docs(examples): create git-ipld example (#993)

Author: magik6k

examples/traverse-ipld-graphs/README.md

@@ -17,6 +17,8 @@ This tutorial goes through several operations over IPLD graphs using the [DAG AP
 
 ## [traverse through a slice of the ethereum blockchain](./eth.js)
 
+## [traverse through a git repo](./git.js)
+
 ## Video of the demos
 
 Find a video with a walkthrough of this examples on Youtube:

examples/traverse-ipld-graphs/git-objects/0f328c91df28c5c01b9e9f9f7e663191fa156593

@@ -0,0 +1,74 @@
+'use strict'
+
+const createNode = require('./create-node.js')
+const asyncEach = require('async/each')
+const path = require('path')
+const multihashing = require('multihashing-async')
+const Block = require('ipfs-block')
+const CID = require('cids')
+const fs = require('fs')
+
+createNode((err, ipfs) => {
+  if (err) {
+    throw err
+  }
+
+  console.log('\nStart of the example:')
+
+  const gitObjects = [
+    path.join(__dirname, '/git-objects/0f328c91df28c5c01b9e9f9f7e663191fa156593'),
+    path.join(__dirname, '/git-objects/177bf18bc707d82b21cdefd0b43b38fc8c5c13fe'),
+    path.join(__dirname, '/git-objects/23cc25f631cb076d5de5036c87678ea713cbaa6a'),
+    path.join(__dirname, '/git-objects/4e425dba7745a781f0712c9a01455899e8c0c249'),
+    path.join(__dirname, '/git-objects/6850c7be7136e6be00976ddbae80671b945c3e9d'),
+    path.join(__dirname, '/git-objects/a5095353cd62a178663dd26efc2d61f4f61bccbe'),
+    path.join(__dirname, '/git-objects/dc9bd15e8b81b6565d3736f9c308bd1bba60f33a'),
+    path.join(__dirname, '/git-objects/e68e6f6e31857877a79fd6b3956898436bb5a76f'),
+    path.join(__dirname, '/git-objects/ee62b3d206cb23f939208898f32d8708c0e3fa3c'),
+    path.join(__dirname, '/git-objects/ee71cef5001b84b0314438f76cf0acd338a2fd21')
+  ]
+
+  asyncEach(gitObjects, (gitObjectsPath, cb) => {
+    const data = fs.readFileSync(gitObjectsPath)
+
+    multihashing(data, 'sha1', (err, multihash) => {
+      if (err) {
+        return cb(err)
+      }
+      const cid = new CID(1, 'git-raw', multihash)
+      console.log(cid.toBaseEncodedString())
+
+      ipfs.block.put(new Block(data, cid), cb)
+    })
+  }, (err) => {
+    if (err) {
+      throw err
+    }
+
+    const v1tag = 'z8mWaGfwSWLMPJ6Q2JdsAjGiXTf61Nbue'
+
+    function errOrLog(comment) {
+      return (err, result) => {
+        if (err) {
+          throw err
+        }
+
+        if (Buffer.isBuffer(result.value)) { //Blobs (files) are returned as buffer instance
+          result.value = result.value.toString()
+        }
+
+        console.log('-'.repeat(80))
+        console.log(comment)
+        console.log(result.value)
+      }
+    }
+
+
+    ipfs.dag.get(v1tag + '/', errOrLog('Tag object:'))
+    ipfs.dag.get(v1tag + '/object/message', errOrLog('Tagged commit message:'))
+    ipfs.dag.get(v1tag + '/object/parents/0/message', errOrLog('Parent of tagged commit:'))
+
+    ipfs.dag.get(v1tag + '/object/tree/src/hash/hello/hash', errOrLog('/src/hello file:'))
+    ipfs.dag.get(v1tag + '/object/parents/0/tree/src/hash/hello/hash', errOrLog('previous version of /src/hello file:'))
+  })
+})