added onlyIfChanged opt

README.md

@@ -62,6 +62,7 @@ Make a new Hyperbee instance. `core` should be a [Hypercore](https://github.com/
 {
   keyEncoding: 'binary', // "binary" (default), "utf-8", "ascii", "json", or an abstract-encoding
   valueEncoding: 'binary' // Same options as keyEncoding like "json", etc
+  onlyIfChanged: true // put a value only if it means a change in the db
 }
 ```
 

index.js

@@ -1,3 +1,4 @@
+const sameData = require('same-data')
 const codecs = require('codecs')
 const { Readable } = require('streamx')
 const mutexify = require('mutexify/promise')
@@ -287,6 +288,7 @@ class Hyperbee extends ReadyResource {
     this.sep = opts.sep || SEP
     this.readonly = !!opts.readonly
     this.prefix = opts.prefix || null
+    this.onlyIfChanged = !!opts.onlyIfChanged
 
     this._unprefixedKeyEncoding = this.keyEncoding
     this._sub = !!this.prefix
@@ -669,7 +671,7 @@ class Batch {
   async put (key, value, opts) {
     const release = this.batchLock ? await this.batchLock() : null
 
-    const cas = (opts && opts.cas) || null
+    const cas = (opts && opts.cas) || (this.tree.onlyIfChanged ? sameData : null)
     const encoding = this._getEncoding(opts)
 
     if (!this.locked) await this.lock()

package.json

@@ -16,6 +16,7 @@
     "protocol-buffers-encodings": "^1.2.0",
     "ready-resource": "^1.0.0",
     "safety-catch": "^1.0.2",
+    "same-data": "^1.0.0",
     "streamx": "^2.12.4"
   },
   "devDependencies": {

test/basic.js

@@ -1,7 +1,6 @@
 const test = require('brittle')
 const b4a = require('b4a')
 const { create, collect, createCore } = require('./helpers')
-
 const Hyperbee = require('..')
 
 test('out of bounds iterator', async function (t) {
@@ -486,3 +485,39 @@ test('supports encodings in snapshot', async function (t) {
   t.alike(await snap1.get('hi'), { seq: 1, key: b4a.from('hi'), value: 'there' })
   t.alike(await snap2.get('hi'), { seq: 1, key: 'hi', value: b4a.from('there') })
 })
+
+test('onlyIfChanged put', async function (t) {
+  {
+    const db = create({ onlyIfChanged: true })
+    await db.ready()
+    await db.put('key', 'value')
+    t.is(db.feed.length, 2)
+    await db.put('key', 'value')
+    t.is(db.feed.length, 2)
+  }
+  {
+    const db = create({ onlyIfChanged: true, valueEncoding: 'binary' })
+    await db.ready()
+    await db.put('key', Buffer.from('buffer'))
+    t.is(db.feed.length, 2)
+    await db.put('key', Buffer.from('buffer'))
+    t.is(db.feed.length, 2)
+  }
+  {
+    const db = create({ onlyIfChanged: true, valueEncoding: 'json' })
+    await db.ready()
+    await db.put('key', { a: 1 })
+    t.is(db.feed.length, 2)
+    await db.put('key', { a: 1 })
+    t.is(db.feed.length, 2)
+  }
+})
+
+test('onlyIfChanged put after del', async function (t) {
+  const db = create({ onlyIfChanged: true })
+  await db.ready()
+  await db.put('key', 'value')
+  await db.del('key')
+  await db.put('key', 'value')
+  t.is(db.feed.length, 4)
+})