      SUBROUTINE DINTRV (XT, LXT, X, ILO, ILEFT, MFLAG)
C
      INTEGER IHI, ILEFT, ILO, ISTEP, LXT, MFLAG, MIDDLE
      DOUBLE PRECISION X, XT
      DIMENSION XT(*)
C***FIRST EXECUTABLE STATEMENT  DINTRV
      IHI = ILO + 1
      IF (IHI.LT.LXT) GO TO 10
      IF (X.GE.XT(LXT)) GO TO 110
      IF (LXT.LE.1) GO TO 90
      ILO = LXT - 1
      IHI = LXT
C
   10 IF (X.GE.XT(IHI)) GO TO 40
      IF (X.GE.XT(ILO)) GO TO 100
C
C *** NOW X .LT. XT(IHI) . FIND LOWER BOUND
      ISTEP = 1
   20 IHI = ILO
      ILO = IHI - ISTEP
      IF (ILO.LE.1) GO TO 30
      IF (X.GE.XT(ILO)) GO TO 70
      ISTEP = ISTEP*2
      GO TO 20
   30 ILO = 1
      IF (X.LT.XT(1)) GO TO 90
      GO TO 70
C *** NOW X .GE. XT(ILO) . FIND UPPER BOUND
   40 ISTEP = 1
   50 ILO = IHI
      IHI = ILO + ISTEP
      IF (IHI.GE.LXT) GO TO 60
      IF (X.LT.XT(IHI)) GO TO 70
      ISTEP = ISTEP*2
      GO TO 50
   60 IF (X.GE.XT(LXT)) GO TO 110
      IHI = LXT
C
C *** NOW XT(ILO) .LE. X .LT. XT(IHI) . NARROW THE INTERVAL
   70 MIDDLE = (ILO+IHI)/2
      IF (MIDDLE.EQ.ILO) GO TO 100
C     NOTE. IT IS ASSUMED THAT MIDDLE = ILO IN CASE IHI = ILO+1
      IF (X.LT.XT(MIDDLE)) GO TO 80
      ILO = MIDDLE
      GO TO 70
   80 IHI = MIDDLE
      GO TO 70
C *** SET OUTPUT AND RETURN
   90 MFLAG = -1
      ILEFT = 1
      RETURN
  100 MFLAG = 0
      ILEFT = ILO
      RETURN
  110 MFLAG = 1
      ILEFT = LXT
      RETURN
      END
