* Note: 't' means 'triangle'

program lab1_v4
implicit none

integer display_actions, chosen_action
real calc_t_area, calc_t_min_angle, calc_t_min_angle_cos

* t_a1 - adjacent angle, t_a2 - opposite angle, t_a3 - third angle
real t_side, t_a1, t_a2, t_a3
logical is_t_configured *1

common /triangle/ t_side, t_a1, t_a2, t_a3

is_t_configured = .false.

10 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 ' //
. 'isn''t set.'
print *

goto 10
endif

* Perform the chosen action
if (chosen_action .eq. 1) then
call set_t_data()
is_t_configured = .true.
elseif (chosen_action .eq. 2) then
print *, 'Area = ', calc_t_area(t_side, t_a1, t_a2, t_a3)
elseif (chosen_action .eq. 3) then
print *, 'Min angle = ', calc_t_min_angle(t_a1, t_a2, t_a3)
elseif (chosen_action .eq. 4) then
print *, 'Cosine of the min angle = ',
. calc_t_min_angle_cos(t_a1, t_a2, t_a3)
endif

* Display the actions menu again
print *
goto 10

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

20 print *, 'Choose an action (enter action number):'
print *, '1) Set 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 20
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.'
print *

validate_chosen_action = .false.
endif

return
end

* Set triangle data
subroutine set_t_data()

real t_side, t_a1, t_a2, t_a3
logical validate_t_side, validate_t_angles

common /triangle/ t_side, t_a1, t_a2, t_a3

print *, '------- Start triangle configuration -------'

* Set triangle adjacent angle
30 print *, 'Enter the adjacent angle (in degrees):'
read *, t_a1

print *, 'Enter the opposite angle (in degrees):'
read *, t_a2

if (.not.(validate_t_angles(t_a1, t_a2))) then
goto 30
endif

t_a3 = 180 - t_a1 - t_a2

* Set triangle side
31 print *, 'Enter the length of the side (in m):'
read *, t_side

if (.not.(validate_t_side(t_side))) then
goto 31
endif

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

return
end

* Validate triangle angle value. Show error if angle isn't correct
logical function validate_t_angles(t_a1, t_a2)

real t_a1, t_a2

validate_t_angles = .true.

if (t_a1 .le. 0 .or. t_a2 .le.0) then
print *
print *, '!!! Angle can''t be less than or equals to 0.'
print *

validate_t_angles = .false.
endif

if (t_a1 + t_a2 .ge. 180) then
print *
print *, '!!! Sum of angles can''t be greater than' //
. 'or equals to 180.'
print *

validate_t_angles = .false.
endif

return
end

* Validate triangle side value. Show error if side isn't correct
logical function validate_t_side(t_side)

real t_side

validate_t_side = .true.

if (t_side .le. 0) then
print *
print *, '!!! Side can''t be less than or equals to 0.'
print *

validate_t_side = .false.
endif

return
end

* Convert degrees to radians
real function deg_to_rad(degrees)

real degrees, PI

parameter (PI = 3.14)

deg_to_rad = degrees * PI / 180

return
end

* Calculate the area (in meters^2) of a triangle
real function calc_t_area(t_side, t_a1, t_a2, t_a3)

real deg_to_rad
real t_side, t_a1, t_a2, t_a3

calc_t_area = t_side ** 2 * sin(deg_to_rad(t_a3))
. * sin(deg_to_rad(t_a2)) / 2 / sin(deg_to_rad(t_a1))

return
end

* Calculate the minimum angle (in degrees) of a triangle
real function calc_t_min_angle(t_a1, t_a2, t_a3)

real t_a1, t_a2, t_a3

calc_t_min_angle = min(t_a1, t_a2, t_a3)

return
end

* Calculate the cosine of the minimum angle (in degrees)
real function calc_t_min_angle_cos(t_a1, t_a2, t_a3)

real calc_t_min_angle, deg_to_rad
real t_a1, t_a2, t_a3

t_min_angle = calc_t_min_angle(t_a1, t_a2, t_a3)
calc_t_min_angle_cos = cos(deg_to_rad(t_min_angle))

return
end