Extract column indexing into reflect2 package

reflect2/fields.go

@@ -0,0 +1,27 @@
+package reflect2
+
+import "reflect"
+
+// TagIndex returns a map that associates tag values with field indices.
+func TagIndex(typ reflect.Type, tag string) map[string]int {
+	tagIndex := map[string]int{}
+	for i := 0; i < typ.NumField(); i++ {
+		tag := typ.Field(i).Tag.Get(tag)
+		if tag != "" {
+			tagIndex[tag] = i
+		}
+	}
+	return tagIndex
+}
+
+// AssociateColumns returns a map that associates column indices with fields.
+func AssociateColumns(typ reflect.Type, tag string, cols []string) map[int]int {
+	tagIndex := TagIndex(typ, tag)
+	colFields := map[int]int{}
+	for i, col := range cols {
+		if fi, ok := tagIndex[col]; ok {
+			colFields[i] = fi
+		}
+	}
+	return colFields
+}