Builds a map from pairs of labels to indexes. This is for caching distances, to avoid recomputing Hamming distances especially when dealing with high-dimensional (large \(P\)) matrices.
buildReverse(N)Sample size, i.e., nrow(\(\mathbf{X}\))
\(N \times N\) matrix whose entries record the index corresponding to the pair of labels (indexed by the matrix dims)
Dependencies: None