C VARIFO    SOURCE    CHAT      05/01/13    04:01:57     5004           
      SUBROUTINE VARIFO(XX,YY,NB,X1,Y1,KERRE)                                   
C----------------------------------------------------------------               
C  ENTREES:                                                                     
C     XX YY NB X1                                                               
C  SORTIE:                                                                      
C     Y1 KERRE                                                                  
C----------------------------------------------------------------               
      IMPLICIT INTEGER(I-N)
      IMPLICIT REAL*8(A-H,O-Z)                                                  
      DIMENSION XX(*),YY(*)                                                     
      KERRE=0                                                                   
*
*   ON VERIFIE QUE LA LISTE DES ABSCISSES EST ORDONNEE
*
      DO 1 I=2,NB
      IF(XX(I).LE.XX(I-1)) GO TO 2
  1   CONTINUE
      JORDO=1
      GO TO 10
*
  2   CONTINUE
      DO 3 I=2,NB
      IF(XX(I).GE.XX(I-1)) GO TO 4
  3   CONTINUE
      JORDO=-1
      GO TO 10
*
  4   CONTINUE
      KERRE=211                                                                 
      RETURN                                                                    
*
*   ON INTERPOLE LINEAIREMENT
*
  10  CONTINUE
      IF (JORDO.EQ.1) THEN
        DO 11 I=2,NB
        IP=I
        IF(XX(I).GT.X1) GO TO 13
  11    CONTINUE
*
      ELSE
        DO 12 I=2,NB
        IP=I
        IF(XX(I).LT.X1) GO TO 13
  12    CONTINUE
      ENDIF
  13  CONTINUE
      IP1=IP-1
      PENTE=(YY(IP)-YY(IP1))/(XX(IP)-XX(IP1))
      Y1=YY(IP1)+PENTE*(X1-XX(IP1))
      RETURN                                                                    
      END