// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package updater checks Gitea releases for new versions and replaces
// the running binary in place, preserving the process ID so systemd
// never notices the restart.
package updater

import (
	"bufio"
	"crypto/sha256"
	"encoding/hex"
	json "encoding/json/v2"
	"fmt"
	"io"
	"math"
	"net/http"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// client is shared by the release check and the download; the timeout
// bounds a hung server.
var client = &http.Client{Timeout: 2 * time.Minute}

// maxBinarySize caps the downloaded binary.
const maxBinarySize = 50 << 20

// ReleaseBase is the Gitea instance hosting volumen releases.
var ReleaseBase = "https://sourcedock.dev"

// syscallExec and executable are the two process-level operations the
// self-update performs; they are variables so a test can observe them
// without replacing the running binary.
var (
	syscallExec = defaultSyscallExec
	executable  = os.Executable
)

// RepoPath identifies the volumen repository on the Gitea instance.
const RepoPath = "/api/v1/repos/petrbalvin/volumen"

// RepoSlug is the repository path used by non-API URLs such as release
// downloads.
const RepoSlug = "/petrbalvin/volumen"

// CheckLatest returns the newest published release tag (without the
// leading "v"), or "" when no release exists.
func CheckLatest() (string, error) {
	req, err := http.NewRequest(http.MethodGet, ReleaseBase+RepoPath+"/releases/latest", nil)
	if err != nil {
		return "", err
	}
	resp, err := client.Do(req)
	if err != nil {
		return "", fmt.Errorf("release check: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return "", fmt.Errorf("release check: HTTP %d", resp.StatusCode)
	}
	var payload struct {
		TagName string `json:"tag_name"`
	}
	if err := json.UnmarshalRead(io.LimitReader(resp.Body, 1<<20), &payload); err != nil {
		return "", fmt.Errorf("release check: %w", err)
	}
	return strings.TrimPrefix(payload.TagName, "v"), nil
}

// UpdateAvailable returns the newer version when one exists, else "".
func UpdateAvailable(current string) string {
	latest, err := CheckLatest()
	if err != nil || latest == "" || latest == current {
		return ""
	}
	if CompareVersions(latest, current) > 0 {
		return latest
	}
	return ""
}

// fetchChecksum reads checksums.txt from the release and returns the
// SHA-256 recorded for name.
func fetchChecksum(version, name string) (string, error) {
	url := fmt.Sprintf("%s%s/releases/download/v%s/checksums.txt", ReleaseBase, RepoSlug, version)
	body, err := fetchBounded(url, 1<<20)
	if err != nil {
		return "", fmt.Errorf("download checksums: %w", err)
	}
	scanner := bufio.NewScanner(strings.NewReader(string(body)))
	for scanner.Scan() {
		fields := strings.Fields(scanner.Text())
		if len(fields) == 2 && strings.TrimPrefix(fields[1], "*") == name {
			return fields[0], nil
		}
	}
	return "", fmt.Errorf("checksums.txt has no entry for %s", name)
}

// fetchBounded downloads url, refusing bodies larger than limit. The
// URL is built from the release base and the version, so it is never
// caller-supplied; redirects are followed by the shared client.
func fetchBounded(url string, limit int64) ([]byte, error) {
	resp, err := client.Get(url)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("fetch %s: HTTP %d", url, resp.StatusCode)
	}
	body, err := io.ReadAll(io.LimitReader(resp.Body, limit+1))
	if err != nil {
		return nil, err
	}
	if int64(len(body)) > limit {
		return nil, fmt.Errorf("body exceeds %d bytes", limit)
	}
	return body, nil
}

// CompareVersions compares two dotted versions numerically per part.
func CompareVersions(a, b string) int {
	as, bs := strings.Split(a, "."), strings.Split(b, ".")
	for i := 0; i < len(as) || i < len(bs); i++ {
		av, bv := part(as, i), part(bs, i)
		if av != bv {
			if av < bv {
				return -1
			}
			return 1
		}
	}
	return 0
}

// part returns the i-th numeric component of a version. A component
// longer than an int is clamped rather than allowed to overflow, because
// an overflowed comparison can invert.
func part(parts []string, i int) int {
	if i >= len(parts) {
		return 0
	}
	value := 0
	for _, r := range parts[i] {
		if r < '0' || r > '9' {
			break
		}
		digit := int(r - '0')
		if value > (math.MaxInt-digit)/10 {
			return math.MaxInt
		}
		value = value*10 + digit
	}
	return value
}

// SelfUpdate downloads the release binary for this platform, replaces
// the running executable atomically, and restarts the process in
// place. It returns the target version.
func SelfUpdate(current string) (string, error) {
	latest, err := CheckLatest()
	if err != nil {
		return "", err
	}
	if latest == "" || CompareVersions(latest, current) <= 0 {
		return latest, fmt.Errorf("already running the latest release")
	}
	exe, err := executable()
	if err != nil {
		return latest, fmt.Errorf("locate executable: %w", err)
	}
	exe, err = filepath.EvalSymlinks(exe)
	if err != nil {
		return latest, fmt.Errorf("resolve executable: %w", err)
	}

	binaryName := fmt.Sprintf("volumen-%s-%s-%s", latest, goos(), goarch())
	asset := fmt.Sprintf("%s%s/releases/download/v%s/%s", ReleaseBase, RepoSlug, latest, binaryName)

	expected, err := fetchChecksum(latest, binaryName)
	if err != nil {
		return latest, err
	}
	body, err := fetchBounded(asset, maxBinarySize)
	if err != nil {
		return latest, fmt.Errorf("download %s: %w", asset, err)
	}
	sum := sha256.Sum256(body)
	if hex.EncodeToString(sum[:]) != expected {
		return latest, fmt.Errorf("checksum mismatch for %s", binaryName)
	}

	dir := filepath.Dir(exe)
	tmp, err := os.CreateTemp(dir, ".volumen-update-*")
	if err != nil {
		return latest, fmt.Errorf("stage download: %w", err)
	}
	tmpPath := tmp.Name()
	if _, err := tmp.Write(body); err != nil {
		tmp.Close()
		os.Remove(tmpPath)
		return latest, fmt.Errorf("stage download: %w", err)
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		os.Remove(tmpPath)
		return latest, fmt.Errorf("stage download: %w", err)
	}
	if err := tmp.Close(); err != nil {
		os.Remove(tmpPath)
		return latest, fmt.Errorf("stage download: %w", err)
	}
	if err := os.Chmod(tmpPath, 0o755); err != nil {
		os.Remove(tmpPath)
		return latest, fmt.Errorf("stage download: %w", err)
	}
	if err := os.Rename(tmpPath, exe); err != nil {
		os.Remove(tmpPath)
		return latest, fmt.Errorf("replace executable: %w", err)
	}
	// Restart in place; the PID stays the same so systemd sees nothing.
	argv := append([]string{exe}, os.Args[1:]...)
	if err := syscallExec(exe, argv); err != nil {
		return latest, fmt.Errorf("restart: %w", err)
	}
	return latest, nil
}