// Package captcha implements generation and verification of image and audio
// CAPTCHAs.
//
// A captcha solution is the sequence of digits 0-9 with the defined length.
// There are two captcha representations: image and audio.
//
// An image representation is a PNG-encoded image with the solution printed on
// it in such a way that makes it hard for computers to solve it using OCR.
//
// An audio representation is a WAVE-encoded (8 kHz unsigned 8-bit) sound
// with the spoken solution (currently in English). To make it hard for
// computers to solve audio captcha, the voice that pronounces numbers has
// random speed and pitch, and there is a randomly generated background noise
// mixed into the sound.
//
// This package doesn't require external files or libraries to generate captcha
// representations; it is self-contained.
//
// To make captchas one-time, the package includes a memory storage that stores
// captcha ids, their solutions, and expiration time. Used captchas are removed
// from the store immediately after calling Verify or VerifyString, while
// unused captchas (user loaded a page with captcha, but didn't submit the
// form) are collected automatically after the predefined expiration time.
// Developers can also provide custom store (for example, which saves captcha
// ids and solutions in database) by implementing Store interface and
// registering the object with SetCustomStore.
//
// Captchas are created by calling New, which returns the captcha id.  Their
// representations, though, are created on-the-fly by calling WriteImage or
// WriteAudio functions. Created representations are not stored anywhere, so
// subsequent calls to these functions with the same id will write the same
// captcha solution, but with a different random representation. Reload
// function will create a new different solution for the provided captcha,
// allowing users to "reload" captcha if they can't solve the displayed one
// without reloading the whole page.  Verify and VerifyString are used to
// verify that the given solution is the right one for the given captcha id.
//
// Server provides an http.Handler which can serve image and audio
// representations of captchas automatically from the URL. It can also be used
// to reload captchas.  Refer to Server function documentation for details, or
// take a look at the example in "example" subdirectory.
package captcha

import (
	"bytes"
	"crypto/rand"
	"github.com/dchest/uniuri"
	"http"
	"io"
	"os"
	"path"
	"strconv"
)

const (
	// Standard number of digits in captcha.
	StdLength = 6
	// The number of captchas created that triggers garbage collection used
	// by default store.
	CollectNum = 100
	// Expiration time of captchas used by default store.
	Expiration = 10 * 60 // 10 minutes

)

var ErrNotFound = os.NewError("captcha with the given id not found")

// globalStore is a shared storage for captchas, generated by New function.
var globalStore = NewMemoryStore(CollectNum, Expiration)

// SetCustomStore sets custom storage for captchas, replacing the default
// memory store. This function must be called before generating any captchas.
func SetCustomStore(s Store) {
	globalStore = s
}

// RandomDigits returns a byte slice of the given length containing random
// digits in range 0-9.
func RandomDigits(length int) []byte {
	d := make([]byte, length)
	if _, err := io.ReadFull(rand.Reader, d); err != nil {
		panic("error reading random source: " + err.String())
	}
	for i := range d {
		d[i] %= 10
	}
	return d
}

// New creates a new captcha of the given length, saves it in the internal
// storage, and returns its id.
func New(length int) (id string) {
	id = uniuri.New()
	globalStore.Set(id, RandomDigits(length))
	return
}

// Reload generates and remembers new digits for the given captcha id.  This
// function returns false if there is no captcha with the given id.
//
// After calling this function, the image or audio presented to a user must be
// refreshed to show the new captcha representation (WriteImage and WriteAudio
// will write the new one).
func Reload(id string) bool {
	old := globalStore.Get(id, false)
	if old == nil {
		return false
	}
	globalStore.Set(id, RandomDigits(len(old)))
	return true
}

// WriteImage writes PNG-encoded image representation of the captcha with the
// given id. The image will have the given width and height.
func WriteImage(w io.Writer, id string, width, height int) os.Error {
	d := globalStore.Get(id, false)
	if d == nil {
		return ErrNotFound
	}
	_, err := NewImage(d, width, height).WriteTo(w)
	return err
}

// WriteAudio writes WAV-encoded audio representation of the captcha with the
// given id.
func WriteAudio(w io.Writer, id string) os.Error {
	d := globalStore.Get(id, false)
	if d == nil {
		return ErrNotFound
	}
	_, err := NewAudio(d).WriteTo(w)
	return err
}

// Verify returns true if the given digits are the ones that were used to
// create the given captcha id.
// 
// The function deletes the captcha with the given id from the internal
// storage, so that the same captcha can't be verified anymore.
func Verify(id string, digits []byte) bool {
	if digits == nil || len(digits) == 0 {
		return false
	}
	reald := globalStore.Get(id, true)
	if reald == nil {
		return false
	}
	return bytes.Equal(digits, reald)
}

// VerifyString is like Verify, but accepts a string of digits.  It removes
// spaces and commas from the string, but any other characters, apart from
// digits and listed above, will cause the function to return false.
func VerifyString(id string, digits string) bool {
	if digits == "" {
		return false
	}
	ns := make([]byte, len(digits))
	for i := range ns {
		d := digits[i]
		switch {
		case '0' <= d && d <= '9':
			ns[i] = d - '0'
		case d == ' ' || d == ',':
			// ignore
		default:
			return false
		}
	}
	return Verify(id, ns)
}

// Collect deletes expired or used captchas from the internal storage. It is
// called automatically by New function every CollectNum generated captchas,
// but still exported to enable freeing memory manually if needed.
//
// Collection is launched in a new goroutine.
func Collect() {
	go globalStore.Collect()
}

type captchaHandler struct {
	imgWidth  int
	imgHeight int
}

// Server returns a handler that serves HTTP requests with image or
// audio representations of captchas. Image dimensions are accepted as
// arguments. The server decides which captcha to serve based on the last URL
// path component: file name part must contain a captcha id, file extension —
// its format (PNG or WAV).
//
// For example, for file name "B9QTvDV1RXbVJ3Ac.png" it serves an image captcha
// with id "B9QTvDV1RXbVJ3Ac", and for "B9QTvDV1RXbVJ3Ac.wav" it serves the
// same captcha in audio format.
//
// To serve an audio captcha as downloadable file, append "?get" to URL.
//
// To reload captcha (get a different solution for the same captcha id), append
// "?reload=x" to URL, where x may be anything (for example, current time or a
// random number to make browsers refetch an image instead of loading it from
// cache).
func Server(w, h int) http.Handler { return &captchaHandler{w, h} }

func (h *captchaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	_, file := path.Split(r.URL.Path)
	ext := path.Ext(file)
	id := file[:len(file)-len(ext)]
	if ext == "" || id == "" {
		http.NotFound(w, r)
		return
	}
	var err os.Error
	if r.FormValue("reload") != "" {
		Reload(id)
	}
	switch ext {
	case ".png", ".PNG":
		w.Header().Set("Content-Type", "image/png")
		err = WriteImage(w, id, h.imgWidth, h.imgHeight)
	case ".wav", ".WAV":
		if r.URL.RawQuery == "get" {
			w.Header().Set("Content-Type", "application/octet-stream")
		} else {
			w.Header().Set("Content-Type", "audio/x-wav")
		}
		//err = WriteAudio(w, id)
		//XXX(dchest) Workaround for Chrome: it wants content-length,
		//or else will start playing NOT from the beginning.
		//File issue: http://code.google.com/p/chromium/issues/detail?id=80565
		d := globalStore.Get(id, false)
		if d == nil {
			err = ErrNotFound
		} else {
			a := NewAudio(d)
			w.Header().Set("Content-Length", strconv.Itoa(a.EncodedLen()))
			_, err = a.WriteTo(w)
		}
	default:
		err = ErrNotFound
	}
	if err != nil {
		if err == ErrNotFound {
			http.NotFound(w, r)
			return
		}
		http.Error(w, "error serving captcha", http.StatusInternalServerError)
	}
}