mri.go

package mangarock

import (
	"image"
	"io"
	"io/ioutil"

	"github.com/pkg/errors"
	"golang.org/x/image/webp"
)

// Decode reads a MRI image from r and returns it as an image.Image.
func Decode(r io.Reader) (image.Image, error) {
	r, _, err := DecodeRaw(r)
	if err != nil {
		return nil, err
	}
	return webp.Decode(r)
}

// DecodeConfig returns the color model and dimensions of a MRI. To recreate
// the original header it has to read the whole stream.
func DecodeConfig(r io.Reader) (image.Config, error) {
	r, _, err := DecodeRaw(r)
	if err != nil {
		return image.Config{}, err
	}
	return webp.DecodeConfig(r)
}

// DecodeRaw reads a MRI image from r, returns its original format (WEBP) as an
// io.Reader and its size in bytes. Can be used if there is no need for an
// image.Image.
func DecodeRaw(r io.Reader) (io.Reader, int, error) {
	bb, err := ioutil.ReadAll(r)
	if err != nil {
		return nil, 0, errors.Wrap(err, "could not read image file")
	}
	pr, pw := io.Pipe()

	// https://developers.google.com/speed/webp/docs/riff_container
	// See drawWebpToCanvas function in the site's client.js file
	// Extracted code: https://jsfiddle.net/6h2sLcs4/30/
	go func() {
		defer pw.Close()

		// The Size of the image excluding the chunk identifier and the field
		// itself.
		size := len(bb) + 7
		pw.Write([]byte("RIFF"))
		pw.Write([]byte{
			byte(size >> 0 & 255),
			byte(size >> 8 & 255),
			byte(size >> 16 & 255),
			byte(size >> 24 & 255),
		})
		pw.Write([]byte("WEBP"))
		pw.Write([]byte("VP8"))

		// The key can be found in MRs client.js file.
		key := byte(101)
		for _, b := range bb {
			pw.Write([]byte{b ^ key})
		}
	}()

	headerSize := 15
	return pr, len(bb) + headerSize, nil
}

func init() {
	image.RegisterFormat("mri", "E", Decode, DecodeConfig)
}