[fix] use correct floating point convertion

reference.c

@@ -1,18 +1,26 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdbool.h>
 
 /* The state must be seeded so that it is not everywhere zero. */
 uint64_t s[ 2 ];
 
-uint64_t xorshift128plus(void) {
+uint64_t xorshift128plus_int(void) {
 	uint64_t s1 = s[ 0 ];
 	const uint64_t s0 = s[ 1 ];
 	s[ 0 ] = s0;
 	s1 ^= s1 << 23;
 	return ( s[ 1 ] = ( s1 ^ s0 ^ ( s1 >> 17 ) ^ ( s0 >> 26 ) ) ) + s0;
 }
 
+double xorshift128plus_double(void) {
+	const uint64_t x = xorshift128plus_int();
+	const uint64_t x_doublefied = UINT64_C(0x3FF) << 52 | x >> 12;
+
+	return *((double *) &x_doublefied) - 1.0;
+}
+
 int main(int argc, char *argv[]) {
   s[0] = 1L;
   s[1] = 2L;
@@ -27,9 +35,17 @@ int main(int argc, char *argv[]) {
     s[1] = atol(argv[3]);
   }
 
+	const bool useDouble = (argc > 4);
+
   printf("[\n");
   for (int i = 0; i < length; i++) {
-    printf("  \"%016llX\"", xorshift128plus());
+
+		if (useDouble) {
+			printf("  \"%.20e\"", xorshift128plus_double());
+		} else {
+			printf("  \"%016llX\"", xorshift128plus_int());
+		}
+
 		if (i < length - 1) {
 			printf(",\n");
 		}