PROGRAM ch1607
IMPLICIT NONE
REAL :: Hi , Hr , Hlow , High , Half , Xl
REAL :: Xh , Xm , D
REAL , PARAMETER :: Tol=1.0E-6
! problem - find hi from expression given 
! in function f 
! F=A*(1.0-0.8*EXP(-0.6*C/A))-B
! HI IS INCIDENT WAVE HEIGHT        (C)
! HR IS REFORMED WAVE HEIGHT        (B)
! D IS WATER DEPTH AT TERRACE EDGE    (A)
  PRINT*,' Give reformed wave height, and water depth'
  READ*,Hr,d
!
! for Hlow- let Hlow=hr 
! for high- let high=Hlow*2.0 
!
! check that signs of function results are different
!
  Hlow = Hr 
  High = Hlow*2.0 
  Xl = F( Hlow, Hr, D)
  Xh = F( High, Hr, D) 
!
  DO WHILE ((XL*XH)  >= 0.0) 
    HIGH = HIGH*2.0
    XH = F(HIGH,HR,D)
  END DO
!
  DO
    HALF=(HLOW+HIGH)*0.5 
    XM=F(HALF,HR,D)
    IF((XL*XM) < 0.0)THEN
      XH=XM
      HIGH=HALF
    ELSE
      XL=XM
      HLOW=HALF
    ENDIF
    IF(ABS(HIGH-HLOW)<= TOL)EXIT
  END DO
  PRINT*,' Incident Wave Height Lies Between'
  PRINT*,Hlow,' and ',High,' metres'
CONTAINS
REAL FUNCTION F(A,B,C)
IMPLICIT NONE
REAL , INTENT (IN) :: A
REAL , INTENT (IN) :: B
REAL , INTENT (IN) :: C
  F=A*(1.0-0.8*EXP(-0.6*C/A))-B
END FUNCTION F
END PROGRAM ch1607