APM.hpp

#pragma once

#include "AdaptiveMap.hpp"

/**
 * An APM maps a probability and a context to a new probability.
 */
class APM : public AdaptiveMap, IPredictor {
private:
  const int N; /**< Number of contexts */
  const int steps;
  int cxt; /**< context index of last prediction */

public:
  int limit; //1..1023
  /**
    * Creates with @ref n contexts using 4*s*n bytes memory.
    * @param n the number of contexts
    * @param s the number of steps
    */
  APM(const Shared* const sh, int n, int s, int limit);
  ~APM() override = default;

  /**
    * a.update() updates probability map. y=(0..1) is the last bit
    */
  void update() override;

  /**
    * Returns a new probability (0..4095) like with @ref StateMap.
    * cx=(0..n-1) is the context.
    * pr=(0..4095) is considered part of the context.
    * The output is computed by interpolating @ref pr into @ref steps ranges nonlinearly
    * with smaller ranges near the ends. The initial output is pr.
    * lim=(0..1023): set a lower limit (like 255) for faster adaptation.
    * @param pr the initial probability. Considered part of the context.
    * @param cx the context
    * @param lim limit; lower limit means faster adaptation
    * @return the adjusted probability
    */
  int p(int pr, int cx);
};