// Package images implements a tiny content-addressed image store: writers upload
// images from the editor, they're saved to disk under the configured directory,
// and served back by hashed filename. Content addressing means the same image
// pasted twice is stored once, and URLs are stable and cacheable forever.
//
// Each stored file also has one row per owner in the `images` table, and a fetch
// joins on the caller. Before that, the store was a flat directory with no
// database presence at all: any authenticated user holding a sha256 could fetch
// anyone else's image. Hashes aren't guessable, so it was never an emergency —
// but "unguessable filename" is not access control, and images pasted into a
// private journal are exactly the content that shouldn't depend on it.
//
// One row per owner (rather than one owner per file) is what keeps deduplication:
// the same picture uploaded by two people is stored once and simply has two rows.
// The file is removed only with its last row.
package images

import (
	"crypto/sha256"
	"database/sql"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"strings"

	"github.com/go-chi/chi/v5"

	"gitea.parodia.dev/drwily/petal/internal/auth"
)

// maxUploadBytes caps a single image at 10 MiB — generous for a writing tool,
// small enough to keep a careless paste from filling the disk.
const maxUploadBytes = 10 << 20

// extByContentType maps the image types we accept to a canonical extension. The
// allowlist doubles as validation: anything not here is rejected.
var extByContentType = map[string]string{
	"image/png":     ".png",
	"image/jpeg":    ".jpg",
	"image/gif":     ".gif",
	"image/webp":    ".webp",
	"image/svg+xml": ".svg",
}

// Handler serves the upload + fetch endpoints, backed by a directory on disk and
// an ownership table.
type Handler struct {
	dir string
	db  *sql.DB
}

// New constructs a Handler, ensuring the storage directory exists and that every
// file already in it has an owner.
func New(dir string, database *sql.DB, backfillOwner string) (*Handler, error) {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return nil, err
	}
	h := &Handler{dir: dir, db: database}
	if err := h.backfill(backfillOwner); err != nil {
		return nil, err
	}
	return h, nil
}

// backfill claims pre-existing files for one user. Images uploaded before
// ownership existed have no row, and a row is now what makes them fetchable —
// so without this every picture already pasted into a document would 404.
// Attributing them to the account that has been the only one until now is the
// only answer the data supports. Idempotent: files that already have an owner
// are left alone.
func (h *Handler) backfill(owner string) error {
	if owner == "" {
		return nil
	}
	entries, err := os.ReadDir(h.dir)
	if err != nil {
		return err
	}
	claimed := 0
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		var exists bool
		if err := h.db.QueryRow(
			`SELECT EXISTS(SELECT 1 FROM images WHERE name = ?)`, e.Name(),
		).Scan(&exists); err != nil {
			return err
		}
		if exists {
			continue
		}
		var size int64
		if info, err := e.Info(); err == nil {
			size = info.Size()
		}
		if _, err := h.db.Exec(
			`INSERT INTO images (name, user_id, content_type, size) VALUES (?, ?, '', ?)
			 ON CONFLICT DO NOTHING`,
			e.Name(), owner, size,
		); err != nil {
			return err
		}
		claimed++
	}
	if claimed > 0 {
		log.Printf("images: claimed %d pre-existing image(s) for %s", claimed, owner)
	}
	return nil
}

// Routes mounts the image endpoints. Mount under "/images" so the full paths are
// POST /api/images (upload), GET /api/images/{name} (fetch) and
// DELETE /api/images/{name} (drop your copy).
func (h *Handler) Routes() chi.Router {
	r := chi.NewRouter()
	r.Post("/", h.upload)
	r.Get("/{name}", h.serve)
	r.Delete("/{name}", h.remove)
	return r
}

// upload accepts a single multipart "image" field, sniffs and validates its
// type, and writes it under a content hash so identical images dedupe. Responds
// with the served URL the editor inserts.
func (h *Handler) upload(w http.ResponseWriter, r *http.Request) {
	r.Body = http.MaxBytesReader(w, r.Body, maxUploadBytes)
	file, _, err := r.FormFile("image")
	if err != nil {
		http.Error(w, "expected an 'image' file field", http.StatusBadRequest)
		return
	}
	defer file.Close()

	data, err := io.ReadAll(file)
	if err != nil {
		http.Error(w, "could not read upload", http.StatusBadRequest)
		return
	}

	// Trust a sniff over the client-declared type. SVG isn't reliably sniffable
	// (DetectContentType returns text/plain or text/xml), so fall back to a
	// lightweight tag check for it.
	ct := http.DetectContentType(data)
	ext, ok := extByContentType[ct]
	if !ok {
		if looksLikeSVG(data) {
			ct, ext, ok = "image/svg+xml", ".svg", true
		}
	}
	if !ok {
		http.Error(w, "unsupported image type", http.StatusUnsupportedMediaType)
		return
	}

	sum := sha256.Sum256(data)
	name := hex.EncodeToString(sum[:])[:32] + ext
	path := filepath.Join(h.dir, name)

	// Skip the write if this exact content is already stored.
	if _, statErr := os.Stat(path); errors.Is(statErr, os.ErrNotExist) {
		if err := os.WriteFile(path, data, 0o644); err != nil {
			http.Error(w, "could not store image", http.StatusInternalServerError)
			return
		}
	}

	// Record the caller as an owner. Re-uploading your own image is a no-op;
	// uploading someone else's identical image adds a second row over one file.
	if _, err := h.db.Exec(
		`INSERT INTO images (name, user_id, content_type, size) VALUES (?, ?, ?, ?)
		 ON CONFLICT (name, user_id) DO NOTHING`,
		name, auth.UserID(r.Context()), ct, len(data),
	); err != nil {
		log.Printf("images: could not record ownership of %s: %v", name, err)
		http.Error(w, "could not store image", http.StatusInternalServerError)
		return
	}

	w.Header().Set("Content-Type", "application/json")
	_ = json.NewEncoder(w).Encode(map[string]string{"url": "/api/images/" + name})
}

// serve returns a stored image by its hashed filename, but only to someone who
// owns it. The filename is validated to be a bare name (no path separators) so
// it can't escape the storage dir, and served with a long-lived cache header
// since content-addressed URLs never change.
//
// Someone else's image is a 404, not a 403: whether a hash exists is itself
// information the caller has no business learning.
func (h *Handler) serve(w http.ResponseWriter, r *http.Request) {
	name, ok := safeName(chi.URLParam(r, "name"))
	if !ok || !h.owns(name, auth.UserID(r.Context())) {
		http.NotFound(w, r)
		return
	}
	path := filepath.Join(h.dir, name)
	if _, err := os.Stat(path); err != nil {
		http.NotFound(w, r)
		return
	}
	// Private: a shared cache must never hand one writer's image to another.
	w.Header().Set("Cache-Control", "private, max-age=31536000, immutable")
	http.ServeFile(w, r, path)
}

// remove drops the caller's claim on an image, and deletes the file itself once
// nobody is left holding it.
func (h *Handler) remove(w http.ResponseWriter, r *http.Request) {
	name, ok := safeName(chi.URLParam(r, "name"))
	if !ok {
		http.NotFound(w, r)
		return
	}
	res, err := h.db.Exec(`DELETE FROM images WHERE name = ? AND user_id = ?`,
		name, auth.UserID(r.Context()))
	if err != nil {
		http.Error(w, "could not delete image", http.StatusInternalServerError)
		return
	}
	if n, _ := res.RowsAffected(); n == 0 {
		http.NotFound(w, r)
		return
	}

	var others bool
	if err := h.db.QueryRow(
		`SELECT EXISTS(SELECT 1 FROM images WHERE name = ?)`, name,
	).Scan(&others); err != nil {
		// The row is gone either way; leaving an orphaned file behind is a
		// wasted block, not a correctness problem.
		log.Printf("images: could not check remaining owners of %s: %v", name, err)
		w.WriteHeader(http.StatusNoContent)
		return
	}
	if !others {
		if err := os.Remove(filepath.Join(h.dir, name)); err != nil && !errors.Is(err, os.ErrNotExist) {
			log.Printf("images: could not remove %s: %v", name, err)
		}
	}
	w.WriteHeader(http.StatusNoContent)
}

// owns reports whether userID has a claim on a stored image.
func (h *Handler) owns(name, userID string) bool {
	var ok bool
	if err := h.db.QueryRow(
		`SELECT EXISTS(SELECT 1 FROM images WHERE name = ? AND user_id = ?)`, name, userID,
	).Scan(&ok); err != nil {
		log.Printf("images: ownership check failed for %s: %v", name, err)
		return false
	}
	return ok
}

// safeName rejects anything that isn't a bare filename, so a request can't walk
// out of the storage directory.
func safeName(name string) (string, bool) {
	if name == "" || name != filepath.Base(name) || strings.ContainsAny(name, `/\`) {
		return "", false
	}
	return name, true
}

// looksLikeSVG does a cheap check for an <svg root tag near the start of the
// file, since DetectContentType doesn't recognize SVG.
func looksLikeSVG(data []byte) bool {
	head := data
	if len(head) > 512 {
		head = head[:512]
	}
	return strings.Contains(strings.ToLower(string(head)), "<svg")
}