# Randomly Generate Base62 for URL Shortening

Last week I wrote a post about how to design an URL shortening service, like TinyURL, and came up with 2 strategies to encode an URL, and one of which is to randomly generate base64 sequence.

Although I knew it works, in theory, I was not so confident deep in mind.

So I wrote some code and made a simulation for generating 10^8 6-digit and 8-digit base62 sequences, respectively.

1 | struct Alphabet { |

The result shows

- for generating 10^8 6-digit sequences, unique ratio of one-time generation is 0.9991
- for the 8-digit case, the unique ratio of one-time generation is high as 0.99999975

Even for 6-digit sequences, probability of two consecutive repeated generation is only 0.000001; and we can generate at least 1 million keys in less than 1 second.

Therefore, this strategy is indeed feasible in practice.

Notwithstanding, there is still one concernn we need to pay: the success ratio of one-time generation may drop dramatically after years, when number of generated combinations exceeds a threshould, like half of the entire combination space.

Unfortunately, I don’t know what this threshould is in probability thoery; but if that day came, we can extend our sample space by extend length of base62 sequence, XD.