strutil: introduce JoinNonEmpty

JoinNonEmpty concatenates non-empty strings with the specified
separator. This overcomes a problem with strings.Join, which
introduces separators for empty strings.

strutil/strutil.go

@@ -355,3 +355,18 @@ func WordWrapPadded(out io.Writer, text []rune, pad string, termWidth int) error
 	}
 	return WordWrap(out, text, indent, indent, termWidth)
 }
+
+// JoinNonEmpty concatenates non-empty strings using sep as separator,
+// and trimming sep from beginning and end of the strings. This
+// overcomes a problem with strings.Join, which will introduce
+// separators for empty strings.
+func JoinNonEmpty(strs []string, sep string) string {
+	nonEmpty := make([]string, 0, len(strs))
+	for _, s := range strs {
+		s = strings.Trim(s, sep)
+		if s != "" {
+			nonEmpty = append(nonEmpty, s)
+		}
+	}
+	return strings.Join(nonEmpty, sep)
+}

strutil/strutil_test.go

@@ -347,3 +347,18 @@ func (strutilSuite) TestWordWrapPadded(c *check.C) {
 		c.Check(buf.String(), check.Equals, t.output)
 	}
 }
+
+func (strutilSuite) TestJoinNonEmpty(c *check.C) {
+	for _, t := range []struct {
+		in  []string
+		out string
+	}{
+		{in: []string{}, out: ""},
+		{in: []string{"", "bar"}, out: "bar"},
+		{in: []string{"", "  ", "bar"}, out: "bar"},
+		{in: []string{"foo", "", "bar"}, out: "foo bar"},
+		{in: []string{" val ", "  ", " boo"}, out: "val boo"},
+	} {
+		c.Check(strutil.JoinNonEmpty(t.in, " "), check.Equals, t.out)
+	}
+}