/*
 * GOOD: Contiguous Array Traversal
 * =================================
 * This program uses a contiguous array for excellent cache locality.
 * The CPU prefetcher can predict sequential access patterns.
 *
 * Compile: make array_sum
 * Run:     ./array_sum
 * Profile: perf stat -e cache-misses,cache-references ./array_sum
 */

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define N 10000000  /* 10 million elements */

double get_time(void) {
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return ts.tv_sec + ts.tv_nsec / 1e9;
}

long sum_array(int *arr, int n) {
    long sum = 0;
    for (int i = 0; i < n; i++) {
        sum += arr[i];
    }
    return sum;
}

int *create_array(int n) {
    int *arr = malloc(n * sizeof(int));
    if (!arr) {
        perror("malloc array");
        exit(1);
    }
    for (int i = 0; i < n; i++) {
        arr[i] = i % 100;
    }
    return arr;
}

int main(void) {
    printf("Contiguous Array Traversal (%d elements)\n", N);
    printf("Sequential memory access - CPU prefetcher works perfectly.\n\n");

    printf("Creating array...\n");
    int *arr = create_array(N);

    /* Warm up */
    sum_array(arr, N);

    double start = get_time();
    long result = sum_array(arr, N);
    double elapsed = get_time() - start;

    printf("Array sum: %ld in %.4f seconds\n\n", result, elapsed);

    printf("To see cache behavior, run:\n");
    printf("  perf stat -e cache-misses,cache-references ./array_sum\n");

    free(arr);
    return 0;
}