Geografic-Information-System/lib/vector/diglib/port_init.c

/*
****************************************************************************
*
* MODULE:       Vector library 
*   	    	
* AUTHOR(S):    Original author CERL, probably Dave Gerdes.
*               Update to GRASS 5.7 Radim Blazek.
*
* PURPOSE:      Lower level functions for reading/writing/manipulating vectors.
*
* COPYRIGHT:    (C) 2001 by the GRASS Development Team
*
*               This program is free software under the GNU General Public
*   	    	License (>=v2). Read the file COPYING that comes with GRASS
*   	    	for details.
*
*****************************************************************************/
#include <stdio.h>
#include <grass/Vect.h>

/*
   **  Written by Dave Gerdes  9/1988
   **  US Army Construction Engineering Research Lab
 */


/* 
   ** 
   **  This code is a quick hack to allow the writing of portable
   **  binary data files.
   **  The approach is to take known values and compare them against
   **  the current machine's internal representation.   A cross reference
   **  table is then built, and then all file reads and writes must go through
   **  through these routines to correct the numbers if need be.
   **
   **  As long as the byte switching is symetrical, the conversion routines
   **  will work both directions.

   **  The integer test patterns are quite simple, and their choice was
   **  arbitrary, but the float and double valued were more critical.

   **  I did not have a specification for IEEE to go by, so it is possible
   **  that I have missed something.  My criteria were:
   **
   **  First, true IEEE numbers had to be chosen to avoid getting an FPE.
   **  Second, every byte in the test pattern had to be unique.   And
   **  finally, the number had to not be sensitive to rounding by the 
   **  specific hardware implementation.
   **
   **  By experimentation it was found that the number  1.3333  met
   **  all these criteria for both floats and doubles

   **  See the discourse at the end of this file for more information
   **  
   **
 */

#define TEST_PATTERN 1.3333
#define LONG_TEST 0x01020304
#define INT_TEST 0x01020304
#define SHORT_TEST 0x0102

static double u_d = TEST_PATTERN;
static float  u_f = TEST_PATTERN;
static long   u_l = LONG_TEST;
static int    u_i = INT_TEST;
static short  u_s = SHORT_TEST;

/* dbl_cmpr holds the bytes of an IEEE representation of  TEST_PATTERN */
static const unsigned char dbl_cmpr [] = {0x3f, 0xf5, 0x55, 0x32, 0x61, 0x7c, 0x1b, 0xda};
/* flt_cmpr holds the bytes of an IEEE representation of  TEST_PATTERN */
static const unsigned char flt_cmpr [] = {0x3f, 0xaa, 0xa9, 0x93};
static const unsigned char lng_cmpr [] = {0x01, 0x02, 0x03, 0x04};
static const unsigned char int_cmpr [] = {0x01, 0x02, 0x03, 0x04};
static const unsigned char shrt_cmpr[] = {0x01, 0x02};

/* Find native sizes */
int nat_dbl  = sizeof(double);
int nat_flt  = sizeof(float);
int nat_lng  = sizeof(long);
int nat_int  = sizeof(int);
int nat_shrt = sizeof(short);

int dbl_order;
int flt_order;
int lng_order;
int int_order;
int shrt_order;

unsigned char dbl_cnvrt [sizeof(double)];
unsigned char flt_cnvrt [sizeof(float )];
unsigned char lng_cnvrt [sizeof(long  )];
unsigned char int_cnvrt [sizeof(int   )];
unsigned char shrt_cnvrt[sizeof(short )];

/*
 * match search_value against each char in basis. 
 * return offset or -1 if not found
 */

static int find_offset(const unsigned char *basis, unsigned char search_value, int size)
{
    int i;

    for (i = 0; i < size; i++)
	if (basis[i] == search_value)
	    return (i);

    return (-1);
}

static int find_offsets(
    const void *pattern, unsigned char *cnvrt, const unsigned char *cmpr,
    int port_size, int nat_size, const char *typename)
{
    int big, ltl;
    int i;

    for (i = 0; i < port_size; i++)
    {
	int off = find_offset(pattern, cmpr[i], nat_size);

	if (off < 0)
	    G_fatal_error("could not find '%x' in %s", cmpr[i], typename);

	cnvrt[i] = off;
    }

    big = ltl = 1;

    for (i = 0; i < port_size; i++)
    {
	if (cnvrt[i] != (nat_size - port_size + i))
	    big = 0;		/* isn't big endian */
	if (cnvrt[i] != (port_size - 1 - i))
	    ltl = 0;		/* isn't little endian */  
    }
 
    if (big)
	return ENDIAN_BIG;

    if (ltl)
	return ENDIAN_LITTLE;

    return ENDIAN_OTHER;
}

void port_init(void)
{
    static int done;

    if (done)
	return;

    done = 1;

    /* Following code checks only if all assumptions are fullfilled */
    /* Check sizes */	      
    if (nat_dbl != PORT_DOUBLE)
	G_fatal_error("sizeof(double) != %d", PORT_DOUBLE);
    if (nat_flt != PORT_FLOAT)
	G_fatal_error("sizeof(float) != %d", PORT_DOUBLE);
    if (nat_lng < PORT_LONG)
	G_fatal_error("sizeof(long) < %d", PORT_LONG);
    if (nat_int < PORT_INT)
	G_fatal_error("sizeof(int) < %d", PORT_INT);
    if (nat_shrt < PORT_SHORT)
	G_fatal_error("sizeof(short) < %d", PORT_SHORT);
      
    /* Find for each byte in big endian test pattern (*_cmpr) 
     * offset of corresponding byte in machine native order.
     * Look if native byte order is little or big or some other (pdp)
     * endian.
     */

    dbl_order  = find_offsets(&u_d, dbl_cnvrt,  dbl_cmpr,  PORT_DOUBLE, nat_dbl,  "double");
    flt_order  = find_offsets(&u_f, flt_cnvrt,  flt_cmpr,  PORT_FLOAT,  nat_flt,  "float" );
    lng_order  = find_offsets(&u_l, lng_cnvrt,  lng_cmpr,  PORT_LONG,   nat_lng,  "long"  );
    int_order  = find_offsets(&u_i, int_cnvrt,  int_cmpr,  PORT_INT,    nat_int,  "int"   );
    shrt_order = find_offsets(&u_s, shrt_cnvrt, shrt_cmpr, PORT_SHORT,  nat_shrt, "short" );
}