processing_options.go

package main

import (
	"context"
	"encoding/base64"
	"errors"
	"fmt"
	"net/http"
	"net/url"
	"regexp"
	"strconv"
	"strings"
	"sync"

	"github.com/imgproxy/imgproxy/structdiff"
)

type urlOption struct {
	Name string
	Args []string
}
type urlOptions []urlOption

type processingHeaders struct {
	Accept        string
	Width         string
	ViewportWidth string
	DPR           string
}

type gravityType int

const (
	gravityUnknown gravityType = iota
	gravityCenter
	gravityNorth
	gravityEast
	gravitySouth
	gravityWest
	gravityNorthWest
	gravityNorthEast
	gravitySouthWest
	gravitySouthEast
	gravitySmart
	gravityFocusPoint
)

var gravityTypes = map[string]gravityType{
	"ce":   gravityCenter,
	"no":   gravityNorth,
	"ea":   gravityEast,
	"so":   gravitySouth,
	"we":   gravityWest,
	"nowe": gravityNorthWest,
	"noea": gravityNorthEast,
	"sowe": gravitySouthWest,
	"soea": gravitySouthEast,
	"sm":   gravitySmart,
	"fp":   gravityFocusPoint,
}

type resizeType int

const (
	resizeFit resizeType = iota
	resizeFill
	resizeCrop
	resizeAuto
)

var resizeTypes = map[string]resizeType{
	"fit":  resizeFit,
	"fill": resizeFill,
	"crop": resizeCrop,
	"auto": resizeAuto,
}

type rgbColor struct{ R, G, B uint8 }

var hexColorRegex = regexp.MustCompile("^([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$")

const (
	hexColorLongFormat  = "%02x%02x%02x"
	hexColorShortFormat = "%1x%1x%1x"
)

type gravityOptions struct {
	Type gravityType
	X, Y float64
}

type extendOptions struct {
	Enabled bool
	Gravity gravityOptions
}

type cropOptions struct {
	Width   int
	Height  int
	Gravity gravityOptions
}

type watermarkOptions struct {
	Enabled   bool
	Opacity   float64
	Replicate bool
	Gravity   gravityOptions
	Scale     float64
}

type processingOptions struct {
	ResizingType resizeType
	Width        int
	Height       int
	Dpr          float64
	Gravity      gravityOptions
	Enlarge      bool
	Extend       extendOptions
	Crop         cropOptions
	Format       imageType
	Quality      int
	MaxBytes     int
	Flatten      bool
	Background   rgbColor
	Blur         float32
	Sharpen      float32

	CacheBuster string

	Watermark watermarkOptions

	PreferWebP  bool
	EnforceWebP bool

	Filename string

	UsedPresets []string
}

const (
	imageURLCtxKey          = ctxKey("imageUrl")
	processingOptionsCtxKey = ctxKey("processingOptions")
	urlTokenPlain           = "plain"
	maxClientHintDPR        = 8

	msgForbidden     = "Forbidden"
	msgInvalidURL    = "Invalid URL"
	msgInvalidSource = "Invalid Source"
)

func (gt gravityType) String() string {
	for k, v := range gravityTypes {
		if v == gt {
			return k
		}
	}
	return ""
}

func (gt gravityType) MarshalJSON() ([]byte, error) {
	for k, v := range gravityTypes {
		if v == gt {
			return []byte(fmt.Sprintf("%q", k)), nil
		}
	}
	return []byte("null"), nil
}

func (rt resizeType) String() string {
	for k, v := range resizeTypes {
		if v == rt {
			return k
		}
	}
	return ""
}

func (rt resizeType) MarshalJSON() ([]byte, error) {
	for k, v := range resizeTypes {
		if v == rt {
			return []byte(fmt.Sprintf("%q", k)), nil
		}
	}
	return []byte("null"), nil
}

var (
	_newProcessingOptions    processingOptions
	newProcessingOptionsOnce sync.Once
)

func newProcessingOptions() *processingOptions {
	newProcessingOptionsOnce.Do(func() {
		_newProcessingOptions = processingOptions{
			ResizingType: resizeFit,
			Width:        0,
			Height:       0,
			Gravity:      gravityOptions{Type: gravityCenter},
			Enlarge:      false,
			Extend:       extendOptions{Enabled: false, Gravity: gravityOptions{Type: gravityCenter}},
			Quality:      conf.Quality,
			MaxBytes:     0,
			Format:       imageTypeUnknown,
			Background:   rgbColor{255, 255, 255},
			Blur:         0,
			Sharpen:      0,
			Dpr:          1,
			Watermark:    watermarkOptions{Opacity: 1, Replicate: false, Gravity: gravityOptions{Type: gravityCenter}},
		}
	})

	po := _newProcessingOptions
	po.UsedPresets = make([]string, 0, len(conf.Presets))

	return &po
}

func (po *processingOptions) isPresetUsed(name string) bool {
	for _, usedName := range po.UsedPresets {
		if usedName == name {
			return true
		}
	}
	return false
}

func (po *processingOptions) presetUsed(name string) {
	po.UsedPresets = append(po.UsedPresets, name)
}

func (po *processingOptions) Diff() structdiff.Entries {
	return structdiff.Diff(newProcessingOptions(), po)
}

func (po *processingOptions) String() string {
	return po.Diff().String()
}

func (po *processingOptions) MarshalJSON() ([]byte, error) {
	return po.Diff().MarshalJSON()
}

func colorFromHex(hexcolor string) (rgbColor, error) {
	c := rgbColor{}

	if !hexColorRegex.MatchString(hexcolor) {
		return c, fmt.Errorf("Invalid hex color: %s", hexcolor)
	}

	if len(hexcolor) == 3 {
		fmt.Sscanf(hexcolor, hexColorShortFormat, &c.R, &c.G, &c.B)
		c.R *= 17
		c.G *= 17
		c.B *= 17
	} else {
		fmt.Sscanf(hexcolor, hexColorLongFormat, &c.R, &c.G, &c.B)
	}

	return c, nil
}

func decodeBase64URL(parts []string) (string, string, error) {
	var format string

	encoded := strings.Join(parts, "")
	urlParts := strings.Split(encoded, ".")

	if len(urlParts[0]) == 0 {
		return "", "", errors.New("Image URL is empty")
	}

	if len(urlParts) > 2 {
		return "", "", fmt.Errorf("Multiple formats are specified: %s", encoded)
	}

	if len(urlParts) == 2 && len(urlParts[1]) > 0 {
		format = urlParts[1]
	}

	imageURL, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(urlParts[0], "="))
	if err != nil {
		return "", "", fmt.Errorf("Invalid url encoding: %s", encoded)
	}

	fullURL := fmt.Sprintf("%s%s", conf.BaseURL, string(imageURL))

	return fullURL, format, nil
}

func decodePlainURL(parts []string) (string, string, error) {
	var format string

	encoded := strings.Join(parts, "/")
	urlParts := strings.Split(encoded, "@")

	if len(urlParts[0]) == 0 {
		return "", "", errors.New("Image URL is empty")
	}

	if len(urlParts) > 2 {
		return "", "", fmt.Errorf("Multiple formats are specified: %s", encoded)
	}

	if len(urlParts) == 2 && len(urlParts[1]) > 0 {
		format = urlParts[1]
	}

	unescaped, err := url.PathUnescape(urlParts[0])
	if err != nil {
		return "", "", fmt.Errorf("Invalid url encoding: %s", encoded)
	}

	fullURL := fmt.Sprintf("%s%s", conf.BaseURL, unescaped)

	return fullURL, format, nil
}

func decodeURL(parts []string) (string, string, error) {
	if len(parts) == 0 {
		return "", "", errors.New("Image URL is empty")
	}

	if parts[0] == urlTokenPlain && len(parts) > 1 {
		return decodePlainURL(parts[1:])
	}

	return decodeBase64URL(parts)
}

func parseDimension(d *int, name, arg string) error {
	if v, err := strconv.Atoi(arg); err == nil && v >= 0 {
		*d = v
	} else {
		return fmt.Errorf("Invalid %s: %s", name, arg)
	}

	return nil
}

func parseBoolOption(str string) bool {
	b, err := strconv.ParseBool(str)

	if err != nil {
		logWarning("`%s` is not a valid boolean value. Treated as false", str)
	}

	return b
}

func isGravityOffcetValid(gravity gravityType, offset float64) bool {
	if gravity == gravityCenter {
		return true
	}

	return offset >= 0 && (gravity != gravityFocusPoint || offset <= 1)
}

func parseGravity(g *gravityOptions, args []string) error {
	nArgs := len(args)

	if nArgs > 3 {
		return fmt.Errorf("Invalid gravity arguments: %v", args)
	}

	if t, ok := gravityTypes[args[0]]; ok {
		g.Type = t
	} else {
		return fmt.Errorf("Invalid gravity: %s", args[0])
	}

	if g.Type == gravitySmart && nArgs > 1 {
		return fmt.Errorf("Invalid gravity arguments: %v", args)
	} else if g.Type == gravityFocusPoint && nArgs != 3 {
		return fmt.Errorf("Invalid gravity arguments: %v", args)
	}

	if nArgs > 1 {
		if x, err := strconv.ParseFloat(args[1], 64); err == nil && isGravityOffcetValid(g.Type, x) {
			g.X = x
		} else {
			return fmt.Errorf("Invalid gravity X: %s", args[1])
		}
	}

	if nArgs > 2 {
		if y, err := strconv.ParseFloat(args[2], 64); err == nil && isGravityOffcetValid(g.Type, y) {
			g.Y = y
		} else {
			return fmt.Errorf("Invalid gravity Y: %s", args[2])
		}
	}

	return nil
}

func applyWidthOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid width arguments: %v", args)
	}

	return parseDimension(&po.Width, "width", args[0])
}

func applyHeightOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid height arguments: %v", args)
	}

	return parseDimension(&po.Height, "height", args[0])
}

func applyEnlargeOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid enlarge arguments: %v", args)
	}

	po.Enlarge = parseBoolOption(args[0])

	return nil
}

func applyExtendOption(po *processingOptions, args []string) error {
	if len(args) > 4 {
		return fmt.Errorf("Invalid extend arguments: %v", args)
	}

	po.Extend.Enabled = parseBoolOption(args[0])

	if len(args) > 1 {
		if err := parseGravity(&po.Extend.Gravity, args[1:]); err != nil {
			return err
		}

		if po.Extend.Gravity.Type == gravitySmart {
			return errors.New("extend doesn't support smart gravity")
		}
	}

	return nil
}

func applySizeOption(po *processingOptions, args []string) (err error) {
	if len(args) > 7 {
		return fmt.Errorf("Invalid size arguments: %v", args)
	}

	if len(args) >= 1 && len(args[0]) > 0 {
		if err = applyWidthOption(po, args[0:1]); err != nil {
			return
		}
	}

	if len(args) >= 2 && len(args[1]) > 0 {
		if err = applyHeightOption(po, args[1:2]); err != nil {
			return
		}
	}

	if len(args) >= 3 && len(args[2]) > 0 {
		if err = applyEnlargeOption(po, args[2:3]); err != nil {
			return
		}
	}

	if len(args) >= 4 && len(args[3]) > 0 {
		if err = applyExtendOption(po, args[3:]); err != nil {
			return
		}
	}

	return nil
}

func applyResizingTypeOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid resizing type arguments: %v", args)
	}

	if r, ok := resizeTypes[args[0]]; ok {
		po.ResizingType = r
	} else {
		return fmt.Errorf("Invalid resize type: %s", args[0])
	}

	return nil
}

func applyResizeOption(po *processingOptions, args []string) error {
	if len(args) > 8 {
		return fmt.Errorf("Invalid resize arguments: %v", args)
	}

	if len(args[0]) > 0 {
		if err := applyResizingTypeOption(po, args[0:1]); err != nil {
			return err
		}
	}

	if len(args) > 1 {
		if err := applySizeOption(po, args[1:]); err != nil {
			return err
		}
	}

	return nil
}

func applyDprOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid dpr arguments: %v", args)
	}

	if d, err := strconv.ParseFloat(args[0], 64); err == nil && d > 0 {
		po.Dpr = d
	} else {
		return fmt.Errorf("Invalid dpr: %s", args[0])
	}

	return nil
}

func applyGravityOption(po *processingOptions, args []string) error {
	return parseGravity(&po.Gravity, args)
}

func applyCropOption(po *processingOptions, args []string) error {
	if len(args) > 5 {
		return fmt.Errorf("Invalid crop arguments: %v", args)
	}

	if err := parseDimension(&po.Crop.Width, "crop width", args[0]); err != nil {
		return err
	}

	if len(args) > 1 {
		if err := parseDimension(&po.Crop.Height, "crop height", args[1]); err != nil {
			return err
		}
	}

	if len(args) > 2 {
		return parseGravity(&po.Crop.Gravity, args[2:])
	}

	return nil
}

func applyQualityOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid quality arguments: %v", args)
	}

	if q, err := strconv.Atoi(args[0]); err == nil && q > 0 && q <= 100 {
		po.Quality = q
	} else {
		return fmt.Errorf("Invalid quality: %s", args[0])
	}

	return nil
}

func applyMaxBytesOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid max_bytes arguments: %v", args)
	}

	if max, err := strconv.Atoi(args[0]); err == nil && max >= 0 {
		po.MaxBytes = max
	} else {
		return fmt.Errorf("Invalid max_bytes: %s", args[0])
	}

	return nil
}

func applyBackgroundOption(po *processingOptions, args []string) error {
	switch len(args) {
	case 1:
		if len(args[0]) == 0 {
			po.Flatten = false
		} else if c, err := colorFromHex(args[0]); err == nil {
			po.Flatten = true
			po.Background = c
		} else {
			return fmt.Errorf("Invalid background argument: %s", err)
		}

	case 3:
		po.Flatten = true

		if r, err := strconv.ParseUint(args[0], 10, 8); err == nil && r <= 255 {
			po.Background.R = uint8(r)
		} else {
			return fmt.Errorf("Invalid background red channel: %s", args[0])
		}

		if g, err := strconv.ParseUint(args[1], 10, 8); err == nil && g <= 255 {
			po.Background.G = uint8(g)
		} else {
			return fmt.Errorf("Invalid background green channel: %s", args[1])
		}

		if b, err := strconv.ParseUint(args[2], 10, 8); err == nil && b <= 255 {
			po.Background.B = uint8(b)
		} else {
			return fmt.Errorf("Invalid background blue channel: %s", args[2])
		}

	default:
		return fmt.Errorf("Invalid background arguments: %v", args)
	}

	return nil
}

func applyBlurOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid blur arguments: %v", args)
	}

	if b, err := strconv.ParseFloat(args[0], 32); err == nil && b >= 0 {
		po.Blur = float32(b)
	} else {
		return fmt.Errorf("Invalid blur: %s", args[0])
	}

	return nil
}

func applySharpenOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid sharpen arguments: %v", args)
	}

	if s, err := strconv.ParseFloat(args[0], 32); err == nil && s >= 0 {
		po.Sharpen = float32(s)
	} else {
		return fmt.Errorf("Invalid sharpen: %s", args[0])
	}

	return nil
}

func applyPresetOption(po *processingOptions, args []string) error {
	for _, preset := range args {
		if p, ok := conf.Presets[preset]; ok {
			if po.isPresetUsed(preset) {
				logWarning("Recursive preset usage is detected: %s", preset)
				continue
			}

			po.presetUsed(preset)

			if err := applyProcessingOptions(po, p); err != nil {
				return err
			}
		} else {
			return fmt.Errorf("Unknown preset: %s", preset)
		}
	}

	return nil
}

func applyWatermarkOption(po *processingOptions, args []string) error {
	if len(args) > 7 {
		return fmt.Errorf("Invalid watermark arguments: %v", args)
	}

	if o, err := strconv.ParseFloat(args[0], 64); err == nil && o >= 0 && o <= 1 {
		po.Watermark.Enabled = o > 0
		po.Watermark.Opacity = o
	} else {
		return fmt.Errorf("Invalid watermark opacity: %s", args[0])
	}

	if len(args) > 1 && len(args[1]) > 0 {
		if args[1] == "re" {
			po.Watermark.Replicate = true
		} else if g, ok := gravityTypes[args[1]]; ok && g != gravityFocusPoint && g != gravitySmart {
			po.Watermark.Gravity.Type = g
		} else {
			return fmt.Errorf("Invalid watermark position: %s", args[1])
		}
	}

	if len(args) > 2 && len(args[2]) > 0 {
		if x, err := strconv.Atoi(args[2]); err == nil {
			po.Watermark.Gravity.X = float64(x)
		} else {
			return fmt.Errorf("Invalid watermark X offset: %s", args[2])
		}
	}

	if len(args) > 3 && len(args[3]) > 0 {
		if y, err := strconv.Atoi(args[3]); err == nil {
			po.Watermark.Gravity.Y = float64(y)
		} else {
			return fmt.Errorf("Invalid watermark Y offset: %s", args[3])
		}
	}

	if len(args) > 4 && len(args[4]) > 0 {
		if s, err := strconv.ParseFloat(args[4], 64); err == nil && s >= 0 {
			po.Watermark.Scale = s
		} else {
			return fmt.Errorf("Invalid watermark scale: %s", args[4])
		}
	}

	return nil
}

func applyFormatOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid format arguments: %v", args)
	}

	if f, ok := imageTypes[args[0]]; ok {
		po.Format = f
	} else {
		return fmt.Errorf("Invalid image format: %s", args[0])
	}

	if !imageTypeSaveSupport(po.Format) {
		return fmt.Errorf("Resulting image format is not supported: %s", po.Format)
	}

	return nil
}

func applyCacheBusterOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid cache buster arguments: %v", args)
	}

	po.CacheBuster = args[0]

	return nil
}

func applyFilenameOption(po *processingOptions, args []string) error {
	if len(args) > 1 {
		return fmt.Errorf("Invalid filename arguments: %v", args)
	}

	po.Filename = args[0]

	return nil
}

func applyProcessingOption(po *processingOptions, name string, args []string) error {
	switch name {
	case "format", "f", "ext":
		return applyFormatOption(po, args)
	case "resize", "rs":
		return applyResizeOption(po, args)
	case "resizing_type", "rt":
		return applyResizingTypeOption(po, args)
	case "size", "s":
		return applySizeOption(po, args)
	case "width", "w":
		return applyWidthOption(po, args)
	case "height", "h":
		return applyHeightOption(po, args)
	case "enlarge", "el":
		return applyEnlargeOption(po, args)
	case "extend", "ex":
		return applyExtendOption(po, args)
	case "dpr":
		return applyDprOption(po, args)
	case "gravity", "g":
		return applyGravityOption(po, args)
	case "crop", "c":
		return applyCropOption(po, args)
	case "quality", "q":
		return applyQualityOption(po, args)
	case "max_bytes", "mb":
		return applyMaxBytesOption(po, args)
	case "background", "bg":
		return applyBackgroundOption(po, args)
	case "blur", "bl":
		return applyBlurOption(po, args)
	case "sharpen", "sh":
		return applySharpenOption(po, args)
	case "watermark", "wm":
		return applyWatermarkOption(po, args)
	case "preset", "pr":
		return applyPresetOption(po, args)
	case "cachebuster", "cb":
		return applyCacheBusterOption(po, args)
	case "filename", "fn":
		return applyFilenameOption(po, args)
	}

	return fmt.Errorf("Unknown processing option: %s", name)
}

func applyProcessingOptions(po *processingOptions, options urlOptions) error {
	for _, opt := range options {
		if err := applyProcessingOption(po, opt.Name, opt.Args); err != nil {
			return err
		}
	}

	return nil
}

func isAllowedSource(imageURL string) bool {
	if len(conf.AllowedSources) == 0 {
		return true
	}
	for _, val := range conf.AllowedSources {
		if strings.HasPrefix(imageURL, string(val)) {
			return true
		}
	}
	return false
}

func parseURLOptions(opts []string) (urlOptions, []string) {
	parsed := make(urlOptions, 0, len(opts))
	urlStart := len(opts) + 1

	for i, opt := range opts {
		args := strings.Split(opt, ":")

		if len(args) == 1 {
			urlStart = i
			break
		}

		parsed = append(parsed, urlOption{Name: args[0], Args: args[1:]})
	}

	var rest []string

	if urlStart < len(opts) {
		rest = opts[urlStart:]
	} else {
		rest = []string{}
	}

	return parsed, rest
}

func defaultProcessingOptions(headers *processingHeaders) (*processingOptions, error) {
	po := newProcessingOptions()

	if strings.Contains(headers.Accept, "image/webp") {
		po.PreferWebP = conf.EnableWebpDetection || conf.EnforceWebp
		po.EnforceWebP = conf.EnforceWebp
	}

	if conf.EnableClientHints && len(headers.ViewportWidth) > 0 {
		if vw, err := strconv.Atoi(headers.ViewportWidth); err == nil {
			po.Width = vw
		}
	}
	if conf.EnableClientHints && len(headers.Width) > 0 {
		if w, err := strconv.Atoi(headers.Width); err == nil {
			po.Width = w
		}
	}
	if conf.EnableClientHints && len(headers.DPR) > 0 {
		if dpr, err := strconv.ParseFloat(headers.DPR, 64); err == nil && (dpr > 0 && dpr <= maxClientHintDPR) {
			po.Dpr = dpr
		}
	}
	if _, ok := conf.Presets["default"]; ok {
		if err := applyPresetOption(po, []string{"default"}); err != nil {
			return po, err
		}
	}

	return po, nil
}

func parsePathAdvanced(parts []string, headers *processingHeaders) (string, *processingOptions, error) {
	po, err := defaultProcessingOptions(headers)
	if err != nil {
		return "", po, err
	}

	options, urlParts := parseURLOptions(parts)

	if err = applyProcessingOptions(po, options); err != nil {
		return "", po, err
	}

	url, extension, err := decodeURL(urlParts)
	if err != nil {
		return "", po, err
	}

	if len(extension) > 0 {
		if err = applyFormatOption(po, []string{extension}); err != nil {
			return "", po, err
		}
	}

	return url, po, nil
}

func parsePathPresets(parts []string, headers *processingHeaders) (string, *processingOptions, error) {
	po, err := defaultProcessingOptions(headers)
	if err != nil {
		return "", po, err
	}

	presets := strings.Split(parts[0], ":")
	urlParts := parts[1:]

	if err = applyPresetOption(po, presets); err != nil {
		return "", nil, err
	}

	url, extension, err := decodeURL(urlParts)
	if err != nil {
		return "", po, err
	}

	if len(extension) > 0 {
		if err = applyFormatOption(po, []string{extension}); err != nil {
			return "", po, err
		}
	}

	return url, po, nil
}

func parsePathBasic(parts []string, headers *processingHeaders) (string, *processingOptions, error) {
	if len(parts) < 6 {
		return "", nil, fmt.Errorf("Invalid basic URL format arguments: %s", strings.Join(parts, "/"))
	}

	po, err := defaultProcessingOptions(headers)
	if err != nil {
		return "", po, err
	}

	po.ResizingType = resizeTypes[parts[0]]

	if err = applyWidthOption(po, parts[1:2]); err != nil {
		return "", po, err
	}

	if err = applyHeightOption(po, parts[2:3]); err != nil {
		return "", po, err
	}

	if err = applyGravityOption(po, strings.Split(parts[3], ":")); err != nil {
		return "", po, err
	}

	if err = applyEnlargeOption(po, parts[4:5]); err != nil {
		return "", po, err
	}

	url, extension, err := decodeURL(parts[5:])
	if err != nil {
		return "", po, err
	}

	if len(extension) > 0 {
		if err := applyFormatOption(po, []string{extension}); err != nil {
			return "", po, err
		}
	}

	return url, po, nil
}

var striderRegex = regexp.MustCompile(`^/(0|16|24|32|64|100|160|256|320|480|800|1024)/(0|16|24|32|64|96|100|1024)/(http.+)$`)

func parsePath(ctx context.Context, r *http.Request) (context.Context, error) {
	path := r.URL.RawPath
	if len(path) == 0 {
		path = r.URL.Path
	}
	match := striderRegex.FindStringSubmatch(path)
	if len(match) > 0 {
		if strings.HasPrefix(match[3], "https://striderusercontent.com") {
			match[3] = "gs" + match[3][5:]
		}
		path = fmt.Sprintf("/insecure/fill/%s/%s/sm/0/plain/%s", match[1], match[2], match[3])
	}
	parts := strings.Split(strings.TrimPrefix(path, "/"), "/")

	if len(parts) < 2 {
		return ctx, newError(404, fmt.Sprintf("Invalid path: %s", path), msgInvalidURL)
	}

	if !conf.AllowInsecure {
		if err := validatePath(parts[0], strings.TrimPrefix(path, fmt.Sprintf("/%s", parts[0]))); err != nil {
			return ctx, newError(403, err.Error(), msgForbidden)
		}
	}

	headers := &processingHeaders{
		Accept:        r.Header.Get("Accept"),
		Width:         r.Header.Get("Width"),
		ViewportWidth: r.Header.Get("Viewport-Width"),
		DPR:           r.Header.Get("DPR"),
	}

	var imageURL string
	var po *processingOptions
	var err error

	if conf.OnlyPresets {
		imageURL, po, err = parsePathPresets(parts[1:], headers)
	} else if _, ok := resizeTypes[parts[1]]; ok {
		imageURL, po, err = parsePathBasic(parts[1:], headers)
	} else {
		imageURL, po, err = parsePathAdvanced(parts[1:], headers)
	}

	if err != nil {
		return ctx, newError(404, err.Error(), msgInvalidURL)
	}

	if !isAllowedSource(imageURL) {
		return ctx, newError(404, fmt.Sprintf("Invalid source"), msgInvalidSource)
	}

	ctx = context.WithValue(ctx, imageURLCtxKey, imageURL)
	ctx = context.WithValue(ctx, processingOptionsCtxKey, po)

	return ctx, nil
}

func getImageURL(ctx context.Context) string {
	return ctx.Value(imageURLCtxKey).(string)
}

func getProcessingOptions(ctx context.Context) *processingOptions {
	return ctx.Value(processingOptionsCtxKey).(*processingOptions)
}