feat: codecs table + impl

package.json

@@ -0,0 +1,34 @@
+{
+  "name": "multicodec-packed",
+  "version": "0.0.0",
+  "description": "JavaScript implementation of the multicodec-packed specification",
+  "main": "src/index.js",
+  "directories": {
+    "test": "test"
+  },
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/multiformats/js-multicodec-packed.git"
+  },
+  "keywords": [
+    "IPFS",
+    "multiformats",
+    "multicodec",
+    "binary",
+    "packed",
+    "the",
+    "data!"
+  ],
+  "author": "David Dias <daviddias@ipfs.io>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/multiformats/js-multicodec-packed/issues"
+  },
+  "homepage": "https://github.com/multiformats/js-multicodec-packed#readme",
+  "dependencies": {
+    "varint": "^4.0.1"
+  }
+}

src/base-table.js

@@ -0,0 +1,20 @@
+const varint = require('varint')
+
+exports = module.exports
+
+// Miscellaneous
+exports.raw = varintBuf(0)          // 0x00
+
+// Multiformats
+exports.multicodec = varintBuf(64)  // 0x40
+exports.multihash = varintBuf(65)   // 0x41
+exports.multiaddr = varintBuf(66)   // 0x42
+
+// Serialization formats
+exports.protobuf = varintBuf(80)    // 0x50
+exports.cbor = varintBuf(81)        // 0x51
+exports.rlp = varintBuf(96)         // 0x60
+
+function varintBuf (n) {
+  return new Buffer(varint.encode(n))
+}

src/index.js

@@ -0,0 +1,36 @@
+'use strict'
+
+const table = require('./base-table')
+const varint = require('varint')
+
+exports = module.exports
+
+exports.addPrefix = (multicodecStrOrCode, data) => {
+  let pfx
+
+  if (Buffer.isBuffer(multicodecStrOrCode)) {
+    pfx = multicodecStrOrCode
+  } else {
+    if (table[multicodecStrOrCode]) {
+      pfx = table[multicodecStrOrCode]
+    } else {
+      throw new Error('multicodec not recognized')
+    }
+  }
+  return Buffer.concat([pfx, data])
+}
+
+exports.rmPrefix = (data) => {
+  varint.decode(data)
+  return data.slice(varint.decode.bytes)
+}
+
+exports.getMulticodec = (prefixedData) => {
+  const code = new Buffer(varint.decode(prefixedData))
+  Object.keys(table)
+        .forEach((multicodec) => {
+          if (Buffer.isEqual(code, table[multicodec])) {
+            return multicodec
+          }
+        })
+}