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A threading riddle

by davidtgoldblatt on 6/29/15, 2:47 PM with 19 comments

• by oconnore on 6/29/15, 8:20 PM

      import Control.Concurrent.STM
      import Control.Monad
      import Data.Vector
  
      type VecInts = Vector (TVar Int)
  
      newVecInts :: Int -> STM (VecInts)
      newVecInts s = generateM s (\_ -> newTVar 0)
  
      modifyVI :: VecInts -> Int -> Int -> STM ()
      modifyVI vi pos val = do
        writeTVar (vi ! pos) val
  
      waitUntilEqual :: VecInts -> Int -> Int -> STM ()
      waitUntilEqual vi p1 p2 = do
        a <- readTVar (vi ! p1)
        b <- readTVar (vi ! p2)
        when (a /= b) retry

• by sdab on 6/30/15, 5:47 PM

  My c++ solution: https://gist.github.com/anonymous/7e6cc8f58e9cde1b6fda

  I believe it follows all of the requirements. There is one glaring flaw which is that it uses a condition variable per 'wait_until_equal' call, though it uses only N mutexes. So this doesn't fall under the N^2 primitives (in the case of many waits), though its unclear to me if thats an official rule.

  Im happy to listen to feedback or if someone sees an error.

• by brlewis on 6/29/15, 9:35 PM

  This is an example of why I see node.js having more value in its single-threaded async architecture than in its capacity to use the same programming language on client and server.

  More generally, when you discover a tricky problem that would make a good interview question, oftentimes there's an architectural decision that would eliminate the problem.