Add support for get/set of a partial hash (#16)

* Add support for get/set of a partial hash

* make actually mergable

README.md

@@ -44,6 +44,14 @@ Update the hash with a new piece of data. `data` should be a buffer or uint8arra
 
 Digest the hash.
 
+#### `hash.getPartialHash()`
+
+Returns the current partial hash.
+
+#### `hash.setPartialHash(data)`
+
+Set the hash to a previously set hash. `data` should be the result of `getPartialHash()` (which returns uint8array)
+
 #### `var promise = blake2b.ready([cb])`
 
 Wait for the WASM code to load. Returns the WebAssembly instance promise as well for convenience.

index.js

@@ -123,6 +123,14 @@ Blake2b.ready = function (cb) {
 
 Blake2b.prototype.ready = Blake2b.ready
 
+Blake2b.prototype.getPartialHash = function () {
+  return wasm.memory.slice(this.pointer, this.pointer + 216);
+}
+
+Blake2b.prototype.setPartialHash = function (ph) {
+  wasm.memory.set(ph, this.pointer);
+}
+
 function noop () {}
 
 function hexSlice (buf, start, len) {

test.js

@@ -55,6 +55,22 @@ blake2b.ready(function () {
     t.end()
   })
 
+  tape('allows getting & setting a partial hash', function (t) {
+    var a = blake2b(64)
+    var b = blake2b(64)
+
+    var partialHash = a
+      .update(Buffer.from('hello'))
+      .update(Buffer.from(' '))
+      .update(Buffer.from('world'))
+      .getPartialHash()
+
+    b.setPartialHash(partialHash)
+
+    t.same(a.digest(), b.digest())
+    t.end()
+  })
+
   vectors.forEach(function (vector, i) {
     tape('test-vectors.json #' + i, function (t) {
       var key = vector.key && Buffer.from(vector.key, 'hex')