bitcoin最长链判断

先看看bitcoin的最长链判断，基于以下代码：

    struct CBlockIndexWorkComparator
    {
        bool operator()(CBlockIndex *pa, CBlockIndex *pb) const {
            // First sort by most total work, ...
            if (pa->nChainWork > pb->nChainWork) return false;
            if (pa->nChainWork < pb->nChainWork) return true;

            // ... then by earliest time received, ...
            if (pa->nSequenceId < pb->nSequenceId) return false;
            if (pa->nSequenceId > pb->nSequenceId) return true;

            // Use pointer address as tie breaker (should only happen with blocks
            // loaded from disk, as those all have id 0).
            if (pa < pb) return false;
            if (pa > pb) return true;

            // Identical blocks.
            return false;
        }
    };

用三个规则进行判断，从1到3如下：

哪个链工作量更大？（注意是工作量，并不是块数量）
哪个接收时间更先？（不同客户端可能不一样）
哪个有更大的指针地址？（不同客户端可能不一样）

第3条规则与抛硬币差不多，判断指针大小，这条规则的目的是为了总能比较出个结果，客户端能选一条链进行工作。
网络在同一时间找到两个不同的块是非常罕见的，如果出现后就可能基于不同链工作，只到下一个区块产生使另一分支变得更长，就会选择更长的分支工作。
真正起作用的还是第1条规则，看哪个条链最长！

比特币白皮书中有一段对此的描述

Nodes always consider the longest chain to be the correct one and will keep working on extending it. If two nodes broadcast different versions of the next block simultaneously, some nodes may receive one or the other first. In that case, they work on the first one they received, but save the other branch in case it becomes longer. The tie will be broken when the next proof-of-work is found and one branch becomes longer; the nodes that were working on the other branch will then switch to the longer one.