/*
   * lib/average.c
   *
   * This source code is licensed under the GNU General Public License,
   * Version 2.  See the file COPYING for more details.
   */
  
  #include <linux/export.h>
  #include <linux/average.h>
  #include <linux/kernel.h>
  #include <linux/bug.h>
  #include <linux/log2.h>
  
  /**
   * DOC: Exponentially Weighted Moving Average (EWMA)
   *
   * These are generic functions for calculating Exponentially Weighted Moving
   * Averages (EWMA). We keep a structure with the EWMA parameters and a scaled
   * up internal representation of the average value to prevent rounding errors.
   * The factor for scaling up and the exponential weight (or decay rate) have to
   * be specified thru the init fuction. The structure should not be accessed
   * directly but only thru the helper functions.
   */
  
  /**
   * ewma_init() - Initialize EWMA parameters
   * @avg: Average structure
   * @factor: Factor to use for the scaled up internal value. The maximum value
   *	of averages can be ULONG_MAX/(factor*weight). For performance reasons
   *	factor has to be a power of 2.
   * @weight: Exponential weight, or decay rate. This defines how fast the
   *	influence of older values decreases. For performance reasons weight has
   *	to be a power of 2.
   *
   * Initialize the EWMA parameters for a given struct ewma @avg.
   */
  void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)
  {
  	WARN_ON(!is_power_of_2(weight) || !is_power_of_2(factor));
  
  	avg->weight = ilog2(weight);
  	avg->factor = ilog2(factor);
  	avg->internal = 0;
  }
  EXPORT_SYMBOL(ewma_init);
  
  /**
   * ewma_add() - Exponentially weighted moving average (EWMA)
   * @avg: Average structure
   * @val: Current value
   *
   * Add a sample to the average.
   */
  struct ewma *ewma_add(struct ewma *avg, unsigned long val)
  {
  	unsigned long internal = ACCESS_ONCE(avg->internal);
  
  	ACCESS_ONCE(avg->internal) = internal ?
  		(((internal << avg->weight) - internal) +
  			(val << avg->factor)) >> avg->weight :
  		(val << avg->factor);
  	return avg;
  }
  EXPORT_SYMBOL(ewma_add);