[WIP] Draft Specification

SPEC.md

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+# Draft IPLD Unixfs Spec
+
+## Basic Structure
+
+A Unixfs is either a file or a directory.
+
+The top level IPLD object has at least two fields: `type` and `data`
+and maybe a few other such as a version string or a set of flags.
+
+The `type` field is either `file` or `dir`.
+
+## IPLD `file`
+
+A file object has the following fields.
+
+  - `type`: String with value of `'file'`.
+  - `data`: Array of `file-data`.
+  - `size`: Integer. Cumulative size of `data`.
+
+The `type` field must be set to `file`.
+
+### `file-data`
+
+File data is an Array. Each element is an Array with only 2 elements (Tuple).
+
+  - 0: Array. Tuple containing two integers, the `start` and `length` offsets of the content.
+    - 0: Integer: start offset.
+    - 1: Integer: length of part(s).
+  - 1: Link. Either a `raw` link or a link to another node formatted as `file-data`.
+
+```javascript
+[
+  [ [0, 1024], Link ],
+  [ [1025, 1002], Link ]
+]
+```
+
+Implementations are encouraged to use nested `file-data` array nodes through links for files
+with many chunks in order to limit the size of the serialized node. No strict boundary is
+set but it is typical to try and limit node size to less than one megabyte.
+
+## IPLD `dir`
+
+A directory object represents a directory.
+
+The key of the map is a filename and is a CBOR text string encoded in UTF-8.
+The value of the map is another CBOR map with the following standard fields:
+
+  - `type`: String with value of `'dir'`.
+  - `data`: TODO: Define structure for directory content data.
+  - `size`: Integer. Cumulative size of all directories and files in `data`.
+
+The `type` field set to `dir`.