/*
    arrays.c
    Tanya Amert, Fall 2023

    How do array accesses get represented in x86-64?

    Compile for debugging via gdb:

        gcc -g -Og -o arrays arrays.c

    Run with no command-line arguments:

        ./arrays
*/
#include <stdio.h>
#include <stdlib.h>

#define NUM_BYTES 40

typedef unsigned char * byte_ptr;
void show_bytes(byte_ptr start, size_t len)
{
    int i;
    for (i = 0; i < len; i++)
    {
        printf(" %.2x", start[i]);
    }

    printf("\n");
}

int main()
{
    // Allocate 40 bytes, which is enough for a char[] with
    // 40 elements, or a short[] with 20 elements, or an int[]
    // with 10 elements, or a long[] with 5 elements
    void *p = malloc(NUM_BYTES);

    // We can cast p to different types
    char *p_char = (char *)p;
    short *p_short = (short *)p;
    int *p_int = (int *)p;
    long *p_long = (long *)p;

    // Set the values such that byte at index b has value b
    for (int i = 0; i < NUM_BYTES; i++)
    {
        p_char[i] = i;
    }

    // Now let's access different elements
    char a30 = p_char[30];   // p + ??
    short s17 = p_short[17]; // p + ??
    int i7 = p_int[7];       // p + ??
    long l3 = p_long[3];     // p + ??

    // Print these values
    printf("\nValues in hex:\n-------------\n");
    printf(" char  at index 30: %x\n", a30 & 0xFF);
    printf("short  at index 17: %x\n", s17 & 0xFFFF);
    printf("   int at index  7: %x\n", i7);
    printf("  long at index  3: %lx\n", l3);

    // Now print the bytes they contain
    printf("\nValues in byte order:\n-------------\n");
    printf(" char  at index 30:");
    show_bytes(&a30, sizeof(char));

    printf("short  at index 17:");
    show_bytes((byte_ptr)&s17, sizeof(short));

    printf("   int at index  7:");
    show_bytes((byte_ptr)&i7, sizeof(int));

    printf("  long at index  3:");
    show_bytes((byte_ptr)&l3, sizeof(long));

    // Now iterate through the bytes as if different arrays
    char sum_char = 0;
    for (int i = 0; i < NUM_BYTES / sizeof(char); i++)
    {
        sum_char += p_char[i];
    }
    printf("\nSum of char values: %x\n", sum_char & 0xFF);

    short sum_short = 0;
    for (int i = 0; i < NUM_BYTES / sizeof(short); i++)
    {
        sum_short += p_short[i];
    }
    printf("\nSum of short values: %x\n", sum_short & 0xFFFF);

    int sum_int = 0;
    for (int i = 0; i < NUM_BYTES / sizeof(int); i++)
    {
        sum_int += p_int[i];
    }
    printf("\nSum of int values: %x\n", sum_int);

    long sum_long = 0;
    for (int i = 0; i < NUM_BYTES / sizeof(long); i++)
    {
        sum_long += p_long[i];
    }
    printf("\nSum of long values: %lx\n", sum_long);

    return 0;
}