/*
 * © Copyright 2014 – Urheberrechtshinweis
 * Alle Rechte vorbehalten / All Rights Reserved
 *
 * Programmcode ist urheberrechtlich geschuetzt.
 * Das Urheberrecht liegt, soweit nicht ausdruecklich anders gekennzeichnet, bei Frank Ebner.
 * Keine Verwendung ohne explizite Genehmigung.
 * (vgl. § 106 ff UrhG / § 97 UrhG)
 */

#ifndef KAHANSUM_H
#define KAHANSUM_H

#include <vector>

template <typename T> class KahanSum {

private:

	const T zero;

	/** current sum */
	T sum;

	/** current compensation */
	T comp;


public:

	/** ctor with zero element */
	KahanSum(const T zero) : zero(zero), sum(zero), comp(zero) {

	}

	KahanSum& operator += (const T val) {
		add(val);
		return *this;
	}

	KahanSum& operator -= (const T val) {
		sub(val);
		return *this;
	}

	T get() const {
		return sum;
	}

private:

#ifdef _MSC_VER 
  #pragma optimize( "", off )
#else
  #pragma GCC push_options
  #pragma GCC optimize ("O0")
#endif

	/** adjust the sum */
	void add(const T val) {
		const T y = val - comp;
		const T t = sum + y;
		comp = (t-sum) - y;				// very important line
		sum = t;
	}

	/** adjust the sum */
	void sub(const T val) {
		const T y = val - comp;
		const T t = sum - y;
		comp = (t-sum) + y;				// very important line
		sum = t;
	}

#ifdef _MSC_VER
  #pragma optimize( "", on ) 
#else
  #pragma GCC pop_options
#endif

public:

	static T get(const std::vector<T>& values, const T zero = T()) {
		return get(values.data(), values.size(), zero);
	}

	static T get(const T* values, const size_t cnt, const T zero = T()) {

		T sum = zero;
		volatile T comp = zero;
		for (size_t i = 0; i < cnt; ++i) {
			const T inp = values[i];
			const volatile T y = inp - comp;
			const volatile T t = sum + y;
			comp = (t-sum) - y;
			sum = t;
		}

		return sum;

	}

};

#endif // KAHANSUM_H