add Block.calculateTarget, use Buffer comparison rather than bigi

package.json

@@ -62,6 +62,7 @@
     "bigi": "^1.4.0",
     "bip66": "^1.1.0",
     "bs58check": "^1.0.5",
+    "buffer-compare": "^1.1.0",
     "buffer-equals": "^1.0.3",
     "buffer-reverse": "^1.0.0",
     "create-hash": "^1.1.0",

src/block.js

@@ -1,6 +1,6 @@
-var BigInteger = require('bigi')
 var bufferutils = require('./bufferutils')
 var bcrypto = require('./crypto')
+var compare = require('buffer-compare')
 
 var Transaction = require('./transaction')
 
@@ -116,14 +116,21 @@ Block.prototype.toHex = function (headersOnly) {
   return this.toBuffer(headersOnly).toString('hex')
 }
 
-Block.prototype.verifyPow = function () {
-  var hash = BigInteger.fromBuffer([].reverse.call(this.getHash()))
-  var mov = ((this.bits >>> 24) - 3) << 3
-  var target = new BigInteger()
-  target.fromInt(this.bits & 0x000ffffff)
-  target = target.shiftLeft(mov)
+var ZEROS = '0000000000000000000000000000000000000000000000000000000000000000'
 
-  return hash.compareTo(target) <= 0
+Block.calculateTarget = function (bits) {
+  var exponent = ((bits & 0xff000000) >> 24) - 3
+  var mantissa = bits & 0x00ffffff
+  var target = mantissa * Math.pow(2, 8 * exponent)
+  var targetHex = Math.floor(target).toString(16)
+
+  return new Buffer(ZEROS.slice(0, 64 - targetHex.length) + targetHex, 'hex')
+}
+
+Block.prototype.verifyPow = function () {
+  var hash = [].reverse.call(this.getHash())
+
+  return compare(hash, Block.calculateTarget(this.bits)) <= 0
 }
 
 module.exports = Block

test/block.js

@@ -1,12 +1,21 @@
 /* global describe, it, beforeEach */
 
 var assert = require('assert')
-
 var Block = require('../src/block')
 
 var fixtures = require('./fixtures/block')
 
 describe('Block', function () {
+  describe('calculateTarget', function () {
+    fixtures.targets.forEach(function (f) {
+      it('returns ' + f.expected + ' for 0x' + f.bits, function () {
+        var bits = parseInt(f.bits, 16)
+
+        assert.equal(Block.calculateTarget(bits).toString('hex'), f.expected)
+      })
+    })
+  })
+
   describe('fromBuffer/fromHex', function () {
     fixtures.valid.forEach(function (f) {
       it('imports the block: ' + f.description + ' correctly', function () {

test/fixtures/block.json

@@ -1,4 +1,22 @@
-
+{
+  "targets": [
+    {
+      "bits": "1d00ffff",
+      "expected": "00000000ffff0000000000000000000000000000000000000000000000000000"
+    },
+    {
+      "bits": "1b80ffff",
+      "expected": "000000000080ffff000000000000000000000000000000000000000000000000"
+    },
+    {
+      "bits": "1b0404cb",
+      "expected": "00000000000404cb000000000000000000000000000000000000000000000000"
+    },
+    {
+      "bits": "1814dd04",
+      "expected": "000000000000000014dd04000000000000000000000000000000000000000000"
+    }
+  ],
   "valid": [
     {
       "description": "Headers only",