ethereum/btcrelay

Hi. Is anyone here, who can explain what exactly is happening in the m_saveAncestors and priv_fastGetBlockHash__ functions in btcChain.se? I understand the idea of the indexing of block ancestors but I'm concerned about the exact details of the implementation.
Specifically, this code block in m_saveAncestors

           with $i = 1:
                while $i < NUM_ANCESTOR_DEPTHS:
                    with $depth = m_getAncDepth($i):
                        if m_getHeight($blockHash) % $depth == 1:
                            m_mwrite32(ref($ancWord) + 4*$i, $prevIbIndex)
                        else:
                            m_mwrite32(ref($ancWord) + 4*$i, m_getAncestor($hashPrevBlock, $i))
                        $i += 1

Also, could it be, that priv_fastGetBlockHash__ fails if the requested block has more than 78125 confirmations? I may be wrong, but it seems to me that the following check will fail in this case:
(excerpt from priv_fastGetBlockHash__):

    blockHash = self.heaviestBlock
    anc_index = NUM_ANCESTOR_DEPTHS - 1 <-- this is equal to 7

    while m_getHeight(blockHash) > blockHeight:
        while m_getHeight(blockHash) - blockHeight < m_getAncDepth(anc_index) && anc_index > 0:
            anc_index -= 1
        blockHash = self.internalBlock[m_getAncestor(blockHash, anc_index)]

    return(blockHash)

Note: m_getAncDepth(7) returns 78125

Thx!