feat: add codec name to codec code helper

Introduce the function `getCodeVarint()` to return a Buffer containing
the Varint encoded codec code of a given codec name. It throws an
error if the codec isn't found.

Reason for this change was the js-cid module that is using the
conversion table directly. If the codec doesn't exist it would throw
error messages that didn't make much sense.

src/index.js

@@ -61,3 +61,16 @@ exports.getCodec = (prefixedData) => {
   const codecName = codeToCodecName[code.toString('hex')]
   return codecName
 }
+
+/**
+ * Get the code as varint of a codec name.
+ * @param {string} codecName
+ * @returns {Buffer}
+ */
+exports.getCodeVarint = (codecName) => {
+  const code = codecNameToCodeVarint[codecName]
+  if (code === undefined) {
+    throw new Error('Codec `' + codecName + '` not found')
+  }
+  return code
+}

test/multicodec.spec.js

@@ -28,4 +28,17 @@ describe('multicodec', () => {
     expect(multicodec.getCodec(prefixedBuf)).to.equal('eth-block')
     expect(buf).to.eql(multicodec.rmPrefix(prefixedBuf))
   })
+
+  it('returns code via codec name', () => {
+    const code = multicodec.getCodeVarint('keccak-256')
+    expect(code).to.eql(Buffer.from('1b', 'hex'))
+  })
+
+  it('throws error on unknown codec name when getting the code', () => {
+    expect(() => {
+      multicodec.getCodeVarint('this-codec-doesnt-exist')
+    }).to.throw(
+      'Codec `this-codec-doesnt-exist` not found'
+    )
+  })
 })