structured_zstd/encoding/

frame_compressor.rs

//! Utilities and interfaces for encoding an entire frame. Allows reusing resources

use alloc::{boxed::Box, vec::Vec};
use core::convert::TryInto;
#[cfg(feature = "hash")]
use twox_hash::XxHash64;

#[cfg(feature = "hash")]
use core::hash::Hasher;

use super::{
    CompressionLevel, Matcher, block_header::BlockHeader, frame_header::FrameHeader, levels::*,
    match_generator::MatchGeneratorDriver,
};
use crate::common::MAX_BLOCK_SIZE;
use crate::fse::fse_encoder::{FSETable, default_ll_table, default_ml_table, default_of_table};

use crate::io::{Read, Write};

/// An interface for compressing arbitrary data with the ZStandard compression algorithm.
///
/// `FrameCompressor` will generally be used by:
/// 1. Initializing a compressor by providing a buffer of data using `FrameCompressor::new()`
/// 2. Starting compression and writing that compression into a vec using `FrameCompressor::begin`
///
/// # Examples
/// ```
/// use structured_zstd::encoding::{FrameCompressor, CompressionLevel};
/// let mock_data: &[_] = &[0x1, 0x2, 0x3, 0x4];
/// let mut output = std::vec::Vec::new();
/// // Initialize a compressor.
/// let mut compressor = FrameCompressor::new(CompressionLevel::Uncompressed);
/// compressor.set_source(mock_data);
/// compressor.set_drain(&mut output);
///
/// // `compress` writes the compressed output into the provided buffer.
/// compressor.compress();
/// ```
pub struct FrameCompressor<R: Read, W: Write, M: Matcher> {
    uncompressed_data: Option<R>,
    compressed_data: Option<W>,
    compression_level: CompressionLevel,
    dictionary: Option<crate::decoding::Dictionary>,
    dictionary_entropy_cache: Option<CachedDictionaryEntropy>,
    source_size_hint: Option<u64>,
    state: CompressState<M>,
    /// When true, emitted frames omit the 4-byte magic number prefix
    /// (`ZSTD_f_zstd1_magicless`). Default false. The caller is
    /// responsible for ensuring the decoder is configured for the
    /// matching format — wire-format only round-trips with a
    /// magicless-aware decoder.
    magicless: bool,
    #[cfg(feature = "hash")]
    hasher: XxHash64,
    /// Block-layout introspection populated at the end of every
    /// successful `compress()`. `None` until the first call.
    /// Behind the `lsm` feature gate.
    #[cfg(feature = "lsm")]
    frame_emit_info: Option<crate::encoding::frame_emit_info::FrameEmitInfo>,
    /// When `true`, `compress()` XXH64-hashes each block's
    /// uncompressed bytes and appends the low-32-bit digest to
    /// `block_checksums`. Default `false` (zero cost). Gated on
    /// `all(lsm, hash)` because XXH64 lives behind the `hash`
    /// feature; an `lsm`-only build has no way to compute digests.
    #[cfg(all(feature = "lsm", feature = "hash"))]
    per_block_checksums_enabled: bool,
    /// Per-block XXH64 (low 32 bits) digests captured during
    /// `compress()` when `per_block_checksums_enabled` is set. Ordered
    /// by block-emit order. `None` until the first call after enabling.
    /// Gated on `all(lsm, hash)` (see `per_block_checksums_enabled`).
    #[cfg(all(feature = "lsm", feature = "hash"))]
    block_checksums: Option<alloc::vec::Vec<u32>>,
}

#[derive(Clone, Default)]
struct CachedDictionaryEntropy {
    huff: Option<crate::huff0::huff0_encoder::HuffmanTable>,
    ll_previous: Option<PreviousFseTable>,
    ml_previous: Option<PreviousFseTable>,
    of_previous: Option<PreviousFseTable>,
}

#[derive(Clone)]
pub(crate) enum PreviousFseTable {
    // Default tables are immutable and already stored alongside the state, so
    // repeating them only needs a lightweight marker instead of cloning FSETable.
    Default,
    Custom(Box<FSETable>),
    Rle(u8),
}

impl PreviousFseTable {
    pub(crate) fn as_table<'a>(&'a self, default: &'a FSETable) -> Option<&'a FSETable> {
        match self {
            Self::Default => Some(default),
            Self::Custom(table) => Some(table),
            Self::Rle(_) => None,
        }
    }
}

pub(crate) struct FseTables {
    /// The three predefined LL/ML/OF tables are functions of
    /// compile-time-constant distributions. The
    /// [`fse_encoder::FseDefaultTable`] type alias resolves to
    /// `&'static FSETable` when a process-wide cache is available
    /// (atomic-pointer targets, or no-atomic targets with the
    /// `critical-section` feature) and to `Box<FSETable>` on the
    /// cache-less no-atomic path (one per-frame allocation, dropped
    /// with the compressor — no `Box::leak`, no unbounded growth).
    /// Both arms `Deref` to `FSETable`, so consumers in
    /// `encoding/blocks/compressed.rs` borrow through `&` uniformly
    /// without seeing the per-target divergence.
    pub(crate) ll_default: crate::fse::fse_encoder::FseDefaultTable,
    pub(crate) ll_previous: Option<PreviousFseTable>,
    pub(crate) ml_default: crate::fse::fse_encoder::FseDefaultTable,
    pub(crate) ml_previous: Option<PreviousFseTable>,
    pub(crate) of_default: crate::fse::fse_encoder::FseDefaultTable,
    pub(crate) of_previous: Option<PreviousFseTable>,
}

impl FseTables {
    pub fn new() -> Self {
        Self {
            ll_default: default_ll_table(),
            ll_previous: None,
            ml_default: default_ml_table(),
            ml_previous: None,
            of_default: default_of_table(),
            of_previous: None,
        }
    }

    /// Borrow the LL default table as `&FSETable`. Abstracts the cfg
    /// split in [`crate::fse::fse_encoder::FseDefaultTable`] —
    /// `&'static FSETable` (atomic / `critical-section`) auto-derefs
    /// directly; `Box<FSETable>` (cache-less no-atomic) derefs
    /// through `Box`. Both arms yield `&FSETable` uniformly so
    /// downstream consumers can stay cfg-agnostic.
    #[inline]
    #[allow(clippy::borrow_deref_ref)]
    pub(crate) fn ll_default_ref(&self) -> &FSETable {
        &*self.ll_default
    }

    /// Borrow the ML default table as `&FSETable`. See [`Self::ll_default_ref`].
    #[inline]
    #[allow(clippy::borrow_deref_ref)]
    pub(crate) fn ml_default_ref(&self) -> &FSETable {
        &*self.ml_default
    }

    /// Borrow the OF default table as `&FSETable`. See [`Self::ll_default_ref`].
    #[inline]
    #[allow(clippy::borrow_deref_ref)]
    pub(crate) fn of_default_ref(&self) -> &FSETable {
        &*self.of_default
    }
}

const PRESPLIT_BLOCK_MIN: usize = 3500;
const PRESPLIT_THRESHOLD_PENALTY_RATE: u64 = 16;
const PRESPLIT_THRESHOLD_BASE: u64 = PRESPLIT_THRESHOLD_PENALTY_RATE - 2;
const PRESPLIT_THRESHOLD_PENALTY: i32 = 3;
const PRESPLIT_CHUNK_SIZE: usize = 8 << 10;
const PRESPLIT_HASH_LOG_MAX: usize = 10;
const PRESPLIT_HASH_TABLE_SIZE: usize = 1 << PRESPLIT_HASH_LOG_MAX;
const PRESPLIT_KNUTH: u32 = 0x9E37_79B9;
/// Donor `SEGMENT_SIZE` in `ZSTD_splitBlock_fromBorders` (`zstd_preSplit.c:201`).
/// Two `SEGMENT_SIZE`-byte fingerprints — one from the start, one from the end —
/// drive the cheap border heuristic; a third one from the middle disambiguates
/// where in the block the transition sits.
const PRESPLIT_BORDERS_SEGMENT: usize = 512;

#[derive(Clone)]
struct PreSplitFingerprint {
    events: [u32; PRESPLIT_HASH_TABLE_SIZE],
    nb_events: usize,
}

impl Default for PreSplitFingerprint {
    fn default() -> Self {
        Self {
            events: [0; PRESPLIT_HASH_TABLE_SIZE],
            nb_events: 0,
        }
    }
}

fn presplit_hash2(bytes: &[u8], hash_log: usize) -> usize {
    debug_assert!(hash_log >= 8);
    if hash_log == 8 {
        return bytes[0] as usize;
    }
    debug_assert!(hash_log <= PRESPLIT_HASH_LOG_MAX);
    let value = u16::from_le_bytes([bytes[0], bytes[1]]) as u32;
    (value.wrapping_mul(PRESPLIT_KNUTH) >> (32 - hash_log)) as usize
}

fn presplit_record_fingerprint(
    fp: &mut PreSplitFingerprint,
    src: &[u8],
    sampling_rate: usize,
    hash_log: usize,
) {
    fp.events.fill(0);
    fp.nb_events = 0;
    if src.len() < 2 {
        return;
    }
    let limit = src.len() - 1;
    let mut n = 0usize;
    while n < limit {
        fp.events[presplit_hash2(&src[n..], hash_log)] += 1;
        n += sampling_rate;
    }
    // Donor parity: zstd_preSplit.c records the integer division, not the
    // rounded-up number of sampled events from the loop above.
    fp.nb_events += limit / sampling_rate;
}

/// Single-byte histogram pass — matches donor `HIST_add` over a small
/// segment with `hashLog == 8` (the `hash2` shortcut at
/// `zstd_preSplit.c:36` returns the raw byte). The byChunks path uses
/// 2-byte hashing for `hashLog >= 9`; this helper exists so the borders
/// heuristic doesn't pay for that wider hash on its 512-byte windows.
fn presplit_record_byte_histogram(fp: &mut PreSplitFingerprint, src: &[u8]) {
    fp.events.fill(0);
    for &b in src {
        fp.events[b as usize] += 1;
    }
    // Donor `HIST_add` returns the maximum symbol; the caller then sets
    // `nbEvents = SEGMENT_SIZE` explicitly (see `zstd_preSplit.c:213`).
    fp.nb_events = src.len();
}

fn presplit_distance(lhs: &PreSplitFingerprint, rhs: &PreSplitFingerprint, hash_log: usize) -> u64 {
    let slots = 1usize << hash_log;
    let mut distance = 0u64;
    for idx in 0..slots {
        let left = lhs.events[idx] as i128 * rhs.nb_events as i128;
        let right = rhs.events[idx] as i128 * lhs.nb_events as i128;
        distance = distance.saturating_add(left.abs_diff(right) as u64);
    }
    distance
}

fn presplit_fingerprints_differ(
    reference: &PreSplitFingerprint,
    new_fp: &PreSplitFingerprint,
    penalty: i32,
    hash_log: usize,
) -> bool {
    debug_assert!(reference.nb_events > 0);
    debug_assert!(new_fp.nb_events > 0);
    let p50 = reference.nb_events as u64 * new_fp.nb_events as u64;
    let deviation = presplit_distance(reference, new_fp, hash_log);
    let threshold = p50.saturating_mul(PRESPLIT_THRESHOLD_BASE + penalty as u64)
        / PRESPLIT_THRESHOLD_PENALTY_RATE;
    deviation >= threshold
}

fn presplit_merge_events(acc: &mut PreSplitFingerprint, new_fp: &PreSplitFingerprint) {
    for idx in 0..PRESPLIT_HASH_TABLE_SIZE {
        acc.events[idx] = acc.events[idx].saturating_add(new_fp.events[idx]);
    }
    acc.nb_events = acc.nb_events.saturating_add(new_fp.nb_events);
}

fn donor_split_block_by_chunks(block: &[u8], level: usize) -> usize {
    debug_assert_eq!(block.len(), MAX_BLOCK_SIZE as usize);
    debug_assert!((1..=4).contains(&level));
    let (sampling_rate, hash_log) = match level - 1 {
        0 => (43, 8),
        1 => (11, 9),
        2 => (5, 10),
        _ => (1, 10),
    };

    let mut past = PreSplitFingerprint::default();
    let mut new_events = PreSplitFingerprint::default();
    let mut penalty = PRESPLIT_THRESHOLD_PENALTY;
    presplit_record_fingerprint(
        &mut past,
        &block[..PRESPLIT_CHUNK_SIZE],
        sampling_rate,
        hash_log,
    );
    let mut pos = PRESPLIT_CHUNK_SIZE;
    while pos <= block.len() - PRESPLIT_CHUNK_SIZE {
        presplit_record_fingerprint(
            &mut new_events,
            &block[pos..pos + PRESPLIT_CHUNK_SIZE],
            sampling_rate,
            hash_log,
        );
        if presplit_fingerprints_differ(&past, &new_events, penalty, hash_log) {
            return pos;
        }
        presplit_merge_events(&mut past, &new_events);
        if penalty > 0 {
            penalty -= 1;
        }
        pos += PRESPLIT_CHUNK_SIZE;
    }
    block.len()
}

/// Donor port of `ZSTD_splitBlock_fromBorders` (`zstd_preSplit.c:198`).
/// Records two 512-byte byte-histograms — one from each end of a 128 KB
/// block — and a third from the middle as a tie-breaker; returns either
/// a quantised split point (32 KB / 64 KB / 96 KB) or the full block
/// size when the two ends look indistinguishable. Cheaper than the
/// chunk-based path because it touches at most 1.5 KB of input
/// regardless of block size.
fn donor_split_block_from_borders(block: &[u8]) -> usize {
    debug_assert_eq!(block.len(), MAX_BLOCK_SIZE as usize);
    let block_size = block.len();
    let mut past = PreSplitFingerprint::default();
    let mut new_fp = PreSplitFingerprint::default();
    presplit_record_byte_histogram(&mut past, &block[..PRESPLIT_BORDERS_SEGMENT]);
    presplit_record_byte_histogram(&mut new_fp, &block[block_size - PRESPLIT_BORDERS_SEGMENT..]);
    // Donor uses `penalty = 0, hash_log = 8` — i.e. raw byte histogram
    // distance with no threshold padding (`zstd_preSplit.c:214`).
    if !presplit_fingerprints_differ(&past, &new_fp, 0, 8) {
        return block_size;
    }

    let mut middle = PreSplitFingerprint::default();
    let mid_start = block_size / 2 - PRESPLIT_BORDERS_SEGMENT / 2;
    presplit_record_byte_histogram(
        &mut middle,
        &block[mid_start..mid_start + PRESPLIT_BORDERS_SEGMENT],
    );

    let dist_from_begin = presplit_distance(&past, &middle, 8);
    let dist_from_end = presplit_distance(&new_fp, &middle, 8);
    // Donor `SEGMENT_SIZE * SEGMENT_SIZE / 3` (`zstd_preSplit.c:221`):
    // if the middle is roughly equidistant from both ends, the change
    // sits near the centre — split at the midpoint.
    let min_distance = (PRESPLIT_BORDERS_SEGMENT as u64) * (PRESPLIT_BORDERS_SEGMENT as u64) / 3;
    if dist_from_begin.abs_diff(dist_from_end) < min_distance {
        return 64 * 1024;
    }
    // Larger `dist_from_begin` (i.e. `middle` farther from the head
    // fingerprint, equivalently closer to the tail) means the new
    // statistics already dominate the centre — the transition
    // happened EARLY → emit a small 32 KB head and let the 96 KB
    // tail absorb the rest. Inverse case: `dist_from_end` larger
    // (middle still resembles the head) means the transition is
    // LATE → emit a 96 KB head so the trailing 32 KB carries the
    // new statistics alone.
    if dist_from_begin > dist_from_end {
        32 * 1024
    } else {
        96 * 1024
    }
}

fn donor_pre_split_level(level: CompressionLevel) -> Option<usize> {
    match level {
        // Donor `ZSTD_blockSplitter_level` table (`clevels.h`): cheap
        // borders heuristic for lazy2 / btlazy2 strategies (levels
        // 11..=15) — the splitter still pays for itself on
        // heterogeneous payloads but the per-block cost stays bounded
        // by two 512-byte histograms.
        CompressionLevel::Level(11..=15) => Some(0),
        // C zstd's default splitter level for btopt/btultra/btultra2 is 4
        // (`ZSTD_splitBlock_byChunks` with internal level 3 — sampling
        // rate 1, `hashLog` 10).
        CompressionLevel::Level(16..=22) => Some(4),
        _ => None,
    }
}

/// XXH64 (low 32 bits, seed 0) over `data`. Shared helper for the
/// per-physical-block checksum sidecar so encoder and decoder hash
/// the exact same byte ranges with the exact same parameters. Gated
/// at `all(lsm, hash)` because the only consumer is the lsm-side
/// `block_checksums` sidecar; non-lsm builds carry no reference to
/// this helper at all.
#[cfg(all(feature = "lsm", feature = "hash"))]
#[inline]
pub(crate) fn xxh64_block_low32(data: &[u8]) -> u32 {
    let mut h = XxHash64::with_seed(0);
    h.write(data);
    h.finish() as u32
}

/// Bench-only entry point for the donor-parity comparator test in
/// `tests/block_splitter_donor_parity.rs`. Dispatches to the same
/// `_from_borders` (split_level == 0) / `_by_chunks` (split_level ∈
/// 1..=4) ports that `donor_optimal_block_size` itself routes
/// through. Caller is responsible for passing exactly
/// `MAX_BLOCK_SIZE` bytes (per donor `ZSTD_splitBlock` contract —
/// "@blockSize must be == 128 KB" in `zstd_preSplit.h`).
#[cfg(feature = "bench_internals")]
pub(crate) fn block_splitter_decision_for_bench(block: &[u8], split_level: usize) -> usize {
    assert_eq!(
        block.len(),
        MAX_BLOCK_SIZE as usize,
        "block_splitter_decision_for_bench expects exactly MAX_BLOCK_SIZE bytes"
    );
    assert!(
        split_level <= 4,
        "block_splitter_decision_for_bench: split_level must be in 0..=4, got {split_level}"
    );
    if split_level == 0 {
        donor_split_block_from_borders(block)
    } else {
        donor_split_block_by_chunks(block, split_level)
    }
}

pub(crate) fn donor_optimal_block_size(
    level: CompressionLevel,
    block: &[u8],
    remaining_src_size: usize,
    block_size_max: usize,
    savings: i64,
) -> usize {
    let Some(split_level) = donor_pre_split_level(level) else {
        return remaining_src_size.min(block_size_max);
    };
    if remaining_src_size < MAX_BLOCK_SIZE as usize || block_size_max < MAX_BLOCK_SIZE as usize {
        return remaining_src_size.min(block_size_max);
    }
    if savings < 3 {
        return MAX_BLOCK_SIZE as usize;
    }
    if block.len() < MAX_BLOCK_SIZE as usize {
        return remaining_src_size.min(block_size_max);
    }
    // Donor `ZSTD_splitBlock` dispatch (`zstd_preSplit.c:234`):
    // `split_level == 0` → cheap borders heuristic;
    // `split_level == 1..=4` → byChunks with internal sampling level
    // `split_level - 1`.
    let raw_split = if split_level == 0 {
        donor_split_block_from_borders(&block[..MAX_BLOCK_SIZE as usize])
    } else {
        donor_split_block_by_chunks(&block[..MAX_BLOCK_SIZE as usize], split_level)
    };
    raw_split
        .max(PRESPLIT_BLOCK_MIN)
        .min(MAX_BLOCK_SIZE as usize)
}

pub(crate) struct CompressState<M: Matcher> {
    pub(crate) matcher: M,
    pub(crate) last_huff_table: Option<crate::huff0::huff0_encoder::HuffmanTable>,
    pub(crate) fse_tables: FseTables,
    pub(crate) block_scratch: crate::encoding::blocks::CompressedBlockScratch,
    /// Offset history for repeat offset encoding: [rep0, rep1, rep2].
    /// Initialized to [1, 4, 8] per RFC 8878 §3.1.2.5.
    pub(crate) offset_hist: [u32; 3],
    /// Strategy tag resolved from the current `CompressionLevel` at every
    /// `matcher.reset()` call. Used by the literal-compression gates
    /// (`min_literals_to_compress`, `min_gain`) in
    /// `encoding::blocks::compressed` to mirror donor's strategy-aware
    /// thresholds (`zstd_compress_literals.c:114-127, 187-188`).
    ///
    /// **Invariant (required of every construction site):** must be
    /// initialized from the active `CompressionLevel` via
    /// `StrategyTag::for_compression_level`, and re-synced from the
    /// active level alongside every `matcher.reset()` call so the
    /// level-aware gates stay correct after a level change. The two
    /// reset sites that own this sync are `FrameCompressor::compress`
    /// and `StreamingEncoder::ensure_frame_started`. There is no
    /// `Default` impl — production constructors
    /// (`FrameCompressor::new`, `new_with_matcher`, the streaming
    /// encoder constructor) plumb this explicitly. Tests that build
    /// `CompressState` by hand must also supply a value.
    pub(crate) strategy_tag: crate::encoding::strategy::StrategyTag,
}

impl<R: Read, W: Write> FrameCompressor<R, W, MatchGeneratorDriver> {
    /// Create a new `FrameCompressor`
    pub fn new(compression_level: CompressionLevel) -> Self {
        Self {
            uncompressed_data: None,
            compressed_data: None,
            compression_level,
            dictionary: None,
            dictionary_entropy_cache: None,
            source_size_hint: None,
            state: CompressState {
                matcher: MatchGeneratorDriver::new(1024 * 128, 1),
                last_huff_table: None,
                fse_tables: FseTables::new(),
                block_scratch: crate::encoding::blocks::CompressedBlockScratch::new(),
                offset_hist: [1, 4, 8],
                strategy_tag: crate::encoding::strategy::StrategyTag::for_compression_level(
                    compression_level,
                ),
            },
            magicless: false,
            #[cfg(feature = "hash")]
            hasher: XxHash64::with_seed(0),
            #[cfg(feature = "lsm")]
            frame_emit_info: None,
            #[cfg(all(feature = "lsm", feature = "hash"))]
            per_block_checksums_enabled: false,
            #[cfg(all(feature = "lsm", feature = "hash"))]
            block_checksums: None,
        }
    }
}

impl<R: Read, W: Write, M: Matcher> FrameCompressor<R, W, M> {
    /// Create a new `FrameCompressor` with a custom matching algorithm implementation
    pub fn new_with_matcher(matcher: M, compression_level: CompressionLevel) -> Self {
        Self {
            uncompressed_data: None,
            compressed_data: None,
            dictionary: None,
            dictionary_entropy_cache: None,
            source_size_hint: None,
            state: CompressState {
                matcher,
                last_huff_table: None,
                fse_tables: FseTables::new(),
                block_scratch: crate::encoding::blocks::CompressedBlockScratch::new(),
                offset_hist: [1, 4, 8],
                strategy_tag: crate::encoding::strategy::StrategyTag::for_compression_level(
                    compression_level,
                ),
            },
            compression_level,
            magicless: false,
            #[cfg(feature = "hash")]
            hasher: XxHash64::with_seed(0),
            #[cfg(feature = "lsm")]
            frame_emit_info: None,
            #[cfg(all(feature = "lsm", feature = "hash"))]
            per_block_checksums_enabled: false,
            #[cfg(all(feature = "lsm", feature = "hash"))]
            block_checksums: None,
        }
    }

    /// Enable or disable magicless frame format (`ZSTD_f_zstd1_magicless`).
    ///
    /// When set to `true`, emitted frames omit the 4-byte magic number
    /// prefix. The matching decoder must be configured to expect a
    /// magicless stream — wire-format only round-trips with a
    /// magicless-aware decoder.
    pub fn set_magicless(&mut self, magicless: bool) {
        self.magicless = magicless;
    }

    /// Before calling [FrameCompressor::compress] you need to set the source.
    ///
    /// This is the data that is compressed and written into the drain.
    pub fn set_source(&mut self, uncompressed_data: R) -> Option<R> {
        self.uncompressed_data.replace(uncompressed_data)
    }

    /// Before calling [FrameCompressor::compress] you need to set the drain.
    ///
    /// As the compressor compresses data, the drain serves as a place for the output to be writte.
    pub fn set_drain(&mut self, compressed_data: W) -> Option<W> {
        self.compressed_data.replace(compressed_data)
    }

    /// Provide a hint about the total uncompressed size for the next frame.
    ///
    /// When set, the encoder selects smaller hash tables and windows for
    /// small inputs, matching the C zstd source-size-class behavior.
    ///
    /// This hint applies only to frame payload bytes (`size`). Dictionary
    /// history is primed separately and does not inflate the hinted size or
    /// advertised frame window.
    /// Must be called before [`compress`](Self::compress).
    pub fn set_source_size_hint(&mut self, size: u64) {
        self.source_size_hint = Some(size);
    }

    /// Compress the uncompressed data from the provided source as one Zstd frame and write it to the provided drain
    ///
    /// This will repeatedly call [Read::read] on the source to fill up blocks until the source returns 0 on the read call.
    /// All compressed blocks are buffered in memory so that the frame header can include the
    /// `Frame_Content_Size` field (which requires knowing the total uncompressed size). The
    /// entire frame — header, blocks, and optional checksum — is then written to the drain
    /// at the end. This means peak memory usage is O(compressed_size).
    ///
    /// To avoid endlessly encoding from a potentially endless source (like a network socket) you can use the
    /// [Read::take] function
    pub fn compress(&mut self) {
        // Reset per-frame introspection state so a re-used compressor
        // doesn't carry over the previous frame's layout/checksums.
        #[cfg(feature = "lsm")]
        {
            self.frame_emit_info = None;
        }
        #[cfg(all(feature = "lsm", feature = "hash"))]
        {
            if self.per_block_checksums_enabled {
                self.block_checksums = Some(alloc::vec::Vec::new());
            } else {
                self.block_checksums = None;
            }
        }
        let initial_size_hint = self.source_size_hint;
        let source_size_hint_known = initial_size_hint.is_some();
        let use_dictionary_state =
            !matches!(self.compression_level, CompressionLevel::Uncompressed)
                && self.state.matcher.supports_dictionary_priming()
                && self.dictionary.is_some();
        if let Some(size_hint) = self.source_size_hint.take() {
            // Keep source-size hint scoped to payload bytes; dictionary priming
            // is applied separately and should not force larger matcher sizing.
            self.state.matcher.set_source_size_hint(size_hint);
        }
        // Clearing buffers to allow re-using of the compressor
        self.state.matcher.reset(self.compression_level);
        self.state.offset_hist = [1, 4, 8];
        // Sync `state.strategy_tag` to the level resolved at this reset so
        // the literal-compression gates (`min_literals_to_compress` /
        // `min_gain` in `encoding::blocks::compressed`) see the correct
        // strategy for the next frame. Frame-by-frame level changes go
        // through this same `compress()` entry point, so re-syncing here
        // covers level switches without touching the matcher dispatch.
        self.state.strategy_tag =
            crate::encoding::strategy::StrategyTag::for_compression_level(self.compression_level);
        let cached_entropy = if use_dictionary_state {
            self.dictionary_entropy_cache.as_ref()
        } else {
            None
        };
        if use_dictionary_state && let Some(dict) = self.dictionary.as_ref() {
            // This state drives sequence encoding, while matcher priming below updates
            // the match generator's internal repeat-offset history for match finding.
            self.state.offset_hist = dict.offset_hist;
            self.state
                .matcher
                .prime_with_dictionary(dict.dict_content.as_slice(), dict.offset_hist);
        }
        if let Some(cache) = cached_entropy {
            self.state.last_huff_table.clone_from(&cache.huff);
        } else {
            self.state.last_huff_table = None;
        }
        // `clone_from` keeps frame-to-frame seeding cheap for reused compressors by
        // reusing existing allocations where possible instead of reallocating every frame.
        if let Some(cache) = cached_entropy {
            self.state
                .fse_tables
                .ll_previous
                .clone_from(&cache.ll_previous);
            self.state
                .fse_tables
                .ml_previous
                .clone_from(&cache.ml_previous);
            self.state
                .fse_tables
                .of_previous
                .clone_from(&cache.of_previous);
        } else {
            self.state.fse_tables.ll_previous = None;
            self.state.fse_tables.ml_previous = None;
            self.state.fse_tables.of_previous = None;
        }
        let ll_entropy = cached_entropy.and_then(|cache| match cache.ll_previous.as_ref() {
            Some(PreviousFseTable::Custom(table)) => Some(table.as_ref()),
            _ => None,
        });
        let ml_entropy = cached_entropy.and_then(|cache| match cache.ml_previous.as_ref() {
            Some(PreviousFseTable::Custom(table)) => Some(table.as_ref()),
            _ => None,
        });
        let of_entropy = cached_entropy.and_then(|cache| match cache.of_previous.as_ref() {
            Some(PreviousFseTable::Custom(table)) => Some(table.as_ref()),
            _ => None,
        });
        self.state.matcher.seed_dictionary_entropy(
            self.state.last_huff_table.as_ref(),
            ll_entropy,
            ml_entropy,
            of_entropy,
        );
        #[cfg(feature = "hash")]
        {
            self.hasher = XxHash64::with_seed(0);
        }
        let source = self.uncompressed_data.as_mut().unwrap();
        let drain = self.compressed_data.as_mut().unwrap();
        let window_size = self.state.matcher.window_size();
        assert!(
            window_size != 0,
            "matcher reported window_size == 0, which is invalid"
        );
        // Accumulate all compressed blocks; the frame header is written
        // after all input has been read so that Frame_Content_Size is
        // known. The default seed is one donor block; smaller seeds for
        // small payloads avoid pinning a full block worth of bytes when
        // the compressed output fits in a few hundred bytes. For larger
        // inputs the default seed amortises the first few `Vec::extend`
        // doublings cheaply and the `peak - default_seed` residue is
        // dominated by internal `compress_block_encoded` buffers anyway,
        // so changing it produces no measurable savings.
        //
        // Seed-size tiers (mirrors donor `ZSTD_CStreamOutSize` naming):
        //
        // * `ALL_BLOCKS_TINY_CAP` — payload ≤ this size, seed equals
        //   payload bound; ≥ everything compressed output could need
        //   for a tiny input.
        // * `ALL_BLOCKS_SMALL_CAP` — small-input seed picked to absorb
        //   one or two doublings without over-allocating.
        // * `ALL_BLOCKS_DEFAULT_CAP` — one donor block; the value the
        //   rest of the encoder is sized around.
        const ALL_BLOCKS_TINY_THRESHOLD: u64 = 4 * 1024;
        const ALL_BLOCKS_SMALL_THRESHOLD: u64 = 64 * 1024;
        const ALL_BLOCKS_TINY_CAP: usize = 4 * 1024;
        const ALL_BLOCKS_SMALL_CAP: usize = 16 * 1024;
        const ALL_BLOCKS_DEFAULT_CAP: usize = 130 * 1024;
        let initial_all_blocks_cap = match initial_size_hint {
            Some(h) if h <= ALL_BLOCKS_TINY_THRESHOLD => ALL_BLOCKS_TINY_CAP,
            Some(h) if h <= ALL_BLOCKS_SMALL_THRESHOLD => ALL_BLOCKS_SMALL_CAP,
            _ => ALL_BLOCKS_DEFAULT_CAP,
        };
        let mut all_blocks: Vec<u8> = Vec::with_capacity(initial_all_blocks_cap);
        let mut total_uncompressed: u64 = 0;
        let mut pending_input: Vec<u8> = Vec::new();
        let mut reached_eof = false;
        let mut savings = 0i64;
        // Compress block by block
        loop {
            // Read up to one donor block. When the pre-block splitter keeps a
            // suffix, top it back up before compressing the next block, matching
            // ZSTD_compress_frameChunk() over a contiguous input buffer.
            let block_capacity = MAX_BLOCK_SIZE as usize;
            let had_pending = !pending_input.is_empty();
            let mut uncompressed_data = if had_pending {
                core::mem::take(&mut pending_input)
            } else {
                self.state.matcher.get_next_space()
            };
            let mut filled = if had_pending {
                uncompressed_data.len()
            } else {
                0
            };
            if uncompressed_data.len() < block_capacity {
                uncompressed_data.resize(block_capacity, 0);
            }
            'read_loop: loop {
                if reached_eof || filled == block_capacity {
                    break 'read_loop;
                }
                let new_bytes = source
                    .read(&mut uncompressed_data[filled..block_capacity])
                    .unwrap();
                if new_bytes == 0 {
                    reached_eof = true;
                    break 'read_loop;
                }
                filled += new_bytes;
                total_uncompressed += new_bytes as u64;
            }
            uncompressed_data.truncate(filled);
            let mut last_block = reached_eof;
            let remaining_for_split = if reached_eof {
                uncompressed_data.len()
            } else {
                block_capacity
            };
            if !matches!(self.compression_level, CompressionLevel::Uncompressed)
                && uncompressed_data.len() == block_capacity
            {
                let block_len = donor_optimal_block_size(
                    self.compression_level,
                    &uncompressed_data,
                    remaining_for_split,
                    block_capacity,
                    savings,
                );
                if block_len < uncompressed_data.len() {
                    pending_input = uncompressed_data.split_off(block_len);
                    // `split_off` returns a Vec whose capacity is typically
                    // close to its length. Next iteration's `had_pending`
                    // branch moves `pending_input` into `uncompressed_data`
                    // and resizes to `block_capacity`, which would reallocate
                    // from scratch on every pre-split. Pre-reserve here so
                    // the resize stays in-place.
                    if pending_input.capacity() < block_capacity {
                        pending_input.reserve_exact(block_capacity - pending_input.len());
                    }
                    last_block = false;
                }
            }
            // As we read, hash that data too
            #[cfg(feature = "hash")]
            self.hasher.write(&uncompressed_data);
            // Per-physical-block XXH64 (low 32 bits) for the optional
            // per-block checksum sidecar. Hashing happens INSIDE the
            // block emitters (RLE / Raw fast-path / Compressed /
            // post-split partitions), so the digests vector has
            // exactly one entry per physical Block_Header written to
            // `all_blocks` — 1:1 with `FrameEmitInfo.blocks`. See
            // `enable_per_block_checksums` rustdoc.
            // Special handling is needed for compression of a totally empty file
            if uncompressed_data.is_empty() {
                let header = BlockHeader {
                    last_block: true,
                    block_type: crate::blocks::block::BlockType::Raw,
                    block_size: 0,
                };
                header.serialize(&mut all_blocks);
                #[cfg(all(feature = "lsm", feature = "hash"))]
                if let Some(checksums) = self.block_checksums.as_mut() {
                    checksums.push(xxh64_block_low32(&[]));
                }
                break;
            }

            match self.compression_level {
                CompressionLevel::Uncompressed => {
                    let header = BlockHeader {
                        last_block,
                        block_type: crate::blocks::block::BlockType::Raw,
                        block_size: uncompressed_data.len().try_into().unwrap(),
                    };
                    header.serialize(&mut all_blocks);
                    #[cfg(all(feature = "lsm", feature = "hash"))]
                    if let Some(checksums) = self.block_checksums.as_mut() {
                        checksums.push(xxh64_block_low32(&uncompressed_data));
                    }
                    all_blocks.extend_from_slice(&uncompressed_data);
                    savings +=
                        uncompressed_data.len() as i64 - (3 + uncompressed_data.len()) as i64;
                }
                CompressionLevel::Fastest
                | CompressionLevel::Default
                | CompressionLevel::Better
                | CompressionLevel::Best
                | CompressionLevel::Level(_) => {
                    let before_len = all_blocks.len();
                    let block_len = uncompressed_data.len();
                    compress_block_encoded(
                        &mut self.state,
                        self.compression_level,
                        last_block,
                        uncompressed_data,
                        &mut all_blocks,
                        #[cfg(all(feature = "lsm", feature = "hash"))]
                        self.block_checksums.as_mut(),
                    );
                    savings += block_len as i64 - (all_blocks.len() - before_len) as i64;
                }
            }
            if last_block && pending_input.is_empty() {
                break;
            }
        }

        // Now that total_uncompressed is known, write the frame header with FCS.
        // Match the donor framing policy for pledged one-shot inputs: use a
        // single-segment frame whenever the source fits the active window.
        let single_segment = !use_dictionary_state
            && source_size_hint_known
            && total_uncompressed >= 512
            && total_uncompressed <= window_size;
        let header = FrameHeader {
            frame_content_size: Some(total_uncompressed),
            single_segment,
            content_checksum: cfg!(feature = "hash"),
            dictionary_id: if use_dictionary_state {
                self.dictionary.as_ref().map(|dict| dict.id as u64)
            } else {
                None
            },
            window_size: if single_segment {
                None
            } else {
                Some(window_size)
            },
            magicless: self.magicless,
        };
        // Write the frame header and compressed blocks separately to avoid
        // shifting the entire `all_blocks` buffer to prepend the header.
        let mut header_buf: Vec<u8> = Vec::with_capacity(14);
        header.serialize(&mut header_buf);
        drain.write_all(&header_buf).unwrap();
        drain.write_all(&all_blocks).unwrap();

        // If the `hash` feature is enabled, then `content_checksum` is set to true in the header
        // and a 32 bit hash is written at the end of the data.
        #[cfg(feature = "hash")]
        {
            // Because we only have the data as a reader, we need to read all of it to calculate the checksum
            // Possible TODO: create a wrapper around self.uncompressed data that hashes the data as it's read?
            let content_checksum = self.hasher.finish();
            drain
                .write_all(&(content_checksum as u32).to_le_bytes())
                .unwrap();
        }

        // FrameEmitInfo population (lsm feature): walk all_blocks to
        // recover per-block layout. Each Block_Header is 3 bytes LE
        // packing `(block_size << 3) | (block_type << 1) | last_block`.
        // Physical body size differs by type: RLE bodies are always 1
        // byte (the repeated byte), Raw/Compressed bodies span
        // `block_size` bytes.
        #[cfg(feature = "lsm")]
        {
            use crate::blocks::block::BlockType as BT;
            use crate::encoding::frame_emit_info::{FrameBlock, FrameEmitInfo};
            // All frame-offset arithmetic below is bounded by u32 on
            // the wire (Block_Size is a 21-bit field, frames bounded
            // by MAX_BLOCK_SIZE * #blocks). A pathologically large
            // frame whose total emitted size exceeds u32::MAX would
            // overflow the cast — bail out by leaving
            // `frame_emit_info` at `None` rather than handing the
            // caller a silently-truncated layout. Checked once for
            // header / all_blocks / cursor up front + once per push;
            // the overflow path is statically unreachable on every
            // realistic frame so the predictor amortises the branch
            // to zero cost on the hot path.
            let frame_header_len: u32 = match u32::try_from(header_buf.len()) {
                Ok(v) => v,
                Err(_) => return,
            };
            let all_blocks_len_u32: u32 = match u32::try_from(all_blocks.len()) {
                Ok(v) => v,
                Err(_) => return,
            };
            let mut blocks: Vec<FrameBlock> = Vec::new();
            let mut cursor: usize = 0;
            while cursor + 3 <= all_blocks.len() {
                let mut header_u32 = [0u8; 4];
                header_u32[..3].copy_from_slice(&all_blocks[cursor..cursor + 3]);
                let raw = u32::from_le_bytes(header_u32);
                let last_block = (raw & 1) != 0;
                let block_type = match (raw >> 1) & 0b11 {
                    0 => BT::Raw,
                    1 => BT::RLE,
                    2 => BT::Compressed,
                    _ => BT::Reserved,
                };
                let block_size_field = raw >> 3;
                // RLE bodies are always 1 byte physical on the wire
                // (the single repeated byte); the spec's Block_Size
                // field carries the logical repeat count. Raw and
                // Compressed bodies physically span block_size_field
                // bytes. Store the physical length in body_size so the
                // 'offset + header + body_size' arithmetic always
                // lands on the next block boundary, and surface the
                // raw spec field separately as block_size_field.
                let physical_body: u32 = match block_type {
                    BT::RLE => 1,
                    _ => block_size_field,
                };
                let cursor_u32: u32 = match u32::try_from(cursor) {
                    Ok(v) => v,
                    Err(_) => return,
                };
                let offset_in_frame = match frame_header_len.checked_add(cursor_u32) {
                    Some(v) => v,
                    None => return,
                };
                blocks.push(FrameBlock {
                    offset_in_frame,
                    header_size: 3,
                    body_size: physical_body,
                    block_size_field,
                    block_type,
                    last_block,
                });
                cursor += 3 + physical_body as usize;
                if last_block {
                    break;
                }
            }
            let checksum_range = if cfg!(feature = "hash") {
                let cs_start = match frame_header_len.checked_add(all_blocks_len_u32) {
                    Some(v) => v,
                    None => return,
                };
                let cs_end = match cs_start.checked_add(4) {
                    Some(v) => v,
                    None => return,
                };
                Some(cs_start..cs_end)
            } else {
                None
            };
            let body_total = match frame_header_len.checked_add(all_blocks_len_u32) {
                Some(v) => v,
                None => return,
            };
            let total_size = if checksum_range.is_some() {
                match body_total.checked_add(4) {
                    Some(v) => v,
                    None => return,
                }
            } else {
                body_total
            };
            self.frame_emit_info = Some(FrameEmitInfo {
                frame_header_range: 0..frame_header_len,
                blocks,
                checksum_range,
                total_size,
            });
        }
    }

    /// Layout of the most recently emitted frame.
    ///
    /// Returns `None` if [`compress`](Self::compress) has not been
    /// called yet on this compressor. After a successful `compress()`
    /// the returned `FrameEmitInfo` describes the frame header range,
    /// every emitted block's offset / size / type, and the optional
    /// trailing content-checksum range — all in frame-absolute byte
    /// offsets matching the bytes written to the drain.
    ///
    /// Behind the `lsm` Cargo feature.
    #[cfg(feature = "lsm")]
    pub fn last_frame_emit_info(&self) -> Option<&crate::encoding::frame_emit_info::FrameEmitInfo> {
        self.frame_emit_info.as_ref()
    }

    /// Opt in to per-block XXH64 checksum computation during
    /// [`compress`](Self::compress). Default off; zero cost when
    /// disabled. The captured digests are accessible via
    /// [`last_frame_block_checksums`](Self::last_frame_block_checksums).
    ///
    /// One checksum is emitted per physical FrameBlock written to
    /// the drain: 1:1 cardinality with
    /// [`last_frame_emit_info`](Self::last_frame_emit_info)'s
    /// `blocks` vector. On the post-split optimization path
    /// (Level 16-22 with large window) the per-partition decompressed
    /// range is hashed inside the partition loop so the digest count
    /// still matches the emitted block count. The decoder collects
    /// per-physical-block digests on the same granularity, so
    /// element-wise equality holds round-trip.
    ///
    /// Behind `all(feature = "lsm", feature = "hash")` — the XXH64
    /// primitive lives behind the `hash` feature, so this method only
    /// compiles when both are enabled.
    #[cfg(all(feature = "lsm", feature = "hash"))]
    pub fn enable_per_block_checksums(&mut self) {
        self.per_block_checksums_enabled = true;
    }

    /// Per-block XXH64 (low 32 bits) digests captured during the most
    /// recent `compress()` call. `None` unless
    /// [`enable_per_block_checksums`](Self::enable_per_block_checksums)
    /// was called before `compress()`.
    ///
    /// Behind `all(feature = "lsm", feature = "hash")`.
    #[cfg(all(feature = "lsm", feature = "hash"))]
    pub fn last_frame_block_checksums(&self) -> Option<&[u32]> {
        self.block_checksums.as_deref()
    }

    /// Get a mutable reference to the source
    pub fn source_mut(&mut self) -> Option<&mut R> {
        self.uncompressed_data.as_mut()
    }

    /// Get a mutable reference to the drain
    pub fn drain_mut(&mut self) -> Option<&mut W> {
        self.compressed_data.as_mut()
    }

    /// Get a reference to the source
    pub fn source(&self) -> Option<&R> {
        self.uncompressed_data.as_ref()
    }

    /// Get a reference to the drain
    pub fn drain(&self) -> Option<&W> {
        self.compressed_data.as_ref()
    }

    /// Retrieve the source
    pub fn take_source(&mut self) -> Option<R> {
        self.uncompressed_data.take()
    }

    /// Retrieve the drain
    pub fn take_drain(&mut self) -> Option<W> {
        self.compressed_data.take()
    }

    /// Before calling [FrameCompressor::compress] you can replace the matcher
    pub fn replace_matcher(&mut self, mut match_generator: M) -> M {
        core::mem::swap(&mut match_generator, &mut self.state.matcher);
        match_generator
    }

    /// Before calling [FrameCompressor::compress] you can replace the compression level
    pub fn set_compression_level(
        &mut self,
        compression_level: CompressionLevel,
    ) -> CompressionLevel {
        let old = self.compression_level;
        self.compression_level = compression_level;
        old
    }

    /// Get the current compression level
    pub fn compression_level(&self) -> CompressionLevel {
        self.compression_level
    }

    /// Attach a pre-parsed dictionary to be used for subsequent compressions.
    ///
    /// In compressed modes, the dictionary id is written only when the active
    /// matcher supports dictionary priming.
    /// Uncompressed mode and non-priming matchers ignore the attached dictionary
    /// at encode time.
    pub fn set_dictionary(
        &mut self,
        dictionary: crate::decoding::Dictionary,
    ) -> Result<Option<crate::decoding::Dictionary>, crate::decoding::errors::DictionaryDecodeError>
    {
        if dictionary.id == 0 {
            return Err(crate::decoding::errors::DictionaryDecodeError::ZeroDictionaryId);
        }
        if let Some(index) = dictionary.offset_hist.iter().position(|&rep| rep == 0) {
            return Err(
                crate::decoding::errors::DictionaryDecodeError::ZeroRepeatOffsetInDictionary {
                    index: index as u8,
                },
            );
        }
        self.dictionary_entropy_cache = Some(CachedDictionaryEntropy {
            huff: dictionary.huf.table.to_encoder_table(),
            ll_previous: dictionary
                .fse
                .literal_lengths
                .to_encoder_table()
                .map(|table| PreviousFseTable::Custom(Box::new(table))),
            ml_previous: dictionary
                .fse
                .match_lengths
                .to_encoder_table()
                .map(|table| PreviousFseTable::Custom(Box::new(table))),
            of_previous: dictionary
                .fse
                .offsets
                .to_encoder_table()
                .map(|table| PreviousFseTable::Custom(Box::new(table))),
        });
        Ok(self.dictionary.replace(dictionary))
    }

    /// Parse and attach a serialized dictionary blob.
    pub fn set_dictionary_from_bytes(
        &mut self,
        raw_dictionary: &[u8],
    ) -> Result<Option<crate::decoding::Dictionary>, crate::decoding::errors::DictionaryDecodeError>
    {
        let dictionary = crate::decoding::Dictionary::decode_dict(raw_dictionary)?;
        self.set_dictionary(dictionary)
    }

    /// Remove the attached dictionary.
    pub fn clear_dictionary(&mut self) -> Option<crate::decoding::Dictionary> {
        self.dictionary_entropy_cache = None;
        self.dictionary.take()
    }
}

#[cfg(test)]
mod tests {
    #[cfg(all(feature = "dict_builder", feature = "std"))]
    use alloc::format;
    use alloc::vec;

    use super::FrameCompressor;
    use crate::blocks::block::BlockType;
    use crate::common::{MAGIC_NUM, MAX_BLOCK_SIZE};
    use crate::decoding::{FrameDecoder, block_decoder, frame::read_frame_header};
    use crate::encoding::{Matcher, Sequence};
    use alloc::vec::Vec;

    fn generate_data(seed: u64, len: usize) -> Vec<u8> {
        let mut state = seed;
        let mut data = Vec::with_capacity(len);
        for _ in 0..len {
            state = state
                .wrapping_mul(6364136223846793005)
                .wrapping_add(1442695040888963407);
            data.push((state >> 33) as u8);
        }
        data
    }

    fn first_block_type(frame: &[u8]) -> BlockType {
        let (_, header_size) = read_frame_header(frame).expect("frame header should parse");
        let mut decoder = block_decoder::new();
        let (header, _) = decoder
            .read_block_header(&frame[header_size as usize..])
            .expect("block header should parse");
        header.block_type
    }

    /// Frame content size is written correctly and C zstd can decompress the output.
    #[cfg(feature = "std")]
    #[test]
    fn fcs_header_written_and_c_zstd_compatible() {
        let levels = [
            crate::encoding::CompressionLevel::Uncompressed,
            crate::encoding::CompressionLevel::Fastest,
            crate::encoding::CompressionLevel::Default,
            crate::encoding::CompressionLevel::Better,
            crate::encoding::CompressionLevel::Best,
        ];
        let fcs_2byte = vec![0xCDu8; 300]; // 300 bytes → 2-byte FCS (256..=65791 range)
        let large = vec![0xABu8; 100_000];
        let inputs: [&[u8]; 5] = [
            &[],
            &[0x00],
            b"abcdefghijklmnopqrstuvwxy\n",
            &fcs_2byte,
            &large,
        ];
        for level in levels {
            for data in &inputs {
                let compressed = crate::encoding::compress_to_vec(*data, level);
                // Verify FCS is present and correct
                let header = crate::decoding::frame::read_frame_header(compressed.as_slice())
                    .unwrap()
                    .0;
                assert_eq!(
                    header.frame_content_size(),
                    data.len() as u64,
                    "FCS mismatch for len={} level={:?}",
                    data.len(),
                    level,
                );
                // Confirm the FCS field is actually present in the header
                // (not just the decoder returning 0 for absent FCS).
                assert_ne!(
                    header.descriptor.frame_content_size_bytes().unwrap(),
                    0,
                    "FCS field must be present for len={} level={:?}",
                    data.len(),
                    level,
                );
                // Verify C zstd can decompress
                let mut decoded = Vec::new();
                zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap_or_else(
                    |e| {
                        panic!(
                            "C zstd decode failed for len={} level={level:?}: {e}",
                            data.len()
                        )
                    },
                );
                assert_eq!(
                    decoded.as_slice(),
                    *data,
                    "C zstd roundtrip failed for len={}",
                    data.len()
                );
            }
        }
    }

    #[cfg(feature = "std")]
    #[test]
    fn source_size_hint_fastest_remains_ffi_compatible_small_input() {
        let data = vec![0xAB; 2047];
        let compressed = {
            let mut compressor = FrameCompressor::new(super::CompressionLevel::Fastest);
            compressor.set_source_size_hint(data.len() as u64);
            compressor.set_source(data.as_slice());
            let mut out = Vec::new();
            compressor.set_drain(&mut out);
            compressor.compress();
            out
        };

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn small_hinted_default_frame_uses_single_segment_header() {
        let data = generate_data(0xD15E_A5ED, 1024);
        let compressed = {
            let mut compressor = FrameCompressor::new(super::CompressionLevel::Default);
            compressor.set_source_size_hint(data.len() as u64);
            compressor.set_source(data.as_slice());
            let mut out = Vec::new();
            compressor.set_drain(&mut out);
            compressor.compress();
            out
        };

        let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
        assert!(
            frame_header.descriptor.single_segment_flag(),
            "small hinted default frames should use single-segment header for Rust/FFI parity"
        );
        assert_eq!(frame_header.frame_content_size(), data.len() as u64);
        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded)
            .expect("ffi decoder must accept single-segment small hinted default frame");
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn small_hinted_numeric_default_levels_use_single_segment_header() {
        let data = generate_data(0xA11C_E003, 1024);
        for level in [
            super::CompressionLevel::Level(0),
            super::CompressionLevel::Level(3),
        ] {
            let compressed = {
                let mut compressor = FrameCompressor::new(level);
                compressor.set_source_size_hint(data.len() as u64);
                compressor.set_source(data.as_slice());
                let mut out = Vec::new();
                compressor.set_drain(&mut out);
                compressor.compress();
                out
            };

            let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
            assert!(
                frame_header.descriptor.single_segment_flag(),
                "small hinted numeric default level frames should use single-segment header (level={level:?})"
            );
            assert_eq!(frame_header.frame_content_size(), data.len() as u64);
            let mut decoded = Vec::new();
            zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap_or_else(|e| {
                panic!(
                    "ffi decoder must accept single-segment small hinted numeric default level frame (level={level:?}): {e}"
                )
            });
            assert_eq!(decoded, data);
        }
    }

    #[cfg(feature = "std")]
    #[test]
    fn source_size_hint_levels_remain_ffi_compatible_small_inputs_matrix() {
        let levels = [
            super::CompressionLevel::Fastest,
            super::CompressionLevel::Default,
            super::CompressionLevel::Better,
            super::CompressionLevel::Best,
            super::CompressionLevel::Level(-1),
            super::CompressionLevel::Level(2),
            super::CompressionLevel::Level(3),
            super::CompressionLevel::Level(4),
            super::CompressionLevel::Level(11),
        ];
        let sizes = [
            511usize, 512, 513, 1023, 1024, 1536, 2047, 2048, 4095, 4096, 8191, 16_384, 16_385,
        ];

        for (seed_idx, seed) in [11u64, 23, 41].into_iter().enumerate() {
            for &size in &sizes {
                let data = generate_data(seed + seed_idx as u64, size);
                for &level in &levels {
                    let compressed = {
                        let mut compressor = FrameCompressor::new(level);
                        compressor.set_source_size_hint(data.len() as u64);
                        compressor.set_source(data.as_slice());
                        let mut out = Vec::new();
                        compressor.set_drain(&mut out);
                        compressor.compress();
                        out
                    };
                    if matches!(size, 511 | 512) {
                        let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
                        assert_eq!(
                            frame_header.descriptor.single_segment_flag(),
                            size == 512,
                            "single_segment 511/512 boundary mismatch: level={level:?} size={size}"
                        );
                    }

                    let mut decoded = Vec::new();
                    zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap_or_else(
                        |e| {
                            panic!(
                                "ffi decode failed with source-size hint: level={level:?} size={size} seed={} err={e}",
                                seed + seed_idx as u64
                            )
                        },
                    );
                    assert_eq!(
                        decoded,
                        data,
                        "hinted ffi roundtrip mismatch: level={level:?} size={size} seed={}",
                        seed + seed_idx as u64
                    );
                }
            }
        }
    }

    #[cfg(feature = "std")]
    #[test]
    fn hinted_levels_use_single_segment_header_symmetrically() {
        let levels = [
            super::CompressionLevel::Fastest,
            super::CompressionLevel::Default,
            super::CompressionLevel::Better,
            super::CompressionLevel::Best,
            super::CompressionLevel::Level(0),
            super::CompressionLevel::Level(2),
            super::CompressionLevel::Level(3),
            super::CompressionLevel::Level(4),
            super::CompressionLevel::Level(11),
        ];
        for (seed_idx, seed) in [7u64, 23, 41].into_iter().enumerate() {
            let size = 1024 + seed_idx * 97;
            let data = generate_data(seed, size);
            for &level in &levels {
                let compressed = {
                    let mut compressor = FrameCompressor::new(level);
                    compressor.set_source_size_hint(data.len() as u64);
                    compressor.set_source(data.as_slice());
                    let mut out = Vec::new();
                    compressor.set_drain(&mut out);
                    compressor.compress();
                    out
                };
                let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
                assert!(
                    frame_header.descriptor.single_segment_flag(),
                    "hinted frame should be single-segment for level={level:?} size={}",
                    data.len()
                );
                assert_eq!(frame_header.frame_content_size(), data.len() as u64);
                let mut decoded = Vec::new();
                zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap_or_else(|e| {
                    panic!(
                        "ffi decode failed for hinted single-segment parity: level={level:?} size={} err={e}",
                        data.len()
                    )
                });
                assert_eq!(decoded, data);
            }
        }
    }

    #[cfg(feature = "std")]
    #[test]
    fn hinted_levels_pin_511_512_single_segment_boundary() {
        let levels = [
            super::CompressionLevel::Fastest,
            super::CompressionLevel::Default,
            super::CompressionLevel::Better,
            super::CompressionLevel::Best,
            super::CompressionLevel::Level(0),
            super::CompressionLevel::Level(2),
            super::CompressionLevel::Level(3),
            super::CompressionLevel::Level(4),
            super::CompressionLevel::Level(11),
        ];
        for (seed_idx, seed) in [7u64, 23, 41].into_iter().enumerate() {
            for &size in &[511usize, 512] {
                let data = generate_data(seed + seed_idx as u64, size);
                for &level in &levels {
                    let compressed = {
                        let mut compressor = FrameCompressor::new(level);
                        compressor.set_source_size_hint(data.len() as u64);
                        compressor.set_source(data.as_slice());
                        let mut out = Vec::new();
                        compressor.set_drain(&mut out);
                        compressor.compress();
                        out
                    };
                    let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
                    assert_eq!(
                        frame_header.descriptor.single_segment_flag(),
                        size == 512,
                        "single_segment 511/512 boundary mismatch: level={level:?} size={size}"
                    );
                    let mut decoded = Vec::new();
                    zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap_or_else(
                        |e| {
                            panic!(
                                "ffi decode failed at single-segment boundary: level={level:?} size={size} seed={} err={e}",
                                seed + seed_idx as u64
                            )
                        },
                    );
                    assert_eq!(decoded, data);
                }
            }
        }
    }

    #[cfg(feature = "std")]
    #[test]
    fn fastest_random_block_uses_raw_fast_path() {
        let data = generate_data(0xC0FF_EE11, 10 * 1024);
        let compressed =
            crate::encoding::compress_to_vec(data.as_slice(), super::CompressionLevel::Fastest);

        assert_eq!(first_block_type(&compressed), BlockType::Raw);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn default_random_block_uses_raw_fast_path() {
        let data = generate_data(0xD15E_A5ED, 10 * 1024);
        let compressed =
            crate::encoding::compress_to_vec(data.as_slice(), super::CompressionLevel::Default);

        assert_eq!(first_block_type(&compressed), BlockType::Raw);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn best_random_block_uses_raw_fast_path() {
        let data = generate_data(0xB35C_AFE1, 10 * 1024);
        let compressed =
            crate::encoding::compress_to_vec(data.as_slice(), super::CompressionLevel::Best);

        assert_eq!(first_block_type(&compressed), BlockType::Raw);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn level2_random_block_uses_raw_fast_path() {
        let data = generate_data(0xA11C_E222, 10 * 1024);
        let compressed =
            crate::encoding::compress_to_vec(data.as_slice(), super::CompressionLevel::Level(2));

        assert_eq!(first_block_type(&compressed), BlockType::Raw);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn better_random_block_uses_raw_fast_path() {
        let data = generate_data(0xBE77_E111, 10 * 1024);
        let compressed =
            crate::encoding::compress_to_vec(data.as_slice(), super::CompressionLevel::Better);

        assert_eq!(first_block_type(&compressed), BlockType::Raw);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
        assert_eq!(decoded, data);
    }

    #[cfg(feature = "std")]
    #[test]
    fn compressible_logs_do_not_fall_back_to_raw_fast_path() {
        let mut data = Vec::with_capacity(16 * 1024);
        const LINE: &[u8] =
            b"ts=2026-04-10T00:00:00Z level=INFO tenant=demo op=flush table=orders\n";
        while data.len() < 16 * 1024 {
            let remaining = 16 * 1024 - data.len();
            data.extend_from_slice(&LINE[..LINE.len().min(remaining)]);
        }

        fn assert_not_raw_for_level(data: &[u8], level: super::CompressionLevel) {
            let compressed = crate::encoding::compress_to_vec(data, level);
            assert_ne!(first_block_type(&compressed), BlockType::Raw);
            assert!(
                compressed.len() < data.len(),
                "compressible input should remain compressible for level={level:?}"
            );
            let mut decoded = Vec::new();
            zstd::stream::copy_decode(compressed.as_slice(), &mut decoded).unwrap();
            assert_eq!(decoded, data);
        }

        assert_not_raw_for_level(data.as_slice(), super::CompressionLevel::Fastest);
        assert_not_raw_for_level(data.as_slice(), super::CompressionLevel::Default);
        assert_not_raw_for_level(data.as_slice(), super::CompressionLevel::Level(3));
        assert_not_raw_for_level(data.as_slice(), super::CompressionLevel::Better);
        assert_not_raw_for_level(data.as_slice(), super::CompressionLevel::Best);
    }

    #[cfg(feature = "std")]
    #[test]
    fn hinted_small_compressible_frames_use_single_segment_across_levels() {
        let mut data = Vec::with_capacity(4 * 1024);
        const LINE: &[u8] =
            b"ts=2026-04-10T00:00:00Z level=INFO tenant=demo op=flush table=orders\n";
        while data.len() < 4 * 1024 {
            let remaining = 4 * 1024 - data.len();
            data.extend_from_slice(&LINE[..LINE.len().min(remaining)]);
        }

        for level in [
            super::CompressionLevel::Fastest,
            super::CompressionLevel::Default,
            super::CompressionLevel::Better,
            super::CompressionLevel::Best,
            super::CompressionLevel::Level(0),
            super::CompressionLevel::Level(3),
            super::CompressionLevel::Level(4),
            super::CompressionLevel::Level(11),
        ] {
            let compressed = {
                let mut compressor = FrameCompressor::new(level);
                compressor.set_source_size_hint(data.len() as u64);
                compressor.set_source(data.as_slice());
                let mut out = Vec::new();
                compressor.set_drain(&mut out);
                compressor.compress();
                out
            };
            let (frame_header, _) = read_frame_header(compressed.as_slice()).unwrap();
            assert!(
                frame_header.descriptor.single_segment_flag(),
                "hinted small compressible frame should use single-segment (level={level:?})"
            );
            assert_ne!(
                first_block_type(&compressed),
                BlockType::Raw,
                "compressible hinted frame should stay off raw fast path (level={level:?})"
            );
            assert!(
                compressed.len() < data.len(),
                "compressible hinted frame should still shrink (level={level:?})"
            );
            let mut decoded = Vec::new();
            zstd::stream::copy_decode(compressed.as_slice(), &mut decoded)
                .unwrap_or_else(|e| panic!("ffi decode failed (level={level:?}): {e}"));
            assert_eq!(decoded, data);
        }
    }

    struct NoDictionaryMatcher {
        last_space: Vec<u8>,
        window_size: u64,
    }

    impl NoDictionaryMatcher {
        fn new(window_size: u64) -> Self {
            Self {
                last_space: Vec::new(),
                window_size,
            }
        }
    }

    impl Matcher for NoDictionaryMatcher {
        fn get_next_space(&mut self) -> Vec<u8> {
            vec![0; self.window_size as usize]
        }

        fn get_last_space(&mut self) -> &[u8] {
            self.last_space.as_slice()
        }

        fn commit_space(&mut self, space: Vec<u8>) {
            self.last_space = space;
        }

        fn skip_matching(&mut self) {}

        fn start_matching(&mut self, mut handle_sequence: impl for<'a> FnMut(Sequence<'a>)) {
            handle_sequence(Sequence::Literals {
                literals: self.last_space.as_slice(),
            });
        }

        fn reset(&mut self, _level: super::CompressionLevel) {
            self.last_space.clear();
        }

        fn window_size(&self) -> u64 {
            self.window_size
        }
    }

    #[test]
    fn frame_starts_with_magic_num() {
        let mock_data = [1_u8, 2, 3].as_slice();
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor.set_source(mock_data);
        compressor.set_drain(&mut output);

        compressor.compress();
        assert!(output.starts_with(&MAGIC_NUM.to_le_bytes()));
    }

    #[test]
    fn very_simple_raw_compress() {
        let mock_data = [1_u8, 2, 3].as_slice();
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor.set_source(mock_data);
        compressor.set_drain(&mut output);

        compressor.compress();
    }

    #[test]
    fn very_simple_compress() {
        let mut mock_data = vec![0; 1 << 17];
        mock_data.extend(vec![1; (1 << 17) - 1]);
        mock_data.extend(vec![2; (1 << 18) - 1]);
        mock_data.extend(vec![2; 1 << 17]);
        mock_data.extend(vec![3; (1 << 17) - 1]);
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor.set_source(mock_data.as_slice());
        compressor.set_drain(&mut output);

        compressor.compress();

        let mut decoder = FrameDecoder::new();
        let mut decoded = Vec::with_capacity(mock_data.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(mock_data, decoded);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(output.as_slice(), &mut decoded).unwrap();
        assert_eq!(mock_data, decoded);
    }

    #[test]
    fn rle_compress() {
        let mock_data = vec![0; 1 << 19];
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor.set_source(mock_data.as_slice());
        compressor.set_drain(&mut output);

        compressor.compress();

        let mut decoder = FrameDecoder::new();
        let mut decoded = Vec::with_capacity(mock_data.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(mock_data, decoded);
    }

    #[test]
    fn aaa_compress() {
        let mock_data = vec![0, 1, 3, 4, 5];
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor.set_source(mock_data.as_slice());
        compressor.set_drain(&mut output);

        compressor.compress();

        let mut decoder = FrameDecoder::new();
        let mut decoded = Vec::with_capacity(mock_data.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(mock_data, decoded);

        let mut decoded = Vec::new();
        zstd::stream::copy_decode(output.as_slice(), &mut decoded).unwrap();
        assert_eq!(mock_data, decoded);
    }

    #[test]
    fn dictionary_compression_sets_required_dict_id_and_roundtrips() {
        let dict_raw = include_bytes!("../../dict_tests/dictionary");
        let dict_for_encoder = crate::decoding::Dictionary::decode_dict(dict_raw).unwrap();
        let dict_for_decoder = crate::decoding::Dictionary::decode_dict(dict_raw).unwrap();

        let mut data = Vec::new();
        for _ in 0..8 {
            data.extend_from_slice(&dict_for_decoder.dict_content[..2048]);
        }

        let mut with_dict = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Fastest);
        let previous = compressor
            .set_dictionary_from_bytes(dict_raw)
            .expect("dictionary bytes should parse");
        assert!(
            previous.is_none(),
            "first dictionary insert should return None"
        );
        assert_eq!(
            compressor
                .set_dictionary(dict_for_encoder)
                .expect("valid dictionary should attach")
                .expect("set_dictionary_from_bytes inserted previous dictionary")
                .id,
            dict_for_decoder.id
        );
        compressor.set_source(data.as_slice());
        compressor.set_drain(&mut with_dict);
        compressor.compress();

        let (frame_header, _) = crate::decoding::frame::read_frame_header(with_dict.as_slice())
            .expect("encoded stream should have a frame header");
        assert_eq!(frame_header.dictionary_id(), Some(dict_for_decoder.id));

        let mut decoder = FrameDecoder::new();
        let mut missing_dict_target = Vec::with_capacity(data.len());
        let err = decoder
            .decode_all_to_vec(&with_dict, &mut missing_dict_target)
            .unwrap_err();
        assert!(
            matches!(
                &err,
                crate::decoding::errors::FrameDecoderError::DictNotProvided { .. }
            ),
            "dict-compressed stream should require dictionary id, got: {err:?}"
        );

        let mut decoder = FrameDecoder::new();
        decoder.add_dict(dict_for_decoder).unwrap();
        let mut decoded = Vec::with_capacity(data.len());
        decoder.decode_all_to_vec(&with_dict, &mut decoded).unwrap();
        assert_eq!(decoded, data);

        let mut ffi_decoder = zstd::bulk::Decompressor::with_dictionary(dict_raw).unwrap();
        let mut ffi_decoded = Vec::with_capacity(data.len());
        let ffi_written = ffi_decoder
            .decompress_to_buffer(with_dict.as_slice(), &mut ffi_decoded)
            .unwrap();
        assert_eq!(ffi_written, data.len());
        assert_eq!(ffi_decoded, data);
    }

    #[cfg(all(feature = "dict_builder", feature = "std"))]
    #[test]
    fn dictionary_compression_roundtrips_with_dict_builder_dictionary() {
        use std::io::Cursor;

        let mut training = Vec::new();
        for idx in 0..256u32 {
            training.extend_from_slice(
                format!("tenant=demo table=orders key={idx} region=eu\n").as_bytes(),
            );
        }
        let mut raw_dict = Vec::new();
        crate::dictionary::create_raw_dict_from_source(
            Cursor::new(training.as_slice()),
            training.len(),
            &mut raw_dict,
            4096,
        )
        .expect("dict_builder training should succeed");
        assert!(
            !raw_dict.is_empty(),
            "dict_builder produced an empty dictionary"
        );

        let dict_id = 0xD1C7_0008;
        let encoder_dict =
            crate::decoding::Dictionary::from_raw_content(dict_id, raw_dict.clone()).unwrap();
        let decoder_dict =
            crate::decoding::Dictionary::from_raw_content(dict_id, raw_dict.clone()).unwrap();

        let mut payload = Vec::new();
        for idx in 0..96u32 {
            payload.extend_from_slice(
                format!(
                    "tenant=demo table=orders op=put key={idx} value=aaaaabbbbbcccccdddddeeeee\n"
                )
                .as_bytes(),
            );
        }

        let mut without_dict = Vec::new();
        let mut baseline = FrameCompressor::new(super::CompressionLevel::Fastest);
        baseline.set_source(payload.as_slice());
        baseline.set_drain(&mut without_dict);
        baseline.compress();

        let mut with_dict = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Fastest);
        compressor
            .set_dictionary(encoder_dict)
            .expect("valid dict_builder dictionary should attach");
        compressor.set_source(payload.as_slice());
        compressor.set_drain(&mut with_dict);
        compressor.compress();

        let (frame_header, _) = crate::decoding::frame::read_frame_header(with_dict.as_slice())
            .expect("encoded stream should have a frame header");
        assert_eq!(frame_header.dictionary_id(), Some(dict_id));
        let mut decoder = FrameDecoder::new();
        decoder.add_dict(decoder_dict).unwrap();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder.decode_all_to_vec(&with_dict, &mut decoded).unwrap();
        assert_eq!(decoded, payload);
        assert!(
            with_dict.len() < without_dict.len(),
            "trained dictionary should improve compression for this small payload"
        );
    }

    #[test]
    fn set_dictionary_from_bytes_seeds_entropy_tables_for_first_block() {
        let dict_raw = include_bytes!("../../dict_tests/dictionary");
        let mut output = Vec::new();
        let input = b"";

        let mut compressor = FrameCompressor::new(super::CompressionLevel::Fastest);
        let previous = compressor
            .set_dictionary_from_bytes(dict_raw)
            .expect("dictionary bytes should parse");
        assert!(previous.is_none());

        compressor.set_source(input.as_slice());
        compressor.set_drain(&mut output);
        compressor.compress();

        assert!(
            compressor.state.last_huff_table.is_some(),
            "dictionary entropy should seed previous huffman table before first block"
        );
        assert!(
            compressor.state.fse_tables.ll_previous.is_some(),
            "dictionary entropy should seed previous ll table before first block"
        );
        assert!(
            compressor.state.fse_tables.ml_previous.is_some(),
            "dictionary entropy should seed previous ml table before first block"
        );
        assert!(
            compressor.state.fse_tables.of_previous.is_some(),
            "dictionary entropy should seed previous of table before first block"
        );
    }

    #[test]
    fn set_dictionary_rejects_zero_dictionary_id() {
        let invalid = crate::decoding::Dictionary {
            id: 0,
            fse: crate::decoding::scratch::FSEScratch::new(),
            huf: crate::decoding::scratch::HuffmanScratch::new(),
            dict_content: vec![1, 2, 3],
            offset_hist: [1, 4, 8],
        };

        let mut compressor: FrameCompressor<
            &[u8],
            Vec<u8>,
            crate::encoding::match_generator::MatchGeneratorDriver,
        > = FrameCompressor::new(super::CompressionLevel::Fastest);
        let result = compressor.set_dictionary(invalid);
        assert!(matches!(
            result,
            Err(crate::decoding::errors::DictionaryDecodeError::ZeroDictionaryId)
        ));
    }

    #[test]
    fn set_dictionary_rejects_zero_repeat_offsets() {
        let invalid = crate::decoding::Dictionary {
            id: 1,
            fse: crate::decoding::scratch::FSEScratch::new(),
            huf: crate::decoding::scratch::HuffmanScratch::new(),
            dict_content: vec![1, 2, 3],
            offset_hist: [0, 4, 8],
        };

        let mut compressor: FrameCompressor<
            &[u8],
            Vec<u8>,
            crate::encoding::match_generator::MatchGeneratorDriver,
        > = FrameCompressor::new(super::CompressionLevel::Fastest);
        let result = compressor.set_dictionary(invalid);
        assert!(matches!(
            result,
            Err(
                crate::decoding::errors::DictionaryDecodeError::ZeroRepeatOffsetInDictionary {
                    index: 0
                }
            )
        ));
    }

    #[test]
    fn uncompressed_mode_does_not_require_dictionary() {
        let dict_id = 0xABCD_0001;
        let dict =
            crate::decoding::Dictionary::from_raw_content(dict_id, b"shared-history".to_vec())
                .expect("raw dictionary should be valid");

        let payload = b"plain-bytes-that-should-stay-raw";
        let mut output = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);
        compressor
            .set_dictionary(dict)
            .expect("dictionary should attach in uncompressed mode");
        compressor.set_source(payload.as_slice());
        compressor.set_drain(&mut output);
        compressor.compress();

        let (frame_header, _) = crate::decoding::frame::read_frame_header(output.as_slice())
            .expect("encoded frame should have a header");
        assert_eq!(
            frame_header.dictionary_id(),
            None,
            "raw/uncompressed frames must not advertise dictionary dependency"
        );

        let mut decoder = FrameDecoder::new();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(decoded, payload);
    }

    #[test]
    fn dictionary_roundtrip_stays_valid_after_output_exceeds_window() {
        use crate::encoding::match_generator::MatchGeneratorDriver;

        let dict_id = 0xABCD_0002;
        let dict = crate::decoding::Dictionary::from_raw_content(dict_id, b"abcdefgh".to_vec())
            .expect("raw dictionary should be valid");
        let dict_for_decoder =
            crate::decoding::Dictionary::from_raw_content(dict_id, b"abcdefgh".to_vec())
                .expect("raw dictionary should be valid");

        // Payload must exceed the encoder's advertised window (512 KiB
        // for Fastest after `window_log = 19` alignment with donor's
        // L1 fast row in `clevels.h`) so the test actually exercises
        // cross-window-boundary behavior.
        let payload = b"abcdefgh".repeat(512 * 1024 / 8 + 64);
        let matcher = MatchGeneratorDriver::new(1024, 1);

        let mut no_dict_output = Vec::new();
        let mut no_dict_compressor =
            FrameCompressor::new_with_matcher(matcher, super::CompressionLevel::Fastest);
        no_dict_compressor.set_source(payload.as_slice());
        no_dict_compressor.set_drain(&mut no_dict_output);
        no_dict_compressor.compress();
        let (no_dict_frame_header, _) =
            crate::decoding::frame::read_frame_header(no_dict_output.as_slice())
                .expect("baseline frame should have a header");
        let no_dict_window = no_dict_frame_header
            .window_size()
            .expect("window size should be present");

        let mut output = Vec::new();
        let matcher = MatchGeneratorDriver::new(1024, 1);
        let mut compressor =
            FrameCompressor::new_with_matcher(matcher, super::CompressionLevel::Fastest);
        compressor
            .set_dictionary(dict)
            .expect("dictionary should attach");
        compressor.set_source(payload.as_slice());
        compressor.set_drain(&mut output);
        compressor.compress();

        let (frame_header, _) = crate::decoding::frame::read_frame_header(output.as_slice())
            .expect("encoded frame should have a header");
        let advertised_window = frame_header
            .window_size()
            .expect("window size should be present");
        assert_eq!(
            advertised_window, no_dict_window,
            "dictionary priming must not inflate advertised window size"
        );
        assert!(
            payload.len() > advertised_window as usize,
            "test must cross the advertised window boundary"
        );

        let mut decoder = FrameDecoder::new();
        decoder.add_dict(dict_for_decoder).unwrap();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(decoded, payload);
    }

    #[test]
    fn source_size_hint_with_dictionary_keeps_roundtrip_and_nonincreasing_window() {
        let dict_id = 0xABCD_0004;
        let dict_content = b"abcd".repeat(1024); // 4 KiB dictionary history
        let dict = crate::decoding::Dictionary::from_raw_content(dict_id, dict_content).unwrap();
        let dict_for_decoder =
            crate::decoding::Dictionary::from_raw_content(dict_id, b"abcd".repeat(1024)).unwrap();
        let payload = b"abcdabcdabcdabcd".repeat(128);

        let mut hinted_output = Vec::new();
        let mut hinted = FrameCompressor::new(super::CompressionLevel::Fastest);
        hinted.set_dictionary(dict).unwrap();
        hinted.set_source_size_hint(1);
        hinted.set_source(payload.as_slice());
        hinted.set_drain(&mut hinted_output);
        hinted.compress();

        let mut no_hint_output = Vec::new();
        let mut no_hint = FrameCompressor::new(super::CompressionLevel::Fastest);
        no_hint
            .set_dictionary(
                crate::decoding::Dictionary::from_raw_content(dict_id, b"abcd".repeat(1024))
                    .unwrap(),
            )
            .unwrap();
        no_hint.set_source(payload.as_slice());
        no_hint.set_drain(&mut no_hint_output);
        no_hint.compress();

        let hinted_window = crate::decoding::frame::read_frame_header(hinted_output.as_slice())
            .expect("encoded frame should have a header")
            .0
            .window_size()
            .expect("window size should be present");
        let no_hint_window = crate::decoding::frame::read_frame_header(no_hint_output.as_slice())
            .expect("encoded frame should have a header")
            .0
            .window_size()
            .expect("window size should be present");
        assert!(
            hinted_window <= no_hint_window,
            "source-size hint should not increase advertised window with dictionary priming",
        );

        let mut decoder = FrameDecoder::new();
        decoder.add_dict(dict_for_decoder).unwrap();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder
            .decode_all_to_vec(&hinted_output, &mut decoded)
            .unwrap();
        assert_eq!(decoded, payload);
    }

    #[test]
    fn source_size_hint_with_dictionary_keeps_roundtrip_for_larger_payload() {
        let dict_id = 0xABCD_0005;
        let dict_content = b"abcd".repeat(1024); // 4 KiB dictionary history
        let dict = crate::decoding::Dictionary::from_raw_content(dict_id, dict_content).unwrap();
        let dict_for_decoder =
            crate::decoding::Dictionary::from_raw_content(dict_id, b"abcd".repeat(1024)).unwrap();
        let payload = b"abcd".repeat(1024); // 4 KiB payload
        let payload_len = payload.len() as u64;

        let mut hinted_output = Vec::new();
        let mut hinted = FrameCompressor::new(super::CompressionLevel::Fastest);
        hinted.set_dictionary(dict).unwrap();
        hinted.set_source_size_hint(payload_len);
        hinted.set_source(payload.as_slice());
        hinted.set_drain(&mut hinted_output);
        hinted.compress();

        let mut no_hint_output = Vec::new();
        let mut no_hint = FrameCompressor::new(super::CompressionLevel::Fastest);
        no_hint
            .set_dictionary(
                crate::decoding::Dictionary::from_raw_content(dict_id, b"abcd".repeat(1024))
                    .unwrap(),
            )
            .unwrap();
        no_hint.set_source(payload.as_slice());
        no_hint.set_drain(&mut no_hint_output);
        no_hint.compress();

        let hinted_window = crate::decoding::frame::read_frame_header(hinted_output.as_slice())
            .expect("encoded frame should have a header")
            .0
            .window_size()
            .expect("window size should be present");
        let no_hint_window = crate::decoding::frame::read_frame_header(no_hint_output.as_slice())
            .expect("encoded frame should have a header")
            .0
            .window_size()
            .expect("window size should be present");
        assert!(
            hinted_window <= no_hint_window,
            "source-size hint should not increase advertised window with dictionary priming",
        );

        let mut decoder = FrameDecoder::new();
        decoder.add_dict(dict_for_decoder).unwrap();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder
            .decode_all_to_vec(&hinted_output, &mut decoded)
            .unwrap();
        assert_eq!(decoded, payload);
    }

    #[test]
    fn custom_matcher_without_dictionary_priming_does_not_advertise_dict_id() {
        let dict_id = 0xABCD_0003;
        let dict = crate::decoding::Dictionary::from_raw_content(dict_id, b"abcdefgh".to_vec())
            .expect("raw dictionary should be valid");
        let payload = b"abcdefghabcdefgh";

        let mut output = Vec::new();
        let matcher = NoDictionaryMatcher::new(64);
        let mut compressor =
            FrameCompressor::new_with_matcher(matcher, super::CompressionLevel::Fastest);
        compressor
            .set_dictionary(dict)
            .expect("dictionary should attach");
        compressor.set_source(payload.as_slice());
        compressor.set_drain(&mut output);
        compressor.compress();

        let (frame_header, _) = crate::decoding::frame::read_frame_header(output.as_slice())
            .expect("encoded frame should have a header");
        assert_eq!(
            frame_header.dictionary_id(),
            None,
            "matchers that do not support dictionary priming must not advertise dictionary dependency"
        );

        let mut decoder = FrameDecoder::new();
        let mut decoded = Vec::with_capacity(payload.len());
        decoder.decode_all_to_vec(&output, &mut decoded).unwrap();
        assert_eq!(decoded, payload);
    }

    #[cfg(feature = "hash")]
    #[test]
    fn checksum_two_frames_reused_compressor() {
        // Compress the same data twice using the same compressor and verify that:
        // 1. The checksum written in each frame matches what the decoder calculates.
        // 2. The hasher is correctly reset between frames (no cross-contamination).
        //    If the hasher were NOT reset, the second frame's calculated checksum
        //    would differ from the one stored in the frame data, causing assert_eq to fail.
        let data: Vec<u8> = (0u8..=255).cycle().take(1024).collect();

        let mut compressor = FrameCompressor::new(super::CompressionLevel::Uncompressed);

        // --- Frame 1 ---
        let mut compressed1 = Vec::new();
        compressor.set_source(data.as_slice());
        compressor.set_drain(&mut compressed1);
        compressor.compress();

        // --- Frame 2 (reuse the same compressor) ---
        let mut compressed2 = Vec::new();
        compressor.set_source(data.as_slice());
        compressor.set_drain(&mut compressed2);
        compressor.compress();

        fn decode_and_collect(compressed: &[u8]) -> (Vec<u8>, Option<u32>, Option<u32>) {
            let mut decoder = FrameDecoder::new();
            let mut source = compressed;
            decoder.reset(&mut source).unwrap();
            while !decoder.is_finished() {
                decoder
                    .decode_blocks(&mut source, crate::decoding::BlockDecodingStrategy::All)
                    .unwrap();
            }
            let mut decoded = Vec::new();
            decoder.collect_to_writer(&mut decoded).unwrap();
            (
                decoded,
                decoder.get_checksum_from_data(),
                decoder.get_calculated_checksum(),
            )
        }

        let (decoded1, chksum_from_data1, chksum_calculated1) = decode_and_collect(&compressed1);
        assert_eq!(decoded1, data, "frame 1: decoded data mismatch");
        assert_eq!(
            chksum_from_data1, chksum_calculated1,
            "frame 1: checksum mismatch"
        );

        let (decoded2, chksum_from_data2, chksum_calculated2) = decode_and_collect(&compressed2);
        assert_eq!(decoded2, data, "frame 2: decoded data mismatch");
        assert_eq!(
            chksum_from_data2, chksum_calculated2,
            "frame 2: checksum mismatch"
        );

        // Same data compressed twice must produce the same checksum.
        // If state leaked across frames, the second calculated checksum would differ.
        assert_eq!(
            chksum_from_data1, chksum_from_data2,
            "frame 1 and frame 2 should have the same checksum (same data, hash must reset per frame)"
        );
    }

    #[cfg(feature = "std")]
    #[test]
    fn fuzz_targets() {
        use std::io::Read;
        fn decode_szstd(data: &mut dyn std::io::Read) -> Vec<u8> {
            let mut decoder = crate::decoding::StreamingDecoder::new(data).unwrap();
            let mut result: Vec<u8> = Vec::new();
            decoder.read_to_end(&mut result).expect("Decoding failed");
            result
        }

        fn decode_szstd_writer(mut data: impl Read) -> Vec<u8> {
            let mut decoder = crate::decoding::FrameDecoder::new();
            decoder.reset(&mut data).unwrap();
            let mut result = vec![];
            while !decoder.is_finished() || decoder.can_collect() > 0 {
                decoder
                    .decode_blocks(
                        &mut data,
                        crate::decoding::BlockDecodingStrategy::UptoBytes(1024 * 1024),
                    )
                    .unwrap();
                decoder.collect_to_writer(&mut result).unwrap();
            }
            result
        }

        fn encode_zstd(data: &[u8]) -> Result<Vec<u8>, std::io::Error> {
            zstd::stream::encode_all(std::io::Cursor::new(data), 3)
        }

        fn encode_szstd_uncompressed(data: &mut dyn std::io::Read) -> Vec<u8> {
            let mut input = Vec::new();
            data.read_to_end(&mut input).unwrap();

            crate::encoding::compress_to_vec(
                input.as_slice(),
                crate::encoding::CompressionLevel::Uncompressed,
            )
        }

        fn encode_szstd_compressed(data: &mut dyn std::io::Read) -> Vec<u8> {
            let mut input = Vec::new();
            data.read_to_end(&mut input).unwrap();

            crate::encoding::compress_to_vec(
                input.as_slice(),
                crate::encoding::CompressionLevel::Fastest,
            )
        }

        fn decode_zstd(data: &[u8]) -> Result<Vec<u8>, std::io::Error> {
            let mut output = Vec::new();
            zstd::stream::copy_decode(data, &mut output)?;
            Ok(output)
        }
        if std::fs::exists("fuzz/artifacts/interop").unwrap_or(false) {
            for file in std::fs::read_dir("fuzz/artifacts/interop").unwrap() {
                if file.as_ref().unwrap().file_type().unwrap().is_file() {
                    let data = std::fs::read(file.unwrap().path()).unwrap();
                    let data = data.as_slice();
                    // Decoding
                    let compressed = encode_zstd(data).unwrap();
                    let decoded = decode_szstd(&mut compressed.as_slice());
                    let decoded2 = decode_szstd_writer(&mut compressed.as_slice());
                    assert!(
                        decoded == data,
                        "Decoded data did not match the original input during decompression"
                    );
                    assert_eq!(
                        decoded2, data,
                        "Decoded data did not match the original input during decompression"
                    );

                    // Encoding
                    // Uncompressed encoding
                    let mut input = data;
                    let compressed = encode_szstd_uncompressed(&mut input);
                    let decoded = decode_zstd(&compressed).unwrap();
                    assert_eq!(
                        decoded, data,
                        "Decoded data did not match the original input during compression"
                    );
                    // Compressed encoding
                    let mut input = data;
                    let compressed = encode_szstd_compressed(&mut input);
                    let decoded = decode_zstd(&compressed).unwrap();
                    assert_eq!(
                        decoded, data,
                        "Decoded data did not match the original input during compression"
                    );
                }
            }
        }
    }

    /// Homogeneous input — every byte the same — must NOT be split:
    /// both border histograms are identical (all 512 hits on a single
    /// slot), so `presplit_fingerprints_differ` returns `false` and the
    /// function takes the early-return path at
    /// `zstd_preSplit.c:214` returning `blockSize`.
    #[test]
    fn donor_split_block_from_borders_keeps_homogeneous_block() {
        let block = vec![0xAAu8; MAX_BLOCK_SIZE as usize];
        let split = super::donor_split_block_from_borders(&block);
        assert_eq!(split, MAX_BLOCK_SIZE as usize);
    }

    /// Heterogeneous input — first half all zeros, second half a
    /// counter sequence — has clearly distinguishable border
    /// histograms, so the borders heuristic decides to split.
    ///
    /// The transition sits at exactly the block midpoint, so the
    /// middle 512-byte sample (`block[mid-256..mid+256]`) is half
    /// zeros + half counter values. That makes it roughly
    /// equidistant from both border fingerprints — the
    /// `abs_diff(dist_from_begin, dist_from_end) < min_distance`
    /// branch fires and the heuristic returns the midpoint (64 KiB)
    /// per `zstd_preSplit.c:222`. The test asserts the exact value
    /// rather than just "one of {32K, 64K, 96K}" so a regression
    /// to a different quantised arm cannot silently slip through.
    #[test]
    fn donor_split_block_from_borders_returns_midpoint_for_centred_transition() {
        let mut block = vec![0u8; MAX_BLOCK_SIZE as usize];
        for (i, byte) in block
            .iter_mut()
            .enumerate()
            .skip(MAX_BLOCK_SIZE as usize / 2)
        {
            *byte = (i % 251 + 1) as u8;
        }
        let split = super::donor_split_block_from_borders(&block);
        assert_eq!(
            split,
            64 * 1024,
            "centred-transition fixture must take the symmetric \
             midpoint arm (`abs_diff < min_distance`), got {split}"
        );
    }

    /// `donor_pre_split_level` maps mid-range levels to the cheap
    /// borders heuristic and high levels to the byChunks path. Levels
    /// below 11 stay unsplit so the splitter never runs on fast /
    /// default presets where its per-block cost would dominate.
    #[test]
    fn donor_pre_split_level_dispatches_by_compression_level() {
        use crate::encoding::CompressionLevel;
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Fastest),
            None
        );
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Default),
            None
        );
        assert_eq!(super::donor_pre_split_level(CompressionLevel::Better), None);
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Level(7)),
            None
        );
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Level(11)),
            Some(0)
        );
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Level(15)),
            Some(0)
        );
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Level(16)),
            Some(4)
        );
        assert_eq!(
            super::donor_pre_split_level(CompressionLevel::Level(22)),
            Some(4)
        );
    }

    /// End-to-end: a 256 KB heterogeneous payload compressed at
    /// Level(13) (borders heuristic active) round-trips through the
    /// crate's own decoder. The pre-split path runs over the first
    /// 128 KB block and emits two consecutive sub-blocks; the second
    /// 128 KB block goes through the splitter on its own. The test
    /// proves the split decisions do not corrupt the frame bitstream.
    #[test]
    fn level_13_borders_split_roundtrips_through_own_decoder() {
        use crate::encoding::CompressionLevel;
        let mut data = vec![0u8; 256 * 1024];
        // First 128 KB: low-entropy repeating run; second 128 KB:
        // counter sequence — clearly distinct border histograms.
        for (i, byte) in data.iter_mut().enumerate() {
            *byte = if i < 128 * 1024 {
                (i & 0x07) as u8
            } else {
                (i % 251 + 1) as u8
            };
        }

        let mut compressed = Vec::new();
        let mut compressor = FrameCompressor::new(CompressionLevel::Level(13));
        compressor.set_source(data.as_slice());
        compressor.set_drain(&mut compressed);
        compressor.compress();

        let mut decoder = FrameDecoder::new();
        let mut source = compressed.as_slice();
        decoder
            .reset(&mut source)
            .expect("frame header should parse");
        while !decoder.is_finished() {
            decoder
                .decode_blocks(&mut source, crate::decoding::BlockDecodingStrategy::All)
                .expect("decode should succeed");
        }
        let mut decoded = Vec::with_capacity(data.len());
        decoder.collect_to_writer(&mut decoded).unwrap();
        assert_eq!(decoded, data, "roundtrip must reproduce the input verbatim");
    }

    /// Regression: `set_compression_level` followed by `compress()` must
    /// refresh `state.strategy_tag` through the reset-time sync so the
    /// literal-compression gates (`min_literals_to_compress`,
    /// `min_gain`) use the NEW level's strategy. Picks a level pair
    /// that genuinely crosses strategy bands — `Fastest` resolves to
    /// `Fast`, `Level(20)` resolves to `BtUltra2` — so a missed sync
    /// would leave the construction-time tag visible and trip the
    /// assertion. `CompressionLevel::Best` would also pass type-wise
    /// but resolves to `Lazy` today, which keeps `min_literals_to_compress`
    /// in the same `shift=3 → 64-byte` band as `Fast` and weakens the
    /// signal that the gate floor actually moved.
    #[cfg(feature = "std")]
    #[test]
    fn set_compression_level_then_compress_refreshes_strategy_tag() {
        use super::CompressionLevel;
        use crate::encoding::strategy::StrategyTag;

        let data = vec![0xABu8; 256];
        let mut out = Vec::new();
        let mut compressor = FrameCompressor::new(CompressionLevel::Fastest);
        let initial_tag = compressor.state.strategy_tag;
        assert_eq!(
            initial_tag,
            StrategyTag::for_compression_level(CompressionLevel::Fastest),
            "construction-time strategy_tag must reflect initial level",
        );

        // Switch to a level whose resolved strategy lives in a different
        // band, then run a full compress cycle — the matcher.reset()
        // inside `compress` is the only site that can refresh the tag.
        let new_level = CompressionLevel::Level(20);
        compressor.set_compression_level(new_level);
        compressor.set_source(data.as_slice());
        compressor.set_drain(&mut out);
        compressor.compress();

        let new_tag = compressor.state.strategy_tag;
        let expected = StrategyTag::for_compression_level(new_level);
        assert_eq!(
            new_tag, expected,
            "strategy_tag must follow set_compression_level → compress, \
             got {new_tag:?} expected {expected:?}",
        );
        assert_eq!(
            expected,
            StrategyTag::BtUltra2,
            "test fixture invariant: Level(20) must resolve to BtUltra2 \
             so the post-switch tag visibly crosses the band boundary",
        );
        assert_ne!(
            new_tag, initial_tag,
            "test fixture invariant: chosen levels must resolve to \
             different StrategyTag variants",
        );
    }

    /// Magicless mode (`ZSTD_f_zstd1_magicless`): encoded frame
    /// MUST NOT start with the 4-byte magic prefix, AND must
    /// round-trip through a magicless-aware decoder.
    #[test]
    fn magicless_frame_omits_magic_and_roundtrips() {
        use crate::common::MAGIC_NUM;
        let input: alloc::vec::Vec<u8> = (0..512u32).map(|i| (i ^ 0xA5) as u8).collect();

        // Encode with magicless = true.
        let mut output: Vec<u8> = Vec::new();
        let mut compressor = FrameCompressor::new(super::CompressionLevel::Default);
        compressor.set_magicless(true);
        compressor.set_source(input.as_slice());
        compressor.set_drain(&mut output);
        compressor.compress();

        // 1. Encoded output must NOT begin with the zstd magic number.
        assert!(
            !output.starts_with(&MAGIC_NUM.to_le_bytes()),
            "magicless frame must omit the 4-byte magic prefix",
        );

        // 2. A magicless-aware decoder must round-trip the payload.
        let mut decoder = crate::decoding::FrameDecoder::new();
        decoder.set_magicless(true);
        let mut cursor: &[u8] = output.as_slice();
        decoder.init(&mut cursor).expect("magicless init");
        decoder
            .decode_blocks(&mut cursor, crate::decoding::BlockDecodingStrategy::All)
            .expect("decode_blocks");
        let mut decoded: Vec<u8> = Vec::new();
        decoder
            .collect_to_writer(&mut decoded)
            .expect("collect_to_writer");
        assert_eq!(decoded, input, "magicless roundtrip must preserve bytes");

        // 3. A standard (magicful) decoder MUST reject a magicless
        //    frame at the header-read step — the first 4 bytes are
        //    the frame-header descriptor + window / dictionary / FCS
        //    metadata, not the magic. We accept either
        //    `BadMagicNumber` (typical case: first 4 bytes don't
        //    match `MAGIC_NUM` and don't fall in the skippable-frame
        //    magic range) or `SkipFrame` (rare: the first 4 bytes
        //    coincidentally land in `0x184D2A50..=0x184D2A5F`). Both
        //    prove the standard decoder did not treat the bytes as a
        //    real magicful frame.
        use crate::decoding::errors::{FrameDecoderError, ReadFrameHeaderError};
        let mut std_decoder = crate::decoding::FrameDecoder::new();
        let std_init = std_decoder.init(output.as_slice());
        match std_init {
            Err(FrameDecoderError::ReadFrameHeaderError(
                ReadFrameHeaderError::BadMagicNumber(_) | ReadFrameHeaderError::SkipFrame { .. },
            )) => {}
            other => panic!(
                "standard decoder must reject a magicless frame with \
                 ReadFrameHeaderError::BadMagicNumber or SkipFrame, got {other:?}",
            ),
        }
    }
}