package attach

import (
	"bytes"
	"context"
	"fmt"
	"image"
	_ "image/gif"
	_ "image/jpeg"
	_ "image/png"
	"io"
	"path"
	"regexp"
	"strings"

	"github.com/disintegration/imaging"
	"gocloud.dev/blob"
	// The webp decoder lets image.DecodeConfig and File.Thumb read .webp
	// originals. imaging cannot encode webp, so a webp thumbnail holds JPEG
	// bytes under the original's .webp name (see defaultEncodeImage).
	_ "golang.org/x/image/webp"
)

const (
	maxThumbEdge         = 4096
	maxThumbSourceBytes  = 32 << 20
	maxThumbSourcePixels = 4096 * 4096
)

// thumbToken is the alphabet allowed for the mode and extension segments of a
// thumb filename. Both are interpolated into a blob key, which fileblob maps
// to a filesystem path, so separators and dots must never reach it.
var thumbToken = regexp.MustCompile(`^[a-z0-9]+$`)

// ThumbFilename is Winter File::getThumbFilename: thumb_<id>_<w>_<h>_<ox>_<oy>_<mode>.<ext>.
// A mode or ext outside [a-z0-9]+ is coerced to "auto" / "jpg" so the result
// is always a single safe path element; File.Thumb rejects such input instead.
func ThumbFilename(id uint, w, h int, offsetX, offsetY int, mode, ext string) string {
	if !thumbToken.MatchString(mode) {
		mode = "auto"
	}
	if !thumbToken.MatchString(ext) {
		ext = "jpg"
	}
	return fmt.Sprintf("thumb_%d_%d_%d_%d_%d_%s.%s", id, w, h, offsetX, offsetY, mode, ext)
}

// PartitionDirectory is Winter File::getPartitionDirectory: first 9 chars of
// disk_name split into 3 groups of 3, joined by '/', with a trailing slash.
func PartitionDirectory(diskName string) string {
	var groups []string
	for i := 0; i < len(diskName) && len(groups) < 3; i += 3 {
		end := i + 3
		if end > len(diskName) {
			end = len(diskName)
		}
		groups = append(groups, diskName[i:end])
	}
	return strings.Join(groups, "/") + "/"
}

// BlobKey is the Winter on-disk key for an original file: partition + disk_name.
func BlobKey(diskName string) string {
	return PartitionDirectory(diskName) + diskName
}

func fileExt(diskName string) string {
	ext := strings.TrimPrefix(path.Ext(diskName), ".")
	if ext == "" {
		return "jpg"
	}
	return strings.ToLower(ext)
}

// PublicURL returns the public URL of a blob key: storage.uploads.
// public_path_prefix and the key joined by exactly one slash. With the
// WinterCMS layout (bucket rooted at storage/app/uploads/public, prefix
// /storage/app/uploads/public) it is Winter's File::getPath() path.
func PublicURL(key string) string {
	prefix := strings.TrimRight(PublicPathPrefix(), "/")
	key = strings.TrimLeft(key, "/")
	if prefix == "" {
		return "/" + key
	}
	return prefix + "/" + key
}

// URL returns the public URL of the original file, Winter's File::getPath():
// PublicURL of BlobKey(DiskName).
func (f *File) URL() string {
	if f == nil {
		return ""
	}
	return PublicURL(BlobKey(f.DiskName))
}

func defaultResizeImage(src image.Image, w, h int, mode string) image.Image {
	switch strings.ToLower(mode) {
	case "crop":
		return imaging.Fill(src, w, h, imaging.Center, imaging.Lanczos)
	case "exact":
		return imaging.Resize(src, w, h, imaging.Lanczos)
	default:
		return imaging.Fit(src, w, h, imaging.Lanczos)
	}
}

var resizeImage = defaultResizeImage

func defaultEncodeImage(w io.Writer, img image.Image, ext string) error {
	format := imaging.JPEG
	switch strings.ToLower(ext) {
	case "png":
		format = imaging.PNG
	case "gif":
		format = imaging.GIF
	}
	return imaging.Encode(w, img, format)
}

var encodeImage = defaultEncodeImage

// Thumb returns the public URL of a lazily generated thumbnail. The second
// call for the same dimensions hits the existing blob and does not resize.
func (f *File) Thumb(ctx context.Context, bucket *blob.Bucket, w, h int, mode string) (string, error) {
	if f == nil {
		return "", fmt.Errorf("attach: file is nil")
	}
	if bucket == nil {
		return "", fmt.Errorf("attach: bucket is nil")
	}
	if mode == "" {
		mode = "auto"
	}
	mode = strings.ToLower(mode)
	if !thumbToken.MatchString(mode) {
		return "", fmt.Errorf("attach: invalid thumb mode %q", mode)
	}
	if w <= 0 || h <= 0 || w > maxThumbEdge || h > maxThumbEdge {
		return "", fmt.Errorf("attach: thumb size %dx%d is out of range", w, h)
	}
	ext := fileExt(f.DiskName)
	if !thumbToken.MatchString(ext) {
		return "", fmt.Errorf("attach: invalid thumb extension %q", ext)
	}
	thumbName := ThumbFilename(f.ID, w, h, 0, 0, mode, ext)
	part := PartitionDirectory(f.DiskName)
	thumbKey := part + thumbName
	exists, err := bucket.Exists(ctx, thumbKey)
	if err != nil {
		return "", fmt.Errorf("attach: thumb exists: %w", err)
	}
	if exists {
		return PublicURL(thumbKey), nil
	}
	origKey := part + f.DiskName
	r, err := bucket.NewReader(ctx, origKey, nil)
	if err != nil {
		return "", fmt.Errorf("attach: read original: %w", err)
	}
	raw, err := io.ReadAll(io.LimitReader(r, maxThumbSourceBytes))
	closeErr := r.Close()
	if err != nil {
		return "", fmt.Errorf("attach: read original: %w", err)
	}
	if closeErr != nil {
		return "", closeErr
	}
	// Check the dimensions from the header before decoding the pixels, so
	// a small file that declares a huge image is refused without
	// allocating it.
	cfg, _, err := image.DecodeConfig(bytes.NewReader(raw))
	if err != nil {
		return "", fmt.Errorf("attach: decode original: %w", err)
	}
	if int64(cfg.Width)*int64(cfg.Height) > maxThumbSourcePixels {
		return "", fmt.Errorf("attach: original image is too large")
	}
	src, _, err := image.Decode(bytes.NewReader(raw))
	if err != nil {
		return "", fmt.Errorf("attach: decode original: %w", err)
	}
	bounds := src.Bounds()
	if int64(bounds.Dx())*int64(bounds.Dy()) > maxThumbSourcePixels {
		return "", fmt.Errorf("attach: original image is too large")
	}
	resized := resizeImage(src, w, h, mode)
	contentType := "image/jpeg"
	switch ext {
	case "png":
		contentType = "image/png"
	case "gif":
		contentType = "image/gif"
	}
	wr, err := bucket.NewWriter(ctx, thumbKey, &blob.WriterOptions{ContentType: contentType})
	if err != nil {
		return "", fmt.Errorf("attach: thumb writer: %w", err)
	}
	encErr := encodeImage(wr, resized, ext)
	closeErr = wr.Close()
	if encErr != nil || closeErr != nil {
		_ = deleteKey(ctx, bucket, thumbKey)
		if encErr != nil {
			return "", encErr
		}
		return "", closeErr
	}
	return PublicURL(thumbKey), nil
}