mstr: add Split function

This is a convenience for strings.Split, with the addition that an empty input
string yields an empty output slice rather than a slice of one empty field.

mstr/mstr.go

@@ -33,14 +33,23 @@ func Trunc(s string, n int) string {
 
 // Lines splits its argument on newlines. It is a convenience function for
 // [strings.Split], except that it returns empty if s == "" and treats a
-// trailing newline as the end of the file.
+// trailing newline as the end of the file rather than an empty line.
 func Lines(s string) []string {
 	if s == "" {
 		return nil
 	}
 	return strings.Split(strings.TrimSuffix(s, "\n"), "\n")
 }
 
+// Split splits its argument on sep. It is a convenience function for
+// [strings.Split], except that it returns empty if s == "".
+func Split(s, sep string) []string {
+	if s == "" {
+		return nil
+	}
+	return strings.Split(s, sep)
+}
+
 // CompareNatural compares its arguments lexicographically, but treats runs of
 // decimal digits as the spellings of natural numbers and compares their values
 // instead of the individual digits.

mstr/mstr_test.go

@@ -7,7 +7,7 @@ import (
 	gocmp "github.com/google/go-cmp/cmp"
 )
 
-func TestString(t *testing.T) {
+func TestTrunc(t *testing.T) {
 	tests := []struct {
 		input string
 		size  int
@@ -59,6 +59,27 @@ func TestLines(t *testing.T) {
 	}
 }
 
+func TestSplit(t *testing.T) {
+	tests := []struct {
+		input, sep string
+		want       []string
+	}{
+		{"", "x", nil},
+		{"y", "x", []string{"y"}},
+		{"x", "x", []string{"", ""}},
+		{"ax", "x", []string{"a", ""}},
+		{"xa", "x", []string{"", "a"}},
+		{"axbxc", "x", []string{"a", "b", "c"}},
+		{"axxc", "x", []string{"a", "", "c"}},
+		{"a,b,c,,d", ",", []string{"a", "b", "c", "", "d"}},
+	}
+	for _, tc := range tests {
+		if diff := gocmp.Diff(mstr.Split(tc.input, tc.sep), tc.want); diff != "" {
+			t.Errorf("Split %q on %q (-got, +want):\n%s", tc.input, tc.sep, diff)
+		}
+	}
+}
+
 func TestCompareNatural(t *testing.T) {
 	tests := []struct {
 		a, b string