feat: w3 index spec

.github/workflows/words-to-ignore.txt

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 2MiB
 ok
 cryptographically
+InterPlanetary
+queryable
+bafy

w3-index.md

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+# W3 Index
+
+![wip](https://img.shields.io/badge/status-wip-orange.svg?style=flat-square)
+
+# Introduction
+
+## Abstract
+
+W3 Indexing protocol allows authorized agents to submit verifiable claims about content-addressable data to be publish on [InterPlanetary Network Indexer (IPNI)][IPNI], making it publicly queryable.
+
+## Concepts
+
+### Space
+
+A namespace, often referred as a "space", is an owned resource that can be shared. It corresponds to a unique asymmetric cryptographic keypair and is identified by a [`did:key`] URI.
+
+# Capabilities
+
+## Index Add
+
+Authorized agent MAY invoke `/space/index/add` capability on the [space] subject to submit [content index] for publishing on [InterPlanetary Network Indexer (IPNI)][IPNI].
+
+### Index Add Invocation Example
+
+Invocation example illustrates Alice requesting to add index under "bafy..blob" to be published on the network.
+
+```js
+{
+  "cmd": "/space/index/add",
+  "sub": "did:key:zAlice",
+  "iss": "did:key:zAlice",
+  "aud": "did:web:web3.storage",
+  "args": {
+    "index": { "/": "bafy...blob" }
+  }
+}
+```
+
+### Index Add Capability
+
+#### Index Add Capability Schema
+
+```ts
+type AddIndex = {
+  cmd: "/space/index/add"
+  sub: SpaceDID
+  args: {
+    // Link is the Content Archive (CAR) containing
+    // the `Index`.
+    index: Link<ContentArchive<Index>>
+  }
+}
+
+// Type describes a CAR format
+type ContentArchive<T> = ByteView<{
+  roots: [Block<T>]
+  blocks: Block[]
+}>
+```
+
+### Index
+
+#### Index Schema
+
+Index schema is variant type keyed by the format descriptor label designed to allow format evolution through versioning and additional schema variants.
+
+```ts
+type Index = Variant<{
+  "index/sharded/[email protected]": ShardedDAGIndex
+}>
+```
+
+#### Sharded DAG Index
+
+Sharded DAG index MUST describe complete set of blocks that make up the `content` DAG in terms of `BlobSlice`s.
+
+##### Sharded DAG Index Schema
+
+```ts
+
+type ShardedDAGIndex = {
+  // content root CID
+  content: Link<any>
+  // links to blob indexes that contain blocks of the content DAG
+  shards: Link<BlobIndex>[]
+}
+
+type BlobIndex = [
+  // hash digest of the blob
+  digest: Multihash
+  // Index of blob slices
+  slices: BlobSlice
+]
+
+type BlobSlice = [
+  // hash digest of the slice
+  digest: Multihash
+  // Slice offset
+  offset: Int
+  // Slice size in bytes
+  length: Int
+]
+
+type Multihash = bytes
+```
+
+ℹ️ Please note that `shards` is a list of `BlobIndex` links. This provide a flexibility of bundling blob indexes or externalizing them by linking to them.
+
+It is RECOMMENDED to bundle all the `BlobIndex`s inside the Content Archive of the `Index`.
+
+##### Sharded DAG Index Example
+
+> For the reader convenience we use `link` function to denote external blocks that should be linked
+
+```js
+{
+  "index/sharded/[email protected]": {
+    "content": { "/": "bafy..dag" },
+    "shards": [
+      link([
+        // blob multihash
+        { "/": { "bytes": "blb...left" } },
+        // sliced within the blob
+        [
+          [{ "/": { "bytes": "block..1"} }, 0, 128],
+          [{ "/": { "bytes": "block..2"} }, 129, 256],
+          [{ "/": { "bytes": "block..3"} }, 257, 384],
+          [{ "/": { "bytes": "block..4"} }, 385, 512]
+        ]
+      ]),
+      link([
+        // blob multihash
+        { "/": { "bytes": "blb...right" } },
+        // sliced within the blob
+        [
+          [{ "/": { "bytes": "block..5"} }, 0, 128],
+          [{ "/": { "bytes": "block..6"} }, 129, 256],
+          [{ "/": { "bytes": "block..7"} }, 257, 384],
+          [{ "/": { "bytes": "block..8"} }, 385, 512]
+        ]
+      ])
+    ]
+  }
+}
+```
+
+[IPNI]:https://github.com/ipni/specs/blob/main/IPNI.md
+[`did:key`]:https://w3c-ccg.github.io/did-method-key/