add getDifficulty method

lib/block/blockheader.js

@@ -9,6 +9,8 @@ var Hash = require('../crypto/hash');
 var JSUtil = require('../util/js');
 var $ = require('../util/preconditions');
 
+var GENESIS_BITS = 0x1d00ffff;
+
 /**
  * Instantiate a BlockHeader from a Buffer, JSON object, or Object with
  * the properties of the BlockHeader
@@ -199,18 +201,36 @@ BlockHeader.prototype.toBufferWriter = function toBufferWriter(bw) {
 };
 
 /**
- * @link https://en.bitcoin.it/wiki/Difficulty
- * @returns {BN} - An instance of BN with the decoded difficulty bits
+ * Returns the target difficulty for this block
+ * @param {Number} bits
+ * @returns {BN} An instance of BN with the decoded difficulty bits
  */
-BlockHeader.prototype.getTargetDifficulty = function getTargetDifficulty(info) {
-  var target = new BN(this.bits & 0xffffff);
-  var mov = 8 * ((this.bits >>> 24) - 3);
+BlockHeader.prototype.getTargetDifficulty = function getTargetDifficulty(bits) {
+  bits = bits || this.bits;
+
+  var target = new BN(bits & 0xffffff);
+  var mov = 8 * ((bits >>> 24) - 3);
   while (mov-- > 0) {
     target = target.mul(new BN(2));
   }
   return target;
 };
 
+/**
+ * @link https://en.bitcoin.it/wiki/Difficulty
+ * @return {Number}
+ */
+BlockHeader.prototype.getDifficulty = function getDifficulty() {
+  var difficulty1TargetBN = this.getTargetDifficulty(GENESIS_BITS).mul(new BN(Math.pow(10, 8)));
+  var currentTargetBN = this.getTargetDifficulty();
+
+  var difficultyString = difficulty1TargetBN.div(currentTargetBN).toString(10);
+  var decimalPos = difficultyString.length - 8;
+  difficultyString = difficultyString.slice(0, decimalPos) + '.' + difficultyString.slice(decimalPos);
+
+  return parseFloat(difficultyString);
+};
+
 /**
  * @returns {Buffer} - The little endian hash buffer of the header
  */

test/block/blockheader.js

@@ -248,6 +248,42 @@ describe('BlockHeader', function() {
 
   });
 
+  describe('#getDifficulty', function() {
+    it('should get the correct difficulty for block 86756', function() {
+      var x = BlockHeader.fromRawBlock(dataRawBlockBuffer);
+      x.bits.should.equal(0x1c3fffc0);
+      x.getDifficulty().should.equal(4);
+    });
+
+    it('should get the correct difficulty for testnet block 552065', function() {
+      var x = new BlockHeader({
+        bits: 0x1b00c2a8
+      });
+      x.getDifficulty().should.equal(86187.62562209);
+    });
+
+    it('should get the correct difficulty for livenet block 373043', function() {
+      var x = new BlockHeader({
+        bits: 0x18134dc1
+      });
+      x.getDifficulty().should.equal(56957648455.01001);
+    });
+
+    it('should get the correct difficulty for livenet block 340000', function() {
+      var x = new BlockHeader({
+        bits: 0x1819012f
+      });
+      x.getDifficulty().should.equal(43971662056.08958);
+    });
+
+    it('should use exponent notation if difficulty is larger than Javascript number', function() {
+      var x = new BlockHeader({
+        bits: 0x0900c2a8
+      });
+      x.getDifficulty().should.equal(1.9220482782645836 * 1e48);
+    });
+  });
+
   it('coverage: caches the "_id" property', function() {
       var blockHeader = BlockHeader.fromRawBlock(dataRawBlockBuffer);
       blockHeader.id.should.equal(blockHeader.id);