Fig. 3

Decision rule tree for classifying a DNA fragment from k-mer statistics \((h,h',g,g',b,x; n)\), meaning number of k-mers of type “host” (h), “weak host” (\(h'\)), “graft” (g), “weak graft” (\(g'\)), “both” (b), and number of k-mers not present in the key-value store (x), respectively; n is the total number of (valid) k-mers in the fragment. We also use weighted scores \(S_\text {host}:= h + \lfloor h'/2 \rfloor \) and \(S_\text {graft}:= g + \lfloor g'/2 \rfloor \) and thresholds \(T_\text {host}:= \lfloor n/4 \rfloor , T_\text {graft}:= \lfloor n/4 \rfloor \) and \(T_\text {both}:= \lfloor n/4 \rfloor \). A fragment is thus classified as “host”, “graft”, “both”, “neither”, or “ambiguous”. Category “ambiguous” is chosen if no other rule applies and no “else” rule is present in a node