package thumbs

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"os"
	"path/filepath"
	"strconv"
)

// Cache keeps finished previews on disk, under Ham Lab's own data directory: one JPEG per
// (path, size, mtime), so a file that changes gets a new preview and an unchanged one is
// never decoded twice. A file ffmpeg could not decode is remembered the same way, with
// ffmpeg's reason beside it, so a folder of odd files does not re-run ffmpeg on every
// visit. Nothing expires: a preview is a few kilobytes, and only files somebody actually
// scrolled past are ever made.
type Cache struct{ Dir string }

// Key identifies one file's preview. "v1" is the preview format: bump it to retire every
// cached preview at once.
func Key(path string, size, mtime int64) string {
	h := sha256.Sum256([]byte("v1\x00" + path + "\x00" + strconv.FormatInt(size, 10) + "\x00" + strconv.FormatInt(mtime, 10)))
	return hex.EncodeToString(h[:])
}

func (c Cache) file(key, ext string) string { return filepath.Join(c.Dir, key[:2], key+ext) }

// Get returns the preview, or the remembered reason it could not be made.
func (c Cache) Get(key string) (jpeg []byte, failure string, found bool) {
	if len(key) < 2 {
		return nil, "", false
	}
	if b, err := os.ReadFile(c.file(key, ".jpg")); err == nil && len(b) > 0 {
		return b, "", true
	}
	if b, err := os.ReadFile(c.file(key, ".err")); err == nil {
		return nil, string(b), true
	}
	return nil, "", false
}

// Put keeps a preview; PutFailure keeps the reason there is none.
func (c Cache) Put(key string, jpeg []byte) error {
	if len(jpeg) == 0 {
		return errors.New("empty preview")
	}
	return c.write(c.file(key, ".jpg"), jpeg)
}

func (c Cache) PutFailure(key, reason string) error {
	return c.write(c.file(key, ".err"), []byte(reason))
}

// write lands the file whole or not at all: a reader never sees a half-written JPEG.
func (c Cache) write(dst string, b []byte) error {
	if err := os.MkdirAll(filepath.Dir(dst), 0o700); err != nil {
		return err
	}
	tmp, err := os.CreateTemp(filepath.Dir(dst), ".tmp-*")
	if err != nil {
		return err
	}
	if _, err := tmp.Write(b); err != nil {
		tmp.Close()
		os.Remove(tmp.Name())
		return err
	}
	if err := tmp.Close(); err != nil {
		os.Remove(tmp.Name())
		return err
	}
	if err := os.Rename(tmp.Name(), dst); err != nil {
		os.Remove(tmp.Name())
		return err
	}
	return nil
}