1Package captcha
  2=====================
  3
  4	import "github.com/dchest/captcha"
  5
  6Package captcha implements generation and verification of image and audio
  7CAPTCHAs.
  8
  9A captcha solution is the sequence of digits 0-9 with the defined length.
 10There are two captcha representations: image and audio.
 11
 12An image representation is a PNG-encoded image with the solution printed on
 13it in such a way that makes it hard for computers to solve it using OCR.
 14
 15An audio representation is a WAVE-encoded (8 kHz unsigned 8-bit) sound
 16with the spoken solution (currently in English). To make it hard for
 17computers to solve audio captcha, the voice that pronounces numbers has
 18random speed and pitch, and there is a randomly generated background noise
 19mixed into the sound.
 20
 21This package doesn't require external files or libraries to generate captcha
 22representations; it is self-contained.
 23
 24To make captchas one-time, the package includes a memory storage that stores
 25captcha ids, their solutions, and expiration time. Used captchas are removed
 26from the store immediately after calling Verify or VerifyString, while
 27unused captchas (user loaded a page with captcha, but didn't submit the
 28form) are collected automatically after the predefined expiration time.
 29Developers can also provide custom store (for example, which saves captcha
 30ids and solutions in database) by implementing Store interface and
 31registering the object with SetCustomStore.
 32
 33Captchas are created by calling New, which returns the captcha id.  Their
 34representations, though, are created on-the-fly by calling WriteImage or
 35WriteAudio functions. Created representations are not stored anywhere, so
 36subsequent calls to these functions with the same id will write the same
 37captcha solution, but with a different random representation. Reload
 38function will create a new different solution for the provided captcha,
 39allowing users to "reload" captcha if they can't solve the displayed one
 40without reloading the whole page.  Verify and VerifyString are used to
 41verify that the given solution is the right one for the given captcha id.
 42
 43Server provides an http.Handler which can serve image and audio
 44representations of captchas automatically from the URL. It can also be used
 45to reload captchas.  Refer to Server function documentation for details, or
 46take a look at the example in "example" subdirectory.
 47
 48
 49Examples
 50--------
 51
 52![Image](https://github.com/dchest/captcha/raw/master/capgen/example.png)
 53
 54[Audio](https://github.com/dchest/captcha/raw/master/capgen/example.wav)
 55
 56
 57Constants
 58---------
 59
 60``` go
 61const (
 62    // Standard number of digits in captcha.
 63    StdLength = 6
 64    // The number of captchas created that triggers garbage collection used
 65    // by default store.
 66    CollectNum = 100
 67    // Expiration time of captchas used by default store.
 68    Expiration = 10 * 60 // 10 minutes
 69
 70)
 71```
 72
 73``` go
 74const (
 75    // Standard width and height of a captcha image.
 76    StdWidth  = 300
 77    StdHeight = 80
 78)
 79```
 80
 81
 82Variables
 83---------
 84
 85``` go
 86var ErrNotFound = os.NewError("captcha with the given id not found")
 87```
 88
 89
 90
 91Functions
 92---------
 93
 94### func Collect
 95
 96	func Collect()
 97	
 98Collect deletes expired or used captchas from the internal storage. It is
 99called automatically by New function every CollectNum generated captchas,
100but still exported to enable freeing memory manually if needed.
101
102Collection is launched in a new goroutine.
103
104### func New
105
106	func New(length int) (id string)
107	
108New creates a new captcha of the given length, saves it in the internal
109storage, and returns its id.
110
111### func RandomDigits
112
113	func RandomDigits(length int) []byte
114	
115RandomDigits returns a byte slice of the given length containing random
116digits in range 0-9.
117
118### func Reload
119
120	func Reload(id string) bool
121	
122Reload generates and remembers new digits for the given captcha id.  This
123function returns false if there is no captcha with the given id.
124
125After calling this function, the image or audio presented to a user must be
126refreshed to show the new captcha representation (WriteImage and WriteAudio
127will write the new one).
128
129### func Server
130
131	func Server(w, h int) http.Handler
132	
133Server returns a handler that serves HTTP requests with image or
134audio representations of captchas. Image dimensions are accepted as
135arguments. The server decides which captcha to serve based on the last URL
136path component: file name part must contain a captcha id, file extension —
137its format (PNG or WAV).
138
139For example, for file name "B9QTvDV1RXbVJ3Ac.png" it serves an image captcha
140with id "B9QTvDV1RXbVJ3Ac", and for "B9QTvDV1RXbVJ3Ac.wav" it serves the
141same captcha in audio format.
142
143To serve an audio captcha as downloadable file, append "?get" to URL.
144
145To reload captcha (get a different solution for the same captcha id), append
146"?reload=x" to URL, where x may be anything (for example, current time or a
147random number to make browsers refetch an image instead of loading it from
148cache).
149
150### func SetCustomStore
151
152	func SetCustomStore(s Store)
153	
154SetCustomStore sets custom storage for captchas, replacing the default
155memory store. This function must be called before generating any captchas.
156
157### func Verify
158
159	func Verify(id string, digits []byte) bool
160	
161Verify returns true if the given digits are the ones that were used to
162create the given captcha id.
163
164The function deletes the captcha with the given id from the internal
165storage, so that the same captcha can't be verified anymore.
166
167### func VerifyString
168
169	func VerifyString(id string, digits string) bool
170	
171VerifyString is like Verify, but accepts a string of digits.  It removes
172spaces and commas from the string, but any other characters, apart from
173digits and listed above, will cause the function to return false.
174
175### func WriteAudio
176
177	func WriteAudio(w io.Writer, id string) os.Error
178	
179WriteAudio writes WAV-encoded audio representation of the captcha with the
180given id.
181
182### func WriteImage
183
184	func WriteImage(w io.Writer, id string, width, height int) os.Error
185	
186WriteImage writes PNG-encoded image representation of the captcha with the
187given id. The image will have the given width and height.
188
189
190Types
191-----
192
193``` go
194type Audio struct {
195    // contains unexported fields
196}
197```
198
199
200### func NewAudio
201
202	func NewAudio(digits []byte) *Audio
203	
204NewImage returns a new audio captcha with the given digits, where each digit
205must be in range 0-9.
206
207### func (*Audio) EncodedLen
208
209	func (a *Audio) EncodedLen() int
210	
211EncodedLen returns the length of WAV-encoded audio captcha.
212
213### func (*Audio) WriteTo
214
215	func (a *Audio) WriteTo(w io.Writer) (n int64, err os.Error)
216	
217WriteTo writes captcha audio in WAVE format into the given io.Writer, and
218returns the number of bytes written and an error if any.
219
220``` go
221type Image struct {
222    *image.NRGBA
223    // contains unexported fields
224}
225```
226
227
228### func NewImage
229
230	func NewImage(digits []byte, width, height int) *Image
231	
232NewImage returns a new captcha image of the given width and height with the
233given digits, where each digit must be in range 0-9.
234
235### func (*Image) WriteTo
236
237	func (img *Image) WriteTo(w io.Writer) (int64, os.Error)
238	
239WriteTo writes captcha image in PNG format into the given writer.
240
241``` go
242type Store interface {
243    // Set sets the digits for the captcha id.
244    Set(id string, digits []byte)
245
246    // Get returns stored digits for the captcha id. Clear indicates
247    // whether the captcha must be deleted from the store.
248    Get(id string, clear bool) (digits []byte)
249
250    // Collect deletes expired captchas from the store.  For custom stores
251    // this method is not called automatically, it is only wired to the
252    // package's Collect function.  Custom stores must implement their own
253    // procedure for calling Collect, for example, in Set method.
254    Collect()
255}
256```
257
258An object implementing Store interface can be registered with SetCustomStore
259function to handle storage and retrieval of captcha ids and solutions for
260them, replacing the default memory store.
261
262### func NewMemoryStore
263
264	func NewMemoryStore(collectNum int, expiration int64) Store
265	
266NewMemoryStore returns a new standard memory store for captchas with the
267given collection threshold and expiration time in seconds. The returned
268store must be registered with SetCustomStore to replace the default one.
269
270
271Bugs
272----
273
274[Not our bug] Google Chrome 10 plays unsigned 8-bit PCM WAVE
275audio on Mac with horrible distortions.  Issue:
276http://code.google.com/p/chromium/issues/detail?id=70730.
277This has been fixed, and version 12 will play them properly.
278
279While Image conforms to io.WriterTo interface, its WriteTo
280method returns 0 instead of the actual bytes written because png.Encode
281doesn't report this.
282
283
284Subdirectories
285--------------
286
287capgen
288example
289generate