Replace SplitInTwo with LeftSplitInTwo/RightSplitInTwo for precise splitting

Replace SplitInTwo calls with LeftSplitInTwo or RightSplitInTwo depending on the intended split direction. In fastseq_json_header.go, extract rank from suffix without splitting; in biosequenceslice.go and taxid.go, use LeftSplitInTwo to split from the left; add RightSplitInTwo utility function for splitting from the right.

pkg/obiformats/fastseq_json_header.go

@@ -296,7 +296,7 @@ func _parse_json_header_(header string, sequence *obiseq.BioSequence) string {
 
 			case strings.HasSuffix(skey, "_taxid"):
 				if dataType == jsonparser.Number || dataType == jsonparser.String {
-					rank, _ := obiutils.SplitInTwo(skey, '_')
+					rank := skey[:len(skey)-len("_taxid")]
 
 					taxid := string(value)
 					sequence.SetTaxid(taxid, rank)

pkg/obiseq/biosequenceslice.go

@@ -195,7 +195,7 @@ func (s *BioSequenceSlice) ExtractTaxonomy(taxonomy *obitax.Taxonomy, seqAsTaxa
 				return nil, fmt.Errorf("sequence %v has no path", s.Id())
 			}
 			last := path[len(path)-1]
-			taxname, _ := obiutils.SplitInTwo(last, ':')
+			taxname, _ := obiutils.LeftSplitInTwo(last, ':')
 			if idx, ok := s.GetIntAttribute("seq_number"); !ok {
 				return nil, errors.New("sequences are not numbered")
 			} else {

pkg/obitax/taxid.go

@@ -31,7 +31,7 @@ func NewTaxidFactory(code string, alphabet obiutils.AsciiSet) *TaxidFactory {
 // It extracts the relevant part of the string after the first colon (':') if present.
 func (f *TaxidFactory) FromString(taxid string) (Taxid, error) {
 	taxid = obiutils.AsciiSpaceSet.TrimLeft(taxid)
-	part1, part2 := obiutils.SplitInTwo(taxid, ':')
+	part1, part2 := obiutils.LeftSplitInTwo(taxid, ':')
 	if len(part2) == 0 {
 		taxid = part1
 	} else {

pkg/obiutils/strings.go

@@ -144,7 +144,7 @@ func (r *AsciiSet) TrimLeft(s string) string {
 	return s[i:]
 }
 
-func SplitInTwo(s string, sep byte) (string, string) {
+func LeftSplitInTwo(s string, sep byte) (string, string) {
 	i := 0
 	for ; i < len(s); i++ {
 		c := s[i]
@@ -157,3 +157,17 @@ func SplitInTwo(s string, sep byte) (string, string) {
 	}
 	return s[:i], s[i+1:]
 }
+
+func RightSplitInTwo(s string, sep byte) (string, string) {
+	i := len(s) - 1
+	for ; i >= 0; i-- {
+		c := s[i]
+		if c == sep {
+			break
+		}
+	}
+	if i == len(s) {
+		return s, ""
+	}
+	return s[:i], s[i+1:]
+}