resources: Add a utility to map from struct fields

For GAPI front ends that want to know what fields they can use and which
they map to, these two functions can be used.

resources/util.go

@@ -22,11 +22,18 @@ import (
 	"encoding/base64"
 	"encoding/gob"
 	"fmt"
+	"reflect"
 	"sort"
+	"strings"
 
 	errwrap "github.com/pkg/errors"
 )
 
+const (
+	// StructTag is the key we use in struct field names for key mapping.
+	StructTag = "lang"
+)
+
 // ResourceSlice is a linear list of resources. It can be sorted.
 type ResourceSlice []Res
 
@@ -77,3 +84,52 @@ func B64ToRes(str string) (Res, error) {
 	}
 	return res, nil
 }
+
+// StructTagToFieldName returns a mapping from recommended alias to actual field
+// name. It returns an error if it finds a collision. It uses the `lang` tags.
+func StructTagToFieldName(res Res) (map[string]string, error) {
+	// TODO: fallback to looking up yaml tags, although harder to parse
+	result := make(map[string]string) // `lang` field tag -> field name
+	st := reflect.TypeOf(res).Elem()  // elem for ptr to res
+	for i := 0; i < st.NumField(); i++ {
+		field := st.Field(i)
+		name := field.Name
+		// TODO: golang 1.7+
+		// if !ok, then nothing is found
+		//if alias, ok := field.Tag.Lookup(StructTag); ok { // golang 1.7+
+		if alias := field.Tag.Get(StructTag); alias != "" { // golang 1.6
+			if val, exists := result[alias]; exists {
+				return nil, fmt.Errorf("field `%s` uses the same key `%s` as field `%s`", name, alias, val)
+			}
+			// empty string ("") is a valid value
+			if alias != "" {
+				result[alias] = name
+			}
+		}
+	}
+	return result, nil
+}
+
+// LowerStructFieldNameToFieldName returns a mapping from the lower case version
+// of each field name to the actual field name. It only returns public fields.
+// It returns an error if it finds a collision.
+func LowerStructFieldNameToFieldName(res Res) (map[string]string, error) {
+	result := make(map[string]string) // lower field name -> field name
+	st := reflect.TypeOf(res).Elem()  // elem for ptr to res
+	for i := 0; i < st.NumField(); i++ {
+		field := st.Field(i)
+		name := field.Name
+
+		if strings.Title(name) != name { // must have been a priv field
+			continue
+		}
+
+		if alias := strings.ToLower(name); alias != "" {
+			if val, exists := result[alias]; exists {
+				return nil, fmt.Errorf("field `%s` uses the same key `%s` as field `%s`", name, alias, val)
+			}
+			result[alias] = name
+		}
+	}
+	return result, nil
+}