Product Advertising API から書籍情報を取得する

paapi.go

package paapi

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"io/ioutil"
	"log"
	"net/http"
	"net/url"
	"strings"
	"time"

	xj "github.com/basgys/goxml2json"
	errors "github.com/pkg/errors"
)

var endpoint = "http://ecs.amazonaws.jp/onca/xml"

func GetJsonFromPaapi(accessKeyId, secretKeyId, associateTag, asin string) (string, error) {
	u, err := url.Parse(endpoint)
	if err != nil {
		return "", err
	}

	delimiter := "?"
	if len(u.Query()) > 0 {
		delimiter = "&"
	}

	v := url.Values{}
	v.Add("AWSAccessKeyId", accessKeyId)
	v.Add("AssociateTag", associateTag)
	v.Add("ItemId", asin)
	v.Add("Operation", "ItemLookup")
	v.Add("ResponseGroup", "ItemAttributes,Images")
	v.Add("Service", "AWSCommerceService")
	v.Add("Timestamp", time.Now().Format("2006-01-02T15:04:05Z"))

	signature := makeHMAC(strings.Join([]string{
		"GET",
		u.Hostname(),
		u.EscapedPath(),
		v.Encode(),
	}, "\n"), secretKeyId)
	v.Add("Signature", signature)

	u, err = url.Parse(u.String() + delimiter + v.Encode())
	if err != nil {
		return "", errors.Wrap(err, "failure to create URL for PAAPI")
	}

	response, err := http.Get(u.String())
	if err != nil {
		return "", errors.Wrap(err, "failure to get XML from PAAPI")
	}
	defer response.Body.Close()

	byteArray, err := ioutil.ReadAll(response.Body)
	if err != nil {
		return "", errors.Wrap(err, "failure to get XML from PAAPI")
	}

	xml := strings.NewReader(string(byteArray))
	json, err := xj.Convert(xml)
	if err != nil {
		return "", errors.Wrap(err, "failure to convert XML to JSON")
		log.Fatal(err)
	}
	return json.String(), nil
}

// http://cipepser.hatenablog.com/entry/2017/05/27/100516
func makeHMAC(msg, key string) string {
	mac := hmac.New(sha256.New, []byte(key))
	mac.Write([]byte(msg))
	return base64.StdEncoding.EncodeToString(mac.Sum(nil))
}