved_offset_hist = state.offset_hist;
        // Snapshot the Huffman table into the scratch's persistent rollback
        // slot: `clone_from` reuses the slot's buffers across blocks (a
        // fresh `.clone()` paid a malloc + free pair on both code containers
        // every block). FSE previous tables are `SharedFseTable` handles —
        // their clone is a refcount bump, no slot needed.
        let mut saved_huff_table = core::mem::take(&mut state.block_scratch.huff_rollback);
        // Only when there IS a table to snapshot. `Option::clone_from` from a
        // `None` source drops what the slot holds, which is every buffer this
        // persistent slot exists to keep: a run of blocks that build a table and
        // then lose the size test would free and rebuild it every time.
        let had_prior_huff_table = state.last_huff_table.is_some();
        if let Some(prior) = state.last_huff_table.as_ref() {
            match saved_huff_table.as_mut() {
                Some(slot) => slot.clone_from(prior),
                None => saved_huff_table = Some(prior.clone()),
            }
        }
        let saved_ll_previous = state.fse_tables.ll_previous.clone();
        let saved_ml_previous = state.fse_tables.ml_previous.clone();
        let saved_of_previous = state.fse_tables.of_previous.clone();
        // Compress directly into `output`: reserve the fixed 3-byte block
        // header, append the payload after it, then backfill the header in
        // place once its length is known — no temp `Vec`, no extend-copy.
        let hdr_off = output.len();
        output.extend_from_slice(&[0u8; 3]);
        let payload_off = output.len();
        compress_block(state, output);
        let payload_len = output.len() - payload_off;
        // Fall back to a raw block when the compressed payload is not
        // smaller than the source (`payload >= block_size`) or exceeds the
        // maximum block size. A compressed block that did not shrink is never
        // the right choice: it wastes bytes and, in a single-segment frame
        // (window == content size), can reference past the declared window
        // and fail to decode in a strict decoder. This mirrors the upstream
        // post-hoc raw fallback and applies to every block, dictionary-primed
        // or not — the pre-compression raw-fast-path only catches blocks that
        // already look incompressible, so small inputs that slip past it but
        // fail to shrink still need this post-hoc store-raw.
        if payload_len >= MAX_BLOCK_SIZE as usize || payload_len >= block_size as usize {
            // Roll back the payload + reserved header and the entropy state.
            output.truncate(hdr_off);
            state.offset_hist = saved_offset_hist;
            // Swap (not move) so the slot keeps owning a reusable table
            // allocation for the next block's snapshot. With no prior table
            // there is nothing to restore, and the table this block built goes
            // into the slot rather than being dropped.
            if had_prior_huff_table {
                core::mem::swap(&mut state.last_huff_table, &mut saved_huff_table);
            } else if let Some(built) = state.last_huff_table.take() {
                saved_huff_table = Some(built);
            }
            // A block that built a table and then chose raw or RLE literals
            // parked it on its way there, so the swap above had nothing to give
            // back and the slot would go into the next block empty — which is
            // the per-block allocation of both code buffers this slot exists to
            // avoid, on exactly the run of blocks that keeps failing the size
            // test. Take the parked table instead: the slot is asked for one
            // per block, the spare's other reader once per frame.
            if saved_huff_table.is_none() {
                saved_huff_table = state.huff_table_spare.take();
            }
            state.fse_tables.roll_back_confirmation([
                saved_ll_previous,
                saved_ml_previous,
                saved_of_previous,
            ]);
            state.block_scratch.huff_rollback = saved_huff_table;
            let header = BlockHeader {
                last_block,
                block_type: BlockType::Raw,
                block_size,
            };
            // Write the header, then the block
            header.serialize(output);
            output.extend_from_slice(state.matcher.get_last_space());
            BlockType::Raw
        } else {
            // Return the snapshot to its slot so the next block's
            // `clone_from` reuses the allocation.
            state.block_scratch.huff_rollback = saved_huff_table;
            let header = BlockHeader {
                last_block,
                block_type: BlockType::Compressed,
                block_size: payload_len as u32,
            };
            // Backfill the reserved 3-byte header in place.
            output[hdr_off..hdr_off + 3].copy_from_slice(&header.to_le_bytes());
            BlockType::Compressed
        }
    }
}

/// Borrowed one-shot variant of [`compress_block_encoded`] for the Fast
/// (Simple) backend: the block bytes live at `[block_start, block_end)`
/// of the matcher's registered borrowed window (`set_borrowed_window`),
/// so nothing is committed into the history. Instead the range
/// is staged via `set_borrowed_block`, which routes the subsequent
/// `start_matching` / `skip_matching_with_hint` to the borrowed scan.
///
/// Mirrors `compress_block_encoded`'s RLE / raw-fast-path / compressed
/// branch selection and shares the heavy `compress_block` machinery; the
/// only differences are how the block is acquired (borrowed slice, no
/// copy) and that raw/RLE bodies are emitted straight from `block`. The
/// post-split branch is unreachable here (it needs an optimal-band
/// strategy, and the borrowed path is gated to the fast one), so it is
/// omitted.
#[allow(clippy::too_many_arguments)]
pub(crate) fn compress_block_encoded_borrowed(
    state: &mut CompressState<MatchGeneratorDriver>,
    compression_level: CompressionLevel,
    last_block: bool,
    block: &[u8],
    block_start: usize,
    block_end: usize,
    output: &mut Vec<u8>,
    dict_active: bool,
    #[cfg(feature = "lsm")] block_decompressed_sizes: Option<&mut Vec<u32>>,
    #[cfg(all(feature = "lsm", feature = "hash"))] block_checksums: Option<&mut Vec<u32>>,
) -> BlockType {
    // The borrowed one-shot path emits ONE block per staged range (no
    // pre-split partition loop). `borrowed_supported()` is the single source
    // of truth for which backend + search configs have a borrowed scan
    // (Simple / Dfast / Row, and HashChain's lazy CHAIN parser + btlazy2); the
    // optimal BT search stays on the owned path. `borrowed_eligible` gates on
    // the same predicate, so this only ever fires on a wiring bug. Checked at
    // entry (not per-branch) so RLE / raw-fast / compressed paths all stage
    // their borrowed range under the same invariant.
    debug_assert!(
        state.matcher.borrowed_supported(),
        "borrowed one-shot path reached for an unsupported backend/search config",
    );
    let block_size = block.len() as u32;
    // Same order as the owned path, cheapest question first: the whole-block
    // grid is asked last and only about blocks the classifier already called
    // incompressible.
    let is_rle = !block.is_empty() && block.iter().all(|x| block[0].eq(x));
    let window_size = state.matcher.window_size();
    // As on the owned path: an attached dictionary keeps the block on the
    // search rather than trusting a sample to say the dictionary is irrelevant.
    let dict_rejects_raw = dict_active;
    // As on the owned path: where no block may go out raw, the grid has nothing
    // to answer and is left alone.
    let raw_skip_reachable = compression_level_allows_raw_fast_path(compression_level, window_size);
    let classifier_asked = raw_skip_worth_asking(
        state.literal_compression_disabled,
        state.workspace.on_fresh_pages(),
        block.len(),
    );
    let looks_incompressible = !is_rle
        && !dict_rejects_raw
        && raw_skip_reachable
        && classifier_asked
        && should_emit_raw_fast_path(compression_level, block);
    let repeats_earlier_content = if looks_incompressible {
        state
            .seen_content
            .record_and_report_repeat(block, window_size as usize)
    } else {
        // As on the owned path: a searched block is recorded, not merely
        // stepped over, except a block of one repeated byte, which nothing
        // will ever ask the grid about, and a frame's last block, which
        // nothing reads the grid after.
        if raw_skip_reachable && (is_rle || last_block) {
            state.seen_content.skip_recording(block.len());
        } else if raw_skip_reachable && !classifier_asked {
            state
                .seen_content
                .record_unclassified(block, window_size as usize);
        } else if raw_skip_reachable {
            state
                .seen_content
                .record_searched(block, window_size as usize);
        }
        false
    };
    if is_rle {
        let rle_byte = block[0];
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            sink.push(crate::encoding::frame_compressor::xxh64_block_low32(block));
        }
        state.matcher.set_borrowed_block(block_start, block_end);
        state.matcher.skip_matching_with_hint(Some(false));
        let header = BlockHeader {
            last_block,
            block_type: BlockType::RLE,
            block_size,
        };
        header.serialize(output);
        output.push(rle_byte);
        BlockType::RLE
    } else if looks_incompressible && !repeats_earlier_content {
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            sink.push(crate::encoding::frame_compressor::xxh64_block_low32(block));
        }
        state.matcher.set_borrowed_block(block_start, block_end);
        state.matcher.skip_matching_with_hint(Some(true));
        let header = BlockHeader {
            last_block,
            block_type: BlockType::Raw,
            block_size,
        };
        header.serialize(output);
        output.extend_from_slice(block);
        BlockType::Raw
    } else {
        // Stage the borrowed range so `compress_block`'s internal
        // `start_matching` scans it in place (no copy into the history).
        state.matcher.set_borrowed_block(block_start, block_end);
        // No post-split branch here: the optimal levels (16-22), the only
        // strategies that post-split, are NOT borrowed-eligible
        // (`borrowed_supported` keeps them owned because the borrowed
        // continuous-index scan yields ratio-worse candidates for their
        // cost-based DP). btlazy2 (L13-15) and every other borrowed backend
        // emit a single block per staged range, handled by the path below.
        #[cfg(feature = "lsm")]
        if let Some(sink) = block_decompressed_sizes {
            sink.push(block_size);
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        if let Some(sink) = block_checksums {
            // Hash the block bytes directly: the staged borrowed range is
            // consumed by the `start_matching` inside `compress_block`
            // below, so hashing `block` is both correct and order-safe.
            sink.push(crate::encoding::frame_compressor::xxh64_block_low32(block));
        }
        let saved_offset_hist = state.offset_hist;
        // Persistent rollback slot — same allocation-reuse rationale as the
        // owned `compress_block_encoded` snapshot above.
        let mut saved_huff_table = core::mem::take(&mut state.block_scratch.huff_rollback);
        // Only when there IS a table to snapshot. `Option::clone_from` from a
        // `None` source drops what the slot holds, which is every buffer this
        // persistent slot exists to keep: a run of blocks that build a table and
        // then lose the size test would free and rebuild it every time.
        let had_prior_huff_table = state.last_huff_table.is_some();
        if let Some(prior) = state.last_huff_table.as_ref() {
            match saved_huff_table.as_mut() {
                Some(slot) => slot.clone_from(prior),
                None => saved_huff_table = Some(prior.clone()),
            }
        }
        let saved_ll_previous = state.fse_tables.ll_previous.clone();
        let saved_ml_previous = state.fse_tables.ml_previous.clone();
        let saved_of_previous = state.fse_tables.of_previous.clone();
        // Compress directly into `output`: reserve the fixed 3-byte block
        // header, append the payload after it, then backfill the header in
        // place once its length is known. Avoids the per-block temp `Vec`
        // plus the `output.extend(compressed)` copy (the dominant per-frame
        // memmove on this hot path).
        let hdr_off = output.len();
        output.extend_from_slice(&[0u8; 3]);
        let payload_off = output.len();
        compress_block(state, output);
        let payload_len = output.len() - payload_off;
        if payload_len >= MAX_BLOCK_SIZE as usize || payload_len >= block_size as usize {
            // Incompressible (compressed payload not smaller than the source,
            // or over the max block size): roll back the payload + reserved
            // header and the entropy state, then emit a stored Raw block. A
            // non-shrinking compressed block wastes bytes and can reference
            // past a single-segment frame's window (== content size); storing
            // raw matches the upstream post-hoc fallback.
            output.truncate(hdr_off);
            state.offset_hist = saved_offset_hist;
            // Swap (not move) so the slot keeps owning a reusable table
            // allocation for the next block's snapshot. With no prior table
            // there is nothing to restore, and the table this block built goes
            // into the slot rather than being dropped.
            if had_prior_huff_table {
                core::mem::swap(&mut state.last_huff_table, &mut saved_huff_table);
            } else if let Some(built) = state.last_huff_table.take() {
                saved_huff_table = Some(built);
            }
            // A block that built a table and then chose raw or RLE literals
            // parked it on its way there, so the swap above had nothing to give
            // back and the slot would go into the next block empty — which is
            // the per-block allocation of both code buffers this slot exists to
            // avoid, on exactly the run of blocks that keeps failing the size
            // test. Take the parked table instead: the slot is asked for one
            // per block, the spare's other reader once per frame.
            if saved_huff_table.is_none() {
                saved_huff_table = state.huff_table_spare.take();
            }
            state.fse_tables.roll_back_confirmation([
                saved_ll_previous,
                saved_ml_previous,
                saved_of_previous,
            ]);
            state.block_scratch.huff_rollback = saved_huff_table;
            let header = BlockHeader {
                last_block,
                block_type: BlockType::Raw,
                block_size,
            };
            header.serialize(output);
            output.extend_from_slice(block);
            BlockType::Raw
        } else {
            // Return the snapshot to its slot so the next block's
            // `clone_from` reuses the allocation.
            state.block_scratch.huff_rollback = saved_huff_table;
            let header = BlockHeader {
                last_block,
                block_type: BlockType::Compressed,
                block_size: payload_len as u32,
            };
            output[hdr_off..hdr_off + 3].copy_from_slice(&header.to_le_bytes());
            BlockType::Compressed
        }
    }
}

/// Whether a compressed block goes through the post-split pass, which may cut
/// it into several blocks along its sequences. Decided by the strategy the
/// frame runs, not by its level, as upstream zstd decides it
/// (`zstd_compress.c`, `ZSTD_resolveBlockSplitterMode`: `strategy >= btopt &&
/// windowLog >= 17`), so a parameter set that moves the strategy moves the
/// pass with it.
///
/// Turning the pass off is not a speed lever. Gated by level it was skipped on
/// the btopt / btultra frames of L13-15, which ran faster there only by paying
/// 6-9% in bytes (z000033[..200000], L13: 90378 against 85017 with the pass).
/// Where the pass costs time, the cost is in the splitter itself, and that is
/// what has to get cheaper.
#[inline]
fn post_split_enabled(strategy_tag: StrategyTag, window_size: u64) -> bool {
    matches!(
        strategy_tag,
        StrategyTag::BtOpt | StrategyTag::BtUltra | StrategyTag::BtUltra2
    ) && window_size >= 1 << 17
}

/// Whether this block may go out raw without being searched.
///
/// The classifier answers from the block's own bytes, which cannot see a repeat
/// that lives in the history, so the caller pairs it with a probe of the match
/// table before acting on it. Callers ask it only after the level and window
/// admit a raw skip at all, and only with no dictionary attached — one keeps the
/// block on the search whatever its own bytes look like.
#[inline]
fn should_emit_raw_fast_path(level: CompressionLevel, block: &[u8]) -> bool {
    if matches!(level, CompressionLevel::Best) {
        return block_looks_incompressible_strict(block);
    }
    block_looks_incompressible(block)
}

#[cfg(test)]
mod tests;