code: func (c *rankCache) Add(id uint64, n uint64) {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	// Ignore if the column count is below the threshold,
  	// unless the count is 0, which is effectively used
  	// to clear the cache value.
  	if n < c.thresholdValue && n > 0 {
  		return
  	}
  
  	c.entries[id] = n
  
  	c.invalidate()
  }
docs: // Add adds a count to the cache.