rust_hdf5/format/messages/

dataspace.rs

//! Dataspace message (type 0x01) — describes dataset dimensionality.
//!
//! Binary layout (version 2):
//!   Byte 0: version = 2
//!   Byte 1: dimensionality (ndims, 0–32)
//!   Byte 2: flags (bit 0 = max dims present)
//!   Byte 3: type (0 = scalar, 1 = simple, 2 = null)
//!   Then ndims * sizeof_size bytes for current dimensions
//!   Then (if flag bit 0) ndims * sizeof_size bytes for max dimensions

use crate::format::bytes::read_le_uint as read_size;
use crate::format::{FormatContext, FormatError, FormatResult};

const VERSION: u8 = 2;
const FLAG_MAX_DIMS: u8 = 0x01;

/// Dataspace type field values.
const DS_TYPE_SCALAR: u8 = 0;
const DS_TYPE_SIMPLE: u8 = 1;
const DS_TYPE_NULL: u8 = 2;

/// Which of the three dataspace classes a message describes (`H5Osdspace.c`'s
/// type byte / `H5S_class_t`).
///
/// `Scalar` (rank 0, exactly one element) and `Null` (no elements at all) both
/// carry an empty `dims`, so the class cannot be inferred from `dims` alone —
/// it must be tracked explicitly, matching upstream's dedicated type field.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DataspaceClass {
    /// Rank 0: a single element, no dimensions.
    Scalar,
    /// Rank >= 1: an ordinary N-dimensional dataspace.
    Simple,
    /// No elements at all (`H5Screate(H5S_NULL)`) — distinct from a scalar
    /// or from a `Simple` dataspace with a zero-length dimension.
    Null,
}

/// Dataspace message payload.
#[derive(Debug, Clone, PartialEq)]
pub struct DataspaceMessage {
    /// Which dataspace class this message describes.
    pub class: DataspaceClass,
    /// Current dimension sizes. Empty for `Scalar` and `Null`.
    pub dims: Vec<u64>,
    /// Maximum dimension sizes; an entry of `u64::MAX` means unlimited.
    ///
    /// `None` is upstream's `H5S_extent_t.max == NULL`: the extent carries no
    /// maximum array at all, which is a different thing from a maximum that
    /// happens to equal the current dimensions. Only decoding a message with
    /// `H5S_VALID_MAX` clear produces it (H5Osdspace.c:188-194) — every extent
    /// the API builds is given one (H5S.c:1293-1299) — and it survives being
    /// rewritten, because `H5S_read` and `H5S_write` decode and re-encode the
    /// one extent (H5S.c:1100, H5S.c:1039) and the encoder raises the flag
    /// only for a non-null array (H5Osdspace.c:271-272).
    ///
    /// Upstream reads the absent array as the current dimensions wherever a
    /// bound is reported (`H5S_extent_get_dims`, H5S.c:968-973), which is what
    /// the crate's consumers do too; `H5S_set_extent` is the exception that
    /// treats it as no bound at all (H5S.c:1777).
    pub max_dims: Option<Vec<u64>>,
}

impl DataspaceMessage {
    // ------------------------------------------------------------------ factories

    /// A scalar dataspace (rank 0, no max dims).
    pub fn scalar() -> Self {
        Self {
            class: DataspaceClass::Scalar,
            dims: Vec::new(),
            max_dims: None,
        }
    }

    /// The NULL dataspace: no elements at all. Unlike [`scalar`](Self::scalar),
    /// a dataset with this dataspace holds zero bytes of data.
    pub fn null() -> Self {
        Self {
            class: DataspaceClass::Null,
            dims: Vec::new(),
            max_dims: None,
        }
    }

    /// A simple dataspace with fixed dimensions (max == current). An empty
    /// `dims` yields a scalar dataspace, matching how upstream never emits a
    /// `Simple`-class dataspace at rank 0.
    ///
    /// The maximum is filled in here, not at encode time, because that is
    /// where upstream fills it in: `H5S_set_extent_simple` allocates
    /// `extent.max` for every simple extent and copies the current dimensions
    /// into it when the caller named no maximum (H5S.c:1293-1299).
    pub fn simple(dims: &[u64]) -> Self {
        let class = Self::class_for_rank(dims.len());
        Self {
            class,
            dims: dims.to_vec(),
            max_dims: match class {
                DataspaceClass::Simple => Some(dims.to_vec()),
                DataspaceClass::Scalar | DataspaceClass::Null => None,
            },
        }
    }

    /// A simple dataspace where every dimension is unlimited.
    pub fn unlimited(current: &[u64]) -> Self {
        Self {
            class: Self::class_for_rank(current.len()),
            dims: current.to_vec(),
            max_dims: Some(vec![u64::MAX; current.len()]),
        }
    }

    /// `Scalar` at rank 0 (matching upstream, which never writes a
    /// `Simple`-class dataspace with zero dimensions), `Simple` otherwise.
    fn class_for_rank(ndims: usize) -> DataspaceClass {
        if ndims == 0 {
            DataspaceClass::Scalar
        } else {
            DataspaceClass::Simple
        }
    }

    /// Whether this is the NULL dataspace (no elements at all).
    pub fn is_null(&self) -> bool {
        self.class == DataspaceClass::Null
    }

    /// The number of elements in the extent (`H5S_GET_EXTENT_NPOINTS`): none
    /// for a null dataspace, one for a scalar, the product of the dimensions
    /// otherwise. `None` when that product does not fit in a `u64`.
    pub fn element_count(&self) -> Option<u64> {
        match self.class {
            DataspaceClass::Null => Some(0),
            DataspaceClass::Scalar => Some(1),
            DataspaceClass::Simple => self.dims.iter().try_fold(1u64, |n, &d| n.checked_mul(d)),
        }
    }

    // ------------------------------------------------------------------ encode

    pub fn encode(&self, ctx: &FormatContext) -> Vec<u8> {
        self.encode_for(ctx, crate::format::ObjectFormat::Modern)
    }

    /// The message version libhdf5 would stamp on this dataspace in a file of
    /// this `format` — `H5S__set_version`: the floor from
    /// `H5O_sdspace_ver_bounds`, raised to 2 when the class needs it. Only the
    /// NULL class does: version 1 has no type field, so it cannot say "no
    /// elements" at all, and upstream `H5S_set_version` raises the same way.
    fn version_for(&self, format: crate::format::ObjectFormat) -> u8 {
        let needed = if self.class == DataspaceClass::Null {
            2
        } else {
            1
        };
        needed.max(format.dataspace_version())
    }

    /// Encode for a file of the given object format.
    ///
    /// A version-1 dataspace differs only in its prefix: no type byte, and
    /// five reserved bytes where version 2 has one, so the header is 8 bytes
    /// rather than 4. The dimensions that follow are identical.
    pub fn encode_for(&self, ctx: &FormatContext, format: crate::format::ObjectFormat) -> Vec<u8> {
        let version = self.version_for(format);
        let ndims = self.dims.len();
        let ss = ctx.sizeof_size as usize;
        // `H5O__sdspace_encode` raises `H5S_VALID_MAX` for a non-null maximum
        // array and for nothing else (H5Osdspace.c:271-272), so an extent that
        // reached us without one is written without one.
        let max_dims = self.max_dims.as_deref();
        let has_max = max_dims.is_some();
        let flags: u8 = if has_max { FLAG_MAX_DIMS } else { 0 };

        let ds_type = match self.class {
            DataspaceClass::Scalar => DS_TYPE_SCALAR,
            DataspaceClass::Simple => DS_TYPE_SIMPLE,
            DataspaceClass::Null => DS_TYPE_NULL,
        };

        let prefix_len = if version == 1 { 8 } else { 4 };
        let body_len = prefix_len + ndims * ss + if has_max { ndims * ss } else { 0 };
        let mut buf = Vec::with_capacity(body_len);

        buf.push(version);
        buf.push(ndims as u8);
        buf.push(flags);
        if version == 1 {
            // reserved byte, then a reserved word: version 1 has no type
            // field, and a rank-0 version-1 dataspace is scalar by definition.
            buf.extend_from_slice(&[0u8; 5]);
        } else {
            buf.push(ds_type);
        }

        // current dimensions
        for &d in &self.dims {
            buf.extend_from_slice(&d.to_le_bytes()[..ss]);
        }

        // max dimensions
        if let Some(maxes) = max_dims {
            for &m in maxes {
                buf.extend_from_slice(&m.to_le_bytes()[..ss]);
            }
        }

        buf
    }

    // ------------------------------------------------------------------ decode

    pub fn decode(buf: &[u8], ctx: &FormatContext) -> FormatResult<(Self, usize)> {
        if buf.len() < 4 {
            return Err(FormatError::BufferTooShort {
                needed: 4,
                available: buf.len(),
            });
        }

        let version = buf[0];
        match version {
            1 => Self::decode_v1(buf, ctx),
            VERSION => Self::decode_v2(buf, ctx),
            _ => Err(FormatError::InvalidVersion(version)),
        }
    }

    /// Decode version 2 dataspace message.
    fn decode_v2(buf: &[u8], ctx: &FormatContext) -> FormatResult<(Self, usize)> {
        let ndims = buf[1] as usize;
        let flags = buf[2];
        let class = match buf[3] {
            DS_TYPE_SCALAR => DataspaceClass::Scalar,
            DS_TYPE_SIMPLE => DataspaceClass::Simple,
            DS_TYPE_NULL => DataspaceClass::Null,
            other => {
                return Err(FormatError::InvalidData(format!(
                    "dataspace type byte {other} is not scalar(0)/simple(1)/null(2)"
                )))
            }
        };
        let has_max = (flags & FLAG_MAX_DIMS) != 0;
        let ss = ctx.sizeof_size as usize;

        let needed = 4 + ndims * ss + if has_max { ndims * ss } else { 0 };
        if buf.len() < needed {
            return Err(FormatError::BufferTooShort {
                needed,
                available: buf.len(),
            });
        }

        let mut pos = 4;

        let mut dims = Vec::with_capacity(ndims);
        for _ in 0..ndims {
            dims.push(read_size(&buf[pos..], ss));
            pos += ss;
        }

        let max_dims = if has_max {
            let mut v = Vec::with_capacity(ndims);
            for _ in 0..ndims {
                v.push(read_size(&buf[pos..], ss));
                pos += ss;
            }
            Some(v)
        } else {
            None
        };

        Ok((
            Self {
                class,
                dims,
                max_dims,
            },
            pos,
        ))
    }

    /// Decode version 1 dataspace message.
    ///
    /// Version 1 layout:
    /// ```text
    /// Byte 0: version = 1
    /// Byte 1: ndims
    /// Byte 2: flags (bit 0 = max dims present, bit 1 = permutation indices present)
    /// Byte 3: reserved
    /// Bytes 4-7: reserved (4 bytes)
    /// Then ndims * sizeof_size bytes for current dimensions
    /// Then (if flag bit 0) ndims * sizeof_size bytes for max dimensions
    /// Then (if flag bit 1) ndims * sizeof_size bytes for permutation indices
    /// ```
    fn decode_v1(buf: &[u8], ctx: &FormatContext) -> FormatResult<(Self, usize)> {
        if buf.len() < 8 {
            return Err(FormatError::BufferTooShort {
                needed: 8,
                available: buf.len(),
            });
        }

        let ndims = buf[1] as usize;
        let flags = buf[2];
        let has_max = (flags & FLAG_MAX_DIMS) != 0;
        let has_perm = (flags & 0x02) != 0;
        let ss = ctx.sizeof_size as usize;

        // Header is 8 bytes for v1 (4 fixed + 4 reserved)
        let mut needed = 8 + ndims * ss;
        if has_max {
            needed += ndims * ss;
        }
        if has_perm {
            needed += ndims * ss;
        }
        if buf.len() < needed {
            return Err(FormatError::BufferTooShort {
                needed,
                available: buf.len(),
            });
        }

        let mut pos = 8; // skip version(1) + ndims(1) + flags(1) + reserved(1) + reserved(4)

        let mut dims = Vec::with_capacity(ndims);
        for _ in 0..ndims {
            dims.push(read_size(&buf[pos..], ss));
            pos += ss;
        }

        let max_dims = if has_max {
            let mut v = Vec::with_capacity(ndims);
            for _ in 0..ndims {
                v.push(read_size(&buf[pos..], ss));
                pos += ss;
            }
            Some(v)
        } else {
            None
        };

        // Skip permutation indices if present
        if has_perm {
            pos += ndims * ss;
        }

        // Version 1 predates the NULL dataspace (added with the version-2
        // message, `H5Osdspace.c`): it carries no type byte, so the class is
        // always inferred from rank, same as `simple()`/`unlimited()`.
        Ok((
            Self {
                class: Self::class_for_rank(ndims),
                dims,
                max_dims,
            },
            pos,
        ))
    }
}

// ======================================================================= tests

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

    fn ctx8() -> FormatContext {
        FormatContext {
            sizeof_addr: 8,
            sizeof_size: 8,
        }
    }

    fn ctx4() -> FormatContext {
        FormatContext {
            sizeof_addr: 4,
            sizeof_size: 4,
        }
    }

    #[test]
    fn roundtrip_scalar() {
        let msg = DataspaceMessage::scalar();
        let encoded = msg.encode(&ctx8());
        assert_eq!(encoded.len(), 4); // version + ndims + flags + type
        let (decoded, consumed) = DataspaceMessage::decode(&encoded, &ctx8()).unwrap();
        assert_eq!(consumed, 4);
        assert_eq!(decoded, msg);
    }

    /// A simple dataspace whose maximum the caller left implicit is written
    /// with the maximum `H5S_set_extent_simple` fills in from the current
    /// dimensions (H5S.c:1292-1299), so the message names them and is that
    /// much longer.
    #[test]
    fn roundtrip_simple_1d() {
        let msg = DataspaceMessage::simple(&[100]);
        let encoded = msg.encode(&ctx8());
        // 4 header + 1*8 dims + 1*8 max dims = 20
        assert_eq!(encoded.len(), 20);
        assert_eq!(encoded[2], FLAG_MAX_DIMS);
        let (decoded, consumed) = DataspaceMessage::decode(&encoded, &ctx8()).unwrap();
        assert_eq!(consumed, 20);
        assert_eq!(decoded.class, msg.class);
        assert_eq!(decoded.dims, msg.dims);
        assert_eq!(decoded.max_dims, Some(vec![100]));
    }

    #[test]
    fn roundtrip_simple_3d_ctx4() {
        let msg = DataspaceMessage::simple(&[10, 20, 30]);
        let encoded = msg.encode(&ctx4());
        // 4 + 3*4 dims + 3*4 max dims = 28
        assert_eq!(encoded.len(), 28);
        let (decoded, consumed) = DataspaceMessage::decode(&encoded, &ctx4()).unwrap();
        assert_eq!(consumed, 28);
        assert_eq!(decoded.class, msg.class);
        assert_eq!(decoded.dims, msg.dims);
        assert_eq!(decoded.max_dims, Some(vec![10, 20, 30]));
    }

    #[test]
    fn roundtrip_unlimited() {
        let msg = DataspaceMessage::unlimited(&[5, 10]);
        let encoded = msg.encode(&ctx8());
        // 4 + 2*8 dims + 2*8 max = 36
        assert_eq!(encoded.len(), 36);
        let (decoded, consumed) = DataspaceMessage::decode(&encoded, &ctx8()).unwrap();
        assert_eq!(consumed, 36);
        assert_eq!(decoded, msg);
        assert_eq!(decoded.max_dims.as_ref().unwrap(), &vec![u64::MAX; 2]);
    }

    #[test]
    fn roundtrip_partial_max() {
        let msg = DataspaceMessage {
            class: DataspaceClass::Simple,
            dims: vec![3, 4],
            max_dims: Some(vec![100, u64::MAX]),
        };
        let encoded = msg.encode(&ctx8());
        let (decoded, _) = DataspaceMessage::decode(&encoded, &ctx8()).unwrap();
        assert_eq!(decoded, msg);
    }

    #[test]
    fn decode_bad_version() {
        let buf = [99u8, 0, 0, 0]; // version 99 — unsupported
        let err = DataspaceMessage::decode(&buf, &ctx8()).unwrap_err();
        match err {
            FormatError::InvalidVersion(99) => {}
            other => panic!("unexpected error: {:?}", other),
        }
    }

    #[test]
    fn decode_v1_simple_1d() {
        // Build a version 1 dataspace: 1D, dims=[100], no max
        let mut buf = vec![
            1, // version 1
            1, // ndims = 1
            0, // flags (no max dims)
            0, // reserved
        ];
        buf.extend_from_slice(&[0u8; 4]); // reserved (4 bytes)
        buf.extend_from_slice(&100u64.to_le_bytes()); // dims[0] = 100

        let (msg, consumed) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert_eq!(consumed, 16); // 8 header + 8 dim
        assert_eq!(msg.dims, vec![100]);
        assert_eq!(msg.max_dims, None);
    }

    /// A message that stores no maximum is re-encoded without one, at either
    /// version. Upstream keeps the state because `H5O__sdspace_encode` raises
    /// `H5S_VALID_MAX` only for a non-null `extent.max` (H5Osdspace.c:271-272)
    /// and the decoder leaves it null when the flag was clear
    /// (H5Osdspace.c:188-194); a re-encode that invented a maximum would make
    /// the message eight bytes per dimension longer than the one libhdf5
    /// writes for the same extent.
    #[test]
    fn a_decoded_message_with_no_maximum_re_encodes_without_one() {
        let mut buf = vec![1, 1, 0, 0];
        buf.extend_from_slice(&[0u8; 4]);
        buf.extend_from_slice(&100u64.to_le_bytes());

        let (msg, _) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert_eq!(msg.max_dims, None);
        assert_eq!(
            msg.encode_for(&ctx8(), crate::format::ObjectFormat::Legacy),
            buf
        );

        let modern = msg.encode_for(&ctx8(), crate::format::ObjectFormat::Modern);
        assert_eq!(modern.len(), 12, "4-byte prefix and one dimension");
        assert_eq!(modern[2], 0, "H5S_VALID_MAX stays clear");
        let (round, _) = DataspaceMessage::decode(&modern, &ctx8()).unwrap();
        assert_eq!(round, msg);
    }

    /// The constructors fill the maximum in the way `H5S_set_extent_simple`
    /// does (H5S.c:1293-1299), so nothing the crate builds reaches the encoder
    /// without one — the absent maximum above is only ever read from a file.
    #[test]
    fn a_constructed_simple_dataspace_carries_its_maximum() {
        assert_eq!(DataspaceMessage::simple(&[3, 4]).max_dims, Some(vec![3, 4]));
        assert_eq!(DataspaceMessage::simple(&[]).max_dims, None);
        assert_eq!(DataspaceMessage::scalar().max_dims, None);
        assert_eq!(DataspaceMessage::null().max_dims, None);
    }

    #[test]
    fn decode_v1_with_max_dims() {
        let mut buf = vec![
            1, // version 1
            2, // ndims = 2
            1, // flags = has max dims
            0, // reserved
        ];
        buf.extend_from_slice(&[0u8; 4]); // reserved
        buf.extend_from_slice(&10u64.to_le_bytes()); // dims[0] = 10
        buf.extend_from_slice(&20u64.to_le_bytes()); // dims[1] = 20
        buf.extend_from_slice(&u64::MAX.to_le_bytes()); // max_dims[0] = unlimited
        buf.extend_from_slice(&100u64.to_le_bytes()); // max_dims[1] = 100

        let (msg, consumed) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert_eq!(consumed, 40); // 8 + 2*8 + 2*8
        assert_eq!(msg.dims, vec![10, 20]);
        assert_eq!(msg.max_dims, Some(vec![u64::MAX, 100]));
    }

    #[test]
    fn decode_buffer_too_short() {
        let buf = [2u8, 1, 0]; // version ok, but too short (need 4 header bytes)
        let err = DataspaceMessage::decode(&buf, &ctx8()).unwrap_err();
        match err {
            FormatError::BufferTooShort { .. } => {}
            other => panic!("unexpected error: {:?}", other),
        }
    }

    #[test]
    fn decode_buffer_too_short_for_dims() {
        // ndims=1, no max, sizeof_size=8 => need 4 + 8 = 12 bytes, give 6
        let buf = [2u8, 1, 0, 1, 0, 0];
        let err = DataspaceMessage::decode(&buf, &ctx8()).unwrap_err();
        match err {
            FormatError::BufferTooShort {
                needed: 12,
                available: 6,
            } => {}
            other => panic!("unexpected error: {:?}", other),
        }
    }

    #[test]
    fn version_byte_is_two() {
        let msg = DataspaceMessage::simple(&[42]);
        let encoded = msg.encode(&ctx8());
        assert_eq!(encoded[0], 2);
    }

    #[test]
    fn roundtrip_null() {
        let msg = DataspaceMessage::null();
        let encoded = msg.encode(&ctx8());
        assert_eq!(encoded.len(), 4); // version + ndims + flags + type, no dims
        assert_eq!(encoded[3], 2); // type byte: null
        let (decoded, consumed) = DataspaceMessage::decode(&encoded, &ctx8()).unwrap();
        assert_eq!(consumed, 4);
        assert_eq!(decoded, msg);
        assert!(decoded.is_null());
    }

    /// A NULL dataspace and a scalar dataspace both have an empty `dims`,
    /// but must not decode to the same message: only the type byte tells
    /// them apart, and it must round-trip distinctly.
    #[test]
    fn null_and_scalar_are_distinct() {
        let null = DataspaceMessage::null();
        let scalar = DataspaceMessage::scalar();
        assert_ne!(null, scalar);
        assert!(null.is_null());
        assert!(!scalar.is_null());

        let null_encoded = null.encode(&ctx8());
        let scalar_encoded = scalar.encode(&ctx8());
        assert_ne!(null_encoded[3], scalar_encoded[3]);

        let (null_decoded, _) = DataspaceMessage::decode(&null_encoded, &ctx8()).unwrap();
        let (scalar_decoded, _) = DataspaceMessage::decode(&scalar_encoded, &ctx8()).unwrap();
        assert!(null_decoded.is_null());
        assert!(!scalar_decoded.is_null());
    }

    #[test]
    fn decode_v2_bad_type_byte() {
        // version 2, ndims=0, flags=0, type=3 (not scalar/simple/null)
        let buf = [2u8, 0, 0, 3];
        let err = DataspaceMessage::decode(&buf, &ctx8()).unwrap_err();
        match err {
            FormatError::InvalidData(_) => {}
            other => panic!("unexpected error: {:?}", other),
        }
    }

    #[test]
    fn decode_v1_is_never_null() {
        // Version 1 has no type byte; rank 0 must decode as Scalar, not Null.
        let buf = [1u8, 0, 0, 0, 0, 0, 0, 0];
        let (msg, _) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert!(!msg.is_null());
        assert_eq!(msg.class, DataspaceClass::Scalar);
    }

    /// The 24 bytes libhdf5 1.14.6 wrote for the dataspace of a shape-(6,)
    /// dataset in a default (superblock-0) file: version 1, an 8-byte prefix,
    /// then current and maximum dimensions.
    #[test]
    fn a_legacy_dataspace_matches_the_bytes_libhdf5_wrote() {
        let ds = DataspaceMessage {
            class: DataspaceClass::Simple,
            dims: vec![6],
            max_dims: Some(vec![6]),
        };
        let buf = ds.encode_for(&ctx8(), crate::format::ObjectFormat::Legacy);
        assert_eq!(
            buf,
            vec![0x01, 0x01, 0x01, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0]
        );
        let (back, consumed) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert_eq!(consumed, buf.len());
        assert_eq!(back, ds);
    }

    /// Version 1 has no type field, so it cannot say "no elements"; upstream
    /// `H5S_set_version` raises a null dataspace to version 2 whatever the
    /// bound says, and so must this.
    #[test]
    fn a_null_dataspace_stays_at_version_2_in_a_legacy_file() {
        let buf = DataspaceMessage::null().encode_for(&ctx8(), crate::format::ObjectFormat::Legacy);
        assert_eq!(buf[0], 2);
        assert_eq!(buf[3], DS_TYPE_NULL);
        let (back, _) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert!(back.is_null());
    }

    /// A scalar has rank 0, so version 1's extra prefix bytes are the whole
    /// message; nothing about the class reaches the file, and the decoder has
    /// to infer it from the rank exactly as libhdf5 does.
    #[test]
    fn a_legacy_scalar_dataspace_round_trips_as_scalar() {
        let buf =
            DataspaceMessage::scalar().encode_for(&ctx8(), crate::format::ObjectFormat::Legacy);
        assert_eq!(buf, vec![1, 0, 0, 0, 0, 0, 0, 0]);
        let (back, _) = DataspaceMessage::decode(&buf, &ctx8()).unwrap();
        assert_eq!(back.class, DataspaceClass::Scalar);
    }
}