libvtrutil/src/vtr_random.cpp

#include <cstddef>

#include "vtr_random.h"
#include "vtr_util.h"
#include "vtr_error.h"

#define CHECK_RAND

namespace vtr {
/* Portable random number generator defined below.  Taken from ANSI C by  *
 * K & R.  Not a great generator, but fast, and good enough for my needs. */

constexpr size_t IA = 1103515245u;
constexpr size_t IC = 12345u;
constexpr size_t IM = 2147483648u;

static unsigned int current_random = 0;

void srandom(int seed) {
	current_random = (unsigned int) seed;
}

/* returns the current_random value */
unsigned int get_current_random(){
	unsigned int result = current_random;
	return result;
}

int irand(int imax) {

	/* Creates a random integer between 0 and imax, inclusive.  i.e. [0..imax] */

	int ival;

	/* current_random = (current_random * IA + IC) % IM; */
	current_random = current_random * IA + IC; /* Use overflow to wrap */
	ival = current_random & (IM - 1); /* Modulus */
	ival = (int) ((float) ival * (float) (imax + 0.999) / (float) IM);

#ifdef CHECK_RAND
	if ((ival < 0) || (ival > imax)) {
		if (ival == imax + 1) {
			/* Due to random floating point rounding, sometimes above calculation gives number greater than ival by 1 */
			ival = imax;
		} else {
            throw VtrError(string_fmt("Bad value in my_irand, imax = %d  ival = %d", imax, ival), __FILE__, __LINE__); 
		}
	}
#endif

	return (ival);
}

float frand(void) {

	/* Creates a random float between 0 and 1.  i.e. [0..1).        */

	float fval;
	int ival;

	current_random = current_random * IA + IC; /* Use overflow to wrap */
	ival = current_random & (IM - 1); /* Modulus */
	fval = (float) ival / (float) IM;

#ifdef CHECK_RAND
	if ((fval < 0) || (fval > 1.)) {
        throw VtrError(string_fmt("Bad value in my_frand, fval = %g", fval), __FILE__, __LINE__);
	}
#endif

	return (fval);
}

} //namespace