feat: immutable block and cached keys

Fixes #10

src/index.js

@@ -14,7 +14,18 @@ function Block (data) {
     throw new Error('Block must be constructed with data')
   }
 
-  this.data = ensureBuffer(data)
+  this._cache = {}
+
+  data = ensureBuffer(data)
+
+  Object.defineProperty(this, 'data', {
+    get () {
+      return data
+    },
+    set () {
+      throw new Error('Tried to change an immutable block')
+    }
+  })
 
   this.key = (hashFunc, callback) => {
     if (typeof hashFunc === 'function') {
@@ -26,7 +37,18 @@ function Block (data) {
       hashFunc = 'sha2-256'
     }
 
-    multihashing(this.data, hashFunc, callback)
+    if (this._cache[hashFunc]) {
+      setImmediate(() => {
+        callback(null, this._cache[hashFunc])
+      })
+    }
+    multihashing(this.data, hashFunc, (err, multihash) => {
+      if (err) {
+        return callback(err)
+      }
+      this._cache[hashFunc] = multihash
+      callback(null, multihash)
+    })
   }
 }
 

test/block.spec.js

@@ -62,13 +62,13 @@ describe('block', () => {
     })
   })
 
-  it.skip('block stays immutable', () => {
-    // it from the original implementation
-    // It doesn't stricly verify the immutability of the Block object
+  it('block stays immutable', () => {
     const block = new Block("Can't change this!")
-    let key = block.key()
-    key = new Buffer('new key')
 
-    expect(key.equals(block.key())).to.equal(false)
+    expect(
+      () => { block.data = 'fail' }
+    ).to.throw(
+      /immutable/
+    )
   })
 })