x014.h

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#pragma once

#include "xmath.h"
#include "xtools.h"
#include <chrono>
#include <cmath>
#include <functional>
#include <iostream>
#include <map>

namespace nmx::apps::x014 {

template<class X, class Y, bool DEBUG = false>
class Cache
{
private:
    //Speicher für (x,f(x))
    std::map<X, Y> _table;
    //Zeiger auf Funktionsobjekt f(x)
    std::function<Y(X)> _fn;

public:
    Cache(Y (*fn)(X))
        : _fn{ fn } {}

    Y get(X x) {
        const auto fitr = _table.find(x);
        if (fitr != _table.end()) {
            // Wert wurde im Speicher gefunden
            if constexpr (DEBUG) {
                std::cout << "from cache:";
            }
            return fitr->second;
        }
        Y val = _fn(x); //Wert wird berechnet
        _table[x] = val; //...und gespeichert
        return val;
    }

    auto size() const { return _table.size(); }
}; //Cache

double fac(size_t n) {
    if (n < 1000) {
        return n == 0 ? 1 : n * fac(n - 1);
    }
    //Stirling-Formel
    return std::sqrt(2 * Math::PI * n) * std::pow(n / Math::E, n);
}

inline void ex1() {
    Cache<size_t, double> c1{ fac }; //ohne Meldung
    for (auto n : { 2, 9, 50, 100, 2, 9 }) {
        std::cout << n << "! =" << c1.get(static_cast<size_t>(n)) << std::endl;
    }
    std::cout << "---------------------------" << std::endl;
    Cache<size_t, double, true> c2{ fac }; //mit Meldung
    for (auto n : { 2, 9, 50, 100, 2, 9 }) {
        std::cout << n << "! =" << c2.get(static_cast<size_t>(n)) << std::endl;
    }
}

inline void ex2() {
    //vermeide lange Bezeichner
    using namespace std::chrono;

    Cache<size_t, double> c1{ fac };
    //Anzahl der Summenterme
    const size_t N = 1e5;
    //Berechnung der Summe mit Zeitmessung
    double sum1 = 0;
    auto start1 = steady_clock::now();
    for (size_t n = 0; n < N; n++) {
        sum1 += 1 / c1.get(n);
    }
    auto end1 = steady_clock::now();
    auto dt1 = duration<double, std::milli>(end1 - start1).count();

    //Wiederholung der Berechnung
    double sum2 = 0;
    auto start2 = steady_clock::now();
    for (size_t n = 0; n < N; n++) {
        sum2 += 1 / c1.get(n);
    }
    auto end2 = steady_clock::now();
    auto dt2 = duration<double, std::milli>(end2 - start2).count();

    //Ausgabe der Ergebnisse auf dem Bildschirm
    std::cout << "dt1=" << dt1 << " ms" << std::endl;
    std::cout << "value=" << sum1 << std::endl;
    std::cout << "---------------------------" << std::endl;
    std::cout << "dt2=" << dt2 << " ms" << std::endl;
    std::cout << "value=" << sum2 << std::endl;
    std::cout << "---------------------------" << std::endl;
    std::cout << "dt2/dt1=" << dt2 / dt1 << std::endl;
    std::cout << "cache size=" << c1.size() << std::endl;
}

} // namespace nmx::apps::x014