Function build_fs_image

src/compilation_pipeline.rs:1478–1726

Serialise `entries` into the on-disk filesystem image format and return the raw bytes. See _`OS_SPECIFICATION.md` for block layout. Layout constants live in [`fs_layout`] and are checked against `fs.hll`.

(entries: &[FsEntry<'_>])

    let inode_offset = |idx: usize| -> usize { BLOCK_SIZE + idx * INODE_SIZE };
    let block_offset = |blk: usize| -> usize { blk * BLOCK_SIZE };

    let write_u16 = |buf: &mut Vec<u8>, off: usize, val: u16| {
        buf[off] = (val & 0xFF) as u8;
        buf[off + 1] = (val >> 8) as u8;
    };
    let write_u32 = |buf: &mut Vec<u8>, off: usize, val: u32| {
        buf[off] = (val & 0xFF) as u8;
        buf[off + 1] = ((val >> 8) & 0xFF) as u8;
        buf[off + 2] = ((val >> 16) & 0xFF) as u8;
        buf[off + 3] = ((val >> 24) & 0xFF) as u8;
    };
    let write_u64 = |buf: &mut Vec<u8>, off: usize, val: u64| {
        for i in 0..8usize {
            buf[off + i] = ((val >> (i * 8)) & 0xFF) as u8;
        }
    };

    // next_inode and next_data_block are mutable state we thread through.
    let mut next_inode: usize = 1; // 0 = root
    let mut next_data_block: usize = 0; // relative to DATA_BLOCK_START

    // Allocate a data block and return its absolute block index.
    let alloc_block = |next: &mut usize| -> usize {
        let blk = DATA_BLOCK_START + *next;
        *next += 1;
        blk
    };

    // Allocate an inode index.
    let alloc_inode = |next: &mut usize| -> usize {
        let idx = *next;
        *next += 1;
        idx
    };

    // Write a name into a 32-byte field at buf[off..off+32].
    let write_name = |buf: &mut Vec<u8>, off: usize, name: &str| {
        let bytes = name.as_bytes();
        let len = bytes.len().min(31);
        buf[off..off + len].copy_from_slice(&bytes[..len]);
        buf[off + len] = 0;
    };

    // Root directory (inode 0)
    let root_data_blk = alloc_block(&mut next_data_block);
    {
        let off = inode_offset(0);
        image[off + IN_TYPE] = 2; // directory
        write_u16(&mut image, off + IN_PARENT, 0); // root's parent = self
        write_u32(&mut image, off + IN_SIZE, 0); // entry count updated later
        write_name(&mut image, off + IN_NAME, "/");
        write_u16(&mut image, off + IN_BLOCKS, root_data_blk as u16);
    }

    // Add a DirEntry to a directory inode's data block
    // Returns false if block is full (not handled for simplicity; 113 entries per block).
    let add_dirent = |image: &mut Vec<u8>, dir_inode: usize, child_inode: usize, name: &str| {