Add functions to generate random numbers in a specified range.

This adds 3 new variants of the random() function:

    random(min integer, max integer) returns integer
    random(min bigint, max bigint) returns bigint
    random(min numeric, max numeric) returns numeric

Each returns a random number x in the range min <= x <= max.

For the numeric function, the number of digits after the decimal point
is equal to the number of digits that "min" or "max" has after the
decimal point, whichever has more.

The main entry points for these functions are in a new C source file.
The existing random(), random_normal(), and setseed() functions are
moved there too, so that they can all share the same PRNG state, which
is kept private to that file.

Dean Rasheed, reviewed by Jian He, David Zhang, Aleksander Alekseev,
and Tomas Vondra.

Discussion: https://postgr.es/m/CAEZATCV89Vxuq93xQdmc0t-0Y2zeeNQTdsjbmV7dyFBPykbV4Q@mail.gmail.com

1 files changed, 36 insertions, 0 deletions

diff --git a/src/common/pg_prng.c b/src/common/pg_prng.c
index c1714a02bd9..15b39411a9b 100644
--- a/src/common/pg_prng.c
+++ b/src/common/pg_prng.c
@@ -185,6 +185,42 @@ pg_prng_int64p(pg_prng_state *state)
 }
 
 /*
+ * Select a random int64 uniformly from the range [rmin, rmax].
+ * If the range is empty, rmin is always produced.
+ */
+int64
+pg_prng_int64_range(pg_prng_state *state, int64 rmin, int64 rmax)
+{
+	int64		val;
+
+	if (likely(rmax > rmin))
+	{
+		uint64		uval;
+
+		/*
+		 * Use pg_prng_uint64_range().  Can't simply pass it rmin and rmax,
+		 * since (uint64) rmin will be larger than (uint64) rmax if rmin < 0.
+		 */
+		uval = (uint64) rmin +
+			pg_prng_uint64_range(state, 0, (uint64) rmax - (uint64) rmin);
+
+		/*
+		 * Safely convert back to int64, avoiding implementation-defined
+		 * behavior for values larger than PG_INT64_MAX.  Modern compilers
+		 * will reduce this to a simple assignment.
+		 */
+		if (uval > PG_INT64_MAX)
+			val = (int64) (uval - PG_INT64_MIN) + PG_INT64_MIN;
+		else
+			val = (int64) uval;
+	}
+	else
+		val = rmin;
+
+	return val;
+}
+
+/*
  * Select a random uint32 uniformly from the range [0, PG_UINT32_MAX].
  */
 uint32