BUG: Fix the shifting of the emulated 128-bit increment.

Author: rkern

.gitignore

@@ -10,3 +10,5 @@
 *.a
 
 test-pcg64-*
+test-native.txt
+test-emulated.txt

Makefile

@@ -29,5 +29,10 @@ pcg64-emulated.o: pcg64.c pcg64.h
 test-pcg64-native: test-pcg64-native.o pcg64-native.o
 test-pcg64-emulated: test-pcg64-emulated.o pcg64-emulated.o
 
+test: test-pcg64-native test-pcg64-emulated
+	./test-pcg64-native > test-native.txt
+	./test-pcg64-emulated > test-emulated.txt
+	diff -u test-native.txt test-emulated.txt
+
 clean:
 	rm -f *.a *.o test-pcg64-native test-pcg64-emulated

pcg64.h

@@ -130,8 +130,11 @@ inline void pcg_setseq_128_srandom_r(pcg_state_setseq_128* rng,
                                      pcg128_t initstate, pcg128_t initseq)
 {
     rng->state = PCG_128BIT_CONSTANT(0ULL, 0ULL);
+    /* Ensure that the increment is odd. */
     rng->inc.high = initseq.high << 1u;
-    rng->inc.high |= initseq.low & 0x800000000000ULL;
+    /* Shift the highest bit from the low word up to the lowest bit of the high
+     * word. */
+    rng->inc.high |= initseq.low >> 63u;
     rng->inc.low = (initseq.low << 1u) | 1u;
     pcg_setseq_128_step_r(rng);
     rng->state = _pcg128_add(rng->state, initstate);

test-pcg64.c

@@ -35,7 +35,8 @@ void print_uint64_hex(uint64_t x)
 int main(int argc, char** argv)
 {
     pcg64_random_t rng;
-    pcg64_srandom_r(&rng, PCG_128BIT_CONSTANT(0ULL, 42ULL), PCG_128BIT_CONSTANT(0ULL, 54ULL));
+    pcg64_srandom_r(&rng, PCG_128BIT_CONSTANT(0ULL, 42ULL),
+	    PCG_128BIT_CONSTANT(0, 0x8000000000000054ULL));
     for (int i=0; i < 100; i++) {
         print_uint64_hex(pcg64_random_r(&rng));
     }