ex1a2.c

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#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <limits.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <assert.h>

#define MAX_NUMS  ((INT32_MAX >> 5) + 1) 
#define BUF_NUMS  4096                   


static inline bool is_little_endian(void)
{
   union { uint32_t u; char c[sizeof(uint32_t)]; } bint = { 0x01020304 };

   return bint.c[0] == 4; 
}

static inline int32_t reverse_int32(int32_t val)
{
    val = ((val << 8) & 0xFF00FF00) | ((val >> 8) & 0xFF00FF);
    
    return (val << 16) | ((val >> 16) & 0xFFFF);
}

int main(int argc, char** argv)
{
   const char* usage = "usage: %s filename\n";

   /* made static, because otherwise, the stack size might be too small.
    */
   static uint32_t have_num[MAX_NUMS];
   
   int32_t buf[BUF_NUMS];
   int     fd;
   int     n;
   int     total_nums = 0;

   /* we assume 32 bit integers, otherwise it will not work
    */
   assert(sizeof(int) >= 4);
   
   /* Check arguments, we need a filename.
    */
   if (argc < 2)
   {
      fprintf(stderr, usage, argv[0]);
      exit(EXIT_FAILURE);
   }
   memset(have_num, 0, sizeof(have_num)); // Proably uneccessary

   /* Open file
    */
   if (0 > (fd = open(argv[1], O_RDONLY)))
   {
      perror(argv[1]);
      exit(EXIT_FAILURE);
   }

   /* Read from file until data exhausted
    */
   while(0 < (n = read(fd, buf, sizeof(buf))))
   {
      int nums_read = n / sizeof(buf[0]); 

      assert(nums_read <= BUF_NUMS);
      
      total_nums += nums_read;

      /* Run through buffer of read numbers and set mark bits
       */
      for(int i = 0; i < nums_read; i++)
      {
         int32_t num = buf[i];

         if (!is_little_endian())
            num = reverse_int32(num);
         
         // fprintf(stderr, "%d\n", num);

         /* Check input: really >= 0 ? Otherwise ignore
          */
         if (num >= 0)
         {
            int      idx = num >> 5;         // n / 32
            uint32_t msk = 1 << (num & 31);  // bit number n mod 32 

            assert(idx >= 0 && idx < MAX_NUMS);
            
            have_num[idx] |= msk;
         }
      }
   }
   if (close(fd))
   {
      perror("close: ");
      exit(EXIT_FAILURE);
   }
   fprintf(stderr, "Total numbers read = %d\n", total_nums);
   
   for(int i = 0; i < MAX_NUMS; i++)
      if (have_num[i])  // just for speed up
         for(int k = 0; k < 31; k++)
            if (have_num[i] & (1 << k))
               printf("%d\n", (i << 5) + k);

   return EXIT_SUCCESS;
}