1 files changed, 247 insertions, 0 deletions
diff --git a/src/common/pg_prng.c b/src/common/pg_prng.c
new file mode 100644
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+/*-------------------------------------------------------------------------
+ *
+ * Pseudo-Random Number Generator
+ *
+ * We use Blackman and Vigna's xoroshiro128** 1.0 algorithm
+ * to have a small, fast PRNG suitable for generating reasonably
+ * good-quality 64-bit data.  This should not be considered
+ * cryptographically strong, however.
+ *
+ * About these generators: https://prng.di.unimi.it/
+ * See also https://en.wikipedia.org/wiki/List_of_random_number_generators
+ *
+ * Copyright (c) 2021-2022, PostgreSQL Global Development Group
+ *
+ * src/common/pg_prng.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+#include <math.h>				/* for ldexp() */
+
+#include "common/pg_prng.h"
+#include "port/pg_bitutils.h"
+
+/* process-wide state vector */
+pg_prng_state pg_global_prng_state;
+
+
+/*
+ * 64-bit rotate left
+ */
+static inline uint64
+rotl(uint64 x, int bits)
+{
+	return (x << bits) | (x >> (64 - bits));
+}
+
+/*
+ * The basic xoroshiro128** algorithm.
+ * Generates and returns a 64-bit uniformly distributed number,
+ * updating the state vector for next time.
+ *
+ * Note: the state vector must not be all-zeroes, as that is a fixed point.
+ */
+static uint64
+xoroshiro128ss(pg_prng_state *state)
+{
+	uint64		s0 = state->s0,
+				sx = state->s1 ^ s0,
+				val = rotl(s0 * 5, 7) * 9;
+
+	/* update state */
+	state->s0 = rotl(s0, 24) ^ sx ^ (sx << 16);
+	state->s1 = rotl(sx, 37);
+
+	return val;
+}
+
+/*
+ * We use this generator just to fill the xoroshiro128** state vector
+ * from a 64-bit seed.
+ */
+static uint64
+splitmix64(uint64 *state)
+{
+	/* state update */
+	uint64		val = (*state += UINT64CONST(0x9E3779B97f4A7C15));
+
+	/* value extraction */
+	val = (val ^ (val >> 30)) * UINT64CONST(0xBF58476D1CE4E5B9);
+	val = (val ^ (val >> 27)) * UINT64CONST(0x94D049BB133111EB);
+
+	return val ^ (val >> 31);
+}
+
+/*
+ * Initialize the PRNG state from a 64-bit integer,
+ * taking care that we don't produce all-zeroes.
+ */
+void
+pg_prng_seed(pg_prng_state *state, uint64 seed)
+{
+	state->s0 = splitmix64(&seed);
+	state->s1 = splitmix64(&seed);
+	/* Let's just make sure we didn't get all-zeroes */
+	(void) pg_prng_seed_check(state);
+}
+
+/*
+ * Initialize the PRNG state from a double in the range [-1.0, 1.0],
+ * taking care that we don't produce all-zeroes.
+ */
+void
+pg_prng_fseed(pg_prng_state *state, double fseed)
+{
+	/* Assume there's about 52 mantissa bits; the sign contributes too. */
+	int64		seed = ((double) ((UINT64CONST(1) << 52) - 1)) * fseed;
+
+	pg_prng_seed(state, (uint64) seed);
+}
+
+/*
+ * Validate a PRNG seed value.
+ */
+bool
+pg_prng_seed_check(pg_prng_state *state)
+{
+	/*
+	 * If the seeding mechanism chanced to produce all-zeroes, insert
+	 * something nonzero.  Anything would do; use Knuth's LCG parameters.
+	 */
+	if (unlikely(state->s0 == 0 && state->s1 == 0))
+	{
+		state->s0 = UINT64CONST(0x5851F42D4C957F2D);
+		state->s1 = UINT64CONST(0x14057B7EF767814F);
+	}
+
+	/* As a convenience for the pg_prng_strong_seed macro, return true */
+	return true;
+}
+
+/*
+ * Select a random uint64 uniformly from the range [0, PG_UINT64_MAX].
+ */
+uint64
+pg_prng_uint64(pg_prng_state *state)
+{
+	return xoroshiro128ss(state);
+}
+
+/*
+ * Select a random uint64 uniformly from the range [rmin, rmax].
+ * If the range is empty, rmin is always produced.
+ */
+uint64
+pg_prng_uint64_range(pg_prng_state *state, uint64 rmin, uint64 rmax)
+{
+	uint64		val;
+
+	if (likely(rmax > rmin))
+	{
+		/*
+		 * Use bitmask rejection method to generate an offset in 0..range.
+		 * Each generated val is less than twice "range", so on average we
+		 * should not have to iterate more than twice.
+		 */
+		uint64		range = rmax - rmin;
+		uint32		rshift = 63 - pg_leftmost_one_pos64(range);
+
+		do
+		{
+			val = xoroshiro128ss(state) >> rshift;
+		} while (val > range);
+	}
+	else
+		val = 0;
+
+	return rmin + val;
+}
+
+/*
+ * Select a random int64 uniformly from the range [PG_INT64_MIN, PG_INT64_MAX].
+ */
+int64
+pg_prng_int64(pg_prng_state *state)
+{
+	return (int64) xoroshiro128ss(state);
+}
+
+/*
+ * Select a random int64 uniformly from the range [0, PG_INT64_MAX].
+ */
+int64
+pg_prng_int64p(pg_prng_state *state)
+{
+	return (int64) (xoroshiro128ss(state) & UINT64CONST(0x7FFFFFFFFFFFFFFF));
+}
+
+/*
+ * Select a random uint32 uniformly from the range [0, PG_UINT32_MAX].
+ */
+uint32
+pg_prng_uint32(pg_prng_state *state)
+{
+	/*
+	 * Although xoroshiro128** is not known to have any weaknesses in
+	 * randomness of low-order bits, we prefer to use the upper bits of its
+	 * result here and below.
+	 */
+	uint64		v = xoroshiro128ss(state);
+
+	return (uint32) (v >> 32);
+}
+
+/*
+ * Select a random int32 uniformly from the range [PG_INT32_MIN, PG_INT32_MAX].
+ */
+int32
+pg_prng_int32(pg_prng_state *state)
+{
+	uint64		v = xoroshiro128ss(state);
+
+	return (int32) (v >> 32);
+}
+
+/*
+ * Select a random int32 uniformly from the range [0, PG_INT32_MAX].
+ */
+int32
+pg_prng_int32p(pg_prng_state *state)
+{
+	uint64		v = xoroshiro128ss(state);
+
+	return (int32) (v >> 33);
+}
+
+/*
+ * Select a random double uniformly from the range [0.0, 1.0).
+ *
+ * Note: if you want a result in the range (0.0, 1.0], the standard way
+ * to get that is "1.0 - pg_prng_double(state)".
+ */
+double
+pg_prng_double(pg_prng_state *state)
+{
+	uint64		v = xoroshiro128ss(state);
+
+	/*
+	 * As above, assume there's 52 mantissa bits in a double.  This result
+	 * could round to 1.0 if double's precision is less than that; but we
+	 * assume IEEE float arithmetic elsewhere in Postgres, so this seems OK.
+	 */
+	return ldexp((double) (v >> (64 - 52)), -52);
+}
+
+/*
+ * Select a random boolean value.
+ */
+bool
+pg_prng_bool(pg_prng_state *state)
+{
+	uint64		v = xoroshiro128ss(state);
+
+	return (bool) (v >> 63);
+}