package downloader

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"mime/multipart"
	"net/http"
	"net/http/cookiejar"
	"net/url"
	"strconv"
	"strings"
	"sync"
	"time"
)

// Client drives one qBittorrent through its WebUI API v2.
type Client struct {
	Config Config

	mu sync.Mutex
	// http carries the SID cookie once logged in; nil until then.
	http *http.Client
}

// AddTorrent hands qBittorrent a magnet or .torrent bytes, filed under the
// configured category with the save path already set.
func (c *Client) AddTorrent(ctx context.Context, a Add) error {
	if a.Magnet == "" && len(a.Torrent) == 0 {
		return fmt.Errorf("add: neither a magnet nor torrent bytes")
	}
	if a.SavePath == "" {
		return fmt.Errorf("add: no save path, which would land the download outside the folder Minstrel imports from")
	}
	body := &bytes.Buffer{}
	form := multipart.NewWriter(body)
	fields := map[string]string{
		"savepath": a.SavePath,
		"category": c.Config.Category,
		// Both, because qBittorrent 5 renamed paused to stopped and an older
		// one ignores the field it does not know.
		"paused":  strconv.FormatBool(a.Paused),
		"stopped": strconv.FormatBool(a.Paused),
	}
	if len(a.Tags) > 0 {
		fields["tags"] = strings.Join(a.Tags, ",")
	}
	if a.StopAtMetadata && !a.Paused {
		fields["stopCondition"] = "MetadataReceived"
	}
	if a.Magnet != "" {
		fields["urls"] = a.Magnet
	}
	for k, v := range fields {
		if v == "" {
			continue
		}
		if err := form.WriteField(k, v); err != nil {
			return fmt.Errorf("add: build form: %w", err)
		}
	}
	if len(a.Torrent) > 0 {
		part, err := form.CreateFormFile("torrents", "release.torrent")
		if err != nil {
			return fmt.Errorf("add: build form: %w", err)
		}
		if _, err := part.Write(a.Torrent); err != nil {
			return fmt.Errorf("add: build form: %w", err)
		}
	}
	if err := form.Close(); err != nil {
		return fmt.Errorf("add: build form: %w", err)
	}
	resp, err := c.post(ctx, "/api/v2/torrents/add", form.FormDataContentType(), body.Bytes())
	if err != nil {
		return err
	}
	// qBittorrent refuses with HTTP 200 and the body "Fails.".
	if strings.HasPrefix(strings.TrimSpace(string(resp)), "Fails") {
		return fmt.Errorf("%w: the client refused the torrent", ErrUnreachable)
	}
	return nil
}

// Torrents lists the configured category's torrents, or the given hashes.
func (c *Client) Torrents(ctx context.Context, hashes ...string) ([]Torrent, error) {
	params := url.Values{}
	if c.Config.Category != "" {
		params.Set("category", c.Config.Category)
	}
	if len(hashes) > 0 {
		params.Set("hashes", strings.Join(hashes, "|"))
	}
	body, err := c.get(ctx, "/api/v2/torrents/info", params)
	if err != nil {
		return nil, err
	}
	var out []Torrent
	if err := json.Unmarshal(body, &out); err != nil {
		return nil, fmt.Errorf("%w: parse torrent list: %v", ErrUnreachable, err)
	}
	for i := range out {
		out[i].Hash = strings.ToLower(out[i].Hash)
	}
	return out, nil
}

// Connected reads qBittorrent's own connection status.
func (c *Client) Connected(ctx context.Context) (bool, error) {
	body, err := c.get(ctx, "/api/v2/transfer/info", nil)
	if err != nil {
		return false, err
	}
	var info struct {
		ConnectionStatus string `json:"connection_status"`
	}
	if err := json.Unmarshal(body, &info); err != nil {
		return false, fmt.Errorf("%w: parse transfer info: %v", ErrUnreachable, err)
	}
	return info.ConnectionStatus == "connected", nil
}

// Stop and Start try qBittorrent 5's names first and fall back to 4's.
func (c *Client) Stop(ctx context.Context, hashes ...string) error {
	return c.command(ctx, hashes, "/api/v2/torrents/stop", "/api/v2/torrents/pause")
}

func (c *Client) Start(ctx context.Context, hashes ...string) error {
	return c.command(ctx, hashes, "/api/v2/torrents/start", "/api/v2/torrents/resume")
}

func (c *Client) Reannounce(ctx context.Context, hashes ...string) error {
	return c.command(ctx, hashes, "/api/v2/torrents/reannounce", "/api/v2/torrents/reannounce")
}

func (c *Client) Recheck(ctx context.Context, hashes ...string) error {
	return c.command(ctx, hashes, "/api/v2/torrents/recheck", "/api/v2/torrents/recheck")
}

func (c *Client) Delete(ctx context.Context, deleteFiles bool, hashes ...string) error {
	if len(hashes) == 0 {
		return nil
	}
	params := url.Values{
		"hashes":      {strings.Join(hashes, "|")},
		"deleteFiles": {strconv.FormatBool(deleteFiles)},
	}
	_, err := c.post(ctx, "/api/v2/torrents/delete", "application/x-www-form-urlencoded", []byte(params.Encode()))
	return err
}

func (c *Client) SkipFiles(ctx context.Context, hash string, indices []int) error {
	return c.filePriority(ctx, hash, indices, "0")
}

func (c *Client) KeepFiles(ctx context.Context, hash string, indices []int) error {
	return c.filePriority(ctx, hash, indices, "1")
}

func (c *Client) filePriority(ctx context.Context, hash string, indices []int, priority string) error {
	if len(indices) == 0 {
		return nil
	}
	ids := make([]string, len(indices))
	for i, n := range indices {
		ids[i] = strconv.Itoa(n)
	}
	params := url.Values{"hash": {hash}, "id": {strings.Join(ids, "|")}, "priority": {priority}}
	_, err := c.post(ctx, "/api/v2/torrents/filePrio", "application/x-www-form-urlencoded", []byte(params.Encode()))
	return err
}

func (c *Client) Files(ctx context.Context, hash string) ([]File, error) {
	raw, err := c.files(ctx, hash)
	if err != nil {
		return nil, err
	}
	out := make([]File, len(raw))
	for i, f := range raw {
		out[i] = File{Index: f.Index, Path: f.Name, Size: f.Size}
	}
	return out, nil
}

func (c *Client) FileStates(ctx context.Context, hash string) ([]FileState, error) {
	raw, err := c.files(ctx, hash)
	if err != nil {
		return nil, err
	}
	out := make([]FileState, len(raw))
	for i, f := range raw {
		done := f.Size
		if f.Progress < 1 {
			// Progress is a float; never round an unfinished file up to whole.
			done = min(int64(f.Progress*float64(f.Size)), f.Size-1)
		}
		out[i] = FileState{Path: f.Name, Size: f.Size, Done: max(done, 0), Skipped: f.Priority == 0}
	}
	return out, nil
}

type qbitFile struct {
	Index    int     `json:"index"`
	Name     string  `json:"name"`
	Size     int64   `json:"size"`
	Progress float64 `json:"progress"`
	Priority int     `json:"priority"`
}

func (c *Client) files(ctx context.Context, hash string) ([]qbitFile, error) {
	body, err := c.get(ctx, "/api/v2/torrents/files", url.Values{"hash": {hash}})
	if err != nil {
		return nil, err
	}
	var raw []qbitFile
	if err := json.Unmarshal(body, &raw); err != nil {
		return nil, fmt.Errorf("%w: parse file list: %v", ErrUnreachable, err)
	}
	return raw, nil
}

func (c *Client) command(ctx context.Context, hashes []string, primary, fallback string) error {
	if len(hashes) == 0 {
		return nil
	}
	params := []byte(url.Values{"hashes": {strings.Join(hashes, "|")}}.Encode())
	_, err := c.post(ctx, primary, "application/x-www-form-urlencoded", params)
	var status statusError
	if errors.As(err, &status) && (status.code == http.StatusNotFound || status.code == http.StatusMethodNotAllowed) {
		_, err = c.post(ctx, fallback, "application/x-www-form-urlencoded", params)
	}
	return err
}

type statusError struct {
	code int
	what string
}

func (e statusError) Error() string { return fmt.Sprintf("%s: HTTP %d", e.what, e.code) }

func (e statusError) Unwrap() error { return ErrUnreachable }

func (c *Client) get(ctx context.Context, path string, params url.Values) ([]byte, error) {
	endpoint := path
	if len(params) > 0 {
		endpoint += "?" + params.Encode()
	}
	return c.do(ctx, http.MethodGet, endpoint, "", nil)
}

func (c *Client) post(ctx context.Context, path, contentType string, body []byte) ([]byte, error) {
	return c.do(ctx, http.MethodPost, path, contentType, body)
}

// do logs in when there is no session, and once more on a 403, which is how
// qBittorrent says a session expired.
func (c *Client) do(ctx context.Context, method, path, contentType string, body []byte) ([]byte, error) {
	if c.Config.BaseURL == "" {
		return nil, ErrUnconfigured
	}
	if err := c.ensureSession(ctx, false); err != nil {
		return nil, err
	}
	out, err := c.call(ctx, method, path, contentType, body)
	var status statusError
	if errors.As(err, &status) && status.code == http.StatusForbidden {
		if err := c.ensureSession(ctx, true); err != nil {
			return nil, err
		}
		return c.call(ctx, method, path, contentType, body)
	}
	return out, err
}

func (c *Client) ensureSession(ctx context.Context, force bool) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.http != nil && !force {
		return nil
	}
	jar, err := cookiejar.New(nil)
	if err != nil {
		return fmt.Errorf("cookie jar: %w", err)
	}
	httpc := &http.Client{Jar: jar}
	params := url.Values{"username": {c.Config.Username}, "password": {c.Config.Password}}
	out, err := c.callWith(ctx, httpc, http.MethodPost, "/api/v2/auth/login",
		"application/x-www-form-urlencoded", []byte(params.Encode()))
	if err != nil {
		var status statusError
		if errors.As(err, &status) && status.code == http.StatusForbidden {
			return fmt.Errorf("%w: too many failed attempts, or this address is banned", ErrAuth)
		}
		return err
	}
	if strings.HasPrefix(strings.TrimSpace(string(out)), "Fails") {
		return fmt.Errorf("%w: wrong username or password", ErrAuth)
	}
	c.http = httpc
	return nil
}

func (c *Client) call(ctx context.Context, method, path, contentType string, body []byte) ([]byte, error) {
	c.mu.Lock()
	httpc := c.http
	c.mu.Unlock()
	if httpc == nil {
		httpc = &http.Client{}
	}
	return c.callWith(ctx, httpc, method, path, contentType, body)
}

func (c *Client) callWith(ctx context.Context, httpc *http.Client, method, path, contentType string, body []byte) ([]byte, error) {
	base := strings.TrimRight(c.Config.BaseURL, "/")
	status, out, err := send(ctx, httpc, c.Config.Timeout, method, base+path, contentType, body, func(h http.Header) {
		// qBittorrent's CSRF check refuses a request without a matching
		// Referer.
		h.Set("Referer", base)
	})
	if err != nil {
		return nil, err
	}
	if status < 200 || status >= 300 {
		return nil, statusError{code: status, what: path}
	}
	return out, nil
}

// send makes one request to a download client under its timeout, lets the
// caller add headers, and reads at most 16 MB. It returns the status for the
// caller to read, because each client's refusals are its own.
func send(ctx context.Context, httpc *http.Client, timeout time.Duration, method, target, contentType string, body []byte, set func(http.Header)) (int, []byte, error) {
	if timeout <= 0 {
		timeout = DefaultTimeout
	}
	ctx, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()
	var reader io.Reader
	if body != nil {
		reader = bytes.NewReader(body)
	}
	req, err := http.NewRequestWithContext(ctx, method, target, reader)
	if err != nil {
		return 0, nil, fmt.Errorf("%w: build request: %v", ErrUnreachable, err)
	}
	req.Header.Set("User-Agent", UserAgent)
	if contentType != "" {
		req.Header.Set("Content-Type", contentType)
	}
	if set != nil {
		set(req.Header)
	}
	resp, err := httpc.Do(req)
	if err != nil {
		return 0, nil, fmt.Errorf("%w: %v", ErrUnreachable, err)
	}
	defer func() { _ = resp.Body.Close() }()
	out, err := io.ReadAll(io.LimitReader(resp.Body, 16<<20))
	if err != nil {
		return 0, nil, fmt.Errorf("%w: read body: %v", ErrUnreachable, err)
	}
	return resp.StatusCode, out, nil
}