add pow retargeting to full chain.

lib/bcoin/fullchain.js

@@ -520,6 +520,69 @@ ChainBlock.prototype.getChainwork = function() {
   return (this.prev ? this.prev.chainwork : new bn(0)).add(this.proof);
 };
 
+Chain.prototype.target = function target(last) {
+  var proofOfWorkLimit = utils.toCompact(network.powLimit);
+  var adjustmentInterval = network.powTargetTimespan / network.powTargetSpacing;
+  var newBlockTs, heightFirst, first;
+
+  adjustmentInterval |= 0;
+
+  if (!last)
+    last = this.getTip();
+
+  // Do not retarget
+  if ((last.height + 1) % adjustmentInterval) {
+    if (network.powAllowMinDifficultyBlocks) {
+      // Special behavior for testnet:
+      newBlockTs = Date.now() / 1000 | 0;
+      if (newBlockTs > last.ts + network.powTargetSpacing * 2)
+        return proofOfWorkLimit;
+
+      while (last.prev
+        && last.height % adjustmentInterval !== 0
+        && last.bits !== proofOfWorkLimit) {
+        last = last.prev;
+      }
+
+      return last.bits;
+    }
+    return last.bits;
+  }
+
+  // Back 2 weeks
+  heightFirst = last.height - (adjustmentInterval - 1);
+  first = this.byHeight(heightFirst);
+
+  if (!first)
+    return 0;
+
+  return this.retarget(last, first.ts);
+};
+
+Chain.prototype.retarget = function retarget(last, firstTs) {
+  var powTargetTimespan = new bn(network.powTargetTimespan);
+  var actualTimespan, powLimit, target;
+
+  if (network.powNoRetargeting)
+    return last.bits;
+
+  actualTimespan = new bn(last.ts).subn(firstTs);
+  if (actualTimespan.cmp(powTargetTimespan.divn(4)) < 0)
+    actualTimespan = powTargetTimespan.divn(4);
+
+  if (actualTimespan.cmp(powTargetTimespan.muln(4)) > 0)
+    actualTimespan = powTargetTimespan.muln(4);
+
+  powLimit = network.powLimit;
+  target = utils.fromCompact(last.bits);
+  target.imul(actualTimespan);
+  target = target.div(powTargetTimespan);
+  if (target.cmp(powLimit) > 0)
+    target = powLimit.clone();
+
+  return utils.toCompact(target);
+};
+
 ChainBlock.prototype.toJSON = function() {
   // return [
   //   this.hash,

lib/bcoin/protocol/network.js

@@ -5,6 +5,7 @@
  */
 
 var bcoin = require('../../bcoin');
+var bn = require('bn.js');
 var utils = bcoin.utils;
 
 /**
@@ -107,6 +108,15 @@ main.magic = 0xd9b4bef9;
 
 main.preload = require('./preload');
 
+main.powLimit = new bn(
+  '00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff',
+  'hex'
+);
+main.powTargetTimespan = 14 * 24 * 60 * 60; // two weeks
+main.powTargetSpacing = 10 * 60;
+main.powAllowMinDifficultyBlocks = false;
+main.powNoRetargeting = false;
+
 /**
  * Testnet (v3)
  * https://en.bitcoin.it/wiki/Testnet
@@ -187,3 +197,12 @@ testnet.preload = {
   'ts': [testnet.genesis.ts],
   'heights': [0]
 };
+
+testnet.powLimit = new bn(
+  '00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff',
+  'hex'
+);
+testnet.powTargetTimespan = 14 * 24 * 60 * 60; // two weeks
+testnet.powTargetSpacing = 10 * 60;
+testnet.powAllowMinDifficultyBlocks = true;
+testnet.powNoRetargeting = false;