wathiede/gist:7982372

## gistfile1.go
package main

import (
    "bytes"
    "io"
    "io/ioutil"
    "log"
    "os"

    "camlistore.org/third_party/github.com/camlistore/goexif/exif"
)

// keepFirstN keeps the first N bytes written to it in Bytes.
type keepFirstN struct {
    N     int
    Bytes []byte
}

func (w *keepFirstN) Write(p []byte) (n int, err error) {
    if n := w.N - len(w.Bytes); n > 0 {
        if n > len(p) {
            n = len(p)
        }
        w.Bytes = append(w.Bytes, p[:n]...)
    }
    return len(p), nil
}

func tryParse(n int, fn string) error {
    f, err := os.Open(fn)
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()

    var imageBuf *keepFirstN
    copyDest := ioutil.Discard

    imageBuf = &keepFirstN{N: n}
    copyDest = io.MultiWriter(copyDest, imageBuf)
    _, err = io.Copy(copyDest, f)
    if err != nil {
        log.Fatal(err)
    }

    _, err = exif.Decode(bytes.NewReader(imageBuf.Bytes))
    return err
}

func main() {
    for _, fn := range os.Args[1:] {
        n := 1 << 30
        for {
            err := tryParse(n, fn)
            if err != nil {
                log.Println(fn, n, "too small")
                break
            }
            n >>= 1
        }
    }
}
	package main

	import (
	"bytes"
	"io"
	"io/ioutil"
	"log"
	"os"

	"camlistore.org/third_party/github.com/camlistore/goexif/exif"
	)

	// keepFirstN keeps the first N bytes written to it in Bytes.
	type keepFirstN struct {
	N int
	Bytes []byte
	}

	func (w *keepFirstN) Write(p []byte) (n int, err error) {
	if n := w.N - len(w.Bytes); n > 0 {
	if n > len(p) {
	n = len(p)
	}
	w.Bytes = append(w.Bytes, p[:n]...)
	}
	return len(p), nil
	}

	func tryParse(n int, fn string) error {
	f, err := os.Open(fn)
	if err != nil {
	log.Fatal(err)
	}
	defer f.Close()

	var imageBuf *keepFirstN
	copyDest := ioutil.Discard

	imageBuf = &keepFirstN{N: n}
	copyDest = io.MultiWriter(copyDest, imageBuf)
	_, err = io.Copy(copyDest, f)
	if err != nil {
	log.Fatal(err)
	}

	_, err = exif.Decode(bytes.NewReader(imageBuf.Bytes))
	return err
	}

	func main() {
	for _, fn := range os.Args[1:] {
	n := 1 << 30
	for {
	err := tryParse(n, fn)
	if err != nil {
	log.Println(fn, n, "too small")
	break
	}
	n >>= 1
	}
	}
	}