Well, there are 6225 == 645345427773512447880377451634304602899218432 different strings of length 25, over an alphabet of 62 characters. If you assume the rand() function was perfect, you want to solve the birthday paradox for a body that has 645345427773512447880377451634304602899218432 days in a year. If you generate N strings, a rough estimation of the probability that all of them are different is: e-N2/1290690855547024895760754903268609205798436864

But that's all irrelevant. Your random number generator is likely to only have 24, 32, 48 or 64 bits of entropy. Which means that most of the possible strings will never be generated. Even with 64 bits of entropy, there are only 18446744073709551616 strings possible. Which makes is far more likely for collisions to happen. In fact, Wikipedia tells us that with a 64 bit hash, there's a one procent chance you have a collision if you have 6.1E8 strings.

For more details, see MathWorld.

In reply to Re: How likely is rand() to repeat? by JavaFan
in thread How likely is rand() to repeat? by desertrat

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