Fix GetTrackDownloadInfo to return signed download URLs

- Pass quality parameter to /tracks/{track_id}/download-info API request
- Add GetTrackDownloadURL method that resolves signed download URLs by
  downloading XML from download_info_url, parsing host/path/ts/s,
  computing MD5 signature (salt: XGRlBW9FXlekgbPrRHuSiA), and
  returning https://{host}/get-mp3/{sign}/{ts}{path}
- Add DownloadInfoURL field to DownloadInfo model for the XML URL
- Update DownloadTrack to use GetDirectLink resolution when URL is empty
- Port signing logic from yandex-music-api-python download_info.py

internal/client/tracks.go

@@ -3,12 +3,47 @@ package client
 
 import (
 	"context"
+	"crypto/md5"
+	"encoding/xml"
 	"fmt"
 
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/models"
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/utils"
 )
 
+const downloadSignSalt = "XGRlBW9FXlekgbPrRHuSiA"
+
+// downloadInfoXML represents the XML response from download_info_url.
+type downloadInfoXML struct {
+	Host string `xml:"host"`
+	Path string `xml:"path"`
+	Ts   string `xml:"ts"`
+	S    string `xml:"s"`
+}
+
+// resolveDownloadURL downloads the XML from downloadInfoURL, parses host/path/ts/s,
+// computes the MD5 signature, and returns the signed download URL:
+// https://{host}/get-mp3/{sign}/{ts}{path}
+func resolveDownloadURL(ctx context.Context, downloadInfoURL string, ri models.RequestInterface) (string, error) {
+	data, err := ri.Download(ctx, downloadInfoURL)
+	if err != nil {
+		return "", err
+	}
+
+	var xmlData downloadInfoXML
+	if err := xml.Unmarshal(data, &xmlData); err != nil {
+		return "", fmt.Errorf("failed to parse download info XML: %w", err)
+	}
+
+	if xmlData.Path == "" || xmlData.S == "" {
+		return "", utils.NewYandexMusicError("invalid download info XML: missing path or s", utils.ErrBadRequest)
+	}
+
+	sign := fmt.Sprintf("%x", md5.Sum([]byte(downloadSignSalt+xmlData.Path[1:]+xmlData.S)))
+
+	return fmt.Sprintf("https://%s/get-mp3/%s/%s%s", xmlData.Host, sign, xmlData.Ts, xmlData.Path), nil
+}
+
 // GetTracks gets tracks by their IDs (GET /tracks).
 // Returns a slice of Track pointers, or nil if no IDs are provided.
 func (c *Client) GetTracks(ctx context.Context, trackIDs []string) ([]*models.Track, error) {
@@ -49,11 +84,12 @@ func (c *Client) GetTrack(ctx context.Context, trackID string) (*models.Track, e
 // GetTrackDownloadInfo gets download info for a track (GET /tracks/{track_id}/download-info).
 // The quality parameter specifies the desired audio quality (e.g., "mp3").
 func (c *Client) GetTrackDownloadInfo(ctx context.Context, trackID, quality string) ([]models.DownloadInfo, error) {
-	// This requires signing - use the sign utility
-	// For now, return basic implementation
 	params := map[string]string{
 		"track_id": trackID,
 	}
+	if quality != "" {
+		params["quality"] = quality
+	}
 
 	var result []models.DownloadInfo
 	if err := c.request.Get(ctx, fmt.Sprintf("/tracks/%s/download-info", trackID), params, &result); err != nil {
@@ -62,8 +98,45 @@ func (c *Client) GetTrackDownloadInfo(ctx context.Context, trackID, quality stri
 	return result, nil
 }
 
+// GetTrackDownloadURL returns a signed direct download URL for a track.
+// It fetches download info, finds a matching entry, downloads the XML from its
+// download_info_url, parses host/path/ts/s, computes the MD5 signature, and
+// returns https://{host}/get-mp3/{sign}/{ts}{path}.
+func (c *Client) GetTrackDownloadURL(ctx context.Context, trackID, format string, bitrate int) (string, error) {
+	info, err := c.GetTrackDownloadInfo(ctx, trackID, format)
+	if err != nil {
+		return "", err
+	}
+
+	// Find matching download info
+	var di *models.DownloadInfo
+	for i := range info {
+		if info[i].Codec == format && (bitrate == 0 || info[i].Bitrate == bitrate) {
+			di = &info[i]
+			break
+		}
+	}
+	if di == nil && len(info) > 0 {
+		di = &info[0]
+	}
+	if di == nil {
+		return "", utils.NewYandexMusicError("no download info available", utils.ErrNotFound)
+	}
+
+	if di.URL != "" {
+		return di.URL, nil
+	}
+
+	if di.DownloadInfoURL == "" {
+		return "", utils.NewYandexMusicError("no download URL available", utils.ErrNotFound)
+	}
+
+	return resolveDownloadURL(ctx, di.DownloadInfoURL, c.request)
+}
+
 // DownloadTrack downloads a track by ID.
-// It first fetches download info, finds a matching format/bitrate, then downloads the file content.
+// It first fetches download info, finds a matching format/bitrate, resolves the
+// signed URL if needed, then downloads the file content.
 func (c *Client) DownloadTrack(ctx context.Context, trackID, format string, bitrate int) ([]byte, error) {
 	info, err := c.GetTrackDownloadInfo(ctx, trackID, format)
 	if err != nil {
@@ -71,20 +144,29 @@ func (c *Client) DownloadTrack(ctx context.Context, trackID, format string, bitr
 	}
 
 	// Find matching download info
-	var url string
-	for _, di := range info {
-		if di.Codec == format && (bitrate == 0 || di.Bitrate == bitrate) {
-			url = di.URL
+	var di *models.DownloadInfo
+	for i := range info {
+		if info[i].Codec == format && (bitrate == 0 || info[i].Bitrate == bitrate) {
+			di = &info[i]
 			break
 		}
 	}
-
-	if url == "" && len(info) > 0 {
-		url = info[0].URL
+	if di == nil && len(info) > 0 {
+		di = &info[0]
+	}
+	if di == nil {
+		return nil, utils.NewYandexMusicError("no download info available", utils.ErrNotFound)
 	}
 
+	url := di.URL
 	if url == "" {
-		return nil, utils.NewYandexMusicError("no download URL available", utils.ErrNotFound)
+		if di.DownloadInfoURL == "" {
+			return nil, utils.NewYandexMusicError("no download URL available", utils.ErrNotFound)
+		}
+		url, err = resolveDownloadURL(ctx, di.DownloadInfoURL, c.request)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	return c.request.Download(ctx, url)

internal/models/track.go

@@ -165,10 +165,12 @@ type TrackLyricsInfo struct {
 
 // DownloadInfo represents file download info for a track.
 type DownloadInfo struct {
-	Quality    string `json:"quality"`
-	Codec      string `json:"codec"`
-	Bitrate    int    `json:"bitrate"`
-	Size       int64  `json:"size"`
-	URL        string `json:"url"`
-	DirectLink bool   `json:"direct_link"`
+	Quality         string `json:"quality"`
+	Codec           string `json:"codec"`
+	Bitrate         int    `json:"bitrate"`
+	Size            int64  `json:"size"`
+	URL             string `json:"url"`
+	DirectLink      bool   `json:"direct_link"`
+	DownloadInfoURL string `json:"download_info_url"`
+	DirectLinkURL   string `json:"-"` // Computed signed download URL
 }