#include "nru.h"

// Attempt to choose a page to replace using an NRU algorithm.  This
// implementation searches all the real pages, and starts each search
// with the page after the last one found.
unsigned long NRUReplacementAlg::fault(const Ref &r) {
	// Stuff we found (by class).  The class 3 is filled in with the
	// starting page.  It may not be class 3, but we won't use the entry
	// unless it happens to be true (in fact, when all pages are class 3).
	bool found[4] = { false, false, false, true };
	unsigned long which[4] = { 0, 0, 0, m_scan_start };

	// Search for one of each class.  If we find a 0, end the search.
	unsigned long scan = m_scan_start;
	do {  // If it rolls around, will find a cleared entry.
		int pageclass = cl(get_PTE_by_frame(scan));
		if(pageclass < 3 && !found[pageclass]) {
			found[pageclass] = true;
			which[pageclass] = scan;
			if(pageclass == 0) break;
		}

		increal(scan);
	} while(scan != m_scan_start);

	// See what we found, and return it.
	for(int cl = 0; cl <= 3; ++cl) {
		// The last entry of found[] is true, so this if will always
		// be true in some iteration.
		if(found[cl]) {
			m_scan_start = which[cl];
			return increal(m_scan_start);
		}
	}
}

// Clear all the referenced bits.
void NRUReplacementAlg::maint()
{
	for(unsigned long frameno = 0; frameno < get_num_frames(); ++frameno) {
		PTE *pte = get_PTE_by_frame(frameno);
		if(pte) pte->clr_ref();
	}
}