hdf5_reader/

global_heap.rs

//! HDF5 Global Heap Collection (GCOL).
//!
//! Global heaps store variable-length data such as variable-length strings
//! and VL arrays. Each collection is a contiguous block in the file that
//! contains multiple heap objects. Objects are referenced by a global heap
//! ID that encodes the collection address and the object index.
//!
//! An object with index 0 marks the free space sentinel and terminates
//! parsing of the collection.

use crate::error::{Error, Result};
use crate::io::Cursor;

/// Signature bytes for a Global Heap Collection: ASCII `GCOL`.
const GCOL_SIGNATURE: [u8; 4] = *b"GCOL";

/// A single object within a global heap collection.
#[derive(Debug, Clone)]
pub struct GlobalHeapObject {
    /// Heap object index (1-based; index 0 is the free-space sentinel).
    pub index: u16,
    /// Reference count.
    pub reference_count: u16,
    /// Raw object data.
    pub data: Vec<u8>,
}

/// A parsed global heap collection containing zero or more heap objects.
#[derive(Debug, Clone)]
pub struct GlobalHeapCollection {
    /// The heap objects in this collection.
    pub objects: Vec<GlobalHeapObject>,
}

impl GlobalHeapCollection {
    /// Parse a global heap collection at the current cursor position.
    ///
    /// Format:
    /// - Signature: `GCOL` (4 bytes)
    /// - Version: 1 (1 byte)
    /// - Reserved: 3 bytes
    /// - Collection size (`length_size` bytes) — total size including header
    /// - Then global heap objects until the collection is exhausted.
    ///
    /// Each global heap object:
    /// - Heap object index (u16 LE)
    /// - Reference count (u16 LE)
    /// - Reserved (4 bytes)
    /// - Object size (`length_size` bytes)
    /// - Object data (padded to 8-byte boundary)
    /// - An index of 0 signals free space / end of objects.
    pub fn parse(cursor: &mut Cursor, _offset_size: u8, length_size: u8) -> Result<Self> {
        let header_start = cursor.position();

        let sig = cursor.read_bytes(4)?;
        if sig != GCOL_SIGNATURE {
            return Err(Error::InvalidGlobalHeapSignature);
        }

        let version = cursor.read_u8()?;
        if version != 1 {
            return Err(Error::UnsupportedGlobalHeapVersion(version));
        }

        // Reserved 3 bytes
        cursor.skip(3)?;

        let collection_size = cursor.read_length(length_size)?;

        // The collection_size includes the header we just read. Calculate the
        // end boundary so we don't read past it.
        let collection_end = header_start + collection_size;

        let mut objects = Vec::new();

        loop {
            // Check if we have enough room for at least an object header
            // (2 + 2 + 4 + length_size bytes minimum).
            let min_obj_header = 8 + length_size as u64;
            if cursor.position() + min_obj_header > collection_end {
                break;
            }

            let index = cursor.read_u16_le()?;

            // Index 0 = free space sentinel — stop parsing.
            if index == 0 {
                break;
            }

            let reference_count = cursor.read_u16_le()?;
            // Reserved 4 bytes
            cursor.skip(4)?;
            let obj_size = cursor.read_length(length_size)?;

            // Guard against reading past the collection.
            if cursor.position() + obj_size > collection_end {
                return Err(Error::UnexpectedEof {
                    offset: cursor.position(),
                    needed: obj_size,
                    available: collection_end.saturating_sub(cursor.position()),
                });
            }

            let data = cursor.read_bytes(obj_size as usize)?.to_vec();

            // Object data is padded to an 8-byte boundary.
            let padded = (obj_size + 7) & !7;
            let padding = padded - obj_size;
            if padding > 0 && cursor.position() + padding <= collection_end {
                cursor.skip(padding as usize)?;
            }

            objects.push(GlobalHeapObject {
                index,
                reference_count,
                data,
            });
        }

        Ok(GlobalHeapCollection { objects })
    }

    /// Look up an object by its index within this collection.
    pub fn get_object(&self, index: u16) -> Option<&GlobalHeapObject> {
        self.objects.iter().find(|o| o.index == index)
    }
}

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

    /// Build a minimal global heap collection with the given objects.
    /// Each object is (index, ref_count, data).
    fn build_gcol(objects: &[(u16, u16, &[u8])], length_size: u8) -> Vec<u8> {
        let mut body = Vec::new();
        for &(index, ref_count, data) in objects {
            body.extend_from_slice(&index.to_le_bytes());
            body.extend_from_slice(&ref_count.to_le_bytes());
            body.extend_from_slice(&[0u8; 4]); // reserved
            match length_size {
                4 => body.extend_from_slice(&(data.len() as u32).to_le_bytes()),
                8 => body.extend_from_slice(&(data.len() as u64).to_le_bytes()),
                _ => panic!("test only supports 4/8"),
            }
            body.extend_from_slice(data);
            // Pad to 8-byte boundary
            let padded = (data.len() + 7) & !7;
            body.resize(body.len() + (padded - data.len()), 0);
        }
        // Free space sentinel (index 0)
        body.extend_from_slice(&0u16.to_le_bytes());

        // Build full collection
        let header_size = 4 + 1 + 3 + length_size as usize; // sig + ver + reserved + size
        let collection_size = header_size + body.len();

        let mut buf = Vec::new();
        buf.extend_from_slice(b"GCOL");
        buf.push(1); // version
        buf.extend_from_slice(&[0, 0, 0]); // reserved
        match length_size {
            4 => buf.extend_from_slice(&(collection_size as u32).to_le_bytes()),
            8 => buf.extend_from_slice(&(collection_size as u64).to_le_bytes()),
            _ => panic!("test only supports 4/8"),
        }
        buf.extend(body);
        buf
    }

    #[test]
    fn test_parse_empty_collection() {
        let data = build_gcol(&[], 8);
        let mut cursor = Cursor::new(&data);
        let col = GlobalHeapCollection::parse(&mut cursor, 8, 8).unwrap();
        assert!(col.objects.is_empty());
    }

    #[test]
    fn test_parse_single_object() {
        let obj_data = b"hello world";
        let data = build_gcol(&[(1, 1, obj_data)], 8);
        let mut cursor = Cursor::new(&data);
        let col = GlobalHeapCollection::parse(&mut cursor, 8, 8).unwrap();

        assert_eq!(col.objects.len(), 1);
        assert_eq!(col.objects[0].index, 1);
        assert_eq!(col.objects[0].reference_count, 1);
        assert_eq!(col.objects[0].data, obj_data);
    }

    #[test]
    fn test_parse_multiple_objects() {
        let data = build_gcol(
            &[
                (1, 1, b"alpha"),
                (2, 3, b"beta"),
                (5, 0, b"gamma123"), // 8 bytes, no padding needed
            ],
            8,
        );
        let mut cursor = Cursor::new(&data);
        let col = GlobalHeapCollection::parse(&mut cursor, 8, 8).unwrap();

        assert_eq!(col.objects.len(), 3);

        let obj1 = col.get_object(1).unwrap();
        assert_eq!(obj1.data, b"alpha");
        assert_eq!(obj1.reference_count, 1);

        let obj2 = col.get_object(2).unwrap();
        assert_eq!(obj2.data, b"beta");
        assert_eq!(obj2.reference_count, 3);

        let obj5 = col.get_object(5).unwrap();
        assert_eq!(obj5.data, b"gamma123");

        assert!(col.get_object(99).is_none());
    }

    #[test]
    fn test_parse_4byte_lengths() {
        let data = build_gcol(&[(1, 2, b"test")], 4);
        let mut cursor = Cursor::new(&data);
        let col = GlobalHeapCollection::parse(&mut cursor, 4, 4).unwrap();

        assert_eq!(col.objects.len(), 1);
        assert_eq!(col.objects[0].data, b"test");
    }

    #[test]
    fn test_bad_signature() {
        let mut data = build_gcol(&[], 8);
        data[0] = b'X';
        let mut cursor = Cursor::new(&data);
        assert!(matches!(
            GlobalHeapCollection::parse(&mut cursor, 8, 8),
            Err(Error::InvalidGlobalHeapSignature)
        ));
    }

    #[test]
    fn test_bad_version() {
        let mut data = build_gcol(&[], 8);
        data[4] = 2; // version 2
        let mut cursor = Cursor::new(&data);
        assert!(matches!(
            GlobalHeapCollection::parse(&mut cursor, 8, 8),
            Err(Error::UnsupportedGlobalHeapVersion(2))
        ));
    }
}