get jpg size without full read. written in Golang

main.go

package main

import (
	"errors"
	"io"
	"os"
)

type ImageSize struct {
	Width  int
	Height int
}

var InvalidImage = errors.New("invalid image")

func readByte(reader io.Reader) (byte, error) {
	b := make([]byte, 1)
	_, err := reader.Read(b)
	if err != nil {
		return 0, err
	}
	return b[0], nil
}

func readBytes(reader io.Reader, size int) ([]byte, error) {
	b := make([]byte, size)
	_, err := reader.Read(b)
	if err != nil {
		return nil, err
	}
	return b, nil
}

func readUInt16BE(data []byte) int {
	return int(data[1]) | (int(data[0]) << 8)
}

func getImageSize(reader io.Reader) (*ImageSize, error) {
	data, err := readBytes(reader, 2)
	if err != nil {
		return nil, err
	}
	if data[0] != 0xFF || data[1] != 0xD8 {
		return nil, InvalidImage
	}

	for {
		code, err := readByte(reader)
		if err != nil {
			return nil, err
		}
		if code != 0xFF {
			return nil, InvalidImage
		}

		code, err = readByte(reader)
		if err != nil {
			return nil, err
		}
		// skip padding bytes
		for code == 0xFF {
			code, err = readByte(reader)
			if err != nil {
				return nil, err
			}
		}

		var length int
		// standalone markers, according to JPEG 1992,
		// http://www.w3.org/Graphics/JPEG/itu-t81.pdf, see Table B.1
		if (0xD0 <= code && code <= 0xD9) || code == 0x01 {
			length = 0
		} else if 0xC0 <= code && code <= 0xFE {
			// the rest of the unreserved markers
			data, err = readBytes(reader, 2)
			if err != nil {
				return nil, err
			}
			length = readUInt16BE(data) - 2
		} else {
			// unknown markers
			return nil, InvalidImage
		}

		if code == 0xD9 || code == 0xDA {
			// end of the data stream
			return nil, InvalidImage
		}

		data, err = readBytes(reader, length)
		if err != nil {
			return nil, err
		}

		if length >= 5 &&
			(0xC0 <= code && code <= 0xCF) &&
			code != 0xC4 && code != 0xC8 && code != 0xCC {
			return &ImageSize{
				Width:  readUInt16BE(data[3:]),
				Height: readUInt16BE(data[1:]),
			}, nil
		}
	}
}

func main() {
	f, err := os.Open("example.jpeg")
	if err != nil {
		panic(err)
	}
	s, err := getImageSize(f)
	if err != nil {
		panic(err)
	}
	println(s.Width, s.Height)
}