From 1bf644c43ff7eda975fbfec1d1ad53db8e0856cf Mon Sep 17 00:00:00 2001
From: Gregory Maxwell <greg@xiph.org>
Date: Sat, 22 Oct 2016 05:33:25 +0000
Subject: [PATCH] IBD check uses minimumchain work instead of checkpoints.

---
 src/chainparams.cpp    | 10 ++++++++++
 src/consensus/params.h |  1 +
 src/main.cpp           |  6 ++++--
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 3d7182423..d4c6e8c59 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -119,6 +119,9 @@ public:
         consensus.nStakeMinConfirmations = 500;
         consensus.nStakeMinAge = 8 * 60 * 60; // 8 hours
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00");
+
         /**
          * The message start string is designed to be unlikely to occur in normal data.
          * The characters are rarely used upper ASCII, not valid as UTF-8, and produce
@@ -215,6 +218,10 @@ public:
         pchMessageStart[3] = 0xef;
         nDefaultPort = 25714;
         nMaxTipAge = 0x7fffffff;
+
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00");
+
         nPruneAfterHeight = 1000;
 
         genesis = CreateGenesisBlock(1393221600, 216178, 0x1f00ffff, 1, 0);
@@ -280,6 +287,9 @@ public:
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 1199145601; // January 1, 2008
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = 1230767999; // December 31, 2008
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00");
+
 //      Deployment of BIP68, BIP112, and BIP113.
 //      consensus.vDeployments[Consensus::DEPLOYMENT_CSV].bit = 0;
 //      consensus.vDeployments[Consensus::DEPLOYMENT_CSV].nStartTime = 999999999999ULL; // never
diff --git a/src/consensus/params.h b/src/consensus/params.h
index d43c865ec..e59236eed 100644
--- a/src/consensus/params.h
+++ b/src/consensus/params.h
@@ -75,6 +75,7 @@ struct Params {
     int nCoinbaseMaturity;
     int nStakeMinConfirmations;
     unsigned int nStakeMinAge;
+    uint256 nMinimumChainWork;
 };
 } // namespace Consensus
 
diff --git a/src/main.cpp b/src/main.cpp
index d8c613f24..0110a8d4b 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1698,7 +1698,9 @@ bool IsInitialBlockDownload()
     LOCK(cs_main);
     if (fImporting || fReindex)
         return true;
-    if (fCheckpointsEnabled && chainActive.Height() < Checkpoints::GetTotalBlocksEstimate(chainParams.Checkpoints()))
+    if (chainActive.Tip() == NULL)
+        return true;
+    if (chainActive.Tip()->nChainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
         return true;
     static bool lockIBDState = false;
     if (lockIBDState)
@@ -1738,7 +1740,7 @@ void CheckForkWarningConditions()
 {
     AssertLockHeld(cs_main);
     // Before we get past initial download, we cannot reliably alert about forks
-    // (we assume we don't get stuck on a fork before the last checkpoint)
+    // (we assume we don't get stuck on a fork before finishing our initial sync)
     if (IsInitialBlockDownload())
         return;