Make a ID for detected face and count the number of people in a minute

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I detected the face in a webcam using "Violo-Jones" face detector in opencv, now I need do the following:

1) I need to make a ID for each face detected say 1,2,3 for person A,B,C.

2) If the person "A" gets out of the frame and come back within some time ( say 1 minute ), then the ID of the Person "A" should be same that is "1" in my case. If the Person "A" comes after a minute he will be counted as a different ID (3).

How can I do the above in opencv? Any ideas or suggestions or links would be great!