NaviWatcher/internal/musicbrainz/client.go

package musicbrainz

import (
	"context"
	"encoding/xml"
	"fmt"
	"io"
	"net/http"
	"time"

	"golang.org/x/time/rate"
	"naviwatcher/internal/config"
)

// MusicBrainzClient wraps net/http.Client with rate limiting and configuration
// for the MusicBrainz Web Service API (version 2).
type MusicBrainzClient struct {
	httpClient  *http.Client
	userAgent   string
	baseURL     string
	rateLimiter *rate.Limiter
}

// NewClient creates a new MusicBrainzClient from the given configuration.
// It initializes the HTTP client with a 30-second timeout and sets up
// a rate limiter for 1 request per second as required by MusicBrainz policy.
func NewClient(cfg config.MusicBrainzConfig) *MusicBrainzClient {
	return &MusicBrainzClient{
		httpClient: &http.Client{
			Timeout: 30 * time.Second,
		},
		userAgent:   cfg.UserAgent,
		baseURL:     "https://musicbrainz.org/ws/2",
		rateLimiter: rate.NewLimiter(rate.Limit(1), 1),
	}
}

// doGet performs a rate-limited HTTP GET request to the MusicBrainz API.
// It blocks until the rate limiter allows the request, then sets the proper
// User-Agent header and returns the response body.
func (c *MusicBrainzClient) doGet(ctx context.Context, path string) ([]byte, error) {
	if err := c.rateLimiter.Wait(ctx); err != nil {
		return nil, fmt.Errorf("rate limiter wait: %w", err)
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.baseURL+path, nil)
	if err != nil {
		return nil, fmt.Errorf("create request: %w", err)
	}
	req.Header.Set("User-Agent", c.userAgent)
	req.Header.Set("Accept", "application/xml")

	resp, err := c.httpClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("execute request: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		body, _ := io.ReadAll(io.LimitReader(resp.Body, 1024))
		return nil, fmt.Errorf("musicbrainz API returned HTTP %d: %s", resp.StatusCode, string(body))
	}

	body, err := io.ReadAll(io.LimitReader(resp.Body, 10*1024*1024)) // 10MB limit
	if err != nil {
		return nil, fmt.Errorf("read response body: %w", err)
	}

	return body, nil
}

// mbArtistRef represents the nested artist element inside a release-group.
type mbArtistRef struct {
	ID   string `xml:"id,attr"`
	Name string `xml:"name"`
}

// mbNameCredit represents the name-credit element inside a release-group.
type mbNameCredit struct {
	Artist mbArtistRef `xml:"artist"`
}

// mbArtistCredit represents the artist-credit element inside a release-group.
type mbArtistCredit struct {
	NameCredit mbNameCredit `xml:"name-credit"`
}

// mbReleaseGroup represents the XML structure of a single release-group
// in the MusicBrainz release-group list response.
type mbReleaseGroup struct {
	ID           string         `xml:"id,attr"`
	Title        string         `xml:"title"`
	Type         string         `xml:"type,attr"`
	Status       string         `xml:"status,attr"`
	ArtistCredit mbArtistCredit `xml:"artist-credit"`
	ReleaseDate  string         `xml:"first-release-date"`
}

// mbReleaseGroupListXML wraps the release-group-list element to properly
// capture both child elements and the count attribute.
type mbReleaseGroupListXML struct {
	ReleaseGroups []mbReleaseGroup `xml:"release-group"`
	Count         int              `xml:"count,attr"`
}

// mbReleaseGroupList represents the XML structure of a release-group list response.
type mbReleaseGroupList struct {
	XMLName          xml.Name              `xml:"metadata"`
	ReleaseGroupList mbReleaseGroupListXML `xml:"release-group-list"`
}

// ParseReleaseGroups parses a MusicBrainz release-group list XML response
// into a ParsedReleaseGroups struct.
func ParseReleaseGroups(data []byte) (*ParsedReleaseGroups, error) {
	var list mbReleaseGroupList
	if err := xml.Unmarshal(data, &list); err != nil {
		return nil, fmt.Errorf("parse release-group XML: %w", err)
	}

	result := &ParsedReleaseGroups{
		Count: list.ReleaseGroupList.Count,
	}
	for _, rg := range list.ReleaseGroupList.ReleaseGroups {
		result.ReleaseGroups = append(result.ReleaseGroups, ReleaseGroup{
			ID:          rg.ID,
			Title:       rg.Title,
			Type:        rg.Type,
			Status:      rg.Status,
			ArtistID:    rg.ArtistCredit.NameCredit.Artist.ID,
			ArtistName:  rg.ArtistCredit.NameCredit.Artist.Name,
			ReleaseDate: rg.ReleaseDate,
		})
	}
	return result, nil
}