feat: preserve base when constructed from a string

BREAKING CHANGE - previously base was not preserved and all CIDs would
be normalised to base58btc when asking for their string representation.

The default will change to base32 in https://github.com/multiformats/js-cid/pull/73/files

The idea behind this change is that we shouldnt lose information when
the user passes us a base encoded string, but keep it and use it as
the default base so toString returns the same string they provided.

I'd like this as a fix for ipld explorer, which currently forces all
CIDs into base58btc, seee: ipfs/ipfs-webui#999

This PR is the smallest change I can see to make it work. Do we want
to also add base as a constructor parameter? If so, i guess it should
go at the end as it's optional, but thats aesthetically displeasing
as the base encoding goes at the front in string form.

License: MIT
Signed-off-by: Oli Evans <oli@tableflip.io>

src/index.js

@@ -62,15 +62,21 @@ class CID {
    *
    */
   constructor (version, codec, multihash) {
+    let base = null
     if (module.exports.isCID(version)) {
       let cid = version
       this.version = cid.version
       this.codec = cid.codec
       this.multihash = Buffer.from(cid.multihash)
+      if (cid.base) {
+        this.base = cid.base
+      }
       return
     }
     if (typeof version === 'string') {
-      if (multibase.isEncoded(version)) { // CID String (encoded with multibase)
+      // base is the base name e.g. 'base32' or false
+      base = multibase.isEncoded(version)
+      if (base) { // CID String (encoded with multibase)
         const cid = multibase.decode(version)
         version = parseInt(cid.slice(0, 1).toString('hex'), 16)
         codec = multicodec.getCodec(cid.slice(1))
@@ -95,6 +101,11 @@ class CID {
       }
     }
 
+    /**
+     * @type {string}
+     */
+    this.base = base || 'base58btc'
+
     /**
      * @type {string}
      */
@@ -197,7 +208,7 @@ class CID {
    * @returns {string}
    */
   toBaseEncodedString (base) {
-    base = base || 'base58btc'
+    base = base || this.base
 
     switch (this.version) {
       case 0: {

test/index.spec.js

@@ -167,6 +167,32 @@ describe('CID', () => {
     })
   })
 
+  describe('.toString', () => {
+    it('returns a CID string', () => {
+      const cid = new CID(hash)
+      expect(cid.toString()).to.equal('QmatYkNGZnELf8cAGdyJpUca2PyY4szai3RHyyWofNY1pY')
+    })
+
+    it('returns a string in the same base as the string passed to the constructor - base64 flavour', () => {
+      const base64Str = 'mAXASIOnrbGCADfkPyOI37VMkbzluh1eaukBqqnl2oFaFnuIt'
+      const cid = new CID(base64Str)
+      expect(cid.toString()).to.equal(base64Str)
+    })
+
+    it('returns a string in the same base as the string passed to the constructor - base16 flavour', () => {
+      const base16Str = 'f01701220e9eb6c60800df90fc8e237ed53246f396e87579aba406aaa7976a056859ee22d'
+      const cid = new CID(base16Str)
+      expect(cid.toString()).to.equal(base16Str)
+    })
+
+    it('returns a string in the base provided', () => {
+      const b58v1Str = 'zdj7Wd8AMwqnhJGQCbFxBVodGSBG84TM7Hs1rcJuQMwTyfEDS'
+      const b32v1Str = 'bafybeidskjjd4zmr7oh6ku6wp72vvbxyibcli2r6if3ocdcy7jjjusvl2u'
+      const cid = new CID(b58v1Str)
+      expect(cid.toString('base32')).to.equal(b32v1Str)
+    })
+  })
+
   describe('utilities', () => {
     const h1 = 'QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n'
     const h2 = 'QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1o'
@@ -207,19 +233,6 @@ describe('CID', () => {
         CID.isCID(new CID(h1).toV1())
       ).to.equal(true)
     })
-
-    it('.toString() outputs default base encoded CID', () => {
-      const mhStr = 'QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n'
-      const cid = new CID(mhStr)
-      expect(`${cid}`).to.equal(mhStr)
-    })
-
-    it('.toString(base) outputs base encoded CID', () => {
-      const b58v1Str = 'zdj7Wd8AMwqnhJGQCbFxBVodGSBG84TM7Hs1rcJuQMwTyfEDS'
-      const b32v1Str = 'bafybeidskjjd4zmr7oh6ku6wp72vvbxyibcli2r6if3ocdcy7jjjusvl2u'
-      const cid = new CID(b58v1Str)
-      expect(cid.toString('base32')).to.equal(b32v1Str)
-    })
   })
 
   describe('throws on invalid inputs', () => {