//! HDF5 Object Header parsing (v1 and v2).

#[cfg(not(feature = "std"))]
use alloc::{boxed::Box, collections::BTreeSet, vec::Vec};
#[cfg(feature = "std")]
use std::collections::BTreeSet;

use byteorder::{ByteOrder, LittleEndian};

use crate::addr::to_usize;
use crate::error::FormatError;
use crate::message_type::MessageType;
use crate::storage::{Storage, Window, len_usize, read_exact_at};

/// OHDR signature for v2 object headers.
const OHDR_SIGNATURE: [u8; 4] = *b"OHDR";

/// OCHK signature for v2 continuation chunks.
const OCHK_SIGNATURE: [u8; 4] = *b"OCHK";

/// A single parsed header message.
#[derive(Debug, Clone)]
pub struct HeaderMessage {
    /// The message type.
    pub msg_type: MessageType,
    /// Size of the message data in bytes.
    pub size: usize,
    /// Message flags byte.
    pub flags: u8,
    /// Creation order (v2 only, when tracking is enabled).
    pub creation_order: Option<u16>,
    /// Raw message data bytes.
    pub data: Vec<u8>,
}

/// Parsed HDF5 object header.
#[derive(Debug, Clone)]
pub struct ObjectHeader {
    /// Header version (1 or 2).
    pub version: u8,
    /// All non-NIL messages collected from all chunks.
    pub messages: Vec<HeaderMessage>,
    /// Object reference count (v1 only).
    pub reference_count: Option<u32>,
    /// Object header flags (v2 only; 0 for v1).
    pub flags: u8,
    /// Access time (v2, when flags bit 2 set).
    pub access_time: Option<u32>,
    /// Modification time (v2, when flags bit 2 set).
    pub modification_time: Option<u32>,
    /// Change time (v2, when flags bit 2 set).
    pub change_time: Option<u32>,
    /// Birth time (v2, when flags bit 2 set).
    pub birth_time: Option<u32>,
}

fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
    match offset.checked_add(needed) {
        Some(end) if end <= data.len() => Ok(()),
        _ => Err(FormatError::UnexpectedEof {
            expected: offset.saturating_add(needed),
            available: data.len(),
        }),
    }
}

fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
    let s = size as usize;
    ensure_len(data, pos, s)?;
    let slice = &data[pos..pos + s];
    Ok(match size {
        2 => LittleEndian::read_u16(slice) as u64,
        4 => LittleEndian::read_u32(slice) as u64,
        8 => LittleEndian::read_u64(slice),
        1 => slice[0] as u64,
        _ => {
            return Err(FormatError::InvalidOffsetSize(size));
        }
    })
}

/// The kind of object an object header describes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ObjectClass {
    /// A group: the header has a Symbol Table or a Link Info message.
    Group,
    /// A dataset: the header has a Datatype and a Dataspace message.
    Dataset,
    /// A committed (named) datatype: a Datatype message, no Dataspace.
    NamedDatatype,
}

impl ObjectHeader {
    /// The kind of object this header describes, decided as libhdf5 decides
    /// it (`H5O__obj_class_real`): group first (a Symbol Table or Link Info
    /// message), then dataset (a Datatype *and* a Dataspace message — not a
    /// Data Layout message), then named datatype (a Datatype message).
    /// `None` when none applies; libhdf5 then cannot open the object
    /// ("unable to determine object type").
    ///
    /// A header with a Datatype and a Data Layout message but no Dataspace
    /// is a named datatype to libhdf5, not a dataset.
    pub fn object_class(&self) -> Option<ObjectClass> {
        let has = |t: MessageType| self.messages.iter().any(|m| m.msg_type == t);
        if has(MessageType::SymbolTable) || has(MessageType::LinkInfo) {
            Some(ObjectClass::Group)
        } else if has(MessageType::Datatype) && has(MessageType::Dataspace) {
            Some(ObjectClass::Dataset)
        } else if has(MessageType::Datatype) {
            Some(ObjectClass::NamedDatatype)
        } else {
            None
        }
    }

    /// Parse an object header at the given offset in the data buffer.
    ///
    /// `offset_size` and `length_size` come from the superblock.
    #[inline]
    pub fn parse(
        data: &[u8],
        offset: usize,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        Self::parse_slice(data, offset as u64, offset_size, length_size)
    }

    /// [`Self::parse`] over any [`Storage`].
    ///
    /// Reads the prefix (at most [`V2_PREFIX_MAX`] bytes, signature
    /// included), then each chunk as one bounded read, continuation chunks
    /// included. A storage with the whole file in memory is parsed as its
    /// slice, by code compiled in this crate (see
    /// [`crate::storage`], "Slice entry points").
    #[inline]
    pub fn parse_in<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        match file.as_contiguous() {
            Some(all) => Self::parse_slice(all, offset, offset_size, length_size),
            None => Self::parse_storage(file, offset, offset_size, length_size),
        }
    }

    /// [`Self::parse_storage`] for the slice, compiled in this crate: the
    /// one copy [`Self::parse`] and [`Self::parse_in`] (in memory) call.
    fn parse_slice(
        data: &[u8],
        offset: u64,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        Self::parse_storage(data, offset, offset_size, length_size)
    }

    fn parse_storage<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        // The first chunk is read once the prefix says how long it is: say
        // so (see `Storage::hint`), for a storage that fetches between
        // attempts.
        if file.as_contiguous().is_none() {
            file.hint(offset, OBJECT_HEADER_HINT_LEN);
        }
        // The longest prefix of either version, in one read. It holds the
        // whole prefix or ends at the end of the file, so its bounds checks
        // are the whole-file ones.
        let prefix = Window::read(file, offset, V2_PREFIX_MAX)?;
        prefix.ensure(0, 4)?;
        if prefix.bytes[..4] == OHDR_SIGNATURE {
            Self::parse_v2(file, offset, &prefix, offset_size, length_size)
        } else {
            Self::parse_v1(file, offset, &prefix, offset_size, length_size)
        }
    }

    fn parse_v1<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        prefix: &Window<'_>,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        // version(1) + reserved(1) + num_messages(2) + ref_count(4) + header_size(4) = 12
        // then pad to 8-byte alignment from start of header
        prefix.ensure(0, 12)?;
        let prefix = &prefix.bytes[..12];

        let version = prefix[0];
        if version != 1 {
            return Err(FormatError::InvalidObjectHeaderVersion(version));
        }

        let num_messages = LittleEndian::read_u16(&prefix[2..4]) as usize;
        let reference_count = LittleEndian::read_u32(&prefix[4..8]);
        let header_data_size = LittleEndian::read_u32(&prefix[8..12]) as usize;

        // libhdf5 (H5O__prefix_deserialize): a header with messages needs room
        // for at least one message header, and one without has an empty chunk.
        if (num_messages > 0 && header_data_size < V1_MSG_HEADER_SIZE)
            || (num_messages == 0 && header_data_size > 0)
        {
            return Err(FormatError::InvalidObjectHeader(
                "bad object header chunk size",
            ));
        }

        // Pad to 8-byte alignment: header prefix is 12 bytes, pad to 16
        let padding = 4; // pad 12-byte prefix to 16-byte alignment
        let msg_start = offset
            .checked_add(12 + padding)
            .ok_or(FormatError::UnexpectedEof {
                expected: usize::MAX,
                available: len_usize(file),
            })?;

        // parse_v1_chunk reads the chunk, with the bounds check that was here.
        // The prefix's count (NIL messages included, capped: it is untrusted)
        // sizes the list once instead of growing it message by message.
        let mut messages = Vec::with_capacity(num_messages.min(64));
        let chunk0_count = Self::parse_v1_chunk(
            file,
            msg_start,
            header_data_size,
            offset_size,
            length_size,
            &mut messages,
        )?;
        // libhdf5 reads every message in the first chunk and refuses a header
        // whose prefix claims fewer than that (continuation chunks are read
        // later and not held to the count). Stopping after the claimed number
        // silently dropped the rest.
        if chunk0_count > num_messages {
            return Err(FormatError::InvalidObjectHeader(
                "bad object header message count",
            ));
        }

        Ok(ObjectHeader {
            version: 1,
            messages,
            reference_count: Some(reference_count),
            flags: 0,
            access_time: None,
            modification_time: None,
            change_time: None,
            birth_time: None,
        })
    }

    /// Parse the messages of one version-1 chunk (`length` bytes at
    /// `offset`, no signature), following continuation messages as they are
    /// met. Returns how many messages (NIL ones included) this chunk itself
    /// holds.
    ///
    /// A version-1 chunk is filled with messages whose sizes are multiples of
    /// 8; libhdf5 refuses a message that is not aligned, that runs past the
    /// end of the chunk, or leftover bytes too few for a message header (a
    /// "gap", which only version 2 allows).
    ///
    /// Continuation chunks are read in the order their messages are found,
    /// as `H5O_protect` loads them (so the messages keep libhdf5's order):
    /// a queue of (address, length) pairs, each chunk read, parsed and
    /// released before the next, so only one chunk buffer is alive at a
    /// time whatever the storage. Every chunk must start at a new address
    /// (else a cycle), and the chunks together may be no larger than the
    /// file, so the bytes read stay within the file's size; a header of
    /// more than [`MAX_V1_CHUNKS`] chunks is refused.
    fn parse_v1_chunk<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        length: usize,
        offset_size: u8,
        length_size: u8,
        messages: &mut Vec<HeaderMessage>,
    ) -> Result<usize, FormatError> {
        // The chunks found so far are also the queue of chunks to read.
        let mut spans = ChunkSpans::new(file.len(), offset, length)?;
        let mut chunk0_count = 0usize;
        let mut next = 0usize;
        let hints = file.as_contiguous().is_none();
        while let Some((chunk_offset, chunk_length)) = spans.get(next) {
            let chunk = read_exact_at(file, chunk_offset, chunk_length)?;
            let known = spans.len;
            let count =
                Self::parse_v1_messages(&chunk, offset_size, length_size, messages, &mut spans)?;
            // The continuation chunks this one names are read next.
            if hints {
                for i in known..spans.len {
                    if let Some((o, l)) = spans.get(i) {
                        file.hint(o, l);
                    }
                }
            }
            // Only the first chunk's messages are held to the prefix count.
            if next == 0 {
                chunk0_count = count;
            }
            next += 1;
        }
        Ok(chunk0_count)
    }

    /// The messages of one version-1 chunk: each checked and appended to
    /// `messages` (NIL ones dropped), each continuation added to `spans`.
    /// Returns how many messages (NIL ones included) the chunk holds.
    ///
    /// Inlined into the chunk loop: kept out of line (`#[inline(never)]`,
    /// 4313917), the call cost `ObjectHeader::parse` about 2.5 ns per
    /// header, 4% on small version-1 headers (A/B builds, 2026-09-27; see
    /// `BENCHMARKS.md`). Without an attribute the compiler keeps it out of
    /// line too.
    #[inline]
    fn parse_v1_messages(
        data: &[u8],
        offset_size: u8,
        length_size: u8,
        messages: &mut Vec<HeaderMessage>,
        spans: &mut ChunkSpans,
    ) -> Result<usize, FormatError> {
        let end = data.len();
        let mut pos = 0usize;
        let mut count = 0usize;
        while pos < end {
            if end - pos < V1_MSG_HEADER_SIZE {
                return Err(FormatError::InvalidObjectHeader(
                    "gap found in early version of file format",
                ));
            }
            let msg_type_raw = LittleEndian::read_u16(&data[pos..pos + 2]);
            let msg_data_size = LittleEndian::read_u16(&data[pos + 2..pos + 4]) as usize;
            let msg_flags = data[pos + 4];
            // reserved(3) at pos+5..pos+8
            pos += V1_MSG_HEADER_SIZE;

            if !msg_data_size.is_multiple_of(8) {
                return Err(FormatError::InvalidObjectHeader("message not aligned"));
            }
            if msg_data_size > end - pos {
                return Err(FormatError::InvalidObjectHeader(
                    "message size exceeds buffer end",
                ));
            }
            let body = &data[pos..pos + msg_data_size];
            check_message(1, msg_type_raw, msg_flags, body, offset_size, length_size)?;
            count += 1;
            let msg_type = MessageType::from_u16(msg_type_raw);
            if msg_type != MessageType::Nil {
                messages.push(HeaderMessage {
                    msg_type,
                    size: msg_data_size,
                    flags: msg_flags,
                    creation_order: None,
                    data: body.to_vec(),
                });
            }
            // Queue continuations (v1 continuation chunks are just raw
            // messages, no signature); check_message has checked the body.
            if msg_type == MessageType::ObjectHeaderContinuation {
                let cont_offset = read_offset(body, 0, offset_size)?;
                let cont_length = to_usize(read_offset(body, offset_size as usize, length_size)?)?;
                spans.add(cont_offset, cont_length)?;
            }
            pos += msg_data_size;
        }
        Ok(count)
    }

    fn parse_v2<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        prefix: &Window<'_>,
        offset_size: u8,
        length_size: u8,
    ) -> Result<ObjectHeader, FormatError> {
        // `ensure_len` checks positions relative to the header against the
        // prefix window and reports them as the whole-file check did, with
        // absolute positions and the file's length.
        let data: &[u8] = &prefix.bytes;
        let file_len = len_usize(file);
        let base = usize::try_from(offset).unwrap_or(usize::MAX);
        let abs = |rel: usize| base.saturating_add(rel);
        let ensure_len = |_: &[u8], rel: usize, needed: usize| prefix.ensure(rel, needed);
        let offset = 0usize;
        // signature(4) + version(1) + flags(1) = 6
        ensure_len(data, offset, 6)?;

        let version = data[offset + 4];
        if version != 2 {
            return Err(FormatError::InvalidObjectHeaderVersion(version));
        }
        let flags = data[offset + 5];
        if flags & !V2_HDR_ALL_FLAGS != 0 {
            return Err(FormatError::InvalidObjectHeader(
                "unknown object header status flag(s)",
            ));
        }

        let mut pos = offset + 6;

        // Optional timestamps (flags bit 5)
        let (access_time, modification_time, change_time, birth_time) = if flags & 0x20 != 0 {
            ensure_len(data, pos, 16)?;
            let at = LittleEndian::read_u32(&data[pos..pos + 4]);
            let mt = LittleEndian::read_u32(&data[pos + 4..pos + 8]);
            let ct = LittleEndian::read_u32(&data[pos + 8..pos + 12]);
            let bt = LittleEndian::read_u32(&data[pos + 12..pos + 16]);
            pos += 16;
            (Some(at), Some(mt), Some(ct), Some(bt))
        } else {
            (None, None, None, None)
        };

        // Optional attribute storage thresholds (flags bit 4)
        if flags & 0x10 != 0 {
            ensure_len(data, pos, 4)?;
            // max_compact_attrs(2) + min_dense_attrs(2) — checked, not stored
            let max_compact = LittleEndian::read_u16(&data[pos..pos + 2]);
            let min_dense = LittleEndian::read_u16(&data[pos + 2..pos + 4]);
            if max_compact < min_dense {
                return Err(FormatError::InvalidObjectHeader(
                    "bad object header attribute phase change values",
                ));
            }
            pos += 4;
        }

        // chunk0 size: width depends on flags bits 0-1
        let chunk_size_width = match flags & 0x03 {
            0 => 1u8,
            1 => 2,
            2 => 4,
            3 => 8,
            _ => unreachable!(),
        };
        ensure_len(data, pos, chunk_size_width as usize)?;
        let chunk0_size = to_usize(read_offset(data, pos, chunk_size_width)?)?;
        pos += chunk_size_width as usize;
        // Bit 2: attribute creation order tracked → messages include creation order field
        let has_creation_order = flags & 0x04 != 0;
        let msg_header_size = if has_creation_order { 6 } else { 4 };
        if chunk0_size > 0 && chunk0_size < msg_header_size {
            return Err(FormatError::InvalidObjectHeader(
                "bad object header chunk size",
            ));
        }

        let chunk0_msg_start = pos;
        let Some(chunk0_abs_end) = abs(pos).checked_add(chunk0_size) else {
            return Err(FormatError::UnexpectedEof {
                expected: usize::MAX,
                available: file_len,
            });
        };
        let chunk0_msg_end = chunk0_abs_end - base;

        // The whole first chunk, prefix to checksum, in one read (its
        // bounds check is the one on the checksum's 4 bytes).
        let chunk0 = read_exact_at(file, base as u64, chunk0_msg_end.saturating_add(4))?;
        let data: &[u8] = &chunk0;

        // Validate checksum: from OHDR signature through all messages (before checksum)
        #[cfg(feature = "checksum")]
        {
            let stored = LittleEndian::read_u32(&data[chunk0_msg_end..chunk0_msg_end + 4]);
            let computed = crate::checksum::jenkins_lookup3(&data[offset..chunk0_msg_end]);
            if computed != stored {
                return Err(FormatError::ChecksumMismatch {
                    expected: stored,
                    computed,
                });
            }
        }

        // Parse messages from chunk0
        let mut messages = Vec::new();
        let mut continuations = Vec::new();
        Self::parse_v2_messages(
            data,
            chunk0_msg_start,
            chunk0_msg_end,
            has_creation_order,
            offset_size,
            length_size,
            &mut messages,
            &mut continuations,
        )?;

        // Follow continuations, one chunk buffer at a time. A chunk address
        // seen twice is a cycle in malformed data, and the chunks may add up
        // to no more than the file; a valid header can have many chunks (libhdf5 adds one
        // whenever a message no longer fits), up to the same bound as a
        // version-1 header.
        let mut spans = ChunkSpans::new(file.len(), base as u64, chunk0_msg_end.saturating_add(4))?;
        // The continuation chunks a chunk names are read next (see
        // `Storage::hint`).
        let hints = file.as_contiguous().is_none();
        if hints {
            for &(o, l) in &continuations {
                file.hint(o as u64, l);
            }
        }
        while let Some((cont_offset, cont_length)) = continuations.pop() {
            spans.add(cont_offset as u64, cont_length)?;
            let known = continuations.len();
            Self::parse_v2_continuation(
                file,
                cont_offset as u64,
                cont_length,
                has_creation_order,
                offset_size,
                length_size,
                &mut messages,
                &mut continuations,
            )?;
            if hints {
                for &(o, l) in &continuations[known..] {
                    file.hint(o as u64, l);
                }
            }
        }

        Ok(ObjectHeader {
            version: 2,
            messages,
            reference_count: None,
            flags,
            access_time,
            modification_time,
            change_time,
            birth_time,
        })
    }

    #[allow(clippy::too_many_arguments)]
    fn parse_v2_messages(
        data: &[u8],
        start: usize,
        end: usize,
        has_creation_order: bool,
        offset_size: u8,
        length_size: u8,
        messages: &mut Vec<HeaderMessage>,
        continuations: &mut Vec<(usize, usize)>,
    ) -> Result<(), FormatError> {
        let msg_header_size = if has_creation_order { 6 } else { 4 };
        let mut pos = start;
        let mut null_count = 0usize;

        while pos < end {
            // Leftover bytes too few for a message header are a gap, which
            // libhdf5 allows only in a chunk without NIL messages (a writer
            // that leaves a gap had no NIL message to put the space in).
            if end - pos < msg_header_size {
                if null_count != 0 {
                    return Err(FormatError::InvalidObjectHeader(
                        "gap in chunk with no null messages",
                    ));
                }
                break;
            }
            let msg_type_raw = data[pos] as u16;
            let msg_data_size = LittleEndian::read_u16(&data[pos + 1..pos + 3]) as usize;
            let msg_flags = data[pos + 3];
            let creation_order = if has_creation_order {
                Some(LittleEndian::read_u16(&data[pos + 4..pos + 6]))
            } else {
                None
            };
            pos += msg_header_size;

            // `end` is where the messages stop and the checksum starts.
            // libhdf5 bounds a message by the chunk including its checksum,
            // but a message that runs into the checksum still fails there:
            // its loop stops at the checksum, and reading the checksum from
            // past its start overruns the chunk ("ran off end of input
            // buffer while decoding"). Both refuse it; only the text
            // differs.
            if msg_data_size > end - pos {
                return Err(FormatError::InvalidObjectHeader(
                    "message size exceeds buffer end",
                ));
            }
            let body = &data[pos..pos + msg_data_size];
            check_message(2, msg_type_raw, msg_flags, body, offset_size, length_size)?;

            let msg_type = MessageType::from_u16(msg_type_raw);
            if msg_type == MessageType::ObjectHeaderContinuation {
                // check_message has checked the body holds both fields.
                let cont_off = to_usize(read_offset(body, 0, offset_size)?)?;
                let cont_len = to_usize(read_offset(body, offset_size as usize, length_size)?)?;
                continuations.push((cont_off, cont_len));
            } else if msg_type == MessageType::Nil {
                null_count += 1;
            } else {
                messages.push(HeaderMessage {
                    msg_type,
                    size: msg_data_size,
                    flags: msg_flags,
                    creation_order,
                    data: body.to_vec(),
                });
            }

            pos += msg_data_size;
        }

        Ok(())
    }

    #[allow(clippy::too_many_arguments)]
    fn parse_v2_continuation<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        length: usize,
        has_creation_order: bool,
        offset_size: u8,
        length_size: u8,
        messages: &mut Vec<HeaderMessage>,
        continuations: &mut Vec<(usize, usize)>,
    ) -> Result<(), FormatError> {
        // OCHK signature(4) + messages + checksum(4)
        let chunk = read_exact_at(file, offset, length)?;
        let data: &[u8] = &chunk;
        let offset = 0usize;
        if length < 8 {
            return Err(FormatError::UnexpectedEof {
                expected: 8,
                available: length,
            });
        }

        ensure_len(data, offset, 4)?;
        if data[offset..offset + 4] != OCHK_SIGNATURE {
            return Err(FormatError::InvalidObjectHeaderSignature);
        }

        let msg_start = offset + 4;
        let checksum_pos = offset + length - 4;

        #[cfg(feature = "checksum")]
        {
            let stored = LittleEndian::read_u32(&data[checksum_pos..checksum_pos + 4]);
            let computed = crate::checksum::jenkins_lookup3(&data[offset..checksum_pos]);
            if computed != stored {
                return Err(FormatError::ChecksumMismatch {
                    expected: stored,
                    computed,
                });
            }
        }

        Self::parse_v2_messages(
            data,
            msg_start,
            checksum_pos,
            has_creation_order,
            offset_size,
            length_size,
            messages,
            continuations,
        )
    }
}

/// What an object header is hinted to take before its prefix is read (see
/// [`Storage::hint`]): the first chunk of a typical dataset's header. A
/// longer header is read all the same.
pub(crate) const OBJECT_HEADER_HINT_LEN: usize = 512;

/// Longest version-2 object header prefix: signature(4) + version(1) +
/// flags(1) + times(16) + attribute phase change(4) + chunk-0 size(8).
const V2_PREFIX_MAX: usize = 34;

/// Size of a version-1 message header: type(2) + size(2) + flags(1) + reserved(3).
const V1_MSG_HEADER_SIZE: usize = 8;

/// The chunks of one object header read so far, in the order they were
/// found (which is the order version-1 chunks are read in). A chunk starting
/// where another did is a cycle. Chunks of a valid header do not overlap, so
/// together they are no larger than the file; a header whose chunks add up
/// to more is refused, which bounds what its chunks can make a reader read
/// (a crafted chain of chunks each nested in the last would otherwise read
/// the file over and over). Overlap itself is not refused: libhdf5 reads
/// such headers (`cve-2025-7067.h5` has one).
///
/// Almost every header has at most a few chunks, and this runs once per
/// header, so the first [`INLINE_CHUNKS`] live in an inline array and are
/// checked for cycles by a scan; only a longer header allocates (the rest
/// of the list, and a set of starts). Allocating a queue and a set for
/// every header made parsing 401 small headers 1.8x slower.
struct ChunkSpans {
    inline: [(u64, usize); INLINE_CHUNKS],
    /// Chunks after the first [`INLINE_CHUNKS`], and every chunk start.
    spill: Option<Box<SpilledSpans>>,
    /// How many chunks there are.
    len: usize,
    /// Bytes of the chunks so far, and the most they may add up to.
    total: u64,
    budget: u64,
}

/// The chunks of a [`ChunkSpans`] beyond its inline ones.
struct SpilledSpans {
    chunks: Vec<(u64, usize)>,
    starts: BTreeSet<u64>,
}

/// How many chunks [`ChunkSpans`] holds without allocating.
const INLINE_CHUNKS: usize = 8;

impl ChunkSpans {
    #[inline]
    fn new(file_len: u64, start: u64, len: usize) -> Result<Self, FormatError> {
        let mut s = Self {
            inline: [(0, 0); INLINE_CHUNKS],
            spill: None,
            len: 0,
            total: 0,
            budget: file_len,
        };
        s.add(start, len)?;
        Ok(s)
    }

    /// Record the chunk `len` bytes at `start`.
    #[inline]
    fn add(&mut self, start: u64, len: usize) -> Result<(), FormatError> {
        self.total = self.total.saturating_add(len as u64);
        if self.len < INLINE_CHUNKS {
            if self.inline[..self.len].iter().any(|&(s, _)| s == start) {
                return Err(FormatError::NestingDepthExceeded);
            }
            self.inline[self.len] = (start, len);
        } else {
            self.add_spilled(start, len)?;
        }
        self.len += 1;
        if self.total > self.budget {
            return Err(FormatError::InvalidObjectHeader(
                "object header chunks larger than the file",
            ));
        }
        Ok(())
    }

    #[cold]
    #[inline(never)]
    fn add_spilled(&mut self, start: u64, len: usize) -> Result<(), FormatError> {
        let inline = &self.inline;
        let spill = self.spill.get_or_insert_with(|| {
            Box::new(SpilledSpans {
                chunks: Vec::new(),
                starts: inline.iter().map(|&(s, _)| s).collect(),
            })
        });
        if !spill.starts.insert(start) || self.len >= MAX_V1_CHUNKS {
            return Err(FormatError::NestingDepthExceeded);
        }
        spill.chunks.push((start, len));
        Ok(())
    }

    /// The `i`th chunk recorded.
    #[inline]
    fn get(&self, i: usize) -> Option<(u64, usize)> {
        if i < INLINE_CHUNKS {
            (i < self.len).then(|| self.inline[i])
        } else {
            self.spill.as_ref()?.chunks.get(i - INLINE_CHUNKS).copied()
        }
    }
}

/// Most chunks a version-1 object header may have (malformed-data guard;
/// libhdf5 has no limit, and a header that gains one continuation chunk per
/// attribute added can have many).
const MAX_V1_CHUNKS: usize = 1 << 16;

/// Every defined version-2 object header status flag (libhdf5
/// `H5O_HDR_ALL_FLAGS`): chunk-0 size width (bits 0-1), attribute creation
/// order tracked/indexed, attribute phase-change values, times stored.
const V2_HDR_ALL_FLAGS: u8 = 0x3F;

// Header message flag bits (libhdf5 `H5O_MSG_FLAG_*`). Bit 0 (constant) needs
// no check. Bit 3 (fail if unknown and the file is opened for writing) never
// fails a read: the parser only ever reads, as libhdf5 ignores it for a
// read-only open.
const MSG_FLAG_SHARED: u8 = 0x02;
const MSG_FLAG_DONTSHARE: u8 = 0x04;
const MSG_FLAG_FAIL_IF_UNKNOWN_AND_OPEN_FOR_WRITE: u8 = 0x08;
const MSG_FLAG_MARK_IF_UNKNOWN: u8 = 0x10;
const MSG_FLAG_WAS_UNKNOWN: u8 = 0x20;
const MSG_FLAG_SHAREABLE: u8 = 0x40;
/// Fail if the message is unknown, whatever the access mode.
const MSG_FLAG_FAIL_IF_UNKNOWN_ALWAYS: u8 = 0x80;

/// Message type ids libhdf5 has a class for (`H5O_msg_class_g`): 0x00-0x18
/// except 0x09 (a test-only "bogus" message). Anything else is an unknown
/// message.
fn is_known_message(id: u16) -> bool {
    id <= 0x18 && id != 0x09
}

/// Message classes that may be shared (`H5O_SHARE_IS_SHARABLE`): dataspace,
/// datatype, the two fill-value messages, filter pipeline and attribute.
fn is_shareable_message(id: u16) -> bool {
    matches!(id, 0x01 | 0x03 | 0x04 | 0x05 | 0x0B | 0x0C)
}

/// Check one header message the way libhdf5 does while it loads an object
/// header (`H5O__chunk_deserialize`), so an object libhdf5 refuses to open is
/// refused here too instead of being read from a corrupt header:
///
/// - contradictory flag combinations;
/// - an unknown message the file says no reader may skip (bit 7). This had
///   bits 3 and 7 the wrong way round once, failing objects libhdf5 reads
///   and reading ones it refuses (`tbogus.h5`);
/// - a known message whose class cannot be shared, flagged shared or
///   shareable (`cve-2016-4332`);
/// - the messages libhdf5 decodes while loading the header, whose decode
///   errors fail the load: continuation, reference count (which a version-1
///   header cannot hold), and both modification-time messages.
fn check_message(
    header_version: u8,
    id: u16,
    flags: u8,
    body: &[u8],
    offset_size: u8,
    length_size: u8,
) -> Result<(), FormatError> {
    let bad_flags = FormatError::InvalidObjectHeader("bad flag combination for message");
    if flags & MSG_FLAG_SHARED != 0 && flags & MSG_FLAG_DONTSHARE != 0 {
        return Err(bad_flags);
    }
    if flags & MSG_FLAG_WAS_UNKNOWN != 0
        && (flags & MSG_FLAG_FAIL_IF_UNKNOWN_AND_OPEN_FOR_WRITE != 0
            || flags & MSG_FLAG_MARK_IF_UNKNOWN == 0)
    {
        return Err(bad_flags);
    }

    if !is_known_message(id) {
        if flags & MSG_FLAG_FAIL_IF_UNKNOWN_ALWAYS != 0 {
            return Err(FormatError::UnsupportedMessage(id));
        }
        return Ok(());
    }
    if flags & (MSG_FLAG_SHARED | MSG_FLAG_SHAREABLE) != 0 && !is_shareable_message(id) {
        return Err(FormatError::InvalidObjectHeader(
            "message of unshareable class flagged as shareable",
        ));
    }

    let overrun = FormatError::InvalidObjectHeader("ran off end of input buffer while decoding");
    match id {
        // Continuation: address + length, and the chunk cannot be empty.
        0x10 => {
            if body.len() < offset_size as usize + length_size as usize {
                return Err(overrun);
            }
            if read_offset(body, offset_size as usize, length_size)? == 0 {
                return Err(FormatError::InvalidObjectHeader(
                    "invalid continuation chunk size (0)",
                ));
            }
        }
        // Reference count: version-2 headers only; version 0 then a u32.
        0x16 => {
            if header_version == 1 {
                return Err(FormatError::InvalidObjectHeader(
                    "object header version does not support reference count message",
                ));
            }
            match body.first() {
                None => return Err(overrun),
                Some(0) => {}
                Some(_) => {
                    return Err(FormatError::InvalidObjectHeader(
                        "bad version number for reference count message",
                    ));
                }
            }
            if body.len() < 5 {
                return Err(overrun);
            }
        }
        // Old modification time: "YYYYMMDDhhmmss" and 2 reserved bytes.
        0x0E => {
            if body.len() < 16 {
                return Err(overrun);
            }
            if !body[..14].iter().all(u8::is_ascii_digit) {
                return Err(FormatError::InvalidObjectHeader(
                    "badly formatted modification time message",
                ));
            }
        }
        // New modification time: version 1, 3 reserved bytes, u32 seconds.
        0x12 => {
            match body.first() {
                None => return Err(overrun),
                Some(1) => {}
                Some(_) => {
                    return Err(FormatError::InvalidObjectHeader(
                        "bad version number for mtime message",
                    ));
                }
            }
            if body.len() < 8 {
                return Err(overrun);
            }
        }
        _ => {}
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;

    fn header_with(types: &[MessageType]) -> ObjectHeader {
        ObjectHeader {
            version: 2,
            messages: types
                .iter()
                .map(|&msg_type| HeaderMessage {
                    msg_type,
                    size: 0,
                    flags: 0,
                    creation_order: None,
                    data: Vec::new(),
                })
                .collect(),
            reference_count: None,
            flags: 0,
            access_time: None,
            modification_time: None,
            change_time: None,
            birth_time: None,
        }
    }

    #[test]
    fn object_class_follows_libhdf5() {
        use MessageType::*;
        let class = |t: &[MessageType]| header_with(t).object_class();
        assert_eq!(
            class(&[Datatype, Dataspace, DataLayout]),
            Some(ObjectClass::Dataset)
        );
        // A Data Layout message does not make a dataset without a dataspace
        // (cve-2024-33874 `/Dset1`: h5py opens it as a named datatype).
        assert_eq!(
            class(&[Datatype, DataLayout]),
            Some(ObjectClass::NamedDatatype)
        );
        assert_eq!(class(&[Datatype]), Some(ObjectClass::NamedDatatype));
        // Group messages win over dataset messages.
        assert_eq!(
            class(&[Datatype, Dataspace, SymbolTable]),
            Some(ObjectClass::Group)
        );
        assert_eq!(class(&[LinkInfo]), Some(ObjectClass::Group));
        // Link messages alone are not a group; nothing is not an object.
        assert_eq!(class(&[Link]), None);
        assert_eq!(class(&[]), None);
    }

    // Helper: build a v1 object header with given messages
    fn build_v1_header(
        messages: &[(u16, &[u8], u8)], // (type, data, flags)
        offset_size: u8,
        length_size: u8,
    ) -> Vec<u8> {
        let _ = (offset_size, length_size);
        // Calculate total header message data size
        let mut msg_bytes = Vec::new();
        for (mtype, mdata, mflags) in messages {
            // v1 message sizes are multiples of 8 (the data is zero-padded).
            let padded = <[u8]>::len(mdata).div_ceil(8) * 8;
            msg_bytes.extend_from_slice(&mtype.to_le_bytes()); // type(2)
            msg_bytes.extend_from_slice(&(padded as u16).to_le_bytes()); // size(2)
            msg_bytes.push(*mflags); // flags(1)
            msg_bytes.extend_from_slice(&[0u8; 3]); // reserved(3)
            msg_bytes.extend_from_slice(mdata); // data
            msg_bytes.resize(msg_bytes.len() + padded - <[u8]>::len(mdata), 0);
        }

        let mut buf = Vec::new();
        buf.push(1); // version
        buf.push(0); // reserved
        buf.extend_from_slice(&(messages.len() as u16).to_le_bytes()); // num_messages
        buf.extend_from_slice(&1u32.to_le_bytes()); // reference_count
        buf.extend_from_slice(&(msg_bytes.len() as u32).to_le_bytes()); // header_data_size
        // Pad to 8-byte alignment (12 bytes so far, pad 4)
        buf.extend_from_slice(&[0u8; 4]);
        buf.extend_from_slice(&msg_bytes);
        buf
    }

    // Helper: build a v2 object header chunk0 with given messages
    fn build_v2_header(
        flags: u8,
        messages: &[(u8, &[u8], u8)], // (type, data, msg_flags)
        timestamps: Option<(u32, u32, u32, u32)>,
    ) -> Vec<u8> {
        let has_creation_order = flags & 0x04 != 0;
        let has_timestamps = flags & 0x20 != 0;
        let mut buf = Vec::new();
        buf.extend_from_slice(&OHDR_SIGNATURE); // 4
        buf.push(2); // version
        buf.push(flags);

        if has_timestamps && let Some((at, mt, ct, bt)) = timestamps {
            buf.extend_from_slice(&at.to_le_bytes());
            buf.extend_from_slice(&mt.to_le_bytes());
            buf.extend_from_slice(&ct.to_le_bytes());
            buf.extend_from_slice(&bt.to_le_bytes());
        }

        if flags & 0x10 != 0 {
            buf.extend_from_slice(&8u16.to_le_bytes()); // max_compact
            buf.extend_from_slice(&6u16.to_le_bytes()); // min_dense
        }

        // Build message bytes to get chunk size
        let mut msg_bytes = Vec::new();
        for (mtype, mdata, mflags) in messages {
            msg_bytes.push(*mtype); // type(1)
            msg_bytes.extend_from_slice(&(mdata.len() as u16).to_le_bytes()); // size(2)
            msg_bytes.push(*mflags); // flags(1)
            if has_creation_order {
                msg_bytes.extend_from_slice(&0u16.to_le_bytes()); // creation_order(2)
            }
            msg_bytes.extend_from_slice(mdata);
        }

        let chunk_size = msg_bytes.len();
        // Write chunk size based on flags bits 0-1
        match flags & 0x03 {
            0 => buf.push(chunk_size as u8),
            1 => buf.extend_from_slice(&(chunk_size as u16).to_le_bytes()),
            2 => buf.extend_from_slice(&(chunk_size as u32).to_le_bytes()),
            3 => buf.extend_from_slice(&(chunk_size as u64).to_le_bytes()),
            _ => unreachable!(),
        }

        buf.extend_from_slice(&msg_bytes);

        // Checksum (CRC32C of everything from OHDR to here)
        let checksum = crate::checksum::jenkins_lookup3(&buf);
        buf.extend_from_slice(&checksum.to_le_bytes());
        buf
    }

    #[test]
    fn parse_v1_zero_messages() {
        let data = build_v1_header(&[], 8, 8);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.version, 1);
        assert_eq!(hdr.messages.len(), 0);
        assert_eq!(hdr.reference_count, Some(1));
        assert_eq!(hdr.flags, 0);
    }

    #[test]
    fn parse_v1_two_messages() {
        let messages = [
            (0x0001u16, &[1u8, 2, 3, 4][..], 0u8), // Dataspace
            (0x0008, &[5u8, 6][..], 0),            // DataLayout
        ];
        let data = build_v1_header(&messages, 8, 8);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 2);
        assert_eq!(hdr.messages[0].msg_type, MessageType::Dataspace);
        // v1 message data is padded to a multiple of 8 bytes.
        assert_eq!(hdr.messages[0].data, vec![1, 2, 3, 4, 0, 0, 0, 0]);
        assert_eq!(hdr.messages[1].msg_type, MessageType::DataLayout);
        assert_eq!(hdr.messages[1].data[..2], [5, 6]);
    }

    /// A version-1 header whose continuation chunks form a chain: chunk k
    /// holds a Dataspace message `[k]` and the continuation to chunk k + 1.
    /// With `cycle`, the last chunk points back at the first continuation
    /// chunk.
    fn v1_chain(n: usize, cycle: bool) -> Vec<u8> {
        // Each continuation chunk: dataspace (8 + 8) + continuation (8 + 16).
        let chunk_len = 40u64;
        let first = 64u64;
        let cont = |addr: u64| {
            let mut b = addr.to_le_bytes().to_vec();
            b.extend_from_slice(&chunk_len.to_le_bytes());
            b
        };
        let mut data = build_v1_header(&[(0x0010, &cont(first)[..], 0)], 8, 8);
        data.resize(first as usize, 0);
        for k in 0..n {
            let mut c = Vec::new();
            c.extend_from_slice(&1u16.to_le_bytes());
            c.extend_from_slice(&8u16.to_le_bytes());
            c.extend_from_slice(&[0; 4]);
            c.extend_from_slice(&(k as u64).to_le_bytes());
            let next = if k + 1 < n {
                first + (k as u64 + 1) * chunk_len
            } else if cycle {
                first
            } else {
                // The last chunk ends in a NIL message instead.
                c.extend_from_slice(&[0, 0, 16, 0, 0, 0, 0, 0]);
                c.extend_from_slice(&[0; 16]);
                data.extend_from_slice(&c);
                continue;
            };
            c.extend_from_slice(&0x10u16.to_le_bytes());
            c.extend_from_slice(&16u16.to_le_bytes());
            c.extend_from_slice(&[0; 4]);
            c.extend_from_slice(&cont(next));
            data.extend_from_slice(&c);
        }
        data
    }

    /// libhdf5 reads any chain of continuation chunks (a header grows one
    /// per attribute added when full); the reader used to stop at 32.
    #[test]
    fn long_v1_continuation_chains_are_read() {
        let data = v1_chain(200, false);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        let spaces: Vec<u8> = hdr
            .messages
            .iter()
            .filter(|m| m.msg_type == MessageType::Dataspace)
            .map(|m| m.data[0])
            .collect();
        assert_eq!(spaces, (0..200).map(|k| k as u8).collect::<Vec<_>>());
    }

    /// A crafted version-1 header whose continuation chunks nest: each
    /// chunk's continuation message points at the rest of that chunk. Read
    /// depth-first with every enclosing chunk kept alive, from storage that
    /// hands out owned buffers, it read n^2 bytes and held them all at once
    /// (a 192 KB file read 768 MB). Chunks adding up to more than the file
    /// are refused, and the bytes read stay within the file's size.
    #[test]
    fn nested_v1_continuation_chunks_are_bounded() {
        use crate::storage::CountingStorage;
        let n = 2000u64;
        let a = 64u64;
        let cont = |addr: u64, len: u64| {
            let mut m = vec![0x10, 0, 16, 0, 0, 0, 0, 0];
            m.extend_from_slice(&addr.to_le_bytes());
            m.extend_from_slice(&len.to_le_bytes());
            m
        };
        // Prefix: version 1, one message, reference count 1, 24 bytes.
        let mut buf = vec![1, 0, 1, 0, 1, 0, 0, 0, 24, 0, 0, 0, 0, 0, 0, 0];
        buf.extend_from_slice(&cont(a, 24 * n));
        buf.resize(a as usize, 0);
        for k in 0..n {
            if k + 1 < n {
                buf.extend_from_slice(&cont(a + 24 * (k + 1), 24 * (n - k - 1)));
            } else {
                buf.extend_from_slice(&[0, 0, 16, 0, 0, 0, 0, 0]);
                buf.extend_from_slice(&[0; 16]);
            }
        }
        let len = buf.len() as u64;
        let s = CountingStorage::new(buf);
        assert!(matches!(
            ObjectHeader::parse_in(&s, 0, 8, 8),
            Err(FormatError::InvalidObjectHeader(
                "object header chunks larger than the file"
            ))
        ));
        assert!(
            s.bytes_read() <= 2 * len,
            "read {} of {len}",
            s.bytes_read()
        );
    }

    /// libhdf5 reads a continuation chunk that overlaps the chunk holding
    /// its message (`cve-2025-7067.h5` has one), and so does this reader.
    #[test]
    fn overlapping_v1_continuation_chunk_is_read() {
        // Chunk 0 (at 16): continuation (24 bytes), then a NIL message at
        // 40; the continuation chunk is that NIL message's 8-byte header.
        let mut cont = 40u64.to_le_bytes().to_vec();
        cont.extend_from_slice(&8u64.to_le_bytes());
        let data = build_v1_header(&[(0x0010, &cont[..], 0), (0x0000, &[][..], 0)], 8, 8);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
    }

    /// A valid chain over owned-buffer storage reads each chunk once.
    #[test]
    fn long_v1_chain_reads_each_chunk_once() {
        use crate::storage::CountingStorage;
        let data = v1_chain(3000, false);
        let len = data.len() as u64;
        let s = CountingStorage::new(data);
        let hdr = ObjectHeader::parse_in(&s, 0, 8, 8).unwrap();
        assert_eq!(
            hdr.messages
                .iter()
                .filter(|m| m.msg_type == MessageType::Dataspace)
                .count(),
            3000
        );
        assert!(s.bytes_read() <= len, "read {} of {len}", s.bytes_read());
    }

    /// Continuation chunks are read in the order their messages are found
    /// (libhdf5's `H5O_protect`), so a chunk's messages follow every
    /// message of the chunk before, not the continuation message.
    #[test]
    fn v1_continuation_messages_keep_libhdf5_order() {
        // Chunk 0: continuation to A, dataspace [1]; A: dataspace [2].
        let a = 64u64;
        let mut cont = a.to_le_bytes().to_vec();
        cont.extend_from_slice(&16u64.to_le_bytes());
        let mut data = build_v1_header(&[(0x0010, &cont[..], 0), (0x0001, &[1; 8][..], 0)], 8, 8);
        data.resize(a as usize, 0);
        data.extend_from_slice(&[1, 0, 8, 0, 0, 0, 0, 0]);
        data.extend_from_slice(&[2; 8]);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        let spaces: Vec<u8> = hdr
            .messages
            .iter()
            .filter(|m| m.msg_type == MessageType::Dataspace)
            .map(|m| m.data[0])
            .collect();
        assert_eq!(spaces, [1, 2]);
    }

    #[test]
    fn v1_continuation_cycles_are_refused() {
        // Within the inline chunk list, and past it (the cycle returns to
        // an inline chunk once the list has spilled).
        for n in [5, 7, 8, 9, 40] {
            let data = v1_chain(n, true);
            assert!(
                matches!(
                    ObjectHeader::parse(&data, 0, 8, 8),
                    Err(FormatError::NestingDepthExceeded)
                ),
                "{n} chunks"
            );
        }
    }

    #[test]
    fn parse_v1_unknown_message_ok() {
        let messages = [(0x00FFu16, &[0xAA, 0xBB][..], 0u8)];
        let data = build_v1_header(&messages, 8, 8);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
        assert_eq!(hdr.messages[0].msg_type, MessageType::Unknown(0x00FF));
    }

    #[test]
    fn parse_v1_unknown_fail_always_errors() {
        // Bit 7 of msg_flags = fail if unknown, whatever the access mode.
        let messages = [(0x00FFu16, &[0xAA][..], 0x80u8)];
        let data = build_v1_header(&messages, 8, 8);
        let err = ObjectHeader::parse(&data, 0, 8, 8).unwrap_err();
        assert_eq!(err, FormatError::UnsupportedMessage(0x00FF));
    }

    #[test]
    fn parse_v1_unknown_fail_on_write_is_ignored_when_reading() {
        // Bit 3 = fail if unknown *and the file is opened for writing*. This
        // parser only reads, so libhdf5 (read-only) opens such an object and
        // so must we. Bits 4/5 (mark if unknown / was unknown) never fail.
        for flags in [0x08u8, 0x10, 0x30] {
            let messages = [(0x00FFu16, &[0xAA][..], flags)];
            let data = build_v1_header(&messages, 8, 8);
            let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
            assert_eq!(hdr.messages[0].msg_type, MessageType::Unknown(0x00FF));
        }
    }

    #[test]
    fn contradictory_message_flags_are_refused() {
        // libhdf5: "bad flag combination for message" for shared + don't
        // share, was-unknown without mark-if-unknown, and was-unknown with
        // fail-if-unknown-on-write.
        for flags in [0x06u8, 0x20, 0x38] {
            let data = build_v1_header(&[(0x00FFu16, &[0xAA][..], flags)], 8, 8);
            assert_eq!(
                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
                FormatError::InvalidObjectHeader("bad flag combination for message"),
                "flags {flags:#x}"
            );
            let data = build_v2_header(0x00, &[(0xF0, &[1, 2], flags)], None);
            assert_eq!(
                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
                FormatError::InvalidObjectHeader("bad flag combination for message"),
                "flags {flags:#x}"
            );
        }
    }

    #[test]
    fn unshareable_message_flagged_shareable_is_refused() {
        // A layout (0x08) or modification time (0x12) message cannot be
        // shared; bit 1 (shared) or bit 6 (shareable) on one is corruption
        // (cve-2016-4332). A datatype (0x03) may be shareable.
        let mtime = [1u8, 0, 0, 0, 0x10, 0x20, 0x30, 0x40];
        for (id, flags) in [(0x08u16, 0x40u8), (0x08, 0x02), (0x12, 0x40)] {
            let data = build_v1_header(&[(id, &mtime[..], flags)], 8, 8);
            assert_eq!(
                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
                FormatError::InvalidObjectHeader(
                    "message of unshareable class flagged as shareable"
                ),
                "id {id:#x} flags {flags:#x}"
            );
        }
        let data = build_v1_header(&[(0x03, &[0u8; 8][..], 0x40)], 8, 8);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
        // An unknown message is never checked for shareability.
        let data = build_v1_header(&[(0x00FF, &[0u8; 8][..], 0x40)], 8, 8);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
    }

    #[test]
    fn v1_message_must_be_aligned() {
        // cve-2018-13873: a v1 message whose size is not a multiple of 8.
        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
        data[16 + 2] = 7; // size field of the only message
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("message not aligned")
        );
    }

    #[test]
    fn message_overrunning_its_chunk_is_refused() {
        // It used to end the chunk quietly, dropping this message and any
        // after it.
        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
        data[16 + 2] = 16;
        data.resize(data.len() + 64, 0);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("message size exceeds buffer end")
        );
        let mut data = build_v2_header(0x00, &[(0x01, &[1, 2], 0)], None);
        data[7 + 1] = 9; // size of the only message (after OHDR, ver, flags, chunk size)
        let chk = crate::checksum::jenkins_lookup3(&data[..data.len() - 4]);
        let n = data.len();
        data[n - 4..].copy_from_slice(&chk.to_le_bytes());
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("message size exceeds buffer end")
        );
    }

    #[test]
    fn v1_gap_after_last_message_is_refused() {
        // Fewer than 8 bytes left over: a gap, which only version 2 allows.
        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
        data[8] += 4; // header_data_size
        data.extend_from_slice(&[0u8; 4]);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("gap found in early version of file format")
        );
    }

    #[test]
    fn v1_chunk_holding_more_messages_than_the_prefix_says_is_refused() {
        // cve-2024-32619: the prefix says 1 message, the chunk holds 2. The
        // second used to be dropped silently.
        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0), (0x03, &[0u8; 8][..], 0)], 8, 8);
        data[2] = 1;
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("bad object header message count")
        );
        // Fewer in the chunk than the prefix says is fine (the rest may be in
        // continuation chunks; libhdf5 only enforces that with strict checks).
        data[2] = 3;
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap().messages.len(),
            2
        );
    }

    #[test]
    fn v1_prefix_chunk_size_must_fit_the_message_count() {
        let mut data = build_v1_header(&[], 8, 8);
        data[8] = 8; // no messages but a non-empty chunk
        data.extend_from_slice(&[0u8; 8]);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("bad object header chunk size")
        );
    }

    #[test]
    fn v1_header_cannot_hold_a_reference_count_message() {
        // cve-2018-11204.
        let data = build_v1_header(&[(0x16, &[0, 2, 0, 0, 0][..], 0)], 8, 8);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader(
                "object header version does not support reference count message"
            )
        );
        let data = build_v2_header(0x00, &[(0x16, &[0, 2, 0, 0, 0], 0)], None);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
    }

    #[test]
    fn modification_time_messages_are_decoded_with_the_header() {
        // cve-2024-33873 (version 0) and cve-2024-33874 (empty message).
        for (body, why) in [
            (
                &[0u8, 0, 0, 0, 1, 2, 3, 4][..],
                "bad version number for mtime message",
            ),
            (&[][..], "ran off end of input buffer while decoding"),
        ] {
            let data = build_v2_header(0x00, &[(0x12, body, 0)], None);
            assert_eq!(
                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
                FormatError::InvalidObjectHeader(why)
            );
        }
        let data = build_v2_header(0x00, &[(0x12, &[1, 0, 0, 0, 1, 2, 3, 4], 0)], None);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
        // The old (0x0E) message is 14 ASCII digits and 2 reserved bytes.
        let data = build_v1_header(&[(0x0E, &b"20110414214255\0\0"[..], 0)], 8, 8);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
        let data = build_v1_header(&[(0x0E, &b"2011041421425x\0\0"[..], 0)], 8, 8);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("badly formatted modification time message")
        );
    }

    #[test]
    fn continuation_message_must_hold_a_nonempty_chunk() {
        let mut cont = [0u8; 16];
        cont[..8].copy_from_slice(&64u64.to_le_bytes());
        let data = build_v2_header(0x00, &[(0x10, &cont, 0)], None);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("invalid continuation chunk size (0)")
        );
        let data = build_v2_header(0x00, &[(0x10, &cont[..8], 0)], None);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("ran off end of input buffer while decoding")
        );
    }

    #[test]
    fn v2_prefix_is_checked() {
        let data = build_v2_header(0x40, &[(0x01, &[1], 0)], None);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("unknown object header status flag(s)")
        );
        // build_v2_header writes max_compact 8, min_dense 6; swap them.
        let mut data = build_v2_header(0x10, &[(0x01, &[1], 0)], None);
        data[6] = 6;
        data[8] = 8;
        let chk = crate::checksum::jenkins_lookup3(&data[..data.len() - 4]);
        let n = data.len();
        data[n - 4..].copy_from_slice(&chk.to_le_bytes());
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("bad object header attribute phase change values")
        );
    }

    #[test]
    fn v2_gap_is_allowed_only_without_nil_messages() {
        // Three bytes after the last message: a gap (a message header is 4).
        let mut data = build_v2_header(0x00, &[(0x01, &[1, 2, 3], 0), (0x03, &[], 0)], None);
        // Turn the empty datatype message (4 header bytes) into a 3-byte gap
        // by shrinking the chunk.
        let n = data.len();
        data.truncate(n - 5);
        data[6] -= 1;
        let chk = crate::checksum::jenkins_lookup3(&data);
        data.extend_from_slice(&chk.to_le_bytes());
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap().messages.len(),
            1
        );

        let mut data = build_v2_header(0x00, &[(0x00, &[1, 2, 3], 0), (0x03, &[], 0)], None);
        let n = data.len();
        data.truncate(n - 5);
        data[6] -= 1;
        let chk = crate::checksum::jenkins_lookup3(&data);
        data.extend_from_slice(&chk.to_le_bytes());
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::InvalidObjectHeader("gap in chunk with no null messages")
        );
    }

    #[test]
    fn parse_v2_unknown_message_flags() {
        let data = build_v2_header(0x00, &[(0xF0, &[1, 2], 0x08)], None);
        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
        let data = build_v2_header(0x00, &[(0xF0, &[1, 2], 0x80)], None);
        assert_eq!(
            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
            FormatError::UnsupportedMessage(0xF0)
        );
    }

    #[test]
    fn parse_v2_no_timestamps_one_message() {
        let data = build_v2_header(0x00, &[(0x01, &[10, 20], 0)], None);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.version, 2);
        assert_eq!(hdr.flags, 0);
        assert_eq!(hdr.messages.len(), 1);
        assert_eq!(hdr.messages[0].msg_type, MessageType::Dataspace);
        assert_eq!(hdr.messages[0].data, vec![10, 20]);
        assert!(hdr.access_time.is_none());
    }

    #[test]
    fn parse_v2_with_timestamps() {
        let data = build_v2_header(0x20, &[(0x01, &[1], 0)], Some((100, 200, 300, 400)));
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.access_time, Some(100));
        assert_eq!(hdr.modification_time, Some(200));
        assert_eq!(hdr.change_time, Some(300));
        assert_eq!(hdr.birth_time, Some(400));
        assert_eq!(hdr.messages.len(), 1);
        // flags bit 5 = timestamps, but bit 2 not set → no creation order in messages
        assert!(hdr.messages[0].creation_order.is_none());
    }

    #[test]
    fn parse_v2_creation_order() {
        // flags bit 2 enables attribute/message creation order tracking
        // flags bit 5 enables timestamps
        // Use 0x24 = bit 2 + bit 5
        let data = build_v2_header(
            0x24,
            &[(0x03, &[9], 0), (0x05, &[8], 0)],
            Some((0, 0, 0, 0)),
        );
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 2);
        assert!(hdr.messages[0].creation_order.is_some());
        assert!(hdr.messages[1].creation_order.is_some());
        assert_eq!(hdr.access_time, Some(0));
    }

    #[test]
    fn parse_v2_checksum_valid() {
        let data = build_v2_header(0x00, &[(0x01, &[1, 2, 3], 0)], None);
        // Should succeed — checksum is valid
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
    }

    #[test]
    fn parse_v2_checksum_invalid() {
        let mut data = build_v2_header(0x00, &[(0x01, &[1, 2, 3], 0)], None);
        // Corrupt checksum
        let len = data.len();
        data[len - 1] ^= 0xFF;
        let err = ObjectHeader::parse(&data, 0, 8, 8).unwrap_err();
        assert!(matches!(err, FormatError::ChecksumMismatch { .. }));
    }

    #[test]
    fn parse_v2_nil_padding_skipped() {
        let data = build_v2_header(
            0x00,
            &[
                (0x00, &[0, 0, 0, 0], 0), // NIL
                (0x01, &[42], 0),         // Dataspace
            ],
            None,
        );
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
        assert_eq!(hdr.messages[0].msg_type, MessageType::Dataspace);
    }

    #[test]
    fn parse_v2_chunk_size_1byte() {
        // flags bits 0-1 = 0 → 1-byte chunk size
        let data = build_v2_header(0x00, &[(0x01, &[1], 0)], None);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
    }

    #[test]
    fn parse_v2_chunk_size_2byte() {
        let data = build_v2_header(0x01, &[(0x01, &[1], 0)], None);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
    }

    #[test]
    fn parse_v2_chunk_size_4byte() {
        let data = build_v2_header(0x02, &[(0x01, &[1], 0)], None);
        let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 1);
    }

    #[test]
    fn parse_v2_continuation() {
        // Build a continuation chunk (OCHK) at a known offset
        let ochk_offset = 256usize;
        let ochk_msg_type = 0x03u8; // Datatype
        let ochk_msg_data = [0xDE, 0xAD];

        // Build the OCHK chunk
        let mut ochk_buf = Vec::new();
        ochk_buf.extend_from_slice(&OCHK_SIGNATURE);
        ochk_buf.push(ochk_msg_type);
        ochk_buf.extend_from_slice(&(ochk_msg_data.len() as u16).to_le_bytes());
        ochk_buf.push(0); // msg flags
        ochk_buf.extend_from_slice(&ochk_msg_data);
        let checksum = crate::checksum::jenkins_lookup3(&ochk_buf);
        ochk_buf.extend_from_slice(&checksum.to_le_bytes());

        let ochk_length = ochk_buf.len();

        // Build continuation message data: offset(8 LE) + length(8 LE)
        let mut cont_data = Vec::new();
        cont_data.extend_from_slice(&(ochk_offset as u64).to_le_bytes());
        cont_data.extend_from_slice(&(ochk_length as u64).to_le_bytes());

        // Build main header with continuation message + a regular message
        let header = build_v2_header(
            0x00,
            &[
                (0x01, &[42], 0),      // Dataspace
                (0x10, &cont_data, 0), // Continuation
            ],
            None,
        );

        // Assemble full "file"
        let total_size = ochk_offset + ochk_buf.len();
        let mut file_data = vec![0u8; total_size];
        file_data[..header.len()].copy_from_slice(&header);
        file_data[ochk_offset..ochk_offset + ochk_buf.len()].copy_from_slice(&ochk_buf);

        let hdr = ObjectHeader::parse(&file_data, 0, 8, 8).unwrap();
        assert_eq!(hdr.messages.len(), 2);
        assert_eq!(hdr.messages[0].msg_type, MessageType::Dataspace);
        assert_eq!(hdr.messages[1].msg_type, MessageType::Datatype);
        assert_eq!(hdr.messages[1].data, vec![0xDE, 0xAD]);
    }

    #[test]
    fn truncated_v1_header() {
        let data = vec![1u8, 0]; // version 1, but too short
        let err = ObjectHeader::parse(&data, 0, 8, 8).unwrap_err();
        assert!(matches!(err, FormatError::UnexpectedEof { .. }));
    }

    #[test]
    fn truncated_v2_header() {
        let data = [b'O', b'H', b'D', b'R', 2]; // signature + version, but no flags
        let err = ObjectHeader::parse(&data, 0, 8, 8).unwrap_err();
        assert!(matches!(err, FormatError::UnexpectedEof { .. }));
    }

    /// Every header, and every truncation of it, parses to the same result
    /// (or the same error) through a `read_at`-only storage as from a slice;
    /// a header in one chunk takes two reads (prefix, chunk).
    #[test]
    fn parse_in_matches_slice_parse() {
        use crate::storage::CountingStorage;
        let mut headers = vec![
            build_v1_header(&[], 8, 8),
            build_v1_header(&[(0x0001, &[1, 2, 3], 0), (0x0003, &[9; 8], 0)], 8, 8),
            build_v2_header(0x00, &[(0x01, &[42], 0)], None),
            build_v2_header(0x03, &[(0x01, &[1, 2], 0), (0x03, &[3], 0)], None),
            build_v2_header(0x24, &[(0x01, &[1], 0)], Some((1, 2, 3, 4))),
            build_v2_header(0x35, &[(0x01, &[1], 0)], Some((5, 6, 7, 8))),
        ];
        // A v2 header with a continuation chunk at 256.
        let mut ochk = OCHK_SIGNATURE.to_vec();
        ochk.extend_from_slice(&[0x03, 2, 0, 0, 0xDE, 0xAD]);
        let sum = crate::checksum::jenkins_lookup3(&ochk);
        ochk.extend_from_slice(&sum.to_le_bytes());
        let mut cont = 256u64.to_le_bytes().to_vec();
        cont.extend_from_slice(&(ochk.len() as u64).to_le_bytes());
        let main = build_v2_header(0x00, &[(0x01, &[42], 0), (0x10, &cont, 0)], None);
        let mut with_cont = vec![0u8; 256 + ochk.len()];
        with_cont[..main.len()].copy_from_slice(&main);
        with_cont[256..].copy_from_slice(&ochk);
        headers.push(with_cont);

        for h in headers {
            for at in [0usize, 3] {
                for cut in 0..=h.len() {
                    let mut f = vec![0u8; at];
                    f.extend_from_slice(&h[..cut]);
                    if at == 0 && cut == h.len() {
                        f.resize(f.len() + 64, 0);
                    }
                    let want = ObjectHeader::parse(&f, at, 8, 8);
                    let storage = CountingStorage::new(f.clone());
                    let got = ObjectHeader::parse_in(&storage, at as u64, 8, 8);
                    assert_eq!(
                        format!("{got:?}"),
                        format!("{want:?}"),
                        "at {at}, cut {cut}"
                    );
                }
            }
        }
        let one_chunk = build_v2_header(0x00, &[(0x01, &[42], 0)], None);
        let storage = CountingStorage::new(one_chunk);
        ObjectHeader::parse_in(&storage, 0, 8, 8).unwrap();
        assert_eq!(storage.reads(), 2);
    }
}