feat: add convenience function for getting the codec code

It's not possible to get the varint encoding from a code as well
es getting the code from a varint encoded array-like structure.

src/index.js

@@ -65,6 +65,15 @@ exports.getCodec = (prefixedData) => {
   return codecName
 }
 
+/**
+ * Get the code of the prefixed data.
+ * @param {Buffer} prefixedData
+ * @returns {number}
+ */
+exports.getCode = (prefixedData) => {
+  return varint.decode(prefixedData)
+}
+
 /**
  * Get the code as varint of a codec name.
  * @param {string} codecName
@@ -78,6 +87,15 @@ exports.getCodeVarint = (codecName) => {
   return code
 }
 
+/**
+ * Get the varint of a code.
+ * @param {Number} code
+ * @returns {Array.<number>}
+ */
+exports.getVarint = (code) => {
+  return varint.encode(code)
+}
+
 // Make the constants top-level constants
 const constants = require('./constants')
 Object.assign(exports, constants)

test/multicodec.spec.js

@@ -34,6 +34,18 @@ describe('multicodec', () => {
     expect(code).to.eql(Buffer.from('1b', 'hex'))
   })
 
+  it('returns code from prefixed data', () => {
+    const buf = Buffer.from('hey')
+    const prefixedBuf = multicodec.addPrefix('dag-cbor', buf)
+    const code = multicodec.getCode(prefixedBuf)
+    expect(code).to.eql(multicodec.DAG_CBOR)
+  })
+
+  it('returns varint from code', () => {
+    const code = multicodec.getVarint(multicodec.KECCAK_256)
+    expect(code).to.eql([0x1b])
+  })
+
   it('throws error on unknown codec name when getting the code', () => {
     expect(() => {
       multicodec.getCodeVarint('this-codec-doesnt-exist')