KOAssignmentScore

基于reverse比对计算出每一个ko中的基因总数

计算KEGG的KO分配得分 S_KO

这个函数直接对应了 KEGG 帮助文档中的公式：

We define a score for each ortholog group in order to assign the best fitting K numbers to the query gene:

 S_KO = S_h - log_2(mn) - log_2( sum_{k=N}^{x} C_k * p^k * (1-p)^{x-k} )

where Sh is the highest score among all ortholog candidates in the ortholog group, m and n are the sequence lengths of the query and the target of BLAST, respectively, N is the number of organisms in ortholog group, x is the number of organisms in the original ortholog group from which this group is derived, and p is the ratio of the size of the original ortholog group versus the size of the entire GENES database. The second term is for the normalization of the first term by sequence lengths, and the third term is a weighting factor to consider the number of ortholog candidates that are found in the original.

Assignment Score (S_KO)

计算二项分布右尾概率的对数 (以2为底) 即计算 log2( sum_{k=N}^{x} C(x,k) p^k (1-p)^(x-k) )

右尾概率的以2为底的对数

计算二项式系数的对数 (以2为底) log2(C(n, k))， 使用 LogGamma 函数来避免计算大数阶乘导致的溢出。

计算以2为底的对数

为单个查询基因分配最佳KO。它会评估所有可能的KO，并选择得分最高的一个。

得分最高的KO分配候选，如果没有则返回Nothing