ignuin.f

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      INTEGER FUNCTION ignuin(low,high)
C**********************************************************************
C
C     INTEGER FUNCTION IGNUIN( LOW, HIGH )
C
C               GeNerate Uniform INteger
C
C
C                              Function
C
C
C     Generates an integer uniformly distributed between LOW and HIGH.
C
C
C                              Arguments
C
C
C     LOW --> Low bound (inclusive) on integer value to be generated
C                         INTEGER LOW
C
C     HIGH --> High bound (inclusive) on integer value to be generated
C                         INTEGER HIGH
C
C
C                              Note
C
C
C     If (HIGH-LOW) > 2,147,483,561 prints error message on * unit and
C     stops the program.
C
C**********************************************************************

C     IGNLGI generates integers between 1 and 2147483562
C     MAXNUM is 1 less than maximum generable value
C     .. Parameters ..
      INTEGER maxnum
      parameter(maxnum=2147483561)
      CHARACTER*(*) err1,err2
      parameter(err1='LOW > HIGH in IGNUIN',
     +          err2=' ( HIGH - LOW ) > 2,147,483,561 in IGNUIN')
C     ..
C     .. Scalar Arguments ..
      INTEGER high,low
C     ..
C     .. Local Scalars ..
      INTEGER err,ign,maxnow,range,ranp1
C     ..
C     .. External Functions ..
      INTEGER ignlgi
      EXTERNAL ignlgi
C     ..
C     .. Intrinsic Functions ..
      INTRINSIC mod
C     ..
C     .. Executable Statements ..
      IF (.NOT. (low.GT.high)) go to 10
      err = 1
C      ABORT-PROGRAM
      go to 80

   10 range = high - low
      IF (.NOT. (range.GT.maxnum)) go to 20
      err = 2
C      ABORT-PROGRAM
      go to 80

   20 IF (.NOT. (low.EQ.high)) go to 30
      ignuin = low
      RETURN

C     Number to be generated should be in range 0..RANGE
C     Set MAXNOW so that the number of integers in 0..MAXNOW is an
C     integral multiple of the number in 0..RANGE

   30 ranp1 = range + 1
      maxnow = (maxnum/ranp1)*ranp1
   40 ign = ignlgi() - 1
      IF (.NOT. (ign.LE.maxnow)) go to 40
      ignuin = low + mod(ign,ranp1)
      RETURN

   80 IF (.NOT. (err.EQ.1)) go to 90
      WRITE (*,*) err1
      go to 100

C     TO ABORT-PROGRAM
   90 WRITE (*,*) err2
  100 WRITE (*,*) ' LOW: ',low,' HIGH: ',high
      WRITE (*,*) ' Abort on Fatal ERROR'
      IF (.NOT. (err.EQ.1)) go to 110
      CALL xstopx('LOW > HIGH in IGNUIN')

  110 CALL xstopx(' ( HIGH - LOW ) > 2,147,483,561 in IGNUIN')

  120 END