--- MITgcm/eesupp/src/different_multiple.F	1998/05/21 18:30:08	1.1
+++ MITgcm/eesupp/src/different_multiple.F	2005/05/15 02:58:40	1.8
@@ -1,29 +1,68 @@
-C     $Header: /home/ubuntu/mnt/e9_copy/MITgcm/eesupp/src/different_multiple.F,v 1.1 1998/05/21 18:30:08 cnh Exp $
+C $Header: /home/ubuntu/mnt/e9_copy/MITgcm/eesupp/src/different_multiple.F,v 1.8 2005/05/15 02:58:40 jmc Exp $
+C $Name:  $
+
 #include "CPP_EEOPTIONS.h"
 
-CStartofinterface
-      LOGICAL FUNCTION DIFFERENT_MULTIPLE( freq, val1, val2 )
-C     /==========================================================\
-C     | LOGICAL FUNCTION DIFFERENT_MULTIPLE                      |
-C     | o Checks two numbers multiple of a third number.         |
-C     |==========================================================|
-C     | This routine is used for diagnostic and other periodic   |
-C     | operations. It is very sensitive to arithmetic precision.|
-C     | For IEEE conforming arithmetic is works well but for     |
-C     | cases where short cut arithmetic  is used it may not work|
-C     | as expected. To overcome this issue compile this routine |
-C     | separately with no optimisation.                         |
-C     \==========================================================/
+CBOP
+C     !ROUTINE: DIFFERENT_MULTIPLE
+
+C     !INTERFACE:
+      LOGICAL FUNCTION DIFFERENT_MULTIPLE( freq, val1, step )
       IMPLICIT NONE
-C     Returns TRUE if val1 and val2 are different multiples of freq.
-      REAL freq, val1, val2
-CEndofinterface
+
+C     !DESCRIPTION:
+C     *==========================================================*
+C     | LOGICAL FUNCTION DIFFERENT\_MULTIPLE                       
+C     | o Checks if a multiple of freq exist
+C     |   around val1 +/- step/2
+C     *==========================================================*
+C     | This routine is used for diagnostic and other periodic    
+C     | operations. It is very sensitive to arithmetic precision. 
+C     | For IEEE conforming arithmetic it works well but for      
+C     | cases where short cut arithmetic  is used it may not work 
+C     | as expected. To overcome this issue compile this routine  
+C     | separately with no optimisation.                          
+C     *==========================================================*
+
+C     !INPUT PARAMETERS:
+C     == Routine arguments ==
+C     freq       :: Frequency by which time is divided.
+C     val1       :: time that is checked 
+C     step       :: length of time interval (around val1) that is checked 
+      _RL  freq, val1, step
+
+C---+----1----+----2----+----3----+----4----+----5----+----6----+----7-|--+----|
+
+C     !LOCAL VARIABLES:
+C     == Local variables ==
+C     v1, v2, v3, v4 :: Temp. for holding time
+C     d1, d2, d3     :: Temp. for hold difference
+      _RL  v1, v2, v3, v4, d1, d2, d3
+CEOP
+
+C     o Do easy cases first.
       DIFFERENT_MULTIPLE = .FALSE.
-      IF ( freq .EQ. 0. ) THEN
-        DIFFERENT_MULTIPLE = .FALSE.
-      ELSE
-        DIFFERENT_MULTIPLE = INT(val1/freq) .NE. INT(val2/freq)
-      ENDIF
-      END
 
+      IF ( freq .NE. 0. ) THEN
+        IF ( ABS(step) .GT. freq ) THEN
+         DIFFERENT_MULTIPLE = .TRUE.
+        ELSE
+
+C         o This case is more complex because of round-off error
+          v1 = val1
+          v2 = val1 - step
+          v3 = val1 + step
+
+C         Test v1 to see if its a "closest multiple"
+          v4 = NINT(v1/freq)*freq
+          d1 = v1-v4
+          d2 = v2-v4
+          d3 = v3-v4
+          IF ( ABS(d1) .LE. ABS(d2) .AND. ABS(d1) .LE. ABS(d3) )
+     &        DIFFERENT_MULTIPLE = .TRUE.
 
+        ENDIF
+      ENDIF
+
+      RETURN
+      END