feat(listenbrainz): add SimilarArtists client method

internal/scrobble/listenbrainz/client.go

@@ -233,3 +233,56 @@ func (c *Client) SimilarRecordings(ctx context.Context, mbid string, limit int)
 		return nil, fmt.Errorf("%w: status %d", ErrPermanent, resp.StatusCode)
 	}
 }
+
+// LB algorithm parameter for /explore/similar-artists/. Hardcoded for v1
+// — matches LB's documented default.
+const lbSimilarArtistsAlgorithm = "session_based_days_7500_session_30_contribution_5_threshold_15_limit_100_filter_True_skip_30"
+
+// SimilarArtist is one entry in the /explore/similar-artists response.
+type SimilarArtist struct {
+	MBID  string  `json:"artist_mbid"`
+	Score float64 `json:"score"`
+}
+
+// SimilarArtists fetches up to `limit` similar artists for the given
+// artist MBID. Public endpoint — no token required. Returns the same
+// typed errors as SubmitListens.
+func (c *Client) SimilarArtists(ctx context.Context, mbid string, limit int) ([]SimilarArtist, error) {
+	base := c.BaseURL
+	if base == "" {
+		base = defaultBaseURL
+	}
+	url := fmt.Sprintf("%s/1/explore/similar-artists/%s?algorithm=%s&count=%d",
+		base, mbid, lbSimilarArtistsAlgorithm, limit)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("listenbrainz: build similar-artists request: %w", err)
+	}
+
+	httpClient := c.HTTP
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrTransient, err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	switch {
+	case resp.StatusCode >= 200 && resp.StatusCode < 300:
+		var out []SimilarArtist
+		if derr := json.NewDecoder(resp.Body).Decode(&out); derr != nil {
+			return nil, fmt.Errorf("%w: decode similar-artists: %v", ErrTransient, derr)
+		}
+		return out, nil
+	case resp.StatusCode == http.StatusUnauthorized:
+		return nil, ErrAuth
+	case resp.StatusCode == http.StatusTooManyRequests:
+		return nil, &RetryAfterError{Wait: parseRetryAfter(resp.Header.Get("Retry-After"))}
+	case resp.StatusCode >= 500:
+		return nil, fmt.Errorf("%w: status %d", ErrTransient, resp.StatusCode)
+	default:
+		return nil, fmt.Errorf("%w: status %d", ErrPermanent, resp.StatusCode)
+	}
+}

internal/scrobble/listenbrainz/client_test.go

@@ -257,3 +257,98 @@ func TestClient_SimilarRecordings_429_ReturnsRetryAfter(t *testing.T) {
 		t.Errorf("Wait = %v, want 120s", ra.Wait)
 	}
 }
+
+func TestClient_SimilarArtists_Success(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte(`[
+			{"artist_mbid": "33333333-3333-3333-3333-333333333333", "score": 0.91},
+			{"artist_mbid": "44444444-4444-4444-4444-444444444444", "score": 0.72}
+		]`))
+	})
+	defer srv.Close()
+	out, err := c.SimilarArtists(context.Background(), "aaaa", 100)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if len(out) != 2 {
+		t.Fatalf("got %d, want 2", len(out))
+	}
+	if out[0].MBID != "33333333-3333-3333-3333-333333333333" || out[0].Score != 0.91 {
+		t.Errorf("first result wrong: %+v", out[0])
+	}
+}
+
+func TestClient_SimilarArtists_AlgorithmParamSet(t *testing.T) {
+	var seenURL string
+	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+		seenURL = r.URL.String()
+		_, _ = w.Write([]byte(`[]`))
+	})
+	defer srv.Close()
+	_, _ = c.SimilarArtists(context.Background(), "abc", 50)
+	if !strings.Contains(seenURL, "algorithm=") {
+		t.Errorf("URL missing algorithm param: %q", seenURL)
+	}
+}
+
+func TestClient_SimilarArtists_LimitParamSet(t *testing.T) {
+	var seenURL string
+	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+		seenURL = r.URL.String()
+		_, _ = w.Write([]byte(`[]`))
+	})
+	defer srv.Close()
+	_, _ = c.SimilarArtists(context.Background(), "abc", 50)
+	if !strings.Contains(seenURL, "count=50") && !strings.Contains(seenURL, "limit=50") {
+		t.Errorf("URL missing count/limit param: %q", seenURL)
+	}
+}
+
+func TestClient_SimilarArtists_401_ReturnsErrAuth(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+	})
+	defer srv.Close()
+	_, err := c.SimilarArtists(context.Background(), "abc", 100)
+	if !errors.Is(err, ErrAuth) {
+		t.Errorf("err = %v, want ErrAuth", err)
+	}
+}
+
+func TestClient_SimilarArtists_400_ReturnsErrPermanent(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+	})
+	defer srv.Close()
+	_, err := c.SimilarArtists(context.Background(), "abc", 100)
+	if !errors.Is(err, ErrPermanent) {
+		t.Errorf("err = %v, want ErrPermanent", err)
+	}
+}
+
+func TestClient_SimilarArtists_503_ReturnsErrTransient(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+	})
+	defer srv.Close()
+	_, err := c.SimilarArtists(context.Background(), "abc", 100)
+	if !errors.Is(err, ErrTransient) {
+		t.Errorf("err = %v, want ErrTransient", err)
+	}
+}
+
+func TestClient_SimilarArtists_429_ReturnsRetryAfter(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Retry-After", "60")
+		w.WriteHeader(http.StatusTooManyRequests)
+	})
+	defer srv.Close()
+	_, err := c.SimilarArtists(context.Background(), "abc", 100)
+	var ra *RetryAfterError
+	if !errors.As(err, &ra) {
+		t.Fatalf("err = %v, want *RetryAfterError", err)
+	}
+	if ra.Wait != 60*time.Second {
+		t.Errorf("Wait = %v, want 60s", ra.Wait)
+	}
+}