#include <stdio.h>
#include <stdlib.h>
#include <limits.h> // For INT_MAX

// Structure to represent a frame and its last used time
typedef struct {
    int page_number;
    int last_used_time;
} Frame;

// Function to find if a page exists in frames and return its index
// Also updates the last_used_time if found
int find_page(Frame frames[], int n_frames, int page, int current_time) {
    for (int i = 0; i < n_frames; i++) {
        if (frames[i].page_number == page) {
            frames[i].last_used_time = current_time; // Update time on hit
            return i; // Page found (Hit)
        }
    }
    return -1; // Page not found (Fault)
}

// Function to find the index of the Least Recently Used (LRU) page
int find_lru_index(Frame frames[], int n_frames) {
    int min_time = INT_MAX;
    int lru_index = 0;
    for (int i = 0; i < n_frames; i++) {
        // Frame must contain a valid page (not -1)
        if (frames[i].page_number != -1 && frames[i].last_used_time < min_time) {
            min_time = frames[i].last_used_time;
            lru_index = i;
        }
    }
    return lru_index;
}

// Function to print the current state of frames
void print_frames(Frame frames[], int n_frames) {
    printf("[");
    for (int i = 0; i < n_frames; i++) {
        if (frames[i].page_number == -1) {
            printf(" - ");
        } else {
            printf(" %d ", frames[i].page_number);
        }
    }
    printf("]\n");
}

int main() {
    int n_frames, n_pages;
    int *pages = NULL; // Reference string
    Frame *frames = NULL; // Frames in memory

    int page_faults = 0;
    int page_hits = 0;
    int current_time = 0; // Counter to track usage time
    int frame_idx = 0; // Index for filling empty frames initially

    // 1. Get Inputs
    printf("Enter the number of frames: ");
    if (scanf("%d", &n_frames) != 1 || n_frames <= 0) {
        fprintf(stderr, "Error: Invalid number of frames.\n");
        return 1;
    }

    printf("Enter the number of pages in the reference string: ");
    if (scanf("%d", &n_pages) != 1 || n_pages <= 0) {
        fprintf(stderr, "Error: Invalid number of pages.\n");
        return 1;
    }

    // 2. Allocate Memory
    pages = (int *)malloc(n_pages * sizeof(int));
    if (pages == NULL) {
        perror("Failed to allocate memory for pages");
        return 1;
    }

    frames = (Frame *)malloc(n_frames * sizeof(Frame));
    if (frames == NULL) {
        perror("Failed to allocate memory for frames");
        free(pages); // Clean up already allocated memory
        return 1;
    }

    printf("Enter the page reference string (space-separated %d integers):\n", n_pages);
    for (int i = 0; i < n_pages; i++) {
        if (scanf("%d", &pages[i]) != 1) {
             fprintf(stderr, "Error reading page reference string.\n");
             free(pages);
             free(frames);
             return 1;
        }
    }

    // 3. Initialize Frames
    for (int i = 0; i < n_frames; i++) {
        frames[i].page_number = -1; // -1 indicates empty frame
        frames[i].last_used_time = -1; // Initialize time
    }

    printf("\n--- LRU Simulation Start ---\n");
    printf("Frames: %d | Reference String Length: %d\n\n", n_frames, n_pages);

    // 4. Process Page References
    for (int i = 0; i < n_pages; i++) {
        current_time++; // Increment time step for each reference
        int current_page = pages[i];
        printf("Ref: %d -> ", current_page);

        int found_index = find_page(frames, n_frames, current_page, current_time);

        if (found_index != -1) {
            // Page Hit
            page_hits++;
            printf("Hit   ");
        } else {
            // Page Fault
            page_faults++;
            printf("Fault ");

            // Find a place for the new page
            int replace_index = -1;

            // Check for an empty frame first
            for(int k=0; k < n_frames; k++){
                if(frames[k].page_number == -1){
                    replace_index = k;
                    break;
                }
            }

            if (replace_index != -1) {
                // Use the empty frame
                frames[replace_index].page_number = current_page;
                frames[replace_index].last_used_time = current_time;
                 printf("(loaded into empty frame %d) ", replace_index);
            } else {
                // No empty frames, find LRU page to replace
                replace_index = find_lru_index(frames, n_frames);
                printf("(replaced P%d in frame %d) ", frames[replace_index].page_number, replace_index);
                frames[replace_index].page_number = current_page;
                frames[replace_index].last_used_time = current_time;
            }
        }
        print_frames(frames, n_frames); // Show frame status after each step
    }

    // 5. Output Results
    printf("\n--- LRU Simulation End ---\n");
    printf("Total Page References: %d\n", n_pages);
    printf("Total Page Hits:    %d\n", page_hits);
    printf("Total Page Faults:  %d\n", page_faults);
    if (n_pages > 0) {
        printf("Hit Rate:           %.2f%%\n", (double)page_hits / n_pages * 100.0);
        printf("Fault Rate:         %.2f%%\n", (double)page_faults / n_pages * 100.0);
    } else {
        printf("Hit Rate:           N/A\n");
        printf("Fault Rate:         N/A\n");
    }


    // 6. Free Memory
    free(pages);
    free(frames);

    return 0;
}