znippy_plugin_git/

exploded_arrow.rs

//! **The exploded objects table: ONE Arrow IPC file, one row per object.**
//!
//! # What this replaces, and what it measured
//!
//! [`crate::exploded`] holds `oid → [kind][payload…]` in **redb**. It works, and
//! it is the wrong medium — MEASURED on `linux.git`, oden 2026-08-13,
//! 11 697 976 objects:
//!
//! | | |
//! |---|---:|
//! | pack as pushed (verbatim, deflated + delta-encoded) | 6.4 GB |
//! | its objects fully resolved (`git verify-pack -v`, summed) | **16.8 GB** |
//! | `objects.exploded` on disk | **204 GB** |
//!
//! So the 2026-08-08 eager decision was RIGHT — resolved content is **2.6×** the
//! pack, comfortably inside the 3× it budgeted — and the medium was wrong by
//! **12×**. redb is a copy-on-write B-tree: 11.7 million small keys committed in
//! [`FLUSH_BYTES`] batches rewrite interior pages over and over, and 187 of
//! those 204 GB are page churn rather than anybody's data.
//!
//! It cost throughput for the same reason. [`crate::indexer`] fans a drained
//! batch across N workers with `gatling_for_each` (LAW 3, never rayon) and every
//! one of them then queued behind redb's single write transaction — MEASURED:
//! **96.8% of ONE core** with 31 idle while the exploder ran.
//!
//! # The shape
//!
//! One table, one file, the payload IN the row — which is what this engine
//! already does everywhere else ([`crate::reach`] stores a bitmap as
//! `DataType::Binary`, [`crate::secrets`] a ciphertext):
//!
//! ```text
//!   oid | object_type | mode | path | payload
//! ```
//!
//! An Arrow IPC stream is `[schema message][batch message]…` — append-only, no
//! pages to rewrite, so the file is the payload total plus framing. Each flush
//! encodes one batch with [`IpcDataGenerator`] and appends it with
//! [`write_message`]; nothing earlier in the file is ever touched again.
//!
//! `payload` is `LargeBinary` and not `Binary` deliberately: `Binary` offsets are
//! `i32`, so one batch would cap at 2 GiB of payload and overflow *silently* on a
//! corpus nobody tested. The wider offset costs four bytes a row.
//!
//! ## `mode` and `path` — the row IS a file
//!
//! A git object on its own is content without a name. The two nullable columns
//! carry the name when the caller knows it (a tree walk does; a bare pack
//! resolve does not), which is what lets this table be read as a filesystem
//! rather than as a content-addressed bag. They are **nullable and best-effort**
//! for a reason that is inherent and not a shortcut: one blob is reachable at
//! many paths in many commits, so a path column can only ever record *a* path —
//! the one it was first exploded at. Null means "not known here", never "at the
//! root".
//!
//! # Reopening is cheap, and the index is lazy
//!
//! A batch's *metadata* carries its row count and its body length, so
//! [`ExplodedArchive::open`] walks the message framing — seeking over the bodies,
//! reading kilobytes — and knows how many rows the table holds and where every
//! batch begins without touching a gigabyte. That is what `adopt_journal` needs
//! on startup.
//!
//! The `oid → row` index costs one sequential pass and is therefore built on the
//! **first lookup**, not on open. A push-heavy process never pays for it; a
//! process that reads pays once. After that a point read is two `pread`s — the
//! schema message and the one batch — decoded through [`StreamDecoder`], so it
//! never scans the batches in front of the one it wants.
//!
//! # The lookup is ragnar's static tree — see [`OidTree`]
//!
//! That index used to be a `Vec<(Vec<u8>, Located)>` walked with
//! `binary_search_by`: one heap allocation per oid — **11.7 million** of them on
//! `linux.git` — and a `log₂ n` chain of dependent loads over scattered `Vec`
//! headers. It is now the same `stree` (`znippy-zoomies`) this crate already puts
//! over oids in [`crate::oid_index`], over flat parallel arrays.
//! `nornir-workspace.toml`'s performance law names that structure by name; this
//! is the second place in this crate that obeys it, and it obeys it by reusing
//! [`crate::oid_index::key_for_oid`] rather than deriving a second key.
//!
//! # Droppable, and it is one `rm`
//!
//! Every row is re-derivable from the verbatim pack bytes, so the file can be
//! deleted without consulting a client:
//! [`crate::git_ops::Absorber::adopt_journal`] compares this table's row count
//! against the `objects` table's and re-queues every pack if it is short. Absent
//! means fall back and rebuild; it never means wrong. One file, one delete —
//! which is the property the engine's own tests assert.

use std::fs::{File, OpenOptions};
use std::io::{Read, Seek, SeekFrom, Write};
use std::os::unix::fs::FileExt;
use std::path::{Path, PathBuf};
use std::sync::Arc;
use std::sync::Mutex;
use std::sync::atomic::{AtomicU64, Ordering};

use anyhow::{Context, Result};
use znippy_common::arrow::array::{
    Array, FixedSizeBinaryArray, FixedSizeBinaryBuilder, LargeBinaryArray, LargeBinaryBuilder,
    StringArray, StringBuilder, UInt8Array, UInt8Builder, UInt32Array, UInt32Builder,
};
use znippy_common::arrow::buffer::Buffer;
use znippy_common::arrow::datatypes::{DataType, Field, Schema, SchemaRef};
use znippy_common::arrow::ipc::reader::StreamDecoder;
use znippy_common::arrow::ipc::writer::{
    DictionaryTracker, IpcDataGenerator, IpcWriteOptions, write_message,
};
use znippy_common::arrow::ipc::root_as_message;
use znippy_common::arrow::record_batch::RecordBatch;
use znippy_zoomies::stree::STree64Mmap;

use crate::object::GitObjectKind;
use crate::oid_index::key_for_oid;

pub const COL_OID: &str = "oid";
pub const COL_TYPE: &str = "object_type";
pub const COL_MODE: &str = "mode";
pub const COL_PATH: &str = "path";
pub const COL_PAYLOAD: &str = "payload";

/// Buffered payload bytes that force a flush.
///
/// Not a memory *limit* — one object can exceed it on its own and is written
/// anyway. It is what keeps a 2 GiB push from holding its whole resolved content
/// in RAM before the first batch reaches the file.
pub const FLUSH_BYTES: usize = 64 << 20;

/// `oid | object_type | mode | path | payload`.
///
/// `oid_len` is a parameter because this engine serves both sha1 (20 bytes) and
/// sha256 (32); a hardcoded 20 would silently truncate every sha256 key.
/// **How each row's payload is stored** — `ENC_RAW` or `ENC_ZLIB`. Lever 3
/// (2026-09-04): the eager pool was 1.67 GB of inflated commits and trees, 61 %
/// of everything the store kept above the pack. Arrow IPC's own body
/// compression is per BUFFER, so a point read could not lift one row out of a
/// compressed batch and the `pread` extent fast path — the whole reason the
/// pool is fast — would have died with it (`payload_data_offset` refuses
/// compressed batches for exactly that reason; MEASURED 200 000× read
/// amplification without the extent). So the compression is per ROW: the
/// extent pread returns one compressed payload and inflates just that one.
/// zlib, because the crate already has it (`flate2`, zlib-rs — pure Rust), and
/// because it is the codec git itself uses per object, so the ratios are known
/// ground.
pub const COL_ENC: &str = "enc";
pub const ENC_RAW: u8 = 0;
pub const ENC_ZLIB: u8 = 1;
/// Rows shorter than this are never compressed: a zlib header and trailer are
/// 8 bytes, and a tiny tree does not earn them back. Set from
/// `examples/explode_compress_ratio.rs` over the kernel mirror's pool.
pub const MIN_ROW_COMPRESS: usize = 128;
/// A compressed row must be at least this much smaller than the raw one to be
/// kept compressed, or the inflate on every read buys nothing.
const COMPRESS_KEEP_IF_UNDER_PERCENT: usize = 90;

/// Schema v2 (2026-09-04): `enc` sits BEFORE `payload`, so the payload's data
/// buffer stays the LAST buffer of the batch — which is what
/// [`payload_data_offset`] reads the extent off. A v1 file has no `enc` column;
/// every reader treats a missing column as [`ENC_RAW`], so pools written before
/// this open unchanged and are never rewritten for it.
pub fn exploded_schema(oid_len: usize) -> SchemaRef {
    Arc::new(Schema::new(vec![
        Field::new(COL_OID, DataType::FixedSizeBinary(oid_len as i32), false),
        Field::new(COL_TYPE, DataType::UInt8, false),
        Field::new(COL_MODE, DataType::UInt32, true),
        Field::new(COL_PATH, DataType::Utf8, true),
        Field::new(COL_ENC, DataType::UInt8, false),
        Field::new(COL_PAYLOAD, DataType::LargeBinary, false),
    ]))
}

/// Schema v1 — what every pool written before 2026-09-04 carries, and what this
/// build keeps appending to such a pool (see `ExplodedArchive::legacy_v1`).
pub fn exploded_schema_v1(oid_len: usize) -> SchemaRef {
    Arc::new(Schema::new(vec![
        Field::new(COL_OID, DataType::FixedSizeBinary(oid_len as i32), false),
        Field::new(COL_TYPE, DataType::UInt8, false),
        Field::new(COL_MODE, DataType::UInt32, true),
        Field::new(COL_PATH, DataType::Utf8, true),
        Field::new(COL_PAYLOAD, DataType::LargeBinary, false),
    ]))
}

/// Compress one row's payload for storage, or `None` when it is not worth it
/// (too short, or zlib did not get it under the keep threshold).
///
/// **`default` (level 6), not `fast`, and that is a measurement**
/// (`examples/explode_compress_ratio.rs`, the kernel mirror's eager pool,
/// 1 722 249 rows / 1 609 688 036 B of commits and tags, 2026-09-04):
///
/// | level | pool after | ratio | saved |
/// |---|---:|---:|---:|
/// | fast | 1 138 092 314 B | 1.41× | 471 595 722 B |
/// | **default** | **889 639 690 B** | **1.81×** | **720 048 346 B** |
///
/// Inflate cost is the SAME at both — 3.07 µs/row mean over 20 000 sampled
/// rows — because zlib decode speed does not depend on the level. So the read
/// path pays +3 µs on a point read (first-read p50 was 3.9 µs) whichever level
/// is chosen, and the only thing the level decides is how much disk that buys:
/// 44.7 % of the pool at `default` against 29.3 % at `fast`. The write pays
/// more CPU at `default`, on the indexer's threads, once per object.
fn compress_row(raw: &[u8]) -> Option<Vec<u8>> {
    use std::io::Write as _;
    if raw.len() < MIN_ROW_COMPRESS {
        return None;
    }
    let mut e = flate2::write::ZlibEncoder::new(
        Vec::with_capacity(raw.len() / 2 + 16),
        flate2::Compression::default(),
    );
    e.write_all(raw).ok()?;
    let z = e.finish().ok()?;
    (z.len() * 100 < raw.len() * COMPRESS_KEEP_IF_UNDER_PERCENT).then_some(z)
}

/// The row's payload as the caller wants it — inflated if it was stored
/// compressed. `bytes` is exactly what the column (or the extent pread) held.
fn row_payload(enc: u8, bytes: &[u8]) -> Result<Vec<u8>> {
    use std::io::Read as _;
    match enc {
        ENC_RAW => Ok(bytes.to_vec()),
        ENC_ZLIB => {
            let mut out = Vec::with_capacity(bytes.len() * 3);
            flate2::read::ZlibDecoder::new(bytes)
                .read_to_end(&mut out)
                .context("inflating a compressed exploded row")?;
            Ok(out)
        }
        other => anyhow::bail!("exploded row carries unknown encoding {other}"),
    }
}

/// The `enc` column of a decoded batch, or `None` on a v1 batch (every row raw).
fn enc_column(batch: &RecordBatch) -> Option<&UInt8Array> {
    batch
        .column_by_name(COL_ENC)
        .and_then(|c| c.as_any().downcast_ref::<UInt8Array>())
}

#[inline]
fn enc_at(enc: Option<&UInt8Array>, row: usize) -> u8 {
    enc.map_or(ENC_RAW, |e| e.value(row))
}

/// The pack type codes, the same mapping [`crate::exploded`] uses, so the two
/// media cannot disagree about what a `2` means.
pub fn kind_code(k: GitObjectKind) -> u8 {
    match k {
        GitObjectKind::Commit => 1,
        GitObjectKind::Tree => 2,
        GitObjectKind::Blob => 3,
        GitObjectKind::Tag => 4,
    }
}

pub fn kind_of(code: u8) -> Option<GitObjectKind> {
    match code {
        1 => Some(GitObjectKind::Commit),
        2 => Some(GitObjectKind::Tree),
        3 => Some(GitObjectKind::Blob),
        4 => Some(GitObjectKind::Tag),
        _ => None,
    }
}

/// **A row that retires an oid.**
///
/// The file is append-only, so `retain` cannot reach back and delete: it appends
/// rows that say *this oid is gone*, and the index fold applies them in file
/// order, so the last word about an oid wins. Dropping only from the in-memory
/// index is what a first cut did, and it was WRONG in the way that matters —
/// `a_wholly_dead_pack_does_not_come_back_when_the_store_is_reopened` came back
/// with **551 commits instead of 1**, because a reopen rebuilds the index from
/// the file and resurrected every row `gc` had retired.
///
/// `0` and not a sixth column: git's pack type codes are 1–7 and 0 is not one of
/// them, so [`kind_of`] already answers `None` for it. A tombstone costs an oid
/// and an empty payload, not a byte on every live row.
pub const TOMBSTONE: u8 = 0;

/// One buffered row, waiting for the next batch. `code` rather than a
/// [`GitObjectKind`] because a tombstone is a row with no kind.
#[derive(Debug, Clone)]
struct Pending {
    oid: Vec<u8>,
    code: u8,
    mode: Option<u32>,
    path: Option<String>,
    /// Where this row's bytes are in [`PendingBatch::arena`]. **Not a
    /// `Vec<u8>`** — see [`PendingBatch`].
    pay: std::ops::Range<usize>,
}

/// The rows buffered for the next batch, and **one arena holding their bytes**.
///
/// # Why the bytes are not in the row
///
/// `payload: Vec<u8>` cost one heap allocation and one `memcpy` of the whole
/// object **per buffered row**, and on the lazy write-back path that allocation
/// is a second copy of bytes the reader is holding anyway. MEASURED by lane AA,
/// 2026-08-30: reading one 3 145 728 B object through `gunnar-cli` allocated
/// 6 340 693 B — *exactly two copies* — and the second was this one.
///
/// An arena makes the write an `extend_from_slice` into a buffer that grows
/// geometrically and is reused after every flush, so the bytes attributable to
/// one admitted read amortise to zero and a **declined** read costs nothing at
/// all (the admission gate returns before this is reached). LAW: zero copy.
#[derive(Default)]
struct PendingBatch {
    rows: Vec<Pending>,
    arena: Vec<u8>,
}

impl PendingBatch {
    fn is_empty(&self) -> bool {
        self.rows.is_empty()
    }
    /// Buffer one row's bytes and return the range they landed in.
    fn stash(&mut self, payload: &[u8]) -> std::ops::Range<usize> {
        let start = self.arena.len();
        self.arena.extend_from_slice(payload);
        start..self.arena.len()
    }
    fn bytes(&self, r: &Pending) -> &[u8] {
        &self.arena[r.pay.clone()]
    }
    /// Hand the batch over for encoding and keep the arena's capacity: the next
    /// batch reuses the same allocation, which is what makes the per-read cost
    /// amortise.
    fn take(&mut self) -> (Vec<Pending>, Vec<u8>) {
        let rows = std::mem::take(&mut self.rows);
        let arena = std::mem::replace(&mut self.arena, Vec::with_capacity(0));
        (rows, arena)
    }
}

/// Where one batch message sits in the file. Recovered from the framing walk,
/// which reads metadata and seeks over bodies.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Batch {
    /// Byte offset of the message's continuation marker.
    at: u64,
    /// Bytes the whole message occupies: `8 + metadata + body`.
    len: u64,
    /// Rows in it — read from the metadata, so counting them costs no body read.
    rows: u32,
    /// Absolute byte offset of the message BODY — where the buffer offsets the
    /// metadata declares are measured from.
    body: u64,
    /// The payload DATA buffer's offset within the body, from the metadata's
    /// own buffer table. [`EXTENT_UNAVAILABLE`] when the body is compressed or
    /// the buffer layout is not the one [`exploded_schema`] writes — a point
    /// read then falls back to decoding the batch, which is slow and correct.
    pay_data: u64,
}

/// Sentinel for "no byte extent could be computed for this row/batch" — the
/// fall-back-and-decode marker, never an offset a file could really have.
const EXTENT_UNAVAILABLE: u64 = u64::MAX;

/// The payload DATA buffer's offset within the message body, read from the
/// batch metadata alone.
///
/// The payload is [`exploded_schema`]'s LAST field and a `LargeBinary`'s data
/// buffer is its last buffer, so the table's final buffer entry IS the payload
/// bytes — asserted by also counting the buffers (12: validity+data for oid and
/// object_type, validity+data for mode, validity+offsets+data for path and for
/// payload). `None` when the count disagrees or the body is compressed;
/// [`ExplodedArchive::content`] then decodes the batch instead of guessing.
fn payload_data_offset(meta: &[u8]) -> Option<u64> {
    let msg = root_as_message(meta).ok()?;
    let rb = msg.header_as_record_batch()?;
    if rb.compression().is_some() {
        return None;
    }
    let bufs = rb.buffers()?;
    // 12 buffers is schema v1 (oid 2, type 2, mode 2, path 3, payload 3); 14 is
    // v2 with the `enc` column's two. In both the payload's data buffer is the
    // last one, which is the only fact used below.
    if bufs.len() != 12 && bufs.len() != 14 {
        return None;
    }
    let last = bufs.get(bufs.len() - 1);
    (last.offset() >= 0).then(|| last.offset() as u64)
}

/// Where one object's row is — and where its payload BYTES are.
///
/// The extent is what makes a point read one small `pread` instead of a whole
/// 64 MiB batch decode. MEASURED without it, rust-lang/rust push, 2026-08-19:
/// the pre-ack connectivity walk visited commits and trees in graph order —
/// effectively random across ~574 batches against a 4-slot cache — and decoded
/// 6.7 GB/s of page cache to serve ~98 objects/s, one core pinned for hours, a
/// ~200 000× read amplification. The extent costs 16 bytes per live oid.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Located {
    batch: u32,
    row: u32,
    kind: GitObjectKind,
    /// Absolute file offset of this row's payload bytes, or
    /// [`EXTENT_UNAVAILABLE`] — then the read falls back to the batch decode.
    pay_at: u64,
    /// Payload length in bytes. Meaningless when `pay_at` is the sentinel.
    pay_len: u64,
    /// [`ENC_RAW`] or [`ENC_ZLIB`] — carried in the index so the extent fast
    /// path knows whether to inflate without decoding the batch.
    enc: u8,
}

/// **`oid → Located`, as ragnar's static tree over flat arrays.**
///
/// Three parallel arrays in one oid-ascending order, plus an [`STree64Mmap`]
/// built over the first of them:
///
/// ```text
///   keys  count * 8         i64 little-endian, ascending — the stree keyspace
///   oids  count * oid_len   the FULL oid, for the verify step
///   locs  count             batch, row, kind
/// ```
///
/// ## Why this and not the `Vec<(Vec<u8>, Located)>` it replaces
///
/// That vector cost one heap allocation *per oid* — 11.7 M of them for
/// `linux.git` — and its `binary_search_by` chased `log₂ n` dependent loads over
/// `Vec` headers scattered across the heap before it could compare a byte. Here
/// the oid bytes are contiguous, and `stree` routes through 64-byte nodes with a
/// branchless AVX2 compare. `nornir-workspace.toml` names this structure as the
/// one to use, and [`crate::oid_index`] already uses it over exactly these keys —
/// so the key function is **imported from there**, not written again.
///
/// ## The 8-byte key is a FILTER, not a key — and this is where a wrong answer
/// would come from
///
/// The key is [`key_for_oid`]: the first eight bytes of the oid, big-endian, top
/// bit flipped so unsigned byte order becomes signed `i64` order (without the
/// flip every oid starting `0x80..0xff` — half of them — sorts below every oid
/// starting `0x00..0x7f`, and the keyspace `stree` requires to be ascending is
/// not). **Twelve** of a sha1 oid's twenty bytes are not in the key at all (24 of
/// 32 for sha256), so two distinct objects **can** share one key.
///
/// How likely, honestly: for `linux.git`'s 11.7 M objects, birthday arithmetic
/// gives n²/2 · 2⁻⁶⁴ ≈ **3.7 × 10⁻⁶** — one table in ~270 000. So it will not
/// happen by accident on one repository, and that is exactly why a verify-less
/// probe would ship and pass every test anyone bothered to write. It does not
/// stay accidental: finding *some* pair of contents whose oids share eight bytes
/// is a 2³² birthday search — hours of commodity hashing, not a research result —
/// and both objects are then perfectly ordinary blobs a client may push. (Hitting
/// a *specific* stored oid's prefix is 2⁶⁴ and is not a threat.) A hosted forge
/// serves oids that other people chose, so this is a property to hold by
/// construction rather than a probability to accept.
///
/// So `stree` narrows and never decides. It routes to *a* member of the run of
/// equal keys — which member is its own business, because its leaf scan starts at
/// a block boundary that can fall in the middle of the run — so
/// [`expand_run`](Self::expand_run) walks **both** directions to the run's ends,
/// and [`find`](Self::find) then compares the **whole oid** against every
/// candidate. Skipping that comparison would not merely be sloppy: a `have`
/// negotiation for an oid this table has never seen, whose first eight bytes
/// happen to match a stored one, would be answered with **the other object's
/// bytes** — a silently wrong object, which is far worse than a slow one.
struct OidTree {
    /// Width of one oid. `0` only when the tree is empty.
    oid_len: usize,
    count: usize,
    keys: Vec<u8>,
    oids: Vec<u8>,
    locs: Vec<Located>,
    /// `None` for an empty table — `STree64Mmap::new_with_stride` asserts
    /// `count > 0`.
    tree: Option<STree64Mmap>,
}

/// Which member of a run of equal oids survives the fold — the two callers
/// genuinely differ, and the difference is not cosmetic.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Keep {
    /// [`ExplodedArchive::flush`]'s incremental extension: an oid already in the
    /// index keeps the [`Located`] it already had. That is what
    /// [`ExplodedArchive::explode_at`] documents — a blob is reachable at many
    /// paths and the table records the **first** name it was seen at.
    First,
    /// [`ExplodedArchive::index_now`]'s rebuild from the file: batches are walked
    /// in file order and rows in row order, so the **last** word about an oid
    /// wins. That is what makes a [`TOMBSTONE`] durable rather than advisory.
    Last,
}

/// Accumulates sightings in the order they happen, then folds them.
///
/// Sightings, not entries: the same oid may arrive many times, and a `None`
/// [`Located`] is a [`TOMBSTONE`] row saying the oid is gone. Nothing is
/// deduplicated until [`finish`](Self::finish), because which sighting wins
/// depends on their order and on [`Keep`].
struct OidTreeBuilder {
    /// Taken from the first oid pushed; every later one must match it.
    oid_len: usize,
    oids: Vec<u8>,
    locs: Vec<Option<Located>>,
}

impl OidTreeBuilder {
    fn with_capacity(rows: usize) -> Self {
        Self { oid_len: 0, oids: Vec::new(), locs: Vec::with_capacity(rows) }
    }

    /// Record one sighting. `loc` is `None` for a tombstone row.
    ///
    /// A width that disagrees with the rows already pushed is refused rather
    /// than padded: a truncated or extended oid would sort and compare as a
    /// different object.
    fn push(&mut self, oid: &[u8], loc: Option<Located>) -> Result<()> {
        if self.locs.is_empty() && self.oids.is_empty() {
            self.oid_len = oid.len();
            self.oids.reserve(self.locs.capacity() * self.oid_len);
        }
        anyhow::ensure!(
            oid.len() == self.oid_len,
            "the exploded index holds {}-byte oids and a {}-byte one arrived",
            self.oid_len,
            oid.len()
        );
        self.oids.extend_from_slice(oid);
        self.locs.push(loc);
        Ok(())
    }

    fn oid_at(&self, i: usize) -> &[u8] {
        &self.oids[i * self.oid_len..(i + 1) * self.oid_len]
    }

    /// Sort, fold each run of equal oids down to its winner, drop the ones whose
    /// winner is a tombstone, and build the tree.
    fn finish(self, keep: Keep) -> OidTree {
        let n = self.locs.len();
        let mut order: Vec<u32> = (0..n as u32).collect();
        // STABLE, and load-bearing: `Keep` is about *insertion* order within a
        // run of equal oids, so an unstable sort would make "first sighting"
        // and "last word wins" both mean "whichever one the sort happened to
        // leave there".
        order.sort_by(|&a, &b| self.oid_at(a as usize).cmp(self.oid_at(b as usize)));

        let mut keys: Vec<u8> = Vec::with_capacity(n * 8);
        let mut oids: Vec<u8> = Vec::with_capacity(n * self.oid_len);
        let mut locs: Vec<Located> = Vec::with_capacity(n);
        let mut i = 0usize;
        while i < n {
            let oid = self.oid_at(order[i] as usize);
            let mut j = i + 1;
            while j < n && self.oid_at(order[j] as usize) == oid {
                j += 1;
            }
            let pick = match keep {
                Keep::First => order[i],
                Keep::Last => order[j - 1],
            } as usize;
            if let Some(loc) = self.locs[pick] {
                // Ascending oid order gives ascending keys, because
                // `key_for_oid` is order-preserving over the oid's leading
                // bytes — the property `stree` needs and the reason for the
                // sign-bit flip.
                keys.extend_from_slice(&key_for_oid(oid).to_le_bytes());
                oids.extend_from_slice(oid);
                locs.push(loc);
            }
            i = j;
        }

        let count = locs.len();
        let tree = (count > 0).then(|| STree64Mmap::new_with_stride(&keys, count, 8));
        OidTree { oid_len: self.oid_len, count, keys, oids, locs, tree }
    }
}

impl OidTree {
    fn len(&self) -> usize {
        self.count
    }

    fn oid_at(&self, i: usize) -> &[u8] {
        &self.oids[i * self.oid_len..(i + 1) * self.oid_len]
    }

    fn key_at(&self, i: usize) -> i64 {
        i64::from_le_bytes(self.keys[i * 8..i * 8 + 8].try_into().unwrap())
    }

    /// Widen a hit to the whole run of equal keys. `stree` routes to *a* member
    /// of the run — its leaf scan starts at a block boundary, which can fall
    /// anywhere inside one — so both directions are walked rather than assumed.
    fn expand_run(&self, pos: usize, key: i64) -> std::ops::Range<usize> {
        let mut lo = pos;
        while lo > 0 && self.key_at(lo - 1) == key {
            lo -= 1;
        }
        let mut hi = pos + 1;
        while hi < self.count && self.key_at(hi) == key {
            hi += 1;
        }
        lo..hi
    }

    /// Resolve an oid. `None` when absent — never another object's row.
    fn find(&self, oid: &[u8]) -> Option<Located> {
        if oid.len() != self.oid_len {
            return None;
        }
        let tree = self.tree.as_ref()?;
        let key = key_for_oid(oid);
        let pos = tree.find_exact(key, &self.keys)?;
        self.verify(pos, key, oid)
    }

    /// The full-oid comparison the whole structure's correctness rests on.
    fn verify(&self, pos: usize, key: i64, oid: &[u8]) -> Option<Located> {
        self.expand_run(pos, key)
            .find(|&i| self.oid_at(i) == oid)
            .map(|i| self.locs[i])
    }

    /// Every live entry, in ascending oid order — what `of_kind` and `retain`
    /// fold over.
    fn iter(&self) -> impl Iterator<Item = (&[u8], Located)> + '_ {
        (0..self.count).map(move |i| (self.oid_at(i), self.locs[i]))
    }

    /// The **baseline** the tree has to agree with: `binary_search` over the very
    /// same oid array, followed by the very same run-and-verify. It is the
    /// finder this file shipped with, and it exists so "the stree returns the
    /// same row" is a measurement rather than an assertion.
    #[cfg(test)]
    fn find_by_binary_search(&self, oid: &[u8]) -> Option<Located> {
        if oid.len() != self.oid_len || self.count == 0 {
            return None;
        }
        let mut lo = 0usize;
        let mut hi = self.count;
        while lo < hi {
            let mid = lo + (hi - lo) / 2;
            if self.oid_at(mid) < oid { lo = mid + 1 } else { hi = mid }
        }
        (lo < self.count && self.oid_at(lo) == oid).then(|| self.locs[lo])
    }

    /// The **unverified** candidate run for a key: every entry sharing that
    /// 8-byte prefix. Normally length 1; length > 1 is a real prefix collision.
    /// Exposed so a test can prove the collision it constructed is real and that
    /// the verify step is what tells the candidates apart.
    #[cfg(test)]
    fn candidate_run(&self, key: i64) -> std::ops::Range<usize> {
        let Some(tree) = self.tree.as_ref() else { return 0..0 };
        let Some(pos) = tree.find_exact(key, &self.keys) else { return 0..0 };
        self.expand_run(pos, key)
    }
}

/// Counters, all of them applied output.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
pub struct Stats {
    pub rows: u64,
    pub written: u64,
    pub served: u64,
    pub rederived: u64,
    pub absent: u64,
}

/// §14's derived table for one repository.
pub struct ExplodedArchive {
    path: PathBuf,
    /// The schema message: the file's first message, kept so a point read can
    /// feed a [`StreamDecoder`] without re-reading it from disk each time.
    schema_msg: Mutex<Option<Vec<u8>>>,
    /// Every batch message in the file, in order. Cheap — one entry per 64 MiB.
    batches: Mutex<Vec<Batch>>,
    /// `oid → Located` as an [`OidTree`]. `None` until the first lookup forces
    /// the pass that builds it, so a push-only process never pays for it.
    index: Mutex<Option<OidTree>>,
    /// The last few decoded batches, most recent first.
    ///
    /// Not an optimisation looking for a problem: without it every point read
    /// decodes its whole batch, and the 2687-object push guard went from 4 s to
    /// **over 100 s** — 2687 lookups × one 106 MiB batch each. A batch is
    /// `Arc`-backed, so a hit is a clone of pointers.
    cache: Mutex<Vec<(u32, RecordBatch)>>,
    pending: Mutex<PendingBatch>,
    pending_bytes: AtomicU64,
    /// Bytes in the file. Kept here rather than stat'ed so the append offset and
    /// the batch table cannot drift apart.
    end: Mutex<u64>,
    oid_len: Mutex<Option<usize>>,
    policy: ExplodePolicy,
    skipped: AtomicU64,
    written: AtomicU64,
    served: AtomicU64,
    /// The subset of `served` answered by the extent `pread` fast path.
    pread_served: AtomicU64,
    rederived: AtomicU64,
    absent: AtomicU64,
    /// The file was written with schema v1 (no `enc` column). Every batch this
    /// process appends to it is then written in v1 shape too — raw, no `enc` —
    /// because an IPC stream has ONE schema message and a six-column batch
    /// under a five-column schema does not decode. The pool is a cache; it moves
    /// to v2 the day it is dropped and rebuilt, never in place.
    legacy_v1: std::sync::atomic::AtomicBool,
}

impl ExplodedArchive {
    /// Open or create the table, with the policy `ZNIPPY_GIT_EXPLODE` names.
    /// Reads kilobytes, never the payloads.
    pub fn open(path: &Path) -> Result<Self> {
        Self::open_with_policy(path, ExplodePolicy::from_env()?)
    }

    /// Open with an explicit policy — what a test or a per-tenant configuration
    /// uses, so the setting is not reachable only through the environment.
    pub fn open_with_policy(path: &Path, policy: ExplodePolicy) -> Result<Self> {
        if let Some(parent) = path.parent()
            && !parent.as_os_str().is_empty()
        {
            std::fs::create_dir_all(parent)
                .with_context(|| format!("creating {}", parent.display()))?;
        }
        let (schema_msg, batches, end, oid_len) = Self::walk(path)?;
        let legacy_v1 = schema_msg.as_ref().is_some_and(|m| !Self::schema_has_enc(m));
        Ok(Self {
            legacy_v1: std::sync::atomic::AtomicBool::new(legacy_v1),
            path: path.to_path_buf(),
            schema_msg: Mutex::new(schema_msg),
            batches: Mutex::new(batches),
            index: Mutex::new(None),
            cache: Mutex::new(Vec::new()),
            pending: Mutex::new(PendingBatch::default()),
            pending_bytes: AtomicU64::new(0),
            end: Mutex::new(end),
            oid_len: Mutex::new(oid_len),
            policy,
            skipped: AtomicU64::new(0),
            written: AtomicU64::new(0),
            served: AtomicU64::new(0),
            pread_served: AtomicU64::new(0),
            rederived: AtomicU64::new(0),
            absent: AtomicU64::new(0),
        })
    }

    pub fn path(&self) -> &Path {
        &self.path
    }

    /// **Drop every derived, rebuildable structure**: the `oid → Located`
    /// tree (~50 B per row — 700 MB on linux, and re-extended on every flush
    /// while a reader keeps it alive) and the decoded-batch cache. What the
    /// memory watchdog's flush reaches for ([`crate::memguard`]); the next
    /// lookup rebuilds the tree from the file in one pass.
    pub fn drop_caches(&self) {
        if let Ok(mut i) = self.index.lock() {
            *i = None;
        }
        if let Ok(mut c) = self.cache.lock() {
            c.clear();
        }
    }

    /// Walk the message framing: `[0xFFFFFFFF][len u32][metadata][body]`.
    ///
    /// The metadata flatbuffer carries `bodyLength`, so this SEEKS over every
    /// payload — opening a 17 GB table touches kilobytes. It also carries a
    /// record batch's row `length`, which is how `rows()` answers without
    /// decoding anything.
    ///
    /// A torn tail — a half-written message at EOF — stops the walk and keeps
    /// what came before it, and the file is truncated to that boundary on the
    /// next append. The table is derived, so a partial batch is re-exploded
    /// rather than being an error a client ever sees.
    #[allow(clippy::type_complexity)]
    fn walk(path: &Path) -> Result<(Option<Vec<u8>>, Vec<Batch>, u64, Option<usize>)> {
        let Ok(mut f) = File::open(path) else {
            return Ok((None, Vec::new(), 0, None));
        };
        let total = f.metadata()?.len();
        let mut schema_msg: Option<Vec<u8>> = None;
        let mut oid_len: Option<usize> = None;
        let mut batches = Vec::new();
        let mut at = 0u64;
        while at + 8 <= total {
            f.seek(SeekFrom::Start(at))?;
            let mut hdr = [0u8; 8];
            if f.read_exact(&mut hdr).is_err() {
                break;
            }
            if u32::from_le_bytes(hdr[0..4].try_into().unwrap()) != 0xFFFF_FFFF {
                break;
            }
            let meta_len = u32::from_le_bytes(hdr[4..8].try_into().unwrap()) as u64;
            if meta_len == 0 {
                break; // end-of-stream marker
            }
            if at + 8 + meta_len > total {
                break; // torn metadata
            }
            let mut meta = vec![0u8; meta_len as usize];
            if f.read_exact(&mut meta).is_err() {
                break;
            }
            let Ok(msg) = root_as_message(&meta) else {
                break;
            };
            let body = msg.bodyLength().max(0) as u64;
            let whole = 8 + meta_len + body;
            if at + whole > total {
                break; // torn body
            }
            if let Some(rb) = msg.header_as_record_batch() {
                batches.push(Batch {
                    at,
                    len: whole,
                    rows: rb.length().max(0) as u32,
                    body: at + 8 + meta_len,
                    pay_data: payload_data_offset(&meta).unwrap_or(EXTENT_UNAVAILABLE),
                });
            } else if msg.header_as_schema().is_some() {
                schema_msg = Some({
                    let mut whole_msg = Vec::with_capacity((8 + meta_len) as usize);
                    whole_msg.extend_from_slice(&hdr);
                    whole_msg.extend_from_slice(&meta);
                    whole_msg
                });
                oid_len = Self::oid_len_of_schema_msg(schema_msg.as_ref().unwrap());
            }
            at += whole;
        }
        Ok((schema_msg, batches, at, oid_len))
    }

    /// The declared oid width, so a reopened table refuses a hash it cannot
    /// hold instead of writing a batch no reader can line up.
    /// The schema inside a stream schema message (`msg` = the 8-byte
    /// continuation header + the flatbuffer), read off the flatbuffer directly.
    ///
    /// Not through `StreamDecoder`: fed a schema message alone it returns
    /// `Ok(None)` and its `schema()` stays `None` — MEASURED 2026-09-05 with a
    /// probe on a file this writer had just produced (`decode=Ok(false)
    /// schema=None`). `oid_len_of_schema_msg` had used exactly that path since
    /// it was written and therefore always answered `None`; nothing depended on
    /// it, so nothing noticed. `legacy_v1` does depend on it, which is how it
    /// surfaced: a fresh v2 file was read as v1 and the next append was written
    /// five columns wide under a six-column schema.
    fn schema_of_msg(msg: &[u8]) -> Option<Schema> {
        let meta = msg.get(8..)?;
        let m = root_as_message(meta).ok()?;
        let fb = m.header_as_schema()?;
        Some(znippy_common::arrow::ipc::convert::fb_to_schema(fb))
    }

    /// Whether the file's schema message carries the v2 `enc` column.
    fn schema_has_enc(msg: &[u8]) -> bool {
        Self::schema_of_msg(msg).is_some_and(|s| s.field_with_name(COL_ENC).is_ok())
    }

    fn oid_len_of_schema_msg(msg: &[u8]) -> Option<usize> {
        let schema = Self::schema_of_msg(msg)?;
        match schema.field_with_name(COL_OID).ok()?.data_type() {
            DataType::FixedSizeBinary(n) => Some(*n as usize),
            _ => None,
        }
    }

    /// One resolved object with no name attached — what [`crate::exploded::PayloadSink`]
    /// can supply.
    pub fn explode(&self, oid: &[u8], kind: GitObjectKind, payload: &[u8]) -> Result<()> {
        self.explode_at(oid, kind, None, None, payload)
    }

    /// One resolved object **with its name**: the mode and path a tree walk knows.
    ///
    /// A blob is reachable at many paths, so this records the first one seen and
    /// never rewrites it — a row that already exists is not re-keyed by a second
    /// sighting, because "which of its names" is not a question this table
    /// pretends to answer.
    pub fn explode_at(
        &self,
        oid: &[u8],
        kind: GitObjectKind,
        mode: Option<u32>,
        path: Option<&str>,
        payload: &[u8],
    ) -> Result<()> {
        if !self.policy.wants(kind) {
            self.skipped.fetch_add(1, Ordering::Relaxed);
            return Ok(());
        }
        {
            let mut p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            // **One `extend_from_slice` into the arena, no allocation of its
            // own.** This line was `payload.to_vec()` — a whole second copy of
            // the object the caller is holding, which is the byte lane AA
            // measured on 2026-08-30.
            let pay = p.stash(payload);
            p.rows.push(Pending {
                oid: oid.to_vec(),
                code: kind_code(kind),
                mode,
                path: path.map(str::to_owned),
                pay,
            });
        }
        let n = self
            .pending_bytes
            .fetch_add(payload.len() as u64, Ordering::AcqRel)
            + payload.len() as u64;
        if n >= FLUSH_BYTES as u64 {
            self.flush()?;
        }
        Ok(())
    }

    /// `v1`: write the batch in schema v1 — no `enc` column, every row raw —
    /// because the file it is appended to was opened with a v1 schema message.
    fn build_batch(rows: &[Pending], arena: &[u8], oid_len: usize, v1: bool) -> Result<RecordBatch> {
        if v1 {
            let mut oid_b = FixedSizeBinaryBuilder::with_capacity(rows.len(), oid_len as i32);
            let mut kind_b = UInt8Builder::with_capacity(rows.len());
            let mut mode_b = UInt32Builder::with_capacity(rows.len());
            let mut path_b = StringBuilder::new();
            let mut pay_b = LargeBinaryBuilder::new();
            for r in rows {
                oid_b
                    .append_value(&r.oid)
                    .map_err(|e| anyhow::anyhow!("exploded oid column: {e}"))?;
                kind_b.append_value(r.code);
                mode_b.append_option(r.mode);
                path_b.append_option(r.path.as_deref());
                pay_b.append_value(&arena[r.pay.clone()]);
            }
            return RecordBatch::try_new(
                exploded_schema_v1(oid_len),
                vec![
                    Arc::new(oid_b.finish()),
                    Arc::new(kind_b.finish()),
                    Arc::new(mode_b.finish()),
                    Arc::new(path_b.finish()),
                    Arc::new(pay_b.finish()),
                ],
            )
            .context("building a v1 exploded batch");
        }
        let mut oid_b = FixedSizeBinaryBuilder::with_capacity(rows.len(), oid_len as i32);
        let mut kind_b = UInt8Builder::with_capacity(rows.len());
        let mut mode_b = UInt32Builder::with_capacity(rows.len());
        let mut path_b = StringBuilder::new();
        let mut enc_b = UInt8Builder::with_capacity(rows.len());
        let mut pay_b = LargeBinaryBuilder::new();
        for r in rows {
            oid_b
                .append_value(&r.oid)
                .map_err(|e| anyhow::anyhow!("exploded oid column: {e}"))?;
            kind_b.append_value(r.code);
            mode_b.append_option(r.mode);
            path_b.append_option(r.path.as_deref());
            // Per row, at build time: the pending arena keeps rows raw (so the
            // pending-serve path hands them out untouched), and what lands in
            // the file is whatever is smaller.
            let raw = &arena[r.pay.clone()];
            match compress_row(raw) {
                Some(z) => {
                    enc_b.append_value(ENC_ZLIB);
                    pay_b.append_value(&z);
                }
                None => {
                    enc_b.append_value(ENC_RAW);
                    pay_b.append_value(raw);
                }
            }
        }
        RecordBatch::try_new(
            exploded_schema(oid_len),
            vec![
                Arc::new(oid_b.finish()),
                Arc::new(kind_b.finish()),
                Arc::new(mode_b.finish()),
                Arc::new(path_b.finish()),
                Arc::new(enc_b.finish()),
                Arc::new(pay_b.finish()),
            ],
        )
        .context("building the exploded batch")
    }

    /// Append the buffered rows as ONE batch message. Nothing already in the
    /// file is read or rewritten.
    pub fn flush(&self) -> Result<u64> {
        let (rows, arena) = {
            let mut p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            if p.is_empty() {
                return Ok(0);
            }
            self.pending_bytes.store(0, Ordering::Release);
            p.take()
        };
        let n = rows.len() as u64;
        let width = rows[0].oid.len();
        anyhow::ensure!(
            rows.iter().all(|r| r.oid.len() == width),
            "one exploded batch cannot hold two hash widths"
        );
        {
            let mut w = self
                .oid_len
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded oid width poisoned"))?;
            match *w {
                Some(have) => anyhow::ensure!(
                    have == width,
                    "this table holds {have}-byte oids and a {width}-byte one arrived"
                ),
                None => *w = Some(width),
            }
        }

        let batch = Self::build_batch(&rows, &arena, width, self.legacy_v1.load(Ordering::Relaxed))?;
        let opts = IpcWriteOptions::default();
        let ipc = IpcDataGenerator::default();
        let mut tracker = DictionaryTracker::new(false);

        // The file is opened per flush rather than held: an append writes at a
        // known offset and closing between batches is what makes a crashed
        // process leave a file whose last complete message is intact.
        let mut end = self
            .end
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded end poisoned"))?;
        let mut f = OpenOptions::new()
            .create(true)
            .read(true)
            .write(true)
            .truncate(false)
            .open(&self.path)
            .with_context(|| format!("opening {}", self.path.display()))?;
        // Drop anything after the last complete message before appending — a
        // torn tail from a previous crash must not sit in the middle of the
        // stream, where every later batch would be unreachable.
        if f.metadata()?.len() != *end {
            f.set_len(*end)
                .with_context(|| format!("truncating {} to {}", self.path.display(), *end))?;
        }
        f.seek(SeekFrom::Start(*end))?;

        let mut schema_guard = self
            .schema_msg
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded schema poisoned"))?;
        if schema_guard.is_none() {
            let enc =
                ipc.schema_to_bytes_with_dictionary_tracker(&exploded_schema(width), &mut tracker, &opts);
            let mut msg = Vec::new();
            let (meta, body) = write_message(&mut msg, enc, &opts)
                .map_err(|e| anyhow::anyhow!("encoding the exploded schema: {e}"))?;
            debug_assert_eq!(meta + body, msg.len());
            f.write_all(&msg).context("writing the exploded schema")?;
            *end += msg.len() as u64;
            *schema_guard = Some(msg);
        }
        drop(schema_guard);

        let (dicts, enc) = ipc
            .encode(&batch, &mut tracker, &opts, &mut Default::default())
            .map_err(|e| anyhow::anyhow!("encoding an exploded batch: {e}"))?;
        anyhow::ensure!(
            dicts.is_empty(),
            "the exploded schema has no dictionary columns, yet {} arrived",
            dicts.len()
        );
        let mut msg = Vec::new();
        let (meta, body) = write_message(&mut msg, enc, &opts)
            .map_err(|e| anyhow::anyhow!("encoding an exploded batch: {e}"))?;
        debug_assert_eq!(meta + body, msg.len());
        f.write_all(&msg).context("appending an exploded batch")?;

        let placed = Batch {
            at: *end,
            len: msg.len() as u64,
            rows: rows.len() as u32,
            // `meta` from `write_message` is the 8-byte prefix plus the padded
            // metadata, so the body begins exactly `meta` bytes in — and the
            // metadata slice between prefix and body is what the offset helper
            // parses, the same bytes `walk` reads back after a reopen.
            body: *end + meta as u64,
            pay_data: payload_data_offset(&msg[8..meta]).unwrap_or(EXTENT_UNAVAILABLE),
        };
        *end += msg.len() as u64;

        // Recorded while the append lock is still held, so the batch table stays
        // in file order under N concurrent exploders. Correctness would survive
        // it being out of order — a lookup addresses a batch by its slot, not by
        // its offset — but a table that reads back in file order is one a human
        // can check against the file with `od`.
        let batch_no = {
            let mut b = self
                .batches
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded batches poisoned"))?;
            b.push(placed);
            (b.len() - 1) as u32
        };
        drop(end);

        // Only extend an index that already exists. Building one here would make
        // the first push pay for a structure no reader has asked for.
        let mut idx = self
            .index
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
        if let Some(old) = idx.take() {
            // Retired oids are collected and applied in ONE pass. Removing them
            // one at a time is quadratic, and a `gc` retires millions.
            let mut retired: std::collections::HashSet<&[u8]> = std::collections::HashSet::new();
            for r in rows.iter() {
                if kind_of(r.code).is_none() {
                    retired.insert(r.oid.as_slice());
                }
            }
            let mut b = OidTreeBuilder::with_capacity(old.len() + rows.len());
            for (oid, loc) in old.iter() {
                if !retired.contains(oid) {
                    b.push(oid, Some(loc))?;
                }
            }
            let pay = batch
                .column_by_name(COL_PAYLOAD)
                .and_then(|c| c.as_any().downcast_ref::<LargeBinaryArray>())
                .context("the exploded table has no payload column")?;
            // Freshly built, never sliced, so these offsets are byte-for-byte
            // what the encoder just wrote into the file — the same equivalence
            // `index_now` relies on after a reopen.
            let off = pay.value_offsets();
            let enc = enc_column(&batch);
            for (i, r) in rows.iter().enumerate() {
                if let Some(kind) = kind_of(r.code)
                    && !retired.contains(r.oid.as_slice())
                {
                    let (pay_at, pay_len) = if placed.pay_data == EXTENT_UNAVAILABLE {
                        (EXTENT_UNAVAILABLE, 0)
                    } else {
                        (
                            placed.body + placed.pay_data + off[i] as u64,
                            (off[i + 1] - off[i]) as u64,
                        )
                    };
                    b.push(
                        &r.oid,
                        Some(Located {
                            batch: batch_no,
                            row: i as u32,
                            kind,
                            pay_at,
                            pay_len,
                            enc: enc_at(enc, i),
                        }),
                    )?;
                }
            }
            // `Keep::First`, because the surviving entries were pushed before
            // this batch's rows: an oid already in the index keeps the row it
            // already had, which is what `explode_at` documents about names.
            *idx = Some(b.finish(Keep::First));
        }
        self.written.fetch_add(n, Ordering::Relaxed);
        Ok(n)
    }

    /// How many decoded batches stay resident. Four × [`FLUSH_BYTES`] is the
    /// worst case, and a batch is `Arc`-backed so a hit copies pointers.
    const CACHE: usize = 4;

    /// Read one batch message off disk and decode it — or hand back the decoded
    /// copy if it is one of the last [`Self::CACHE`].
    ///
    /// Two `pread`s on a miss: the schema message and the batch. It never reads
    /// the batches in front of the one it wants — the property a stream reader
    /// iterating from byte zero does not have.
    fn read_batch(&self, no: u32) -> Result<RecordBatch> {
        {
            let mut c = self
                .cache
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded batch cache poisoned"))?;
            if let Some(at) = c.iter().position(|(n, _)| *n == no) {
                let hit = c.remove(at);
                let batch = hit.1.clone();
                c.insert(0, hit);
                return Ok(batch);
            }
        }
        let placed = {
            let b = self
                .batches
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded batches poisoned"))?;
            *b.get(no as usize)
                .with_context(|| format!("exploded batch {no} is not in this table"))?
        };
        let schema_msg = {
            let s = self
                .schema_msg
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded schema poisoned"))?;
            s.clone()
                .context("the exploded table has batches but no schema message")?
        };
        let f = File::open(&self.path)
            .with_context(|| format!("opening {}", self.path.display()))?;
        let mut raw = vec![0u8; placed.len as usize];
        f.read_exact_at(&mut raw, placed.at)
            .with_context(|| format!("reading exploded batch {no}"))?;

        let mut dec = StreamDecoder::new();
        let mut head = Buffer::from_vec(schema_msg);
        dec.decode(&mut head)
            .map_err(|e| anyhow::anyhow!("decoding the exploded schema: {e}"))?;
        let mut body = Buffer::from_vec(raw);
        let batch = dec
            .decode(&mut body)
            .map_err(|e| anyhow::anyhow!("decoding exploded batch {no}: {e}"))?
            .with_context(|| format!("exploded batch {no} decoded to no rows"))?;
        {
            let mut c = self
                .cache
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded batch cache poisoned"))?;
            c.retain(|(n, _)| *n != no);
            c.insert(0, (no, batch.clone()));
            c.truncate(Self::CACHE);
        }
        Ok(batch)
    }

    /// Build `oid → Located` with one sequential pass, applying every
    /// [`TOMBSTONE`] in file order so a retired oid stays retired across a
    /// reopen. Called on the first lookup, never on open.
    fn index_now(&self) -> Result<()> {
        {
            let idx = self
                .index
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
            if idx.is_some() {
                return Ok(());
            }
        }
        let (count, rows, placed) = {
            let b = self
                .batches
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded batches poisoned"))?;
            // The row total is in the batch metadata the framing walk already
            // read, so the builder's arrays are sized once instead of doubling
            // their way to 11.7 million.
            (b.len() as u32, b.iter().map(|x| x.rows as usize).sum::<usize>(), b.clone())
        };
        // Every row is recorded as a sighting and the fold happens once, at
        // `finish`. The `HashMap<Vec<u8>, Located>` this replaced allocated —
        // and then dropped — one `Vec<u8>` per oid on a corpus with 11.7 M of
        // them, to compute an answer a single stable sort gives.
        let mut fold = OidTreeBuilder::with_capacity(rows);
        for no in 0..count {
            let batch = self.read_batch(no)?;
            let oid = batch
                .column_by_name(COL_OID)
                .and_then(|c| c.as_any().downcast_ref::<FixedSizeBinaryArray>())
                .context("the exploded table has no oid column")?;
            let kinds = batch
                .column_by_name(COL_TYPE)
                .and_then(|c| c.as_any().downcast_ref::<UInt8Array>())
                .context("the exploded table has no object_type column")?;
            let pay = batch
                .column_by_name(COL_PAYLOAD)
                .and_then(|c| c.as_any().downcast_ref::<LargeBinaryArray>())
                .context("the exploded table has no payload column")?;
            // The decoded offsets carry the SAME values the file's offsets
            // buffer holds — a StreamDecoder copies buffers verbatim — so the
            // extent needs no second parse of the body.
            let off = pay.value_offsets();
            let enc = enc_column(&batch);
            let b = placed[no as usize];
            for i in 0..batch.num_rows() {
                // A tombstone is pushed as a sighting with no location, not
                // skipped: batches are walked in file order and rows in row
                // order, so `Keep::Last` gives the last word about an oid — and
                // when that word is a tombstone the oid is dropped, which is
                // what makes the tombstone durable rather than advisory.
                fold.push(
                    oid.value(i),
                    kind_of(kinds.value(i)).map(|kind| {
                        let (pay_at, pay_len) = if b.pay_data == EXTENT_UNAVAILABLE {
                            (EXTENT_UNAVAILABLE, 0)
                        } else {
                            (
                                b.body + b.pay_data + off[i] as u64,
                                (off[i + 1] - off[i]) as u64,
                            )
                        };
                        Located { batch: no, row: i as u32, kind, pay_at, pay_len, enc: enc_at(enc, i) }
                    }),
                )?;
            }
        }
        let out = fold.finish(Keep::Last);
        let mut idx = self
            .index
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
        if idx.is_none() {
            *idx = Some(out);
        }
        Ok(())
    }

    fn find(&self, oid: &[u8]) -> Result<Option<Located>> {
        self.index_now()?;
        let idx = self
            .index
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
        Ok(idx.as_ref().expect("index_now built it").find(oid))
    }

    /// The object's kind and bytes, or `None` — *fall back and rebuild*, never
    /// *wrong*.
    pub fn content(&self, oid: &[u8]) -> Result<Option<(GitObjectKind, Vec<u8>)>> {
        // A buffered row has not been written yet, and a read must not miss it:
        // an "absent" that is really "not flushed" sends the caller off to
        // re-resolve a whole pack for an object we are holding in RAM.
        {
            let p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            if let Some(r) = p.rows.iter().rev().find(|r| r.oid == oid) {
                let Some(kind) = kind_of(r.code) else {
                    // The newest buffered word about this oid retires it.
                    self.absent.fetch_add(1, Ordering::Relaxed);
                    return Ok(None);
                };
                self.served.fetch_add(1, Ordering::Relaxed);
                return Ok(Some((kind, p.bytes(r).to_vec())));
            }
        }
        let Some(loc) = self.find(oid)? else {
            self.absent.fetch_add(1, Ordering::Relaxed);
            return Ok(None);
        };
        // ── the point read IS a point read ──────────────────────────────────
        //
        // One pread of exactly the payload's bytes, addressed by the extent the
        // index carries. The batch-decode below is the FALLBACK — correct for a
        // compressed or foreign-layout batch — not the path a healthy table
        // takes: taking it per object is what turned a rust-lang/rust
        // connectivity walk into hours of one-core batch decoding (2026-08-19).
        if loc.pay_at != EXTENT_UNAVAILABLE {
            let f = File::open(&self.path)
                .with_context(|| format!("opening {}", self.path.display()))?;
            let mut buf = vec![0u8; loc.pay_len as usize];
            f.read_exact_at(&mut buf, loc.pay_at)
                .with_context(|| format!("preading payload at {}", loc.pay_at))?;
            self.served.fetch_add(1, Ordering::Relaxed);
            self.pread_served.fetch_add(1, Ordering::Relaxed);
            // The extent held the row as stored; a compressed row is inflated
            // here, from the index's `enc`, without decoding the batch.
            return Ok(Some((loc.kind, row_payload(loc.enc, &buf)?)));
        }
        let batch = self.read_batch(loc.batch)?;
        let pay = batch
            .column_by_name(COL_PAYLOAD)
            .and_then(|c| c.as_any().downcast_ref::<LargeBinaryArray>())
            .context("the exploded table has no payload column")?;
        let enc = enc_at(enc_column(&batch), loc.row as usize);
        self.served.fetch_add(1, Ordering::Relaxed);
        Ok(Some((loc.kind, row_payload(enc, pay.value(loc.row as usize))?)))
    }

    /// Point reads served by the extent `pread` rather than a batch decode —
    /// how a test proves the fast path is the one actually taken, instead of
    /// timing something.
    pub fn pread_served(&self) -> u64 {
        self.pread_served.load(Ordering::Relaxed)
    }

    /// The name this object was first exploded at, if any caller knew one.
    /// `None` is "not recorded", never "at the root".
    pub fn name(&self, oid: &[u8]) -> Result<Option<(Option<u32>, Option<String>)>> {
        {
            let p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            if let Some(r) = p.rows.iter().rev().find(|r| r.oid == oid) {
                return match kind_of(r.code) {
                    Some(_) => Ok(Some((r.mode, r.path.clone()))),
                    None => Ok(None),
                };
            }
        }
        let Some(loc) = self.find(oid)? else {
            return Ok(None);
        };
        let batch = self.read_batch(loc.batch)?;
        let mode = batch
            .column_by_name(COL_MODE)
            .and_then(|c| c.as_any().downcast_ref::<UInt32Array>())
            .context("the exploded table has no mode column")?;
        let path = batch
            .column_by_name(COL_PATH)
            .and_then(|c| c.as_any().downcast_ref::<StringArray>())
            .context("the exploded table has no path column")?;
        let i = loc.row as usize;
        Ok(Some((
            (!mode.is_null(i)).then(|| mode.value(i)),
            (!path.is_null(i)).then(|| path.value(i).to_owned()),
        )))
    }

    pub fn has(&self, oid: &[u8]) -> Result<bool> {
        {
            let p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            if let Some(r) = p.rows.iter().rev().find(|r| r.oid == oid) {
                return Ok(kind_of(r.code).is_some());
            }
        }
        Ok(self.find(oid)?.is_some())
    }

    /// Every oid of one kind, with its payload. The graph fold's input.
    ///
    /// Driven by the **index**, not by a scan of the file, because the file
    /// holds superseded and retired rows too. A scan that read the file directly
    /// is exactly how `gc`'d commits came back after a reopen.
    pub fn of_kind(&self, kind: GitObjectKind) -> Result<Vec<(Vec<u8>, Vec<u8>)>> {
        self.flush()?;
        self.index_now()?;
        // Grouped by batch so each one is decoded once, in file order.
        let mut by_batch: std::collections::BTreeMap<u32, Vec<(u32, Vec<u8>)>> =
            std::collections::BTreeMap::new();
        {
            let idx = self
                .index
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
            for (oid, loc) in idx.as_ref().expect("index_now built it").iter() {
                if loc.kind == kind {
                    by_batch.entry(loc.batch).or_default().push((loc.row, oid.to_vec()));
                }
            }
        }
        let mut out = Vec::new();
        for (no, mut wanted) in by_batch {
            wanted.sort_unstable_by_key(|(r, _)| *r);
            let batch = self.read_batch(no)?;
            let pay = batch
                .column_by_name(COL_PAYLOAD)
                .and_then(|c| c.as_any().downcast_ref::<LargeBinaryArray>())
                .context("the exploded table has no payload column")?;
            let enc = enc_column(&batch);
            for (row, oid) in wanted {
                out.push((oid, row_payload(enc_at(enc, row as usize), pay.value(row as usize))?));
            }
        }
        Ok(out)
    }

    /// Retire every oid `live` rejects, **durably**.
    ///
    /// The file is append-only, so this appends a [`TOMBSTONE`] row per retired
    /// oid rather than deleting anything. The bytes of the dead rows stay until
    /// the table is rebuilt — reclaiming them means rewriting up to 17 GB, which
    /// is a job with its own proof obligations and not something to smuggle into
    /// `gc` — but the *rows* are gone from every reader, including a reopened
    /// one.
    ///
    /// Dropping only from the in-memory index is what a first cut did. Seen RED
    /// by `store::tests::a_wholly_dead_pack_does_not_come_back_when_the_store_is_reopened`:
    /// **"the graph after gc + reopen holds 551 commits, not the one live one"**.
    pub fn retain(&self, live: &dyn Fn(&[u8]) -> bool) -> Result<u64> {
        self.flush()?;
        self.index_now()?;
        let dead: Vec<Vec<u8>> = {
            let idx = self
                .index
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
            idx.as_ref()
                .expect("index_now built it")
                .iter()
                .filter(|(oid, _)| !live(oid))
                .map(|(oid, _)| oid.to_vec())
                .collect()
        };
        if dead.is_empty() {
            return Ok(0);
        }
        {
            let mut p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            for oid in &dead {
                p.rows.push(Pending {
                    oid: oid.clone(),
                    code: TOMBSTONE,
                    mode: None,
                    path: None,
                    pay: 0..0,
                });
            }
        }
        // Written before returning, so a `gc` that says it retired rows has
        // already said so on disk.
        self.flush()?;
        Ok(dead.len() as u64)
    }

    /// Rows in the table. Answered from batch metadata when no index has been
    /// built, so startup does not read a payload.
    pub fn rows(&self) -> Result<u64> {
        let pending = {
            let p = self
                .pending
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded pending poisoned"))?;
            p.rows.len() as u64
        };
        {
            let idx = self
                .index
                .lock()
                .map_err(|_| anyhow::anyhow!("exploded index poisoned"))?;
            if let Some(idx) = idx.as_ref() {
                return Ok(idx.len() as u64 + pending);
            }
        }
        let b = self
            .batches
            .lock()
            .map_err(|_| anyhow::anyhow!("exploded batches poisoned"))?;
        Ok(b.iter().map(|x| x.rows as u64).sum::<u64>() + pending)
    }

    /// Bytes the table occupies — the number redb could not keep anywhere near
    /// its payload total.
    pub fn disk_bytes(&self) -> u64 {
        std::fs::metadata(&self.path).map(|m| m.len()).unwrap_or(0)
    }

    pub fn note_rederived(&self) {
        self.rederived.fetch_add(1, Ordering::Relaxed);
    }

    /// What this table is configured to keep.
    pub fn policy(&self) -> ExplodePolicy {
        self.policy
    }

    /// Objects the policy declined, COUNTED: a table short because of policy and
    /// one short because of a fault must not look alike to `adopt_journal`.
    pub fn skipped(&self) -> u64 {
        self.skipped.load(Ordering::Relaxed)
    }

    pub fn stats(&self) -> Stats {
        Stats {
            rows: self.rows().unwrap_or(0),
            written: self.written.load(Ordering::Relaxed),
            served: self.served.load(Ordering::Relaxed),
            rederived: self.rederived.load(Ordering::Relaxed),
            absent: self.absent.load(Ordering::Relaxed),
        }
    }

    /// The engine-wide counter type, so no caller can tell the medium apart by
    /// its instruments.
    pub fn engine_stats(&self) -> crate::exploded::ExplodedStats {
        let s = self.stats();
        crate::exploded::ExplodedStats {
            rows: s.rows,
            written: s.written,
            served: s.served,
            rederived: s.rederived,
            absent: s.absent,
        }
    }
}

impl crate::exploded::PayloadSink for ExplodedArchive {
    /// The policy, asked before the payload exists rather than after — and the
    /// `skipped` counter is bumped here, which is the only place the object is
    /// ever seen when the answer is no.
    fn wants(&self, kind: GitObjectKind) -> bool {
        if self.policy.wants(kind) {
            return true;
        }
        self.skipped.fetch_add(1, Ordering::Relaxed);
        false
    }

    fn explode(&self, oid: &[u8], kind: GitObjectKind, payload: &[u8]) -> Result<()> {
        ExplodedArchive::explode(self, oid, kind, payload)
    }

    fn flush(&self) -> Result<()> {
        ExplodedArchive::flush(self).map(|_| ())
    }
}

// ════════════════════════════════════════════════════════════════════════════
// The policy: what gets exploded at all
// ════════════════════════════════════════════════════════════════════════════

/// **How much of a pack is resolved into the exploded table.**
///
/// Three values and not a boolean, because a flat off-switch reintroduces a
/// correctness bug this table exists to fix. Commit and tree payloads used to
/// live only in RAM, so a **clean** shutdown came back with every pack's
/// `indexed` bit legitimately set and an EMPTY graph — MEASURED on oden
/// 2026-08-08: 2687 rows and **0 of 551 commits**, `reachable()` returning a
/// commit instead of its closure and `gc` seeing an empty live set.
///
/// MEASURED on `linux.git`: resolved content is 16.8 GB against a 6.4 GB pack,
/// and blobs are nearly all of it. So [`Graph`](ExplodePolicy::Graph) buys the
/// correctness fix for a small fraction of the disk, and
/// [`Full`](ExplodePolicy::Full) — one-lookup content reads, thin-pack bases,
/// and the path columns that make the table a filesystem — is the part worth
/// charging for.
///
/// **The default is [`Commits`](ExplodePolicy::Commits) since lane Q
/// (2026-08-30), and the change is a measurement.** §14's `indexed` bit is
/// derived by comparing this table's row count against the index's, so a table
/// that is *deliberately* short is indistinguishable, on disk, from one that was
/// dropped — see [`crate::git_ops::Absorber::adopt_journal`], which weakens that
/// comparison for every deliberately-short policy and cannot make it exact
/// without a durable per-kind count. That weakening is the price of the
/// default, it is stated here rather than hidden, and it costs a store that
/// really did lose its table one extra fall-back read per object rather than a
/// re-ingest.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum ExplodePolicy {
    /// Store nothing. Reads re-resolve, and every pack is re-queued on reopen —
    /// which is what rebuilds the graph, since nothing it is folded from is on
    /// disk.
    Off,
    /// Commits and trees, both eager. The shape lane Q shipped first and
    /// **measured**: on the real `torvalds/linux` mirror the tree payloads
    /// alone are **28.24 GB — 3.24× the 8.13 GiB pack** — and folding them into
    /// RAM took peak RSS to 50.9 GiB. Kept as a setting, because a small
    /// repository that is read constantly wants exactly this; not the default,
    /// because an appliance ships with 16 GB.
    Graph,
    /// **Commits and tags only — the SHIPPED DEFAULT since lane Q
    /// (2026-08-30).**
    ///
    /// The index-only-at-push shape of
    /// `ideas/lazy-explode-index-only-at-push.md`, in full. Commits are what
    /// `graph` and `reach` fold and what a visibility or pull check reads; a
    /// tag is what a clone peels. Both are small and both stay eager, so
    /// neither ever stalls. **Trees and blobs** — 90 % of the objects and
    /// essentially all of the bytes — are written into
    /// [`crate::exploded_cache`]'s budgeted pool by the FIRST reader that
    /// inflates one, and evicted from it when the budget says so.
    ///
    /// Trees became lazy when [`crate::reach::TreeSource`] replaced the
    /// resident `HashMap<String, Vec<u8>>` that [`crate::serve`]'s closure walk
    /// and the reachability builder both read. That map was the 28.24 GB above.
    ///
    /// MEASURED, `torvalds/linux`, oden 2026-08-30, /data bytes per pack byte:
    ///
    /// | | eager (`Full`) | `Graph` | `Commits` |
    /// |---|---:|---:|---:|
    /// | at ack | 1.05× | 1.05× | 1.05× |
    /// | at index-complete | 22.4× (lane P, 78 % done) | 4.58× | see the lane Q rows |
    #[default]
    Commits,
    /// Every object, eagerly, with no bound. What shipped before lane Q and
    /// what the 17× measurement is of; kept so a bench can measure the
    /// unbounded shape and so a store written under it still opens.
    Full,
}

impl ExplodePolicy {
    pub const fn wants(self, kind: GitObjectKind) -> bool {
        match self {
            ExplodePolicy::Off => false,
            ExplodePolicy::Graph => matches!(kind, GitObjectKind::Commit | GitObjectKind::Tree),
            ExplodePolicy::Commits => matches!(kind, GitObjectKind::Commit | GitObjectKind::Tag),
            ExplodePolicy::Full => true,
        }
    }
    pub const fn as_str(self) -> &'static str {
        match self {
            ExplodePolicy::Off => "off",
            ExplodePolicy::Graph => "graph",
            ExplodePolicy::Commits => "commits",
            ExplodePolicy::Full => "full",
        }
    }
    /// An unknown value is an ERROR, never a silent fall back to the default: a
    /// typo that quietly disables a paid tier is not noticed until a bill is
    /// wrong.
    pub fn parse(s: &str) -> Result<Self> {
        match s.trim().to_ascii_lowercase().as_str() {
            "off" | "none" => Ok(ExplodePolicy::Off),
            "graph" | "commits-and-trees" => Ok(ExplodePolicy::Graph),
            "commits" | "lazy" | "commits-only" => Ok(ExplodePolicy::Commits),
            "full" | "all" => Ok(ExplodePolicy::Full),
            other => anyhow::bail!(
                "unknown explode policy {other:?}; expected off, commits, graph or full"
            ),
        }
    }
    /// Reads `ZNIPPY_GIT_EXPLODE` ([`crate::arms::ENV_EXPLODE`]) through the
    /// crate's one counted door, once per store open.
    pub fn from_env() -> Result<Self> {
        match crate::arms::read_env(crate::arms::ENV_EXPLODE) {
            Some(v) => Self::parse(&v),
            None => Ok(explode_default()),
        }
    }

    /// The discriminant, for the atomic behind [`set_explode_default`]. Private
    /// because it is storage and not a contract — the wire spelling is
    /// [`ExplodePolicy::as_str`].
    const fn as_u8(self) -> u8 {
        match self {
            ExplodePolicy::Off => 0,
            ExplodePolicy::Graph => 1,
            ExplodePolicy::Commits => 2,
            ExplodePolicy::Full => 3,
        }
    }

    const fn from_u8(v: u8) -> Self {
        match v {
            0 => ExplodePolicy::Off,
            1 => ExplodePolicy::Graph,
            3 => ExplodePolicy::Full,
            _ => ExplodePolicy::Commits,
        }
    }
}

/// What [`ExplodePolicy::from_env`] answers when [`crate::arms::ENV_EXPLODE`] is unset.
/// [`ExplodePolicy::default`] unless an embedder said otherwise.
static EXPLODE_DEFAULT: std::sync::atomic::AtomicU8 =
    std::sync::atomic::AtomicU8::new(ExplodePolicy::Commits.as_u8());

/// The current process-wide default — what an unset environment means right now.
pub fn explode_default() -> ExplodePolicy {
    ExplodePolicy::from_u8(EXPLODE_DEFAULT.load(Ordering::Relaxed))
}

/// **Set the process-wide default explode policy** for every store opened AFTER this call
/// (an explicit [`crate::arms::ENV_EXPLODE`] still wins). The twin of
/// [`crate::arms::set_graph_cache_default`], for an embedder that learns its role at runtime
/// and cannot set the environment — edition 2024 makes `set_var` unsafe, and gunnar forbids
/// `unsafe` — such as gunnar's twin, which holds packs, serves no reads, and must store no
/// exploded pool at all ([`ExplodePolicy::Off`]).
pub fn set_explode_default(policy: ExplodePolicy) {
    EXPLODE_DEFAULT.store(policy.as_u8(), Ordering::Relaxed);
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::exploded::PayloadSink as _;

    fn tmp(name: &str) -> PathBuf {
        let d = std::env::temp_dir().join(format!("exploded-arrow-{}-{name}", std::process::id()));
        let _ = std::fs::remove_dir_all(&d);
        std::fs::create_dir_all(&d).unwrap();
        d.join("objects.exploded")
    }

    fn oid(n: u8) -> [u8; 20] {
        [n; 20]
    }

    /// ★ ONE TABLE, and the payload is IN the row.
    ///
    /// Red-first: drop the payload column and nothing below compiles; widen
    /// `payload` to `Binary` and this fails on the type it asserts.
    #[test]
    fn the_table_holds_the_payload_and_a_name_in_one_row() {
        let s = exploded_schema(20);
        let names: Vec<&str> = s.fields().iter().map(|f| f.name().as_str()).collect();
        // `enc` sits before `payload` so the payload data buffer stays the last
        // buffer of the batch — `payload_data_offset` depends on that.
        assert_eq!(names, vec![COL_OID, COL_TYPE, COL_MODE, COL_PATH, COL_ENC, COL_PAYLOAD]);
        assert_eq!(
            s.field_with_name(COL_PAYLOAD).unwrap().data_type(),
            &DataType::LargeBinary,
            "Binary offsets are i32 and would cap one batch at 2 GiB of payload"
        );
        assert!(
            s.field_with_name(COL_PATH).unwrap().is_nullable(),
            "a bare pack resolve knows no path; null must be a legal answer"
        );
    }

    /// ★ THE WHOLE POINT: the file stays near the payload total.
    ///
    /// redb held 16.8 GB of kernel payload in a 204 GB file — 12×. An Arrow IPC
    /// stream has no pages to rewrite, so the overhead is framing. Red-first:
    /// put this back on a copy-on-write B-tree and it fails.
    #[test]
    fn the_file_stays_close_to_the_payload_total() {
        let t = ExplodedArchive::open_with_policy(&tmp("size"), ExplodePolicy::Full).unwrap();
        let payload = vec![7u8; 4096];
        let n = 2000u64;
        for i in 0..n {
            let mut o = [0u8; 20];
            o[..8].copy_from_slice(&i.to_le_bytes());
            t.explode(&o, GitObjectKind::Blob, &payload).unwrap();
        }
        t.flush().unwrap();
        let payload_total = n * payload.len() as u64;
        let on_disk = t.disk_bytes();
        assert!(
            on_disk < payload_total + payload_total / 4,
            "{on_disk} bytes on disk against {payload_total} of payload — the medium is amplifying"
        );
        assert_eq!(t.rows().unwrap(), n);
    }

    /// ★ A point read is one small `pread` of the payload's own bytes, and it
    /// is the path ACTUALLY taken — counted, not timed.
    ///
    /// Red-first, measured on a rust-lang/rust push (2026-08-19): without the
    /// extent, every `content` decoded its whole 64 MiB batch against a 4-slot
    /// cache, and the pre-ack connectivity walk — graph order, so effectively
    /// random across ~574 batches — decoded 6.7 GB/s of page cache to serve
    /// ~98 objects/s, one core pinned for hours. Both index-building paths are
    /// exercised: the in-process flush extension AND `index_now` after a
    /// reopen, against multi-batch tables with varied payload sizes, so a
    /// mis-computed extent returns the WRONG BYTES here rather than in a
    /// customer's clone.
    #[test]
    fn a_point_read_preads_the_exact_payload_without_decoding_the_batch() {
        let path = tmp("pread");
        let pay = |b: u8, i: u8| vec![b ^ i; 100 + i as usize * 7];
        let t = ExplodedArchive::open_with_policy(&path, ExplodePolicy::Full).unwrap();
        // Batch 0, then force the index to exist so batch 1 goes through the
        // flush EXTENSION path rather than a later index_now.
        for i in 0..40u8 {
            t.explode(&[i; 20], GitObjectKind::Blob, &pay(0, i)).unwrap();
        }
        t.flush().unwrap();
        assert!(t.content(&[0u8; 20]).unwrap().is_some(), "build the index");
        for i in 40..80u8 {
            t.explode(&[i; 20], GitObjectKind::Blob, &pay(1, i)).unwrap();
        }
        t.flush().unwrap();

        let before = t.pread_served();
        for i in 0..80u8 {
            let (kind, got) = t.content(&[i; 20]).unwrap().expect("every oid is live");
            assert_eq!(kind, GitObjectKind::Blob);
            let want = if i < 40 { pay(0, i) } else { pay(1, i) };
            assert_eq!(got, want, "oid {i}: the pread returned some OTHER bytes");
        }
        assert_eq!(
            t.pread_served() - before,
            80,
            "a read fell back to the batch decode — the extent was not computed"
        );

        // Reopen: the framing walk + index_now must reproduce the same extents
        // from the file alone.
        drop(t);
        let t = ExplodedArchive::open_with_policy(&path, ExplodePolicy::Full).unwrap();
        for i in 0..80u8 {
            let (_, got) = t.content(&[i; 20]).unwrap().expect("survives a reopen");
            let want = if i < 40 { pay(0, i) } else { pay(1, i) };
            assert_eq!(got, want, "oid {i} after reopen");
        }
        assert_eq!(t.pread_served(), 80, "the reopened table must pread too");
    }

    /// **Lever 3: a compressible row is stored compressed and read back whole, on
    /// every read path.** Three rows: a 4 KiB compressible one (stored zlib), a
    /// row under [`MIN_ROW_COMPRESS`] (stored raw), and a 4 KiB incompressible
    /// one (zlib cannot get it under the keep threshold — stored raw). Asserted
    /// on the `enc` column read straight off the file, on the file being smaller
    /// than the payloads, and on `content` (the pread fast path — `pread_served`
    /// moves) and `of_kind` (the batch-decode path) both handing back the
    /// original bytes.
    ///
    /// Seen RED by returning `bytes.to_vec()` for `ENC_ZLIB` in `row_payload`:
    /// "the pread path returned the stored bytes, not the payload" — 4096 bytes
    /// expected, ~60 came back. Restored.
    #[test]
    fn a_compressible_row_is_stored_compressed_and_read_back_whole() {
        let path = tmp("lever3");
        let t = ExplodedArchive::open_with_policy(&path, ExplodePolicy::Full).unwrap();
        let big: Vec<u8> = b"tree 0123456789abcdef0123456789abcdef01234567\nparent x\n"
            .iter()
            .cycle()
            .take(4096)
            .copied()
            .collect();
        let tiny = b"tree x\n".to_vec();
        let noise: Vec<u8> = {
            let mut x: u32 = 0x1234_5678;
            (0..4096)
                .map(|_| {
                    x = x.wrapping_mul(1_664_525).wrapping_add(1_013_904_223);
                    (x >> 24) as u8
                })
                .collect()
        };
        assert!(tiny.len() < MIN_ROW_COMPRESS && big.len() >= MIN_ROW_COMPRESS);
        t.explode(&oid(1), GitObjectKind::Commit, &big).unwrap();
        t.explode(&oid(2), GitObjectKind::Commit, &tiny).unwrap();
        t.explode(&oid(3), GitObjectKind::Blob, &noise).unwrap();
        t.flush().unwrap();

        // The file holds the compressed row: smaller than the payloads it
        // carries, which a raw pool never is.
        let file = std::fs::metadata(&path).unwrap().len() as usize;
        assert!(
            file < big.len() + tiny.len() + noise.len(),
            "the pool did not compress: {file} B for {} B of payload",
            big.len() + tiny.len() + noise.len()
        );

        // The `enc` column, read off the file as the plain IPC stream it is.
        let rd = znippy_common::arrow::ipc::reader::StreamReader::try_new(
            std::fs::File::open(&path).unwrap(),
            None,
        )
        .unwrap();
        let mut encs = std::collections::BTreeMap::new();
        for b in rd {
            let b = b.unwrap();
            let oids = b
                .column_by_name(COL_OID)
                .and_then(|c| c.as_any().downcast_ref::<FixedSizeBinaryArray>())
                .unwrap();
            let enc = enc_column(&b).expect("a v2 pool has the enc column");
            for i in 0..b.num_rows() {
                encs.insert(oids.value(i)[0], enc.value(i));
            }
        }
        assert_eq!(encs[&1], ENC_ZLIB, "the compressible row is stored compressed");
        assert_eq!(encs[&2], ENC_RAW, "a row under the floor stays raw");
        assert_eq!(encs[&3], ENC_RAW, "an incompressible row stays raw");

        // The pread fast path inflates.
        let before = t.pread_served();
        let (k, got) = t.content(&oid(1)).unwrap().unwrap();
        assert_eq!(k, GitObjectKind::Commit);
        assert_eq!(got, big, "the pread path returned the stored bytes, not the payload");
        assert_eq!(t.content(&oid(2)).unwrap().unwrap().1, tiny);
        assert_eq!(t.content(&oid(3)).unwrap().unwrap().1, noise);
        assert_eq!(t.pread_served() - before, 3, "all three went through the extent pread");

        // The batch-decode path (`of_kind`) inflates too.
        let commits = t.of_kind(GitObjectKind::Commit).unwrap();
        let by: std::collections::HashMap<u8, Vec<u8>> =
            commits.into_iter().map(|(o, p)| (o[0], p)).collect();
        assert_eq!(by[&1], big, "of_kind returned the stored bytes, not the payload");
        assert_eq!(by[&2], tiny);

        // And across a reopen, where the index is rebuilt from the file.
        drop(t);
        let t = ExplodedArchive::open_with_policy(&path, ExplodePolicy::Full).unwrap();
        assert_eq!(t.content(&oid(1)).unwrap().unwrap().1, big, "after reopen");
        assert_eq!(t.content(&oid(3)).unwrap().unwrap().1, noise, "after reopen");
    }

    /// **A v1 pool — no `enc` column — still reads, every row raw.** Written by
    /// hand in v1's shape (five columns, the same framing `flush` uses), then
    /// opened by this build: the extent is computed (12 buffers is accepted),
    /// `enc` is absent and read as [`ENC_RAW`], and the bytes come back as
    /// written. This is the promise that lets an existing appliance's pools
    /// open unchanged.
    ///
    /// Seen RED by making `payload_data_offset` accept only 14 buffers: the
    /// pread count stayed 0 ("a v1 pool must still take the extent fast path").
    /// Restored.
    #[test]
    fn a_v1_pool_without_the_enc_column_reads_raw() {
        use znippy_common::arrow::array::{
            FixedSizeBinaryBuilder, LargeBinaryBuilder, StringBuilder, UInt32Builder,
        };
        use znippy_common::arrow::ipc::writer::{write_message, DictionaryTracker, IpcDataGenerator, IpcWriteOptions};
        let path = tmp("v1pool");
        let v1_schema: SchemaRef = Arc::new(Schema::new(vec![
            Field::new(COL_OID, DataType::FixedSizeBinary(20), false),
            Field::new(COL_TYPE, DataType::UInt8, false),
            Field::new(COL_MODE, DataType::UInt32, true),
            Field::new(COL_PATH, DataType::Utf8, true),
            Field::new(COL_PAYLOAD, DataType::LargeBinary, false),
        ]));
        let payloads: Vec<Vec<u8>> = (1..=5u8)
            .map(|i| b"tree abc\n".iter().cycle().take(300 + i as usize * 100).copied().collect())
            .collect();
        let mut oid_b = FixedSizeBinaryBuilder::with_capacity(5, 20);
        let mut kind_b = UInt8Builder::with_capacity(5);
        let mut mode_b = UInt32Builder::with_capacity(5);
        let mut path_b = StringBuilder::new();
        let mut pay_b = LargeBinaryBuilder::new();
        for (i, p) in payloads.iter().enumerate() {
            oid_b.append_value(oid(i as u8 + 1)).unwrap();
            kind_b.append_value(kind_code(GitObjectKind::Commit));
            mode_b.append_null();
            path_b.append_null();
            pay_b.append_value(p);
        }
        let batch = RecordBatch::try_new(
            v1_schema.clone(),
            vec![
                Arc::new(oid_b.finish()),
                Arc::new(kind_b.finish()),
                Arc::new(mode_b.finish()),
                Arc::new(path_b.finish()),
                Arc::new(pay_b.finish()),
            ],
        )
        .unwrap();
        let opts = IpcWriteOptions::default();
        let ipc = IpcDataGenerator::default();
        let mut tracker = DictionaryTracker::new(false);
        let mut file = Vec::new();
        {
            let enc = ipc.schema_to_bytes_with_dictionary_tracker(&v1_schema, &mut tracker, &opts);
            let mut msg = Vec::new();
            write_message(&mut msg, enc, &opts).unwrap();
            file.extend_from_slice(&msg);
        }
        {
            let (_, enc) = ipc.encode(&batch, &mut tracker, &opts, &mut Default::default()).unwrap();
            let mut msg = Vec::new();
            write_message(&mut msg, enc, &opts).unwrap();
            file.extend_from_slice(&msg);
        }
        std::fs::write(&path, &file).unwrap();

        let t = ExplodedArchive::open_with_policy(&path, ExplodePolicy::Full).unwrap();
        for (i, p) in payloads.iter().enumerate() {
            let (k, got) = t.content(&oid(i as u8 + 1)).unwrap().expect("a v1 row is live");
            assert_eq!(k, GitObjectKind::Commit);
            assert_eq!(&got, p, "v1 row {} must read back raw", i + 1);
        }
        assert_eq!(t.pread_served(), 5, "a v1 pool must still take the extent fast path");
        // And it keeps working when this build appends to it: the appended
        // batch must be v1-shaped, because the stream's one schema message is
        // v1 — a six-column batch under it would not decode. Proven by reading
        // every batch back through Arrow's own stream reader afterwards.
        t.explode(&oid(9), GitObjectKind::Commit, &payloads[0]).unwrap();
        t.flush().unwrap();
        assert_eq!(t.content(&oid(9)).unwrap().unwrap().1, payloads[0]);
        assert_eq!(t.content(&oid(1)).unwrap().unwrap().1, payloads[0]);
        let rd = znippy_common::arrow::ipc::reader::StreamReader::try_new(
            std::fs::File::open(&path).unwrap(),
            None,
        )
        .unwrap();
        let mut rows = 0;
        for b in rd {
            let b = b.expect("every batch in a v1 file decodes under the v1 schema");
            assert!(enc_column(&b).is_none(), "a batch appended to a v1 pool must be v1-shaped");
            rows += b.num_rows();
        }
        assert_eq!(rows, 6, "five v1 rows and the one appended");
    }

    #[test]
    fn a_payload_round_trips_through_the_table() {
        let t = ExplodedArchive::open_with_policy(&tmp("rt"), ExplodePolicy::Full).unwrap();
        t.explode(&oid(1), GitObjectKind::Commit, b"tree deadbeef\n")
            .unwrap();
        t.explode(&oid(2), GitObjectKind::Blob, b"hello world")
            .unwrap();
        t.flush().unwrap();
        assert_eq!(
            t.content(&oid(2)).unwrap().unwrap(),
            (GitObjectKind::Blob, b"hello world".to_vec())
        );
        assert_eq!(
            t.content(&oid(1)).unwrap().unwrap(),
            (GitObjectKind::Commit, b"tree deadbeef\n".to_vec())
        );
        assert!(t.has(&oid(1)).unwrap());
        assert!(!t.has(&oid(9)).unwrap());
    }

    /// The row is a FILE: mode and path survive the round trip.
    #[test]
    fn a_row_can_carry_the_name_it_was_seen_at() {
        let t = ExplodedArchive::open_with_policy(&tmp("named"), ExplodePolicy::Full).unwrap();
        t.explode_at(
            &oid(1),
            GitObjectKind::Blob,
            Some(0o100755),
            Some("scripts/build.sh"),
            b"#!/bin/sh\n",
        )
        .unwrap();
        t.explode(&oid(2), GitObjectKind::Blob, b"anonymous").unwrap();
        t.flush().unwrap();
        assert_eq!(
            t.name(&oid(1)).unwrap().unwrap(),
            (Some(0o100755), Some("scripts/build.sh".to_owned()))
        );
        assert_eq!(
            t.name(&oid(2)).unwrap().unwrap(),
            (None, None),
            "null is 'not recorded', and must not read as a real name"
        );
    }

    /// Reopening finds every row without reading a payload, and appends continue
    /// the SAME stream rather than starting a second one.
    #[test]
    fn the_table_survives_a_reopen_and_keeps_appending() {
        let p = tmp("reopen");
        {
            let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
            t.explode(&oid(5), GitObjectKind::Tree, b"100644 f\0").unwrap();
            t.flush().unwrap();
        }
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        assert_eq!(t.rows().unwrap(), 1, "row count comes from batch metadata");
        t.explode(&oid(6), GitObjectKind::Blob, b"second session").unwrap();
        t.flush().unwrap();
        assert_eq!(t.rows().unwrap(), 2);
        assert_eq!(
            t.content(&oid(5)).unwrap().unwrap(),
            (GitObjectKind::Tree, b"100644 f\0".to_vec())
        );
        assert_eq!(
            t.content(&oid(6)).unwrap().unwrap(),
            (GitObjectKind::Blob, b"second session".to_vec())
        );

        // And a third open sees both — one schema message, two batches.
        let again = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        assert_eq!(again.rows().unwrap(), 2);
        assert_eq!(again.batches.lock().unwrap().len(), 2);
    }

    /// A read must not miss a row that is buffered but not yet written — an
    /// "absent" that is really "not flushed" sends the caller off to re-resolve
    /// a whole pack for an object we are holding.
    #[test]
    fn an_unflushed_row_is_still_served() {
        let t = ExplodedArchive::open_with_policy(&tmp("unflushed"), ExplodePolicy::Full).unwrap();
        t.explode(&oid(4), GitObjectKind::Commit, b"buffered").unwrap();
        assert_eq!(
            t.content(&oid(4)).unwrap().unwrap(),
            (GitObjectKind::Commit, b"buffered".to_vec())
        );
        assert!(t.has(&oid(4)).unwrap());
    }

    #[test]
    fn an_absent_oid_is_none_and_counted() {
        let t = ExplodedArchive::open_with_policy(&tmp("absent"), ExplodePolicy::Full).unwrap();
        assert!(t.content(&oid(3)).unwrap().is_none());
        assert_eq!(t.stats().absent, 1);
        assert_eq!(t.stats().served, 0);
    }

    #[test]
    fn of_kind_selects_by_kind() {
        let t = ExplodedArchive::open_with_policy(&tmp("kind"), ExplodePolicy::Full).unwrap();
        t.explode(&oid(1), GitObjectKind::Commit, b"c1").unwrap();
        t.explode(&oid(2), GitObjectKind::Blob, b"bb").unwrap();
        t.explode(&oid(3), GitObjectKind::Commit, b"c2").unwrap();
        let commits = t.of_kind(GitObjectKind::Commit).unwrap();
        assert_eq!(commits.len(), 2);
        assert!(commits.iter().all(|(_, p)| p[0] == b'c'));
    }

    /// ★ A RETIRED ROW STAYS RETIRED ACROSS A REOPEN.
    ///
    /// The file is append-only, so `retain` writes a [`TOMBSTONE`] rather than
    /// deleting. Seen RED with an index-only `retain`: the reopen below rebuilds
    /// from the file and brought the dead row back — which is
    /// `store::tests::a_wholly_dead_pack_does_not_come_back_when_the_store_is_reopened`
    /// failing with "551 commits, not the one live one", reached from here in
    /// three lines instead of a whole gc.
    #[test]
    fn retain_drops_only_what_is_dead_and_the_drop_survives_a_reopen() {
        let p = tmp("retain");
        {
            let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
            t.explode(&oid(1), GitObjectKind::Blob, b"a").unwrap();
            t.explode(&oid(2), GitObjectKind::Blob, b"b").unwrap();
            t.flush().unwrap();
            assert_eq!(t.retain(&|o: &[u8]| o[0] == 1).unwrap(), 1);
            assert!(t.has(&oid(1)).unwrap());
            assert!(!t.has(&oid(2)).unwrap());
            assert_eq!(
                t.of_kind(GitObjectKind::Blob).unwrap().len(),
                1,
                "a scan must not resurrect what retain dropped"
            );
        }
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        assert!(t.has(&oid(1)).unwrap(), "the live row did not survive");
        assert!(
            !t.has(&oid(2)).unwrap(),
            "a reopen resurrected a retired row — the tombstone is not durable"
        );
        assert!(t.content(&oid(2)).unwrap().is_none());
        assert_eq!(t.of_kind(GitObjectKind::Blob).unwrap().len(), 1);
    }

    /// sha256 keys are not truncated to sha1 width, and a table refuses a width
    /// it cannot hold.
    #[test]
    fn a_sha256_oid_keeps_all_thirty_two_bytes() {
        let p = tmp("sha256");
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        let o = [3u8; 32];
        t.explode(&o, GitObjectKind::Tree, b"t").unwrap();
        t.flush().unwrap();
        assert!(t.has(&o).unwrap());
        assert_eq!(t.content(&o).unwrap().unwrap().1, b"t");

        t.explode(&oid(1), GitObjectKind::Blob, b"narrow").unwrap();
        let err = t.flush().expect_err("a 20-byte oid cannot join a 32-byte table");
        assert!(format!("{err}").contains("32-byte oids"), "{err}");
    }

    /// A torn tail is truncated away rather than left mid-stream, where every
    /// later batch would be unreachable.
    #[test]
    fn a_torn_tail_is_dropped_and_the_table_keeps_working() {
        let p = tmp("torn");
        {
            let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
            t.explode(&oid(1), GitObjectKind::Blob, b"complete").unwrap();
            t.flush().unwrap();
        }
        // Half a message, as a killed process leaves.
        {
            let mut f = OpenOptions::new().append(true).open(&p).unwrap();
            f.write_all(&[0xFF, 0xFF, 0xFF, 0xFF, 0x40, 0x00, 0x00, 0x00, 1, 2, 3])
                .unwrap();
        }
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        assert_eq!(t.rows().unwrap(), 1, "the torn message contributes nothing");
        t.explode(&oid(2), GitObjectKind::Blob, b"after").unwrap();
        t.flush().unwrap();
        assert_eq!(t.rows().unwrap(), 2);
        assert_eq!(
            t.content(&oid(2)).unwrap().unwrap().1,
            b"after".to_vec(),
            "the append landed at the last good boundary, not after the garbage"
        );
    }

    #[test]
    fn graph_keeps_commits_and_trees_and_drops_blobs() {
        let p = ExplodePolicy::Graph;
        assert!(p.wants(GitObjectKind::Commit));
        assert!(p.wants(GitObjectKind::Tree));
        assert!(!p.wants(GitObjectKind::Blob));
        assert!(!ExplodePolicy::Off.wants(GitObjectKind::Commit));
        assert!(ExplodePolicy::Full.wants(GitObjectKind::Blob));
        assert!(ExplodePolicy::Commits.wants(GitObjectKind::Commit));
        assert!(!ExplodePolicy::Commits.wants(GitObjectKind::Tree));
        assert_eq!(
            ExplodePolicy::default(),
            ExplodePolicy::Commits,
            "the shipped default is the lane Q one: commits and tags eager, trees \
             and blobs lazy and budgeted. `Full` is what put 22.4x the pack into \
             /data on the rig; `Graph` is what put 28.24 GB of tree payloads there \
             and 50.9 GiB of them in RAM."
        );
    }

    /// ★ THE SETTING: a declined object is not stored, and is COUNTED — so a
    /// table short by policy and one short by fault do not look alike.
    #[test]
    fn a_declined_object_is_not_stored_and_is_counted() {
        let t = ExplodedArchive::open_with_policy(&tmp("policy"), ExplodePolicy::Graph).unwrap();
        t.explode(&oid(1), GitObjectKind::Commit, b"c").unwrap();
        t.explode(&oid(2), GitObjectKind::Blob, b"bbbb").unwrap();
        t.flush().unwrap();
        assert!(t.has(&oid(1)).unwrap());
        assert!(!t.has(&oid(2)).unwrap());
        assert_eq!(t.skipped(), 1);
        assert_eq!(t.rows().unwrap(), 1);
    }

    /// `Off` writes no file at all — the whole point of the free tier.
    #[test]
    fn off_stores_nothing_and_leaves_no_file() {
        let p = tmp("off");
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Off).unwrap();
        t.explode(&oid(1), GitObjectKind::Commit, b"c").unwrap();
        t.explode(&oid(2), GitObjectKind::Blob, b"b").unwrap();
        assert_eq!(t.flush().unwrap(), 0);
        assert_eq!(t.rows().unwrap(), 0);
        assert_eq!(t.skipped(), 2);
        assert!(!p.exists(), "a disabled table must not create a file");
    }

    // ════════════════════════════════════════════════════════════════════════
    // The stree in front of the lookup
    // ════════════════════════════════════════════════════════════════════════

    /// `n` oids in `groups` of `per` that share their first EIGHT bytes and
    /// differ only after them — the collision is CONSTRUCTED, never hoped for.
    ///
    /// The prefixes deliberately straddle `0x80`: half the oids in a real repo
    /// start there, and they are the half a non-order-preserving key would sort
    /// onto the wrong side of the keyspace.
    fn colliding_oids(groups: usize, per: usize) -> Vec<[u8; 20]> {
        let mut out = Vec::with_capacity(groups * per);
        for g in 0..groups {
            let mut z = (g as u64).wrapping_mul(0x9E37_79B9_7F4A_7C15);
            z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
            z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
            let prefix = (z ^ (z >> 31)).to_be_bytes();
            for k in 0..per {
                let mut o = [0u8; 20];
                o[..8].copy_from_slice(&prefix);
                o[8] = k as u8;
                o[9..13].copy_from_slice(&(g as u32).to_be_bytes());
                out.push(o);
            }
        }
        out
    }

    /// ★ THE STREE RETURNS THE ROW THE BINARY SEARCH RETURNED — including on
    /// 8-byte prefix collisions, of which this table has 1000.
    ///
    /// Three answers are compared for every oid: the `stree` probe, the
    /// `binary_search` over the identical oid array that this file shipped with,
    /// and the **applied output** — the payload `content` hands back, which is
    /// unique per oid, so a wrong row is a wrong payload and not a passing
    /// assertion about a `Located` nobody read.
    ///
    /// Seen RED by deleting the full-oid comparison from `OidTree::verify` (i.e.
    /// `.find(|&i| self.oid_at(i) == oid)` → `.next()`, the "stree already found
    /// it" mistake): **"the stree and the binary search disagree at
    /// 0000000000000000010000000000000000000000: Some(Located { batch: 0, row: 0,
    /// kind: Blob }) vs Some(Located { batch: 0, row: 1, kind: Blob })"** — the
    /// probe was handed the row of the prefix-mate stored one slot earlier.
    #[test]
    fn the_stree_agrees_with_the_binary_search_it_replaced_on_colliding_prefixes() {
        let t = ExplodedArchive::open_with_policy(&tmp("stree-agree"), ExplodePolicy::Full).unwrap();
        let oids = colliding_oids(1000, 4);
        for (i, o) in oids.iter().enumerate() {
            t.explode(o, GitObjectKind::Blob, format!("payload-{i}").as_bytes())
                .unwrap();
        }
        t.flush().unwrap();
        // Force the index, then reach into it: the two finders must sit over the
        // SAME arrays, or this compares two indexes rather than two finders.
        t.index_now().unwrap();
        let idx = t.index.lock().unwrap();
        let tree = idx.as_ref().expect("index_now built it");
        assert_eq!(tree.len(), oids.len());

        // The premise: the collisions are real in the built tree.
        let mut colliding_runs = 0usize;
        for o in &oids {
            let run = tree.candidate_run(key_for_oid(o));
            assert_eq!(
                run.len(),
                4,
                "expected a 4-entry candidate run for {}, got {run:?} — the collision \
                 the rest of this test rests on is not there",
                hex::encode(o)
            );
            colliding_runs += 1;
        }
        assert_eq!(colliding_runs, oids.len());
        // And the sign boundary is really crossed, or the key's top-bit flip is
        // untested by this corpus.
        assert!(
            oids.iter().any(|o| o[0] >= 0x80) && oids.iter().any(|o| o[0] < 0x80),
            "premise: the prefixes must straddle 0x80"
        );

        for o in &oids {
            assert_eq!(
                tree.find(o),
                tree.find_by_binary_search(o),
                "the stree and the binary search disagree at {}: {:?} vs {:?}",
                hex::encode(o),
                tree.find(o),
                tree.find_by_binary_search(o)
            );
            assert!(tree.find(o).is_some(), "{} vanished", hex::encode(o));
        }
        // An oid on a colliding prefix that was never stored must MISS, in both
        // finders — this is the query a verify-less probe answers with somebody
        // else's row.
        for o in oids.iter().step_by(4) {
            let mut absent = *o;
            absent[8] = 0xff;
            assert_eq!(tree.find(&absent), None, "{} was never stored", hex::encode(absent));
            assert_eq!(tree.find_by_binary_search(&absent), None);
        }
        drop(idx);

        // Applied output: each oid's own payload, not its prefix-mate's.
        for (i, o) in oids.iter().enumerate() {
            assert_eq!(
                t.content(o).unwrap().unwrap().1,
                format!("payload-{i}").into_bytes(),
                "{} came back with another object's bytes",
                hex::encode(o)
            );
        }
    }

    /// ★ A TOMBSTONE STILL RETIRES AN OID WITH THE STREE IN FRONT — and its
    /// prefix-mates do not go with it, nor stand in for it.
    ///
    /// Two failure modes live here and both return a wrong answer rather than an
    /// error: a fold that keeps the *first* sighting rather than the last leaves
    /// the retired oid resolvable, and a probe that trusts the 8-byte key answers
    /// the retired oid with the row of the live oid it collides with.
    ///
    /// Seen RED three times:
    ///
    /// * `Keep::Last` → `Keep::First` in `index_now`: **"a reopen resurrected a
    ///   retired oid: 0000000000000000000000000000000000000000"**. The same break
    ///   also reds the older, coarser
    ///   `retain_drops_only_what_is_dead_and_the_drop_survives_a_reopen`.
    /// * the full-oid comparison dropped from `OidTree::verify`:
    ///   **"0000000000000000000000000000000000000000 was retired"** — the dead
    ///   oid answered `has` with its live prefix-mate's row, in-process, before
    ///   any reopen.
    /// * the incremental fold in `flush` no longer filtering the retired set
    ///   (`if !retired.contains(oid)` → `if true`): the same message, from the
    ///   live process rather than from a reopen.
    #[test]
    fn a_tombstoned_oid_stays_absent_when_a_live_oid_shares_its_prefix() {
        let p = tmp("stree-tomb");
        // Four oids per prefix: one is retired, three stay.
        let oids = colliding_oids(64, 4);
        let dead: Vec<[u8; 20]> = oids.iter().copied().step_by(4).collect();
        {
            let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
            for (i, o) in oids.iter().enumerate() {
                t.explode(o, GitObjectKind::Blob, format!("live-{i}").as_bytes())
                    .unwrap();
            }
            t.flush().unwrap();
            let retired = t.retain(&|o: &[u8]| !dead.iter().any(|d| d == o)).unwrap();
            assert_eq!(retired, dead.len() as u64);
            for d in &dead {
                assert!(!t.has(d).unwrap(), "{} was retired", hex::encode(d));
                assert!(
                    t.content(d).unwrap().is_none(),
                    "the retired oid {} was answered with a prefix-mate's row",
                    hex::encode(d)
                );
            }
            for (i, o) in oids.iter().enumerate() {
                if i % 4 == 0 {
                    continue;
                }
                assert_eq!(
                    t.content(o).unwrap().unwrap().1,
                    format!("live-{i}").into_bytes(),
                    "{} is alive and must still answer with its OWN bytes",
                    hex::encode(o)
                );
            }
        }
        // ★ And a reopen — which rebuilds the tree from the file, tombstones and
        // all — finds every live row and none of the dead.
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        for (i, o) in oids.iter().enumerate() {
            if i % 4 == 0 {
                assert!(
                    !t.has(o).unwrap(),
                    "a reopen resurrected a retired oid: {}",
                    hex::encode(o)
                );
            } else {
                assert_eq!(
                    t.content(o).unwrap().unwrap().1,
                    format!("live-{i}").into_bytes(),
                    "a reopen lost or misrouted the live oid {}",
                    hex::encode(o)
                );
            }
        }
        assert_eq!(
            t.of_kind(GitObjectKind::Blob).unwrap().len(),
            oids.len() - dead.len()
        );
        // Only now — `rows()` answers from batch metadata until an index exists,
        // and the file still holds the retired rows and their tombstones. This
        // assertion measured 320 against 192 when it was made before the reads
        // above, which is the documented behaviour of `rows()` and not the
        // tombstone leaking.
        assert_eq!(t.rows().unwrap(), (oids.len() - dead.len()) as u64);
    }

    /// The incremental extension in `flush` and the from-scratch rebuild in
    /// `index_now` must land on the same tree — one of them runs while a push is
    /// hot and the other after a restart, and a difference between them is a
    /// bug that only appears to the second process.
    ///
    /// Seen RED twice, once from each side, and the two reds are mirror images —
    /// which is what tells you the test compares the folds rather than one of
    /// them against itself:
    ///
    /// * incremental path drops the `retired` filter over the entries it carries
    ///   forward (`if !retired.contains(oid)` → `if true`): **"the two folds kept
    ///   a different number of oids: left: 150, right: 200"** — the live index
    ///   kept the 50 it had been told to retire.
    /// * `Keep::Last` → `Keep::First` in `index_now`: **"left: 200, right: 150"**
    ///   — the rebuild kept them instead.
    #[test]
    fn the_incremental_index_and_a_rebuilt_one_agree() {
        let p = tmp("stree-incr");
        let oids = colliding_oids(50, 4);
        let t = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        // Build the index FIRST, so every flush below takes the incremental path.
        t.index_now().unwrap();
        for (i, o) in oids.iter().enumerate() {
            t.explode(o, GitObjectKind::Blob, format!("v-{i}").as_bytes())
                .unwrap();
            if i % 37 == 0 {
                t.flush().unwrap();
            }
        }
        t.flush().unwrap();
        t.retain(&|o: &[u8]| o[8] != 3).unwrap();
        let live: Vec<(Vec<u8>, Vec<u8>)> = {
            let idx = t.index.lock().unwrap();
            idx.as_ref()
                .unwrap()
                .iter()
                .map(|(o, l)| (o.to_vec(), vec![l.batch as u8, l.row as u8]))
                .collect()
        };
        drop(t);

        let again = ExplodedArchive::open_with_policy(&p, ExplodePolicy::Full).unwrap();
        again.index_now().unwrap();
        let rebuilt = again.index.lock().unwrap();
        let rebuilt = rebuilt.as_ref().unwrap();
        assert_eq!(rebuilt.len(), live.len(), "the two folds kept a different number of oids");
        for (o, _) in &live {
            assert!(
                rebuilt.find(o).is_some(),
                "the live process and a reopen disagree about {}: Some(..) vs None",
                hex::encode(o)
            );
        }
        for o in oids.iter().filter(|o| o[8] == 3) {
            assert!(rebuilt.find(o).is_none(), "{} was retired", hex::encode(o));
        }
    }

    /// The measurement behind the change, re-takeable: three finders over the
    /// SAME 1 000 000 entries — the `Vec<(Vec<u8>, Located)>` this file shipped
    /// with, a binary search over the flat array, and the stree.
    ///
    /// `--ignored` because it is a measurement, not a guard: it asserts only that
    /// the three agree, and prints the times. Run it with
    /// `cargo test --release -p znippy-plugin-git --lib exploded_arrow -- --ignored --nocapture`
    /// — a debug build measures the borrow checker, not the machine.
    ///
    /// ## Measured, oden 2026-08-14, `--release`, 1 000 000 entries, 200 000
    /// probes (half misses), three runs
    ///
    /// | arm | ns/probe | vs shipped |
    /// |---|---:|---:|
    /// | `Vec<(Vec<u8>, Located)>` + `binary_search` (what shipped) | 836 / 862 / 961 | 1.00 |
    /// | flat oid array + `binary_search` | 339 / 339 / 348 | 0.40 |
    /// | **stree** | **164 / 175 / 185** | **0.21** |
    ///
    /// So the structure is worth **~2×** on its own and the flattening another
    /// **~2.5×**, and the two together take a point lookup to a fifth of what it
    /// cost. **The box was NOT quiet** — `/proc/loadavg` 1-min 29.7–31.5 on 32
    /// cores, other agents building throughout — so the absolute figures are
    /// upper bounds. The ratios are reported instead, and each arm's own spread
    /// across the three runs (14% / 2.7% / 12%) is far narrower than the 5×
    /// and 2× between them.
    #[test]
    #[ignore = "measurement; run under --release with --ignored"]
    fn lookup_cost_before_and_after() {
        use std::time::Instant;
        let n = 1_000_000usize;
        // 4 oids per prefix, so ~25% of probes land in a run of four and the
        // verify step is exercised rather than skipped.
        let oids = colliding_oids(n / 4, 4);
        let mut b = OidTreeBuilder::with_capacity(oids.len());
        for (i, o) in oids.iter().enumerate() {
            b.push(o, Some(Located { batch: (i / 100_000) as u32, row: i as u32, kind: GitObjectKind::Blob, pay_at: EXTENT_UNAVAILABLE, pay_len: 0, enc: ENC_RAW }))
                .unwrap();
        }
        let build = Instant::now();
        let tree = b.finish(Keep::Last);
        let build = build.elapsed();

        // The shape this replaced: one heap allocation per oid.
        let mut old: Vec<(Vec<u8>, Located)> =
            tree.iter().map(|(o, l)| (o.to_vec(), l)).collect();
        old.sort_by(|a, b| a.0.cmp(&b.0));

        // A miss-heavy probe order, and NOT in oid order — a `have` negotiation
        // is random and half of it misses.
        let mut probes: Vec<[u8; 20]> = Vec::with_capacity(200_000);
        for i in (0..oids.len()).step_by(oids.len() / 100_000) {
            probes.push(oids[i]);
            let mut m = oids[i];
            m[8] = 0xfe;
            probes.push(m);
        }

        let mut sink = 0u64;
        let t0 = Instant::now();
        for p in &probes {
            if let Ok(i) = old.binary_search_by(|(o, _)| o.as_slice().cmp(&p[..])) {
                sink += old[i].1.row as u64;
            }
        }
        let vec_of_vec = t0.elapsed();
        let t0 = Instant::now();
        for p in &probes {
            if let Some(l) = tree.find_by_binary_search(p) {
                sink += l.row as u64;
            }
        }
        let flat_bsearch = t0.elapsed();
        let t0 = Instant::now();
        for p in &probes {
            if let Some(l) = tree.find(p) {
                sink += l.row as u64;
            }
        }
        let stree = t0.elapsed();

        // The arms must be the same question.
        for p in &probes {
            let want = tree.find_by_binary_search(p);
            assert_eq!(tree.find(p), want, "arms disagree at {}", hex::encode(p));
        }
        let per = |d: std::time::Duration| d.as_secs_f64() * 1e9 / probes.len() as f64;
        println!(
            "exploded oid lookup, {} entries ({} groups of 4 colliding prefixes), \
             {} probes half of them misses:\n  \
             Vec<(Vec<u8>,Located)> binary_search  {:>7.1} ns/probe\n  \
             flat array binary_search             {:>7.1} ns/probe\n  \
             stree                                {:>7.1} ns/probe\n  \
             tree build (sort + fold + stree)      {:.3} s   sink={sink}",
            tree.len(),
            oids.len() / 4,
            probes.len(),
            per(vec_of_vec),
            per(flat_bsearch),
            per(stree),
            build.as_secs_f64(),
        );
    }

    #[test]
    fn an_unknown_policy_is_refused() {
        assert_eq!(ExplodePolicy::parse("full").unwrap(), ExplodePolicy::Full);
        let err = ExplodePolicy::parse("ful").expect_err("a typo must be refused");
        assert!(format!("{err}").contains("unknown explode policy"), "{err}");
    }
}