// Write a C program to solve the readers and writers Problem.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>

#define NUM_READERS 3
#define NUM_WRITERS 2

// shared data structure - simple integer
int shared_data = 0;

// sync variables
pthread_mutex_t mutex;  // mutex for read_count protection
sem_t wrt;             // semaphore for writer access control
int read_count = 0;    // tracks active readers

void* reader(void* arg) {
    int id = *((int*)arg);
    while (1) {
        // read delay sim
        sleep(1);

        // critical section entry
        pthread_mutex_lock(&mutex);
        read_count++;
        if (read_count == 1) {
            // first reader blocks writers
            sem_wait(&wrt);
        }
        pthread_mutex_unlock(&mutex);

        // critical section
        printf("Reader %d is reading shared_data = %d\n", id, shared_data);
        sleep(2);  // read time sim

        // critical section exit
        pthread_mutex_lock(&mutex);
        read_count--;
        if (read_count == 0) {
            // last reader unblocks writers
            sem_post(&wrt);
        }
        pthread_mutex_unlock(&mutex);
    }
    pthread_exit(NULL);
}

void* writer(void* arg) {
    int id = *((int*)arg);
    while (1) {
        // write delay sim
        sleep(3);

        // critical section entry
        sem_wait(&wrt);  // exclusive access control

        // critical section
        shared_data += 1;
        printf("Writer %d is writing: shared_data becomes %d\n", id, shared_data);
        sleep(2);  // write time sim

        // critical section exit
        sem_post(&wrt);
    }
    pthread_exit(NULL);
}

int main(void) {
    pthread_t readers[NUM_READERS], writers[NUM_WRITERS];
    int reader_ids[NUM_READERS], writer_ids[NUM_WRITERS];
    int i;

    // sync init
    pthread_mutex_init(&mutex, NULL);
    sem_init(&wrt, 0, 1);  // binary semaphore init

    // reader thread creation
    for (i = 0; i < NUM_READERS; i++) {
        reader_ids[i] = i + 1;
        if (pthread_create(&readers[i], NULL, reader, &reader_ids[i]) != 0) {
            perror("Failed to create reader thread");
            exit(EXIT_FAILURE);
        }
    }

    // writer thread creation
    for (i = 0; i < NUM_WRITERS; i++) {
        writer_ids[i] = i + 1;
        if (pthread_create(&writers[i], NULL, writer, &writer_ids[i]) != 0) {
            perror("Failed to create writer thread");
            exit(EXIT_FAILURE);
        }
    }

    // thread joining
    for (i = 0; i < NUM_READERS; i++) {
        pthread_join(readers[i], NULL);
    }
    for (i = 0; i < NUM_WRITERS; i++) {
        pthread_join(writers[i], NULL);
    }

    // cleanup
    pthread_mutex_destroy(&mutex);
    sem_destroy(&wrt);

    return 0;
}