// 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"
	"io"
	"os"
)

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()
}