golang runtime struct fields filtering

fields_filter.go

package main

import (
	"fmt"
	"net/http"
	"reflect"
	"strings"
)

type Person struct {
	ID      string
	Name    string
	Age     int
	Address string
}

func main() {
	var personStore = map[string]*Person{
		"1": {
			ID:      "1",
			Name:    "Person A",
			Age:     42,
			Address: "RU",
		},
		"2": {
			ID:      "2",
			Name:    "Person B",
			Age:     24,
			Address: "UR",
		},
	}
	fieldsIdx := newStructFieldsIdx(Person{})
	http.HandleFunc("/persons/", func(w http.ResponseWriter, r *http.Request) {
		personID := r.URL.Path[9:]
		p, ok := personStore[personID]
		if !ok {
			w.WriteHeader(http.StatusNotFound)
			return
		}
		fieldsToSelect := r.URL.Query()["field"]
		if len(fieldsToSelect) == 0 {
			fmt.Fprintf(w, "%+v\n", p)
			return
		}
		fmt.Fprintf(w, "%+v\n", fieldsIdx.Filter(p, fieldsToSelect))
	})
	http.ListenAndServe(":8080", nil)
}

type structFieldsIndex struct {
	t              reflect.Type
	fieldsByLcName map[string]reflect.StructField
}

func newStructFieldsIdx(structIntstance interface{}) *structFieldsIndex {
	t := reflect.TypeOf(structIntstance)
	idx := make(map[string]reflect.StructField)
	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		idx[strings.ToLower(f.Name)] = f
	}
	return &structFieldsIndex{
		t:              t,
		fieldsByLcName: idx,
	}
}
func (idx *structFieldsIndex) Filter(structInstance interface{}, fields []string) interface{} {
	elem := reflect.ValueOf(structInstance).Elem()
	ff := make([]reflect.StructField, 0, len(fields))
	vv := make([]reflect.Value, 0, len(fields))
	for _, name := range fields {
		f, ok := idx.fieldsByLcName[strings.ToLower(name)]
		if !ok {
			continue
		}
		ff = append(ff, f)
		vv = append(vv, elem.FieldByIndex(f.Index))
	}
	r := reflect.New(reflect.StructOf(ff)).Elem()
	for i, v := range vv {
		r.Field(i).Set(v)
	}
	return r.Addr().Interface()
}