Can phone apps detect what phones around them are running the same app?

I would like a cell phone app to be able to determine how many cell phones around it are running the same app.

I'm not sure what direction to start researching (API's,concepts) to understand how my phone would figure out what 开发者_JS百科other phones are running the same app in a specific distance.

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There are 2 ways.

1.) Implement bluetooth detection. This is extremely battery intensive and a huge security risk.

2.) Have the app update a server with its location, have the server determine what phones are around it and send that information back to the phone.

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I don't know of any API's, but you may be able to get some ideas from the Bump app (for iPhone and Android) which has the ability to communicate in this fashion.