// Package update implements gpu-turnstile's self-updater: it polls the
// Gitea releases API, downloads the Windows binary of newer releases, and
// verifies its Ed25519 signature (produced by CI with OpenSSL) before
// swapping it in next to the running executable.
package update

import (
	"context"
	"crypto/ed25519"
	"crypto/sha256"
	"crypto/x509"
	"encoding/hex"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"os"
	"strconv"
	"strings"
	"time"
)

// maxAssetSize bounds release asset downloads.
const maxAssetSize = 512 << 20

// Updater checks one Gitea repository for releases.
type Updater struct {
	Repo    string // e.g. https://git.rambossek.at/PUBLIC/gpu-turnstile
	Asset   string // e.g. gpu-turnstile.exe
	Version string // current binary version, e.g. v0.1.2 ("dev" cannot be compared)
	// Desired is the APP_VER policy: "dev" disables updates, "stable" (or
	// empty) tracks the latest release, anything else is an exact vX.Y.Z
	// release tag to pin — staging it even when that means a downgrade.
	Desired string
	Log     *slog.Logger
	Client  *http.Client
}

type release struct {
	TagName string `json:"tag_name"`
	Assets  []struct {
		Name               string `json:"name"`
		BrowserDownloadURL string `json:"browser_download_url"`
	} `json:"assets"`
}

func (u *Updater) logger() *slog.Logger {
	if u.Log != nil {
		return u.Log
	}
	return slog.Default()
}

func (u *Updater) httpClient() *http.Client {
	if u.Client != nil {
		return u.Client
	}
	return &http.Client{Timeout: 5 * time.Minute}
}

// apiURL derives <scheme>://<host>/api/v1/repos/<owner>/<name> from Repo.
func (u *Updater) apiURL() (string, error) {
	repoURL, err := url.Parse(u.Repo)
	if err != nil || repoURL.Scheme == "" || repoURL.Host == "" {
		return "", fmt.Errorf("invalid UPDATE_REPO %q", u.Repo)
	}
	ownerName := strings.Trim(repoURL.Path, "/")
	if len(strings.Split(ownerName, "/")) != 2 {
		return "", fmt.Errorf("UPDATE_REPO %q: expected path /<owner>/<name>", u.Repo)
	}
	return fmt.Sprintf("%s://%s/api/v1/repos/%s", repoURL.Scheme, repoURL.Host, ownerName), nil
}

// newerVersion reports whether latest is a higher vX.Y.Z version than
// current. Both may carry a leading "v".
func newerVersion(current, latest string) (bool, error) {
	parse := func(s string) ([3]int, error) {
		var v [3]int
		parts := strings.Split(strings.TrimPrefix(s, "v"), ".")
		if len(parts) != 3 {
			return v, fmt.Errorf("not a vX.Y.Z version: %q", s)
		}
		for i, p := range parts {
			n, err := strconv.Atoi(p)
			if err != nil {
				return v, fmt.Errorf("not a vX.Y.Z version: %q", s)
			}
			v[i] = n
		}
		return v, nil
	}
	cur, err := parse(current)
	if err != nil {
		return false, err
	}
	lat, err := parse(latest)
	if err != nil {
		return false, err
	}
	for i := 0; i < 3; i++ {
		if lat[i] != cur[i] {
			return lat[i] > cur[i], nil
		}
	}
	return false, nil
}

func (u *Updater) get(ctx context.Context, url string) ([]byte, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}
	resp, err := u.httpClient().Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		io.Copy(io.Discard, resp.Body)
		return nil, fmt.Errorf("GET %s: %s", url, resp.Status)
	}
	return io.ReadAll(io.LimitReader(resp.Body, maxAssetSize))
}

func verifySignature(pubKeyPEM string, data, sig []byte) error {
	block, _ := pem.Decode([]byte(pubKeyPEM))
	if block == nil {
		return fmt.Errorf("invalid embedded public key PEM")
	}
	key, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		return fmt.Errorf("parse public key: %w", err)
	}
	pub, ok := key.(ed25519.PublicKey)
	if !ok {
		return fmt.Errorf("public key is not Ed25519")
	}
	if !ed25519.Verify(pub, data, sig) {
		return fmt.Errorf("signature verification failed")
	}
	return nil
}

// stage swaps data into place at exePath: the running executable is
// renamed aside (allowed on Windows) and the new file takes its name.
func stage(exePath string, data []byte) error {
	newPath := exePath + ".new"
	oldPath := exePath + ".old"
	os.Remove(oldPath) // leftover from a previous update
	if err := os.WriteFile(newPath, data, 0o755); err != nil {
		return err
	}
	if err := os.Rename(exePath, oldPath); err != nil {
		os.Remove(newPath)
		return err
	}
	if err := os.Rename(newPath, exePath); err != nil {
		os.Rename(oldPath, exePath) // roll back
		return err
	}
	return nil
}

// CleanupOld removes the .old binary left behind by a staged update.
// Call once at startup.
func CleanupOld(exePath string) {
	os.Remove(exePath + ".old")
	os.Remove(exePath + ".new")
}

// Check performs a single update check. staged is true when a
// signature-verified binary has been swapped into place at exePath; the
// caller should then restart the process. to is the release tag the check
// resolved (the latest release or the pinned tag), set once the release
// fetch succeeded — even when staging afterwards fails. A nil error with
// staged=false means "no action" (up to date, APP_VER=dev, or no embedded
// public key); a non-nil error means the check failed and the running
// binary is untouched.
func (u *Updater) Check(ctx context.Context, exePath string) (staged bool, to string, err error) {
	log := u.logger()
	desired := u.Desired
	if desired == "" {
		desired = "stable"
	}
	if desired == "dev" {
		log.Debug("auto-update: APP_VER=dev, skipping")
		return false, "", nil
	}
	if publicKeyPEM == "" {
		log.Debug("auto-update: no public key embedded, skipping")
		return false, "", nil
	}
	api, err := u.apiURL()
	if err != nil {
		return false, "", err
	}

	pinned := desired != "stable"
	endpoint := api + "/releases/latest"
	if pinned {
		endpoint = api + "/releases/tags/" + desired
	}
	body, err := u.get(ctx, endpoint)
	if err != nil {
		return false, "", fmt.Errorf("fetch release: %w", err)
	}
	var rel release
	if err := json.Unmarshal(body, &rel); err != nil {
		return false, "", fmt.Errorf("parse release: %w", err)
	}
	to = rel.TagName
	if pinned {
		// Pin mode: any difference from the target tag means stage it —
		// including downgrades and replacing a dev binary.
		if u.Version == rel.TagName {
			log.Debug("auto-update: already on pinned version", "version", u.Version)
			return false, to, nil
		}
	} else if u.Version != "" && u.Version != "dev" {
		// Stable mode: only strictly newer releases count; a dev binary
		// cannot be compared and is always replaced by the latest release.
		newer, err := newerVersion(u.Version, rel.TagName)
		if err != nil {
			return false, to, err
		}
		if !newer {
			log.Debug("auto-update: up to date", "version", u.Version, "latest", rel.TagName)
			return false, to, nil
		}
	}

	urls := make(map[string]string, len(rel.Assets))
	for _, a := range rel.Assets {
		urls[a.Name] = a.BrowserDownloadURL
	}
	assetURL, ok := urls[u.Asset]
	if !ok {
		return false, to, fmt.Errorf("release %s has no asset %q", rel.TagName, u.Asset)
	}
	sigURL, ok := urls[u.Asset+".sig"]
	if !ok {
		return false, to, fmt.Errorf("release %s has no signature asset %q", rel.TagName, u.Asset+".sig")
	}

	data, err := u.get(ctx, assetURL)
	if err != nil {
		return false, to, fmt.Errorf("download %s: %w", u.Asset, err)
	}
	sig, err := u.get(ctx, sigURL)
	if err != nil {
		return false, to, fmt.Errorf("download signature: %w", err)
	}

	if sumURL, ok := urls[u.Asset+".sha256"]; ok {
		sumText, err := u.get(ctx, sumURL)
		if err != nil {
			return false, to, fmt.Errorf("download checksum: %w", err)
		}
		want := strings.Fields(string(sumText))[0]
		got := hex.EncodeToString(sha256Bytes(data))
		if !strings.EqualFold(want, got) {
			return false, to, fmt.Errorf("sha256 mismatch: got %s, want %s", got, want)
		}
	}
	if err := verifySignature(publicKeyPEM, data, sig); err != nil {
		return false, to, err
	}

	if err := stage(exePath, data); err != nil {
		return false, to, fmt.Errorf("stage update: %w", err)
	}
	log.Info("auto-update: new version staged", "from", u.Version, "to", rel.TagName)
	return true, to, nil
}

func sha256Bytes(data []byte) []byte {
	sum := sha256.Sum256(data)
	return sum[:]
}