APM1.hpp

#ifndef PAQ8PX_APM1_HPP
#define PAQ8PX_APM1_HPP

#include <cstdint>
#include <cassert>
#include "Shared.hpp"
#include "UpdateBroadcaster.hpp"
#include "Squash.hpp"
#include "Stretch.hpp"

/**
 * APM1 maps a probability and a context into a new probability that bit y will next be 1.
 * After each guess it updates its state to improve future guesses.
 * Probabilities are scaled 12 bits (0-4095).
 */
class APM1 : IPredictor {
private:
    const Shared * const shared;
    int index; /**< last p, context */
    const int n; /**< number of contexts */
    Array<uint16_t> t; /**< [n][33]:  p, context -> p */
    const int rate;

public:
    /**
     * Creates an instance with @ref n contexts and learning rate @ref r.
     * @ref r determines the learning rate. Smaller = faster, must be in the range (0, 32).
     * Uses 66*n bytes memory.
     * @param n the number of contexts
     * @param r the learning rate
     */
    APM1(const Shared* const sh, int n, int r);
    /**
     * Returns adjusted probability in context @ref cx (0 to n-1).
     * @param pr initial (pre-adjusted) probability
     * @param cxt the context
     * @return adjusted probability
     */
    int p(int pr, int cxt);
    void update() override;
};

#endif //PAQ8PX_APM1_HPP