format: read symbol table nodes over Storage

SymbolTableNode::parse_in reads the node's 8-byte header, checks the
entries against the end of the file as before, and reads them in one
bounded read. parse is a wrapper. New test: nodes with 4- and 8-byte
offsets, valid and with a bad version, at two offsets and cut at every
length, parse identically through a read_at-only CountingStorage.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/symbol_table.rs

@@ -4,6 +4,7 @@
 use alloc::vec::Vec;
 
 use crate::error::FormatError;
+use crate::storage::{Storage, len_usize, read_exact_at};
 
 /// Symbol Table message (type 0x0011) found in v1 group object headers.
 #[derive(Debug, Clone, PartialEq)]
@@ -79,48 +80,53 @@ impl SymbolTableNode {
         offset: usize,
         offset_size: u8,
     ) -> Result<SymbolTableNode, FormatError> {
-        // signature(4) + version(1) + reserved(1) + number_of_symbols(2) = 8
-        if offset
-            .checked_add(8)
-            .is_none_or(|end| end > file_data.len())
-        {
-            return Err(FormatError::UnexpectedEof {
-                expected: offset.saturating_add(8),
-                available: file_data.len(),
-            });
-        }
+        Self::parse_in(&file_data, offset as u64, offset_size)
+    }
 
-        if &file_data[offset..offset + 4] != b"SNOD" {
+    /// [`Self::parse`] over any [`Storage`]: one read of the node's header,
+    /// one of its entries.
+    pub fn parse_in(
+        file: &dyn Storage,
+        offset: u64,
+        offset_size: u8,
+    ) -> Result<SymbolTableNode, FormatError> {
+        let file_len = len_usize(file);
+        // signature(4) + version(1) + reserved(1) + number_of_symbols(2) = 8
+        let header = read_exact_at(file, offset, 8)?;
+
+        if &header[..4] != b"SNOD" {
             return Err(FormatError::InvalidSymbolTableNodeSignature);
         }
 
-        let version = file_data[offset + 4];
+        let version = header[4];
         if version != 1 {
             return Err(FormatError::InvalidSymbolTableNodeVersion(version));
         }
 
-        let num_symbols =
-            u16::from_le_bytes([file_data[offset + 6], file_data[offset + 7]]) as usize;
+        let num_symbols = u16::from_le_bytes([header[6], header[7]]) as usize;
 
         let os = offset_size as usize;
         // Each entry: link_name_offset(os) + obj_hdr_addr(os) + cache_type(4) + reserved(4) + scratch(16)
         let entry_size = os + os + 4 + 4 + 16;
-        let entries_start = offset + 8;
+        // `offset + 8` fits: the header's read checked it.
+        let entries_start = offset as usize + 8;
         let needed = entries_start.checked_add(num_symbols * entry_size).ok_or(
             FormatError::UnexpectedEof {
                 expected: usize::MAX,
-                available: file_data.len(),
+                available: file_len,
             },
         )?;
-        if needed > file_data.len() {
+        if needed > file_len {
             return Err(FormatError::UnexpectedEof {
                 expected: needed,
-                available: file_data.len(),
+                available: file_len,
             });
         }
+        let body = read_exact_at(file, entries_start as u64, num_symbols * entry_size)?;
+        let file_data: &[u8] = &body;
 
         let mut entries = Vec::with_capacity(num_symbols);
-        let mut pos = entries_start;
+        let mut pos = 0usize;
         for _ in 0..num_symbols {
             let link_name_offset = read_offset(file_data, pos, offset_size)?;
             pos += os;
@@ -256,4 +262,28 @@ mod tests {
         let result = SymbolTableNode::parse(&data, usize::MAX / 2, 8);
         assert!(result.is_err());
     }
+
+    /// Nodes, cut at every length and at an offset, parse identically
+    /// through a `read_at`-only storage.
+    #[test]
+    fn storage_parse_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        for os in [4u8, 8] {
+            let node = build_snod(&[(0, 0x100, 0), (8, 0x200, 1), (16, 0x300, 2)], os);
+            let mut bad = node.clone();
+            bad[4] = 2;
+            for full in [node, bad] {
+                for at in [0usize, 7] {
+                    for cut in 0..=full.len() {
+                        let mut f = vec![0u8; at];
+                        f.extend_from_slice(&full[..cut]);
+                        let storage = CountingStorage::new(f.clone());
+                        let want = SymbolTableNode::parse(&f, at, os);
+                        let got = SymbolTableNode::parse_in(&storage, at as u64, os);
+                        assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                    }
+                }
+            }
+        }
+    }
 }