DOUBLE PRECISION FUNCTION ACORN()
C
C     FORTRAN IMPLEMENTATION OF ACORN RANDOM NUMBER GENERATOR OF ORDER
C     LESS THAN OR EQUAL TO 120 (HIGHER ORDERS CAN BE OBTAINED BY
C     INCREASING THE PARAMETER VALUE MAXORD) AND MODULUS LESS THAN OR
C     EQUAL TO 2^60.
C
C     AFTER APPROPRIATE INITIALIZATION OF THE COMMON BLOCK /IACO2/ EACH
C     CALL TO ACORN GENERATES A SINGLE VARIATE DRAWN FROM A UNIFORM
C     DISTRIBUTION OVER THE UNIT INTERVAL.
C
      PARAMETER (MAXORD=120, MAXOP1=MAXORD+1)
      COMMON /IACO2/ KORDEJ, MAXJNT, IXV1(MAXOP1), IXV2(MAXOP1)

      DO 10 I = 1, KORDEJ
      IXV1(I + 1) = (IXV1(I + 1) + IXV1(I))
      IXV2(I + 1) = (IXV2(I + 1) + IXV2(I))

      IF (IXV2(I + 1) .GE. MAXJNT) THEN
        IXV2(I + 1) = IXV2(I + 1) - MAXJNT
        IXV1(I + 1) = IXV1(I + 1) + 1
      END IF
      IF (IXV1(I + 1) .GE. MAXJNT) IXV1(I + 1) = IXV1(I + 1) - MAXJNT
   10 CONTINUE

      ACORN = (DBLE(IXV1(KORDEJ + 1)) +
     &         DBLE(IXV2(KORDEJ + 1)) / MAXJNT) / MAXJNT
      END
C     ******************************************************************
      SUBROUTINE SACORN(ISEED)
C
C     INITIALISES THE ACORN GENERATOR:
C
C     KORDEJ - LESS OR EQUAL MAXORD.
C     MAXJNT - SHOULD BE 2**30.
C     IXV1   - ARRAY OF ARBITRARY ODD INTEGERS, EACH LESS THAN MAXJNT.
C     IXV2   - ARRAY OF ARBITRARY INTEGERS, EACH LESS THAN MAXJNT.
C
      PARAMETER (MAXORD=120, MAXOP1=MAXORD+1)
      COMMON /IACO2/ KORDEJ, MAXJNT, IXV1(MAXOP1), IXV2(MAXOP1)

      KORDEJ = 120
      MAXJNT = 2**30
      IFIRST = MOD(ISEED, MAXJNT)
      IF (MOD(IFIRST, 2) .EQ. 0) IFIRST = IFIRST - 1
      IXV1(1) = IFIRST
      END