expose simple string slice based sorting to top-level bleve

this change means simple sort requirements no longer require
importing the search package (high-level API goal)

also the sort test at the top-level was changed to use this form

index_test.go

@@ -743,10 +743,7 @@ func TestSortMatchSearch(t *testing.T) {
 	}
 
 	req := NewSearchRequest(NewMatchQuery("One"))
-	req.SortBy(search.SortOrder{
-		&search.SortField{Field: "Day"},
-		&search.SortField{Field: "Name"},
-	})
+	req.SortBy([]string{"Day", "Name"})
 	req.Fields = []string{"*"}
 	sr, err := index.Search(req)
 	if err != nil {

search.go

@@ -223,7 +223,17 @@ func (r *SearchRequest) AddFacet(facetName string, f *FacetRequest) {
 }
 
 // SortBy changes the request to use the requested sort order
-func (r *SearchRequest) SortBy(order search.SortOrder) {
+// this form uses the simplified syntax with an array of strings
+// each string can either be a field name
+// or the magic value _id and _score which refer to the doc id and search score
+// any of these values can optionally be prefixed with - to reverse the order
+func (r *SearchRequest) SortBy(order []string) {
+	so := search.ParseSortOrderStrings(order)
+	r.Sort = so
+}
+
+// SortByCustom changes the request to use the requested sort order
+func (r *SearchRequest) SortByCustom(order search.SortOrder) {
 	r.Sort = order
 }
 

search/sort.go

@@ -98,7 +98,7 @@ func ParseSearchSortObj(input map[string]interface{}) (SearchSort, error) {
 	return nil, fmt.Errorf("unknown search sort by: %s", by)
 }
 
-func ParseSearchSortString(input string) (SearchSort, error) {
+func ParseSearchSortString(input string) SearchSort {
 	descending := false
 	if strings.HasPrefix(input, "-") {
 		descending = true
@@ -109,16 +109,16 @@ func ParseSearchSortString(input string) (SearchSort, error) {
 	if input == "_id" {
 		return &SortDocID{
 			Descending: descending,
-		}, nil
+		}
 	} else if input == "_score" {
 		return &SortScore{
 			Descending: descending,
-		}, nil
+		}
 	}
 	return &SortField{
 		Field:      input,
 		Descending: descending,
-	}, nil
+	}
 }
 
 func ParseSearchSortJSON(input json.RawMessage) (SearchSort, error) {
@@ -133,7 +133,16 @@ func ParseSearchSortJSON(input json.RawMessage) (SearchSort, error) {
 		}
 		return ParseSearchSortObj(sortObj)
 	}
-	return ParseSearchSortString(sortString)
+	return ParseSearchSortString(sortString), nil
+}
+
+func ParseSortOrderStrings(in []string) SortOrder {
+	rv := make(SortOrder, 0, len(in))
+	for _, i := range in {
+		ss := ParseSearchSortString(i)
+		rv = append(rv, ss)
+	}
+	return rv
 }
 
 func ParseSortOrderJSON(in []json.RawMessage) (SortOrder, error) {