!***********************************************************************************************************************************
!
!                                                         P U R S U I T
!
!  Program:      PURSUIT
!
!  Programmer:   David G. Simpson (based on HP-25 program by Larry L. Simpson)
!                Laurel, Maryland
!
!  Date:         September 24, 2006
!
!  Language:     Fortran-90
!
!  Version:      1.00a
!
!  Description:  This is a Fortran version of the original game "Pursuit" for the HP-25 calculator by
!                L.L. Simpson, Dec. 1976.
!
!  Files:        Source file:
!
!                   pursuit.f90                   Main program
!
!  Notes:        You are in pursuit of a faster ship that is spotted when 566 m from your ship.  You fire
!                torpedoes at it but it maneuvers to avoid being hit.  When it is 1000 m from you, it is
!                out of range of the torpedoes.  When given an indication of the error in your aim, you
!                correct the angle and fire a torpedo.  If you score a hit (within 5 m of target), the
!                program displays "Hit!".  If the ship is out of range, "Out of range" is displayed.
!
!***********************************************************************************************************************************

      PROGRAM PURSUIT

      IMPLICIT NONE

      DOUBLE PRECISION, PARAMETER :: PI = 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803D0

      REAL, PARAMETER :: X0 = 400.0
      REAL, PARAMETER :: Y0 = X0
      REAL, PARAMETER :: DIST_ERROR = 5.0
      REAL, PARAMETER :: DIR_ERROR = DIST_ERROR * 180.0/PI
      REAL, PARAMETER :: RANGE = 1000.0

      REAL :: RN, R, THETA, X, Y, THETA0, THETA_ERR, CORR

      CHARACTER :: ANS

!-----------------------------------------------------------------------------------------------------------------------------------

!
!     Print introductory message.
!

      WRITE (UNIT=*, FMT='(/A)') ' Welcome to Pursuit.  You are in pursuit of a faster ship that'
      WRITE (UNIT=*, FMT='(A)')  ' is spotted when 566 m from your ship.  You fire torpedoes at'
      WRITE (UNIT=*, FMT='(A)')  ' it but it maneuvers to avoid being hit.  When it is 1000 m'
      WRITE (UNIT=*, FMT='(A)')  ' from you, it is out of range of the torpedoes.  When given an'
      WRITE (UNIT=*, FMT='(A)')  ' indication of the error in your aim, you correct the angle'
      WRITE (UNIT=*, FMT='(A)')  ' and fire a torpedo.  If you score a hit (within 5 m of target),'
      WRITE (UNIT=*, FMT='(A)')  ' the program displays "Hit!".  If the ship is out of range, '
      WRITE (UNIT=*, FMT='(A)')  ' "Out of range" is displayed.'

!
!     Initialize data.
!

      CALL RANDOM_SEED                                                              ! initialize random number seed

   10 X = X0                                                                        ! initialize data
      Y = Y0
      THETA0 = 45.0

      DO                                                                            ! begin main loop

         CALL RANDOM_NUMBER (RN)                                                    ! generate random number between 0 and 1
         X = X + RN*100.0                                                           ! X coordinate of target

         CALL RANDOM_NUMBER (RN)                                                    ! generate random number between 0 and 1
         Y = Y + RN*100.0                                                           ! Y coordinate of target

         R = SQRT(X**2+Y**2)                                                        ! distance to target

         IF (R .GT. RANGE) THEN                                                     ! check if out of range
            WRITE (UNIT=*, FMT='(A)') ' Out of range.'
            EXIT
         END IF

         THETA = ATAN2(Y,X) * 180.0/PI                                              ! direction to target

         THETA_ERR = THETA0 - THETA                                                 ! direction error

         IF (R*ABS(THETA_ERR) .LE. DIR_ERROR) THEN                                  ! check if hit
            WRITE (UNIT=*, FMT='(A)') ' Hit!'
            EXIT
         END IF

         WRITE (UNIT=*, FMT='(/A,F7.2,A)') ' Range:  ', R, ' meters'                ! print range to target
         WRITE (UNIT=*, FMT='(A,F7.2,A)') ' Error:  ', THETA_ERR, ' degrees'        ! print direction error

         WRITE (UNIT=*, FMT='(A)', ADVANCE='NO') ' Enter correction (deg):  '       ! get user input (correction)
         READ (UNIT=*, FMT=*) CORR

         CALL RANDOM_NUMBER (RN)                                                    ! generate random number between 0 and 1
         THETA0 = THETA0 + CORR + RN                                                ! compute torpedo direction

      END DO                                                                        ! end main loop


      WRITE (UNIT=*, FMT='(/A)', ADVANCE='NO') ' Play again?  (Y/N):  '             ! ask if player wants to play again
      READ (UNIT=*, FMT=*) ANS
      IF ((ANS .EQ. 'Y') .OR. (ANS .EQ. 'y')) GO TO 10

      STOP

      END PROGRAM PURSUIT