Module codec 

MessagePack encoding/decoding for tree nodes

Blobs are stored raw (not wrapped) for efficiency. Tree nodes are MessagePack-encoded.

Determinism: Unlike CBOR, MessagePack doesn’t have a built-in canonical encoding. We ensure deterministic output by:

1. Using fixed struct field order (Rust declaration order via serde)
2. Converting HashMap metadata to BTreeMap before encoding (sorted keys)
3. Sorting directory links by BUD-16 name order before encoding

File-node link order is preserved because chunk order is semantic.

Format uses short keys for compact encoding:

• t: type (1 = File, 2 = Dir, 3 = Fanout) - node type
• l: links array
• h: hash (in link)
• t: type (in link, 0 = Blob, 1 = File, 2 = Dir, 3 = Fanout)
• n: name (in link, optional)
• s: size (in link)
• m: metadata (optional)

Enums

CodecError

Error type for codec operations

Functions

decode_tree_node

Decode MessagePack to a tree node

encode_and_hash

Encode a tree node and compute its hash

encode_tree_node

Encode a tree node to MessagePack

get_node_type

Get the type of data (Blob, File, Dir, or Fanout) Returns LinkType::Blob for raw blobs that aren’t tree nodes

is_directory_node

Check if data is a directory-like tree node (node_type == Dir or Fanout) Note: Prefer try_decode_tree_node() to avoid double decoding

is_tree_node

Check if data is a MessagePack-encoded tree node (vs raw blob) Tree-shaped data with unsupported types still counts as a tree node so callers that subsequently decode it surface a codec error instead of reinterpreting the bytes as a raw blob. Note: Prefer try_decode_tree_node() to avoid double decoding

try_decode_tree_node

Try to decode data as a tree node Returns Some(TreeNode) if valid tree node, None otherwise This is preferred over is_tree_node() to avoid double decoding