Builds a map from indexes to pairs of labels. This is for caching distances, to avoid recomputing Hamming distances especially when dealing with high-dimensional (large \(P\)) matrices.
buildForward(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