Add realtime safe random number generator

libs/pbd/pbd/pcg_rand.h

@@ -0,0 +1,96 @@
+/*
+ * Copyright (C) 2021 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef _pbd_pcg_rand_
+#define _pbd_pcg_rand_
+
+#include <assert.h>
+#include <stdint.h>
+#include <time.h>
+
+namespace PBD {
+
+/**
+ * *Really* minimal PCG32 code / (c) 2014 M.E. O'Neill / pcg-random.org
+ *
+ * To be used in cases where an efficient and realtime-safe random
+ * generator is needed.
+ */
+class PCGRand {
+public:
+  PCGRand ()
+  {
+    int foo = 0;
+    uint64_t initseq = (intptr_t)&foo;
+    _state = 0;
+    _inc = (initseq << 1) | 1;
+    rand_u32 ();
+    _state += time (NULL) ^ (intptr_t)this;
+    rand_u32 ();
+  }
+
+  /* unsigned float [0..1] */
+  float rand_uf ()
+  {
+    return rand_u32 () / 4294967295.f;
+  }
+
+  /* signed float [-1..+1] */
+  float rand_sf ()
+  {
+    return (rand_u32 () / 2147483647.5f) - 1.f;
+  }
+
+  /* uniform integer min <= rand <= max */
+	int rand (int min, int max)
+	{
+		int range;
+		if (min > max) {
+			range = 1 + min - max;
+			min = max;
+		} else {
+			range = 1 + max - min;
+		}
+
+		assert (range < 4294967296);
+
+		while (true) {
+			uint32_t value = rand_u32 ();
+			if (value < 4294967296 - 4294967296 % range) {
+				return min + value % range;
+			}
+		}
+	}
+
+  uint32_t rand_u32 ()
+  {
+    uint64_t oldstate = _state;
+    _state = oldstate * 6364136223846793005ULL + _inc;
+    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
+    uint32_t rot = oldstate >> 59u;
+    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
+  }
+
+private:
+  uint64_t _state;
+  uint64_t _inc;
+};
+
+} // namespace PBD
+
+#endif