* t - triangle

      program lab1_v4
      implicit none

      integer display_actions, chosen_action
      real calc_t_area, calc_t_min_angle, calc_t_min_angle_cos

      integer t_type
      real t_side, t_angle
      logical is_t_configured *1

      common /triangle/ t_type, t_side, t_angle

      is_t_configured = .false.

1     chosen_action = display_actions()
      print *

      if (chosen_action .eq. 5) stop

* Block the chosen action if the triangle data is not set, and display the actions menu again
      if (chosen_action .ne. 1 .and. .not.(is_t_configured)) then
         print *, '!!! Can''t calculate because triangle data ' //
     .            'is not specified. Specify it.'
         print *
         goto 1
      endif

* Perform the chosen action
      if (chosen_action .eq. 1) then
         call customize_t()
         is_t_configured = .true.
      elseif (chosen_action .eq. 2) then
         print *, 'Area = ', calc_t_area(t_type, t_side, t_angle)
      elseif (chosen_action .eq. 3) then
         print *, 'Min angle = ', calc_t_min_angle(t_type, t_angle)
      elseif (chosen_action .eq. 4) then
         print *, 'Cosine of the min angle = ',
     .            calc_t_min_angle_cos(t_type, t_angle)
      endif

* Display the actions menu again
      print *
      goto 1

      end


* Displays the action menu so that the user choices the one of them
* returns: number of the chosen menu action
      integer function display_actions()

      logical validate_chosen_action

2     print *, 'Choose an action (enter action number):'
      print *, '1) Configure triangle data;'
      print *, '2) Calculate the area of a triangle;'
      print *, '3) Calculate the minimum angle of a triangle;'
      print *, '4) Calculate the cosine of the minimum angle;'
      print *, '5) End the program.'

      read *, display_actions

      if (.not.(validate_chosen_action(display_actions))) then
         goto 2
      endif

      return
      end

* Validate chosen actions menu item by it number. Show error if action isn't valid
      logical function validate_chosen_action(chosen_action)

      integer chosen_action

      validate_chosen_action = .true.

      if (chosen_action .lt. 1 .or. chosen_action .gt. 5) then
         print *
         print *, 'Chosen action is undefined. Try again.'
         print *

         validate_chosen_action = .false.
      endif

      return
      end

* Configure triangle data
      subroutine customize_t()

      integer t_type
      real t_side, t_angle

      common /triangle/ t_type, t_side, t_angle

      print *, '------- Triangle configuration -------'
      print *, 'Choose the type of triangle (enter type number):'
      print *, '1) Equilateral triangle;'
      print *, '2) Right triangle.'

      read *, t_type

      if (t_type .eq. 1) then
         t_angle = 60

         print *, 'Enter the length of the side (in cm):'
      elseif (t_type .eq. 2) then
         print *, 'Enter the opposite angle (in degrees):'
         read *, t_angle

         print *, 'Enter the length of the leg (in cm):'
      endif

      read *, t_side


      print *, '------- End of triangle configuration -------'

      return
      end

* Validate triangle data. Show error if data isn't valid.
      logical subroutine validate_t_data(t_type, t_side, t_angle)

      integer t_type
      real t_side, t_angle

      if (t_type .eq. 1)

      return
      end

* Calculate the area of a triangle
      real function calc_t_area(t_type, t_side, t_angle)

      integer t_type
      real t_side, t_angle

      if (t_type .eq. 1) then
         calc_t_area = sqrt(3.0) / 4.0 * t_side ** 2
      else
         calc_t_area = 1
      endif

      return
      end

* Calculate the minimum angle of a triangle
      real function calc_t_min_angle(t_type, t_angle)

      integer t_type
      real t_angle

      if (t_type .eq. 1) then
         calc_t_min_angle = 60
      else
         calc_t_min_angle = min(t_angle, 90.0 - t_angle)
      endif

      return
      end

* Calculate the cosine of the minimum angle
      real function calc_t_min_angle_cos(t_type, t_angle)

      real calc_t_min_angle
      integer t_type
      real t_angle, t_min_angle, PI

      parameter PI = 3.141592

      t_min_angle = calc_t_min_angle(t_type, t_angle)
      calc_t_min_angle_cos = cos(t_min_angle * PI / 180.0)

      return
      end