Algorithms

Received = Tally of all monthly payments on the policy.
Expected = Policy age in months x monthly premium.
If Received < Expected, policy is in arrears.
If Received > Expected, policy is paid in advance.
If Received == Expected, policy is current.

volatile int WorkingBuff = 0;
volatile int NextBuff = 1;
volatile int RenderBuff = 1;

//Thread A
void stuff()
{
  NextBuff = WorkingBuff;
  int usedId = 0;
  do
  {
    usedId = RenderBuff;
    for(int i=0; i<3; i++)
    {
      if(i != NextBuff && i != usedId)
        WorkingBuff = i;
    }
  } while(RenderBuff != usedId);
}

//Thread B
void render()
{
  RenderBuff = NextBuff;
  //do stuff with RenderBuff
}