// Package exchange talks to FabledExchange's native API for the two jobs that
// are not downloading (M502 #5518): searching its DHT index (/api/search) and
// describing one torrent (/api/torrents/{infohash}). Downloading is
// internal/downloader's. Ported from Fabled Librarian's internal/exchange.
//
// The native search rather than Exchange's Torznab feed, because it reports
// whether a swarm was measured and how many files a torrent holds, which the
// feed cannot say.
package exchange

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"strings"
	"time"

	"git.fabledsword.com/bvandeusen/minstrel/internal/indexer"
)

// ErrUnconfigured is an Exchange with no address.
var ErrUnconfigured = errors.New("exchange is not configured")

// ErrUnreachable is a call that could not be made or was not understood.
var ErrUnreachable = errors.New("exchange unreachable")

// ErrAuth is an API key Exchange refused, or a secured Exchange asked with
// none.
var ErrAuth = errors.New("exchange refused the API key")

// ErrUnknown is a torrent Exchange has not crawled: a JSON 404, which is an
// answer and not a fault.
var ErrUnknown = errors.New("exchange has not crawled this torrent")

// UserAgent names Minstrel in Exchange's logs.
const UserAgent = "Minstrel (self-hosted music server; release search)"

// DefaultTimeout is what an unset Timeout means. Exchange answers from its
// own database.
const DefaultTimeout = 20 * time.Second

// Config is one Exchange.
type Config struct {
	BaseURL string
	// APIKey is optional and sent only when set: an empty bearer makes a
	// secured Exchange answer 401.
	APIKey  string
	Timeout time.Duration
}

// Client asks one Exchange.
type Client struct {
	Config Config
	// HTTP is injectable for tests. Nil takes http.DefaultClient.
	HTTP *http.Client
	// Code is the acquisition_services row, stamped onto search results.
	Code   string
	Logger *slog.Logger
}

func (c *Client) log() *slog.Logger {
	if c == nil || c.Logger == nil {
		return slog.Default()
	}
	return c.Logger
}

// Detail is what Exchange knows about one torrent.
type Detail struct {
	InfoHash      string       `json:"infohash"`
	Name          string       `json:"name"`
	Size          int64        `json:"size"`
	FilesCount    int          `json:"files_count"`
	Seeders       int          `json:"seeders"`
	SwarmKnown    bool         `json:"swarm_known"`
	Leechers      int          `json:"leechers"`
	LeechersKnown bool         `json:"leechers_known"`
	ContentType   string       `json:"content_type"`
	Files         []DetailFile `json:"files"`
}

// DetailFile is one file inside a torrent.
type DetailFile struct {
	Index int    `json:"index"`
	Path  string `json:"path"`
	Size  int64  `json:"size"`
}

// Torrent asks Exchange about one v1 infohash.
func (c *Client) Torrent(ctx context.Context, infohash string) (Detail, error) {
	if strings.TrimSpace(c.Config.BaseURL) == "" {
		return Detail{}, ErrUnconfigured
	}
	hash := indexer.NormalizeHash(infohash)
	if len(hash) != 40 || !indexer.IsInfoHash(hash) {
		return Detail{}, fmt.Errorf("%w: %q is not a 40-character infohash", ErrUnreachable, infohash)
	}
	body, err := c.get(ctx, "/api/torrents/"+hash)
	if err != nil {
		return Detail{}, err
	}
	var out Detail
	if err := json.Unmarshal(body, &out); err != nil {
		return Detail{}, fmt.Errorf("%w: parse torrent detail: %v", ErrUnreachable, err)
	}
	if out.InfoHash == "" {
		return Detail{}, ErrUnknown
	}
	return out, nil
}

func (c *Client) get(ctx context.Context, path string) ([]byte, error) {
	timeout := c.Config.Timeout
	if timeout <= 0 {
		timeout = DefaultTimeout
	}
	ctx, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()

	target := strings.TrimRight(c.Config.BaseURL, "/") + path
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrUnreachable, err)
	}
	req.Header.Set("User-Agent", UserAgent)
	req.Header.Set("Accept", "application/json")
	if key := strings.TrimSpace(c.Config.APIKey); key != "" {
		req.Header.Set("Authorization", "Bearer "+key)
	}
	client := c.HTTP
	if client == nil {
		client = http.DefaultClient
	}
	res, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrUnreachable, err)
	}
	defer func() { _ = res.Body.Close() }()
	body, err := io.ReadAll(io.LimitReader(res.Body, 1<<20))
	if err != nil {
		return nil, fmt.Errorf("%w: read answer: %v", ErrUnreachable, err)
	}
	switch {
	case res.StatusCode == http.StatusNotFound:
		// A 404 that is not JSON came from something else at this address,
		// which must not look like a healthy Exchange that lacks a torrent.
		if !looksLikeJSON(body) {
			return nil, fmt.Errorf("%w: something at this address answered 404, "+
				"but it was not FabledExchange", ErrUnreachable)
		}
		return nil, ErrUnknown
	case res.StatusCode == http.StatusUnauthorized || res.StatusCode == http.StatusForbidden:
		return nil, fmt.Errorf("%w: %s", ErrAuth,
			message(body, "set this FabledExchange's API key in Minstrel's settings"))
	case res.StatusCode == http.StatusServiceUnavailable:
		return nil, fmt.Errorf("%w: %s", ErrUnreachable, message(body, "no index is configured"))
	case res.StatusCode < 200 || res.StatusCode >= 300:
		return nil, fmt.Errorf("%w: %s", ErrUnreachable,
			message(body, fmt.Sprintf("returned %d", res.StatusCode)))
	}
	return body, nil
}

// message is Exchange's own {"error": "..."} text, or fallback.
func message(body []byte, fallback string) string {
	var env struct {
		Error string `json:"error"`
	}
	if err := json.Unmarshal(body, &env); err == nil && strings.TrimSpace(env.Error) != "" {
		return strings.TrimSpace(env.Error)
	}
	return fallback
}

func looksLikeJSON(body []byte) bool {
	t := strings.TrimSpace(string(body))
	return strings.HasPrefix(t, "{") || strings.HasPrefix(t, "[")
}