add: io reader composition

interfaces/15-io-reader-composition/database.go

@@ -0,0 +1,112 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"reflect"
+)
+
+type database struct {
+	list  *list
+	types map[string]item
+}
+
+// load the list by decoding the data from any kind of data source.
+func (db *database) load(r io.Reader) error {
+	return json.NewDecoder(r).Decode(db)
+}
+
+func (db *database) MarshalJSON() ([]byte, error) {
+	type encodable struct {
+		Type string
+		Item item
+	}
+
+	var e []encodable
+
+	for _, it := range *db.list {
+		t := reflect.TypeOf(it).Elem().Name()
+		e = append(e, encodable{t, it})
+	}
+
+	return json.Marshal(e)
+}
+
+func (db *database) UnmarshalJSON(data []byte) error {
+	var decodables []struct {
+		Type string
+		Item json.RawMessage
+	}
+
+	if err := json.Unmarshal(data, &decodables); err != nil {
+		return err
+	}
+
+	for _, d := range decodables {
+		it, err := db.newItem(d.Item, d.Type)
+		if err != nil {
+			return err
+		}
+
+		*db.list = append(*db.list, it)
+	}
+
+	return nil
+}
+
+func (db *database) newItem(data []byte, typ string) (item, error) {
+	it, ok := db.types[typ]
+	if !ok {
+		return nil, fmt.Errorf("newItem: type (%q) does not exist", typ)
+	}
+
+	t := reflect.TypeOf(it)
+	e := t.Elem()
+	v := reflect.New(e)
+	it = v.Interface().(item)
+
+	return it, json.Unmarshal(data, &it)
+}
+
+func (db *database) register(typ string, it item) {
+	if db.types == nil {
+		db.types = make(map[string]item)
+	}
+	db.types[typ] = it
+}
+
+// I put it here as a reference implementation of manually reading from a reader.
+// func (db *database) loadX(r io.Reader) error {
+// 	buf := make([]byte, 64)
+//
+// 	var (
+// 		data []byte
+// 		n    int
+// 		err  error
+// 	)
+//
+// 	for {
+// 		n, err = r.Read(buf)
+//
+// 		if n > 0 {
+// 			data = append(data, buf[:n]...)
+//		}
+//
+// 		if err != nil {
+// 			break
+// 		}
+// 	}
+//
+// 	if err != io.EOF {
+// 		return err
+// 	}
+// 	return json.Unmarshal(data, db)
+// }