BIP113(Median Time Past)

// https://github.com/btcsuite/btcd/blob/7d2daa5bfef28c5e282571bc06416516936115ee/blockchain/blockindex.go#L185

// CalcPastMedianTime calculates the median time of the previous few blocks
// prior to, and including, the block node.
//
// This function is safe for concurrent access.
func (node *blockNode) CalcPastMedianTime() time.Time {
    // Create a slice of the previous few block timestamps used to calculate
    // the median per the number defined by the constant medianTimeBlocks.
    timestamps := make([]int64, medianTimeBlocks)
    numNodes := 0
    iterNode := node
    for i := 0; i < medianTimeBlocks && iterNode != nil; i++ {
            timestamps[i] = iterNode.timestamp
            numNodes++

            iterNode = iterNode.parent
    }

    // Prune the slice to the actual number of available timestamps which
    // will be fewer than desired near the beginning of the block chain
    // and sort them.
    timestamps = timestamps[:numNodes]
    sort.Sort(timeSorter(timestamps))

    // NOTE: The consensus rules incorrectly calculate the median for even
    // numbers of blocks.  A true median averages the middle two elements
    // for a set with an even number of elements in it.   Since the constant
    // for the previous number of blocks to be used is odd, this is only an
    // issue for a few blocks near the beginning of the chain.  I suspect
    // this is an optimization even though the result is slightly wrong for
    // a few of the first blocks since after the first few blocks, there
    // will always be an odd number of blocks in the set per the constant.
    //
    // This code follows suit to ensure the same rules are used, however, be
    // aware that should the medianTimeBlocks constant ever be changed to an
    // even number, this code will be wrong.
    medianTimestamp := timestamps[numNodes/2]
    return time.Unix(medianTimestamp, 0)