ratint.f

      SUBROUTINE RATINT(XA,YA,N,X,Y,DY)
      PARAMETER (NMAX=10,TINY=1.E-25)
      DIMENSION XA(N),YA(N),C(NMAX),D(NMAX)
      NS=1
      HH=ABS(X-XA(1))
      DO 11 I=1,N
        H=ABS(X-XA(I))
        IF (H.EQ.0.)THEN
          Y=YA(I)
          DY=0.0
          RETURN
        ELSE IF (H.LT.HH) THEN
          NS=I
          HH=H
        ENDIF
        C(I)=YA(I)
        D(I)=YA(I)+TINY
11    CONTINUE
      Y=YA(NS)
      NS=NS-1
      DO 13 M=1,N-1
        DO 12 I=1,N-M
          W=C(I+1)-D(I)
          H=XA(I+M)-X
          T=(XA(I)-X)*D(I)/H
          DD=T-C(I+1)
          IF(DD.EQ.0.)PAUSE
          DD=W/DD
          D(I)=C(I+1)*DD
          C(I)=T*DD
12      CONTINUE
        IF (2*NS.LT.N-M)THEN
          DY=C(NS+1)
        ELSE
          DY=D(NS)
          NS=NS-1
        ENDIF
        Y=Y+DY
13    CONTINUE
      RETURN
      END