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slalib/random.F

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+#include <sla_config.h>
+      REAL FUNCTION sla_RANDOM (SEED)
+*+
+*     - - - - - - -
+*      R A N D O M
+*     - - - - - - -
+*
+*  Generate pseudo-random real number in the range 0 <= X < 1.
+*  (single precision)
+*
+*
+*  Given:
+*     SEED     real     an arbitrary real number
+*
+*  Notes:
+*
+*  1)  The result is a pseudo-random REAL number in the range
+*      0 <= sla_RANDOM < 1.
+*
+*  2)  SEED is used first time through only.
+*
+*  Called:  RAN or RAND (a REAL function returning a random variate --
+*           the precise function which is called depends on which functions
+*           are available when the library is built).  If neither of these
+*           is available, we use the local substitute RANDOM defined
+*           in rtl_random.c
+*
+*  P.T.Wallace   Starlink   14 October 1991
+*
+*  License:
+*    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 (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*-
+
+      IMPLICIT NONE
+
+      REAL SEED
+
+#if HAVE_RAND
+      REAL RAND
+#elif HAVE_RANDOM
+      REAL RANDOM
+#else
+ error "Can't find random-number function"
+#endif
+
+      REAL AS
+      INTEGER ISEED
+      LOGICAL FIRST
+      SAVE FIRST
+      DATA FIRST /.TRUE./
+
+
+
+*  If first time, turn SEED into a large, odd integer
+      IF (FIRST) THEN
+         AS=ABS(SEED)+1.0
+         ISEED=NINT(AS/10.0**(NINT(ALOG10(AS))-6))
+         IF (MOD(ISEED,2).EQ.0) ISEED=ISEED+1
+         FIRST=.FALSE.
+#if HAVE_RAND
+         AS = RAND(ISEED)
+#endif
+      ELSE
+         ISEED=0
+      END IF
+
+*  Next pseudo-random number
+#if HAVE_RAND
+      sla_RANDOM=RAND(0)
+#elif HAVE_RANDOM
+      sla_RANDOM=RANDOM(ISEED)
+#endif
+
+      END