// Package gitsync keeps bootapi's template Set in step with the templates repo.
// It fetches the repo's branch tarball over plain HTTP (Gitea's
// /archive/<branch>.tar.gz) every interval (default 3m, like argocd), holds the
// template files in memory, and atomically swaps the Engine's active Set when
// the content changes. A parse failure keeps the last-good Set and is only
// logged/counted, so a bad template push can never take bootapi down. The
// embedded defaults remain the fallback when the repo is unreachable at startup.
//
// The repo is only ever read as a file tree plus a change signal, so no git
// binary is involved: the runtime image stays distroless and the pod needs no
// writable volume.
package gitsync

import (
	"archive/tar"
	"compress/gzip"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"io/fs"
	"log/slog"
	"net/http"
	"path"
	"sort"
	"strings"
	"sync/atomic"
	"testing/fstest"
	"time"

	"git.unkin.net/unkin/bootapi/internal/render"
)

// maxArchiveBytes caps the downloaded tarball. The templates repo is a handful
// of text files (~12KB compressed); this only exists to bound a hostile or
// broken response.
const maxArchiveBytes = 32 << 20

// maxFileBytes caps a single extracted template.
const maxFileBytes = 1 << 20

// Options configures the syncer.
type Options struct {
	URL      string // repo URL, e.g. https://git.unkin.net/unkin/bootapi-templates.git
	Branch   string
	Token    string // optional; sent as a Gitea token header for a private repo
	Interval time.Duration
	Client   *http.Client // optional; defaults to a 30s-timeout client
}

// Syncer fetches a templates repo and reloads an Engine on change.
type Syncer struct {
	opt      Options
	embedded fs.FS
	engine   *render.Engine

	digest     string       // content digest of the last fetched tree
	syncs      atomic.Int64 // successful reloads (Set swapped)
	failures   atomic.Int64 // fetch or parse failures (last-good kept)
	generation atomic.Int64 // increments on every successful swap
}

// New builds a Syncer. embedded is the fallback template FS. Call SetEngine
// before Run so reloads have an Engine to swap into (the Engine needs the
// initial Set from Bootstrap first, hence the two-step wiring).
func New(opt Options, embedded fs.FS) *Syncer {
	if opt.Branch == "" {
		opt.Branch = "main"
	}
	if opt.Interval <= 0 {
		opt.Interval = 3 * time.Minute
	}
	if opt.Client == nil {
		opt.Client = &http.Client{Timeout: 30 * time.Second}
	}
	return &Syncer{opt: opt, embedded: embedded}
}

// SetEngine points the syncer at the live Engine whose Set it swaps on reload.
func (s *Syncer) SetEngine(e *render.Engine) { s.engine = e }

// Syncs/Failures/Generation are exported for the server's metrics collector.
func (s *Syncer) Syncs() int64      { return s.syncs.Load() }
func (s *Syncer) Failures() int64   { return s.failures.Load() }
func (s *Syncer) Generation() int64 { return s.generation.Load() }

// ArchiveURL is the branch tarball URL derived from the repo URL.
func (o Options) ArchiveURL() string {
	base := strings.TrimSuffix(strings.TrimRight(o.URL, "/"), ".git")
	return base + "/archive/" + o.Branch + ".tar.gz"
}

// Bootstrap fetches the repo and builds the initial Set from embedded + the
// fetched tree. On any fetch/parse failure it returns an embedded-only Set plus
// a non-nil error (which the caller logs but treats as non-fatal, so bootapi
// always starts with at least the embedded defaults).
func (s *Syncer) Bootstrap(ctx context.Context) (*render.Set, error) {
	tree, digest, commit, err := s.fetch(ctx)
	if err != nil {
		return s.embeddedSet(fmt.Errorf("template fetch failed, using embedded defaults: %w", err))
	}
	s.digest = digest
	set, err := render.BuildSet(s.embedded, tree)
	if err != nil {
		return s.embeddedSet(fmt.Errorf("fetched templates failed to parse, using embedded defaults: %w", err))
	}
	s.generation.Add(1)
	slog.Info("templates loaded", "commit", commit, "digest", digest)
	return set, nil
}

// embeddedSet returns the embedded-only Set alongside the degrade reason. A
// broken embedded set is genuinely fatal.
func (s *Syncer) embeddedSet(reason error) (*render.Set, error) {
	set, err := render.BuildSet(s.embedded, nil)
	if err != nil {
		return nil, err
	}
	return set, reason
}

// Run polls the repo every interval until ctx is cancelled.
func (s *Syncer) Run(ctx context.Context) {
	t := time.NewTicker(s.opt.Interval)
	defer t.Stop()
	slog.Info("template sync started", "url", s.opt.ArchiveURL(), "interval", s.opt.Interval)
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			s.pollOnce(ctx)
		}
	}
}

func (s *Syncer) pollOnce(ctx context.Context) {
	tree, digest, commit, err := s.fetch(ctx)
	if err != nil {
		s.failures.Add(1)
		slog.Error("template fetch failed; keeping last-good set", "err", err)
		return
	}
	if digest == s.digest {
		return
	}
	// Record the new digest before parsing so an unchanged bad push is counted
	// once, not on every poll.
	s.digest = digest
	set, err := render.BuildSet(s.embedded, tree)
	if err != nil {
		s.failures.Add(1)
		slog.Error("template reload failed to parse; keeping last-good set", "commit", commit, "err", err)
		return
	}
	s.engine.Swap(set)
	s.syncs.Add(1)
	s.generation.Add(1)
	slog.Info("templates reloaded", "commit", commit, "digest", digest, "generation", s.generation.Load())
}

// fetch downloads the branch tarball and extracts it into an in-memory FS. The
// digest is taken over the extracted tree (not the gzip bytes) so a
// re-compressed but identical archive is not treated as a change. commit is the
// source commit Gitea advertises in its immutable Link header, for logging only.
func (s *Syncer) fetch(ctx context.Context) (fs.FS, string, string, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, s.opt.ArchiveURL(), nil)
	if err != nil {
		return nil, "", "", err
	}
	if s.opt.Token != "" {
		req.Header.Set("Authorization", "token "+s.opt.Token)
	}
	resp, err := s.opt.Client.Do(req)
	if err != nil {
		return nil, "", "", err
	}
	defer func() { _ = resp.Body.Close() }()
	if resp.StatusCode != http.StatusOK {
		return nil, "", "", fmt.Errorf("GET %s: %s", s.opt.ArchiveURL(), resp.Status)
	}

	tree, err := extract(io.LimitReader(resp.Body, maxArchiveBytes))
	if err != nil {
		return nil, "", "", err
	}
	if len(tree) == 0 {
		return nil, "", "", fmt.Errorf("archive contained no files")
	}
	return tree, digest(tree), commitFromLink(resp.Header.Get("Link")), nil
}

// extract reads a gzipped tar and returns its regular files keyed by path with
// the archive's single top-level directory stripped (Gitea prefixes every entry
// with "<repo>/"). Entries that would escape the tree are skipped rather than
// trusted: the archive is a network input.
func extract(r io.Reader) (fstest.MapFS, error) {
	gz, err := gzip.NewReader(r)
	if err != nil {
		return nil, fmt.Errorf("gzip: %w", err)
	}
	defer func() { _ = gz.Close() }()

	out := fstest.MapFS{}
	tr := tar.NewReader(gz)
	for {
		h, err := tr.Next()
		if err == io.EOF {
			return out, nil
		}
		if err != nil {
			return nil, fmt.Errorf("tar: %w", err)
		}
		if h.Typeflag != tar.TypeReg {
			continue
		}
		name := stripRoot(h.Name)
		if name == "" || !fs.ValidPath(name) {
			continue
		}
		b, err := io.ReadAll(io.LimitReader(tr, maxFileBytes))
		if err != nil {
			return nil, fmt.Errorf("tar %s: %w", h.Name, err)
		}
		out[name] = &fstest.MapFile{Data: b, Mode: 0o444}
	}
}

// stripRoot removes the archive's leading directory component.
func stripRoot(name string) string {
	clean := path.Clean(strings.TrimPrefix(name, "./"))
	if strings.HasPrefix(clean, "/") || strings.HasPrefix(clean, "..") {
		return ""
	}
	_, rest, ok := strings.Cut(clean, "/")
	if !ok {
		return ""
	}
	return rest
}

// digest hashes the extracted tree: every path and its contents, in path order.
func digest(tree fstest.MapFS) string {
	names := make([]string, 0, len(tree))
	for n := range tree {
		names = append(names, n)
	}
	sort.Strings(names)

	h := sha256.New()
	for _, n := range names {
		_, _ = fmt.Fprintf(h, "%s\x00%d\x00", n, len(tree[n].Data))
		_, _ = h.Write(tree[n].Data)
	}
	return hex.EncodeToString(h.Sum(nil))[:16]
}

// commitFromLink pulls the commit SHA out of Gitea's immutable-archive Link
// header: <.../archive/<sha>.tar.gz?rev=<sha>>; rel="immutable".
func commitFromLink(link string) string {
	_, rev, ok := strings.Cut(link, "rev=")
	if !ok {
		return ""
	}
	sha, _, _ := strings.Cut(rev, ">")
	return strings.TrimSpace(sha)
}