package backend

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"slices"
	"strconv"
	"strings"
	"unicode"
)

// coverMatch is a cover found by a source.
type coverMatch struct {
	ExternalID string
	ImageURL   string
}

// coverSource finds cover images in an external database.
type coverSource interface {
	// Name is stored as the cover's source, e.g. "steamgriddb".
	Name() string
	// Search returns the cover for the first of names that the source knows, or nil when
	// none matched. An error means the source could not be asked.
	Search(ctx context.Context, names []string) (*coverMatch, error)
	// ByID returns the cover of the entry with the given ID, or ErrNotFound.
	ByID(ctx context.Context, id string) (*coverMatch, error)
}

// getJSON sends req and decodes a JSON response into v. A 404 gives ErrNotFound, other
// failures ErrUpstream.
func getJSON(client *http.Client, req *http.Request, v any) error {
	resp, err := client.Do(req)
	if err != nil {
		return fmt.Errorf("%w: %v", ErrUpstream, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode == http.StatusNotFound {
		return ErrNotFound
	}
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("%w: %s answered %s", ErrUpstream, req.URL.Host, resp.Status)
	}
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(v); err != nil {
		return fmt.Errorf("%w: invalid response from %s: %v", ErrUpstream, req.URL.Host, err)
	}
	return nil
}

// steamGridDB finds game box art ("grids") on SteamGridDB. It needs an API key.
type steamGridDB struct {
	baseURL string
	key     string
	client  *http.Client
}

const steamGridDBURL = "https://www.steamgriddb.com/api/v2"

func (s *steamGridDB) Name() string { return "steamgriddb" }

func (s *steamGridDB) get(ctx context.Context, path string, query url.Values, v any) error {
	u := s.baseURL + path
	if len(query) > 0 {
		u += "?" + query.Encode()
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
	if err != nil {
		return err
	}
	req.Header.Set("Authorization", "Bearer "+s.key)
	var wrapper struct {
		Success bool            `json:"success"`
		Data    json.RawMessage `json:"data"`
	}
	if err := getJSON(s.client, req, &wrapper); err != nil {
		return err
	}
	if !wrapper.Success {
		return fmt.Errorf("%w: SteamGridDB request failed", ErrUpstream)
	}
	return json.Unmarshal(wrapper.Data, v)
}

type steamGridDBGame struct {
	ID   int    `json:"id"`
	Name string `json:"name"`
}

func (s *steamGridDB) Search(ctx context.Context, names []string) (*coverMatch, error) {
	for _, name := range names {
		var games []steamGridDBGame
		if err := s.get(ctx, "/search/autocomplete/"+url.PathEscape(name), nil, &games); err != nil {
			return nil, err
		}
		if len(games) == 0 {
			continue
		}
		match, err := s.ByID(ctx, strconv.Itoa(bestGame(name, games).ID))
		if errors.Is(err, ErrNotFound) {
			continue // the game has no box art
		}
		return match, err
	}
	return nil, nil
}

func (s *steamGridDB) ByID(ctx context.Context, id string) (*coverMatch, error) {
	if _, err := strconv.Atoi(id); err != nil {
		return nil, ErrNotFound
	}
	query := url.Values{
		"dimensions": {"600x900"}, // portrait box art
		"types":      {"static"},
		"mimes":      {"image/png,image/jpeg"},
		"nsfw":       {"false"},
		"humor":      {"false"},
	}
	var grids []struct {
		URL string `json:"url"`
	}
	if err := s.get(ctx, "/grids/game/"+id, query, &grids); err != nil {
		return nil, err
	}
	if len(grids) == 0 {
		return nil, ErrNotFound
	}
	return &coverMatch{ExternalID: id, ImageURL: grids[0].URL}, nil
}

// bestGame picks the search result for name:
//  1. a result with exactly the words of name, in any order ("Lemmings 3D" → "3D Lemmings");
//  2. else a result that ends with the words of name, as words added in front are usually a
//     publisher or series ("Civilization IV" → "Sid Meier's Civilization IV");
//  3. else SteamGridDB's own first result. Words added after the name can be a subtitle, a
//     sequel or an expansion ("Joe & Mac: Caveman Ninja", "Joe & Mac Returns"), which only
//     SteamGridDB's ranking can tell apart.
func bestGame(name string, games []steamGridDBGame) steamGridDBGame {
	want := titleWords(name)
	for _, game := range games {
		if sameWords(titleWords(game.Name), want) {
			return game
		}
	}
	for _, game := range games {
		if have := titleWords(game.Name); len(have) > len(want) && slices.Equal(have[len(have)-len(want):], want) {
			return game
		}
	}
	return games[0]
}

// sameWords reports whether a and b have the same words, in any order.
func sameWords(a, b []string) bool {
	return len(a) == len(b) && slices.Equal(slices.Sorted(slices.Values(a)), slices.Sorted(slices.Values(b)))
}

// romanNumerals are read as numbers, so "Octopath Traveler 2" matches "Octopath Traveler II".
var romanNumerals = map[string]string{
	"i": "1", "ii": "2", "iii": "3", "iv": "4", "v": "5", "vi": "6", "vii": "7", "viii": "8", "ix": "9", "x": "10",
}

// titleWords returns the words of a title in lower case, without punctuation.
func titleWords(title string) []string {
	words := strings.FieldsFunc(strings.ToLower(title), func(r rune) bool {
		return !unicode.IsLetter(r) && !unicode.IsDigit(r)
	})
	for i, word := range words {
		if number, ok := romanNumerals[word]; ok {
			words[i] = number
		}
	}
	return words
}

// aniList finds anime covers on AniList. It needs no key. Its search is loose, so a
// result is only used when one of its titles is the searched name.
type aniList struct {
	url    string
	client *http.Client
}

const aniListURL = "https://graphql.anilist.co"

const aniListFields = `id title { romaji english native } coverImage { extraLarge large }`

type aniListMedia struct {
	ID    int `json:"id"`
	Title struct {
		Romaji  string `json:"romaji"`
		English string `json:"english"`
		Native  string `json:"native"`
	} `json:"title"`
	CoverImage struct {
		ExtraLarge string `json:"extraLarge"`
		Large      string `json:"large"`
	} `json:"coverImage"`
}

func (m aniListMedia) match() *coverMatch {
	image := m.CoverImage.ExtraLarge
	if image == "" {
		image = m.CoverImage.Large
	}
	if image == "" {
		return nil
	}
	return &coverMatch{ExternalID: strconv.Itoa(m.ID), ImageURL: image}
}

func (a *aniList) Name() string { return "anilist" }

func (a *aniList) query(ctx context.Context, query string, variables map[string]any) (*aniListMedia, error) {
	body, err := json.Marshal(map[string]any{"query": query, "variables": variables})
	if err != nil {
		return nil, err
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, a.url, bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Accept", "application/json")
	var resp struct {
		Data struct {
			Media *aniListMedia `json:"Media"`
		} `json:"data"`
	}
	if err := getJSON(a.client, req, &resp); err != nil {
		return nil, err
	}
	if resp.Data.Media == nil {
		return nil, ErrNotFound
	}
	return resp.Data.Media, nil
}

func (a *aniList) Search(ctx context.Context, names []string) (*coverMatch, error) {
	for _, name := range names {
		media, err := a.query(ctx, `query ($search: String) { Media(search: $search, type: ANIME) { `+aniListFields+` } }`,
			map[string]any{"search": name})
		if errors.Is(err, ErrNotFound) {
			continue
		}
		if err != nil {
			return nil, err
		}
		want := normalizeTitle(name)
		for _, title := range []string{media.Title.Romaji, media.Title.English, media.Title.Native} {
			if title != "" && normalizeTitle(title) == want {
				if match := media.match(); match != nil {
					return match, nil
				}
			}
		}
	}
	return nil, nil
}

func (a *aniList) ByID(ctx context.Context, id string) (*coverMatch, error) {
	n, err := strconv.Atoi(id)
	if err != nil {
		return nil, ErrNotFound
	}
	media, err := a.query(ctx, `query ($id: Int) { Media(id: $id) { `+aniListFields+` } }`, map[string]any{"id": n})
	if err != nil {
		return nil, err
	}
	if match := media.match(); match != nil {
		return match, nil
	}
	return nil, ErrNotFound
}

// normalizeTitle keeps only the letters and digits of a title, in lower case, so
// "Cowboy Bebop: The Movie" and "cowboy bebop the movie" compare equal.
func normalizeTitle(title string) string {
	var b strings.Builder
	for _, r := range strings.ToLower(title) {
		if unicode.IsLetter(r) || unicode.IsDigit(r) {
			b.WriteRune(r)
		}
	}
	return b.String()
}